Differential regulation of effector- and central-memory responses to Toxoplasma gondii Infection by IL-12 revealed by tracking of Tgd057-specific CD8+ T cells

Severe malaria enforces short-lived effector cell differentiation but does not prevent effective secondary responses by memory CD8 T cells

Parasitic infections are a major worldwide health burden, yet most studies of CD8 T cell differentiation focus on acute viral and bacterial infections. To understand effector and memory CD8 T cell responses during erythrocytic malaria infection in mice, we utilized transgenic OT-I T cells and compared CD8 T cell responses between infection with OVA-expressing strains of Listeria monocytogenes (Lm) and Plasmodium berghei ANKA (PbA). We find that CD8 T cells expand vigorously during both infections. However, in contrast to Lm infection, PbA infection induces T cells that are heavily biased toward an IL-7Ra-deficient and KLRG1+ short-lived effector cell (SLEC) phenotype at the expense of memory precursor effector cell (MPECs) formation. PbA-induced inflammation, including IFNγ, is partially responsible for this outcome. Following treatment with antimalarial drugs and T cell contraction, PbA-primed memory T cells are rarely found in the blood and peripheral tissues but do maintain a low presence in the spleen and bone marrow. Despite these poor numbers, PbA memory T cells robustly expand upon vaccination or viral infection, control pathogen burden, and form secondary memory pools. Thus, despite PbA enforced SLEC formation and limited memory, effective secondary responses can still proceed.

In a model of parasite-mediated exhaustion, stem-like CD8 T cells differentiate into an unconventional intermediate effector memory subset

CD8 T cell exhaustion has been reported in mice susceptible to Toxoplasma gondii infection. While the differentiation of CD8 exhausted subsets has been extensively reported, most of these studies have been conducted in chronic viral and cancer models. During chronic T. gondii infection, phenotypic and transcriptomic analyses of the polyclonal antigen-specific CD8 T cell response characterize four populations based on KLRG1 and CD62L expression. Pop1 (KLRG1 + CD62L lo ) bears the attributes of a terminal effector subset, and pop2 (KLRG1 - CD62L lo ) is similar to effector memory CD8 T cells. Akin to chronic viral infection and cancer systems, pop3 (KLRG1 - CD62L hi ) exhibits the characteristics of stem-like progenitor CD8 T cells (high Tcf7, Slamf6, and Cxcr5 expression), whereas pop4 (KLRG1 + CD62L hi ) closely resembles a transitory subset (elevated Tbx21, low Tcf1, and Tox expression). During chronic viral infection, the stem-like progenitor CD8 T cells transition into a terminally differentiated exhausted subset via an intermediate population. However, in our system, pop3 generates pop4, which does not convert into a conventional terminally differentiated exhausted subset but instead transitions into effector pop1. Notably, during the chronic phase of the infection, pop1 cannot retain its functionality, irrespective of its origin, which may hamper its ability to control reactivation. Our observations emphasize that the differentiation of exhausted CD8 T cells in non-viral infections, like chronic toxoplasmosis, follows a different pattern than established models and highlights the need to develop new immune strategies better tailored for a broad range of pathogens.

T lymphocyte-dependent IL-10 down-regulates a cytokine storm driven by Toxoplasma gondii GRA24

As a model organism in the study of immunity to infection, Toxoplasma gondii has been instrumental in establishing key principles of host anti-microbial defense and its regulation. Here, we employed an attenuated uracil auxotroph strain of Type I Toxoplasma designated OMP to further untangle the early immune response to this parasitic pathogen. Experiments using αβ T cell-deficient Tcrb-/- mice unexpectedly revealed that an intact αβ T lymphocyte compartment was essential to survive infection with OMP. Subsequent antibody depletion and knockout mouse experiments demonstrated contributions from CD4+ T cells and most predominantly CD8+ T cells in resistance. Using transgenic knockout mice, we found only a partial requirement for IFN-γ and a lack of requirement for Toll-like receptor (TLR) adaptor MyD88 in resistance. In contrast to other studies on Toxoplasma, the ability to survive OMP infection did not require IL-12p40. Surprisingly, T cell-dependent IL-10 was found to be critical for survival, and deficiency of this cytokine triggered an abnormally high systemic inflammatory response. We also found that parasite molecule GRA24, a dense granule protein that triggers TLR-independent IL-12 production, acts as a virulence factor contributing to death of OMP-infected Tcrb-/- and IL-10-/- mice. Furthermore, resistance against OMP was restored in Tcrb-/- mice via monoclonal depletion of IL-12p40, suggesting that GRA24-induced IL-12 underlies the fatal immunopathology observed. Collectively, our studies provide insight into a novel and rapidly arising T lymphocyte-dependent anti-inflammatory response to T. gondii which operates independently of MyD88 and IL-12 and that depends on the function of parasite-dense granule protein GRA24.IMPORTANCEAs a model infectious microbe and an important human pathogen, the apicomplexan Toxoplasma gondii has provided many important insights into innate and adaptive immunity to infection. We show here that a low virulence uracil auxotrophic Toxoplasma strain emerges as a virulent parasite in the absence of an intact T cell compartment. Both CD4+ and CD8+ T lymphocytes are required for optimal protection, in line with previous findings in other models of Toxoplasma infection. Nevertheless, several novel aspects of the response were identified in our study. Protection occurs independently of IL-12 and MyD88 and only partially requires IFN-γ. This is noteworthy particularly because the cytokines IL-12 and IFN-γ have previously been regarded as essential for protective immunity to T. gondii. Instead, we identified the anti-inflammatory effects of T cell-dependent IL-10 as the critical factor enabling host survival. The parasite dense granule protein GRA24, a host-directed mitogen-activated protein kinase activator, was identified as a major virulence factor in T cell-deficient hosts. Collectively, our results provide new and unexpected insights into host resistance to Toxoplasma.

Endogenous IL-27 during toxoplasmosis limits early monocyte responses and their inflammatory activation by pathological T cells

Mice that lack the genes for IL-27, or the IL-27 receptor, and infected with Toxoplasma gondii develop T cell-mediated pathology. Here, studies were performed to determine the impact of endogenous IL-27 on the immune response to T. gondii in wild-type (WT) mice. Analysis of infected mice revealed the early production of IL-27p28 by a subset of Ly6Chi, inflammatory monocytes, and sustained IL-27p28 production at sites of acute and chronic infection. Administration of anti-IL-27p28 prior to infection resulted in an early (day 5) increase in levels of macrophage and granulocyte activation, as well as enhanced effector T cell responses, as measured by both cellularity, cytokine production, and transcriptional profiling. This enhanced acute response led to immune pathology, while blockade during the chronic phase of infection resulted in enhanced T cell responses but no systemic pathology. In the absence of IL-27, the enhanced monocyte responses observed at day 10 were a secondary consequence of activated CD4+ T cells. Thus, in WT mice, IL-27 has distinct suppressive effects that impact innate and adaptive immunity during different phases of this infection.

IMPORTANCE

The molecule IL-27 is critical in limiting the immune response to the parasite Toxoplasma gondii. In the absence of IL-27, a lethal, overactive immune response develops during infection. However, when exactly in the course of infection this molecule is needed was unclear. By selectively inhibiting IL-27 during this parasitic infection, we discovered that IL-27 was only needed during, but not prior to, infection. Additionally, IL-27 is only needed in the active areas in which the parasite is replicating. Finally, our work found that a previously unstudied cell type, monocytes, was regulated by IL-27, which contributes further to our understanding of the regulatory networks established by this molecule.

Growth differentiation factor-15 is an IFN-γ regulated mediator of infection-induced weight loss and the hepatic FGF21 response

Infections are often accompanied by weight loss caused by alterations in host behavior and metabolism, also known as sickness behaviors. Recent studies have revealed that sickness behaviors can either promote or impede survival during infections depending on factors such as the type of infectious pathogen. Nevertheless, we have an incomplete understanding of the underlying mechanisms of sickness behaviors. Furthermore, although the host immune responses to infections have long been known to contribute to the induction of sickness behaviors, recent studies have identified emerging cytokines that are also key regulators of host metabolism during infection and inflammation, such as growth differentiation factor 15 (GDF-15). GDF-15 is a distant member of the TGF-β superfamily that causes weight loss by suppressing appetite and food consumption and causing emesis. These effects require activation of neurons that express the only known GDF-15 receptor, the GFRAL receptor. GDF-15 also functions in the periphery including the induction of ketogenesis and immunoregulation. Nevertheless, the functions and regulation of GDF-15 during live infections is not yet known. Murine infection with avirulent Toxoplasma gondii is an established model to understand infection-induced weight loss. Past studies have determined that acute T. gondii infection causes weight loss due to diminished food consumption and increased energy expenditure through unknown mechanisms. Additionally, our lab previously demonstrated that T. gondii causes upregulation in serum GDF-15 in an IFN-γ-dependent manner during the post-acute phase of the infection. In this study, we interrogated the in-vivo functions and immune regulation of GDF-15 during Toxoplasma gondii infection. First, we found that in wild-type mice, acute T. gondii infection caused a significant weight loss that is preceded by elevation of serum levels of IFN-γ and GDF-15. To determine whether IFN-γ regulates GDF-15, we neutralized IFN-γ on days 5 and 6 and measured GDF-15 on day 7 and found that serum but not tissue levels of GDF-15 decreased after IFN-γ neutralization. Additionally, exogenous IFN-γ was sufficient to elevate serum GDF-15 in the absence of infection. Next, we compared the outcomes of T. gondii infection between WT and Gdf15-/- mice. We observed that the weight trajectories were declining in WT mice while they were increasing in Gdf15-/-mice during the acute phase of the infection. This difference in trajectories extended throughout the chronic infection resulting to an overall weight loss relative to initial weights in WT mice but not Gdf15-/-mice. Then, we determined that GDF-15 is not essential for survival and immunoregulation during T. gondii infection. We also demonstrated that GDF-15 is required for the induction of FGF21, stress-induced cytokine with prominent roles in regulating host metabolism. Finally, we discovered a cytokine cascade IFN-γ-GDF-15-FGF21 that is likely involved in the regulation of host metabolism. Overall, our study provides evidence that IFN-γ contributes to the regulation of host metabolism during infection by inducing GDF-15 and FGF21. GDF-15 orchestrates changes in host metabolism that supports the host immune response in clearing the infection. These physiological alterations induce FGF21, which in turn, orchestrates the adaptive responses to the effects of GDF-15, which can be detrimental when protracted.

Animal Models for Toxoplasma gondii Infection

Toxoplasma gondii is an obligate intracellular protozoan parasite that commonly infects mammals and birds throughout the world. This protocol describes murine models of acute T. gondii infection, toxoplasmic encephalitis and toxoplasma retinochoroiditis. T. gondii infection in severe combined immunodeficient (SCID) mice, deficient in T and B cells, has allowed for the study of T cell-independent mechanisms of defense against intracellular organisms, as described here. The uracil auxotroph strain cps1-1 and temperature-sensitive mutant strains of T. gondii induce protection against challenge with virulent strains of the parasite. They have allowed studies of immunization and adoptive-transfer experiments. A protocol is provided for infection with these mutant strains. The EGS strain of T. gondii has the unique feature of spontaneously forming tissue cysts in cell culture. Dual fluorescent reporter stains of this strain have allowed the study of tachyzoite to bradyzoite transitions in vitro and in vivo. A protocol for in vitro and in vivo growth of this strain and tissue cyst isolation is provided. Genetic manipulation of T. gondii and mice has led to the development of parasites that express fluorescent proteins as well as mice with fluorescently labeled leukocytes. This together with the use of T. gondii that express model antigens and transgenic mice that express the appropriate T cell receptor have facilitated the in vivo study of parasite host-interaction. In addition, parasites that express bioluminescent markers have made it possible to study the dynamics of infection in real time using bioluminescence imaging. Support protocols present methodology for evaluation of progression of infection and immune response to the parasite that includes these newer methodologies. In addition, support protocols address the maintenance of T. gondii tissue cysts and tachyzoites, as well as preparation of T. gondii lysate antigens. © 2023 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Induction of acute T. gondii infection in mice Basic Protocol 2: Model of toxoplasmic encephalitis and toxoplasma retinochoroiditis in chronically infected mice Basic Protocol 3: Assessment of T. gondii invasion into neural tissue Basic Protocol 4: T. gondii infection in scid/scid (SCID) mice Basic Protocol 5: Infection with the uracil auxotroph strain CPS1-1 or the temperature-sensitive TS-4 strain of T. gondii Basic Protocol 6: In vivo and in vitro maintenance of the EGS strain of T. gondii Support Protocol 1: Assessment of progression of infection and immune response to T. gondii Support Protocol 2: Maintenance of a bank of T. gondii cysts of the ME49 strain Support Protocol 3: Maintenance of T. gondii tachyzoites using human foreskin fibroblasts Support Protocol 4: Maintenance of T. gondii tachyzoites in mice Support Protocol 5: Preparation of T. gondii lysate antigens Support Protocol 6: Isolation of T. gondii tissue cysts from brain.

Variation in CD8 T cell IFNγ differentiation to strains of Toxoplasma gondii is characterized by small effect QTLs with contribution from ROP16

Introduction

Toxoplasma gondii induces a strong CD8 T cell response characterized by the secretion of IFNγ that promotes host survival during infection. The initiation of CD8 T cell IFNγ responses in vitro differs widely between clonal lineage strains of T. gondii, in which type I strains are low inducers, while types II and III strains are high inducers. We hypothesized this phenotype is due to a polymorphic "Regulator Of CD8 T cell Response" (ROCTR).

Methods

Therefore, we screened F1 progeny from genetic crosses between the clonal lineage strains to identify ROCTR. Naïve antigen-specific CD8 T cells (T57) isolated from transnuclear mice, which are specific for the endogenous and vacuolar TGD057 antigen, were measured for their ability to become activated, transcribe Ifng and produce IFNγ in response to T. gondii infected macrophages.

Results

Genetic mapping returned four non-interacting quantitative trait loci (QTL) with small effect on T. gondii chromosomes (chr) VIIb-VIII, X and XII. These loci encompass multiple gene candidates highlighted by ROP16 (chrVIIb-VIII), GRA35 (chrX), TgNSM (chrX), and a pair of uncharacterized NTPases (chrXII), whose locus we report to be significantly truncated in the type I RH background. Although none of the chromosome X and XII candidates bore evidence for regulating CD8 T cell IFNγ responses, type I variants of ROP16 lowered Ifng transcription early after T cell activation. During our search for ROCTR, we also noted the parasitophorous vacuole membrane (PVM) targeting factor for dense granules (GRAs), GRA43, repressed the response suggesting PVM-associated GRAs are important for CD8 T cell activation. Furthermore, RIPK3 expression in macrophages was an absolute requirement for CD8 T cell IFNγ differentiation implicating the necroptosis pathway in T cell immunity to T. gondii.

Discussion

Collectively, our data suggest that while CD8 T cell IFNγ production to T. gondii strains vary dramatically, it is not controlled by a single polymorphism with strong effect. However, early in the differentiation process, polymorphisms in ROP16 can regulate commitment of responding CD8 T cells to IFNγ production which may have bearing on immunity to T. gondii.

IL-18BP mediates the balance between protective and pathological immune responses to Toxoplasma gondii

Interleukin-18 (IL-18) promotes natural killer (NK) and T cell production of interferon (IFN)-γ, a key factor in resistance to Toxoplasma gondii, but previous work has shown a limited role for endogenous IL-18 in control of this parasite. Although infection with T. gondii results in release of IL-18, the production of IFN-γ induces high levels of the IL-18 binding protein (IL-18BP). Antagonism of IL-18BP with a "decoy-to-the-decoy" (D2D) IL-18 construct that does not signal but rather binds IL-18BP results in enhanced innate lymphoid cell (ILC) and T cell responses and improved parasite control. In addition, the use of IL-18 resistant to IL-18BP ("decoy-resistant" IL-18 [DR-18]) is more effective than exogenous IL-18 at promoting innate resistance to infection. DR-18 enhances CD4+ T cell production of IFN-γ but results in CD4+ T cell-mediated pathology. Thus, endogenous IL-18BP restrains aberrant immune pathology, and this study highlights strategies that can be used to tune this regulatory pathway for optimal anti-pathogen responses.

Early T-bet promotes LFA1 upregulation required for CD8+ effector and memory T cell development

The T-box transcription factor T-bet is regarded as a "master regulator" of CD4+ Th1 differentiation and IFN-γ production. However, in multiple models of infection, T-bet appears less critical for CD8+ T cell expansion and effector function. Here, we show that following vaccination with a replication-deficient strain of Toxoplasma gondii, CD8+ T cell expression of T-bet is required for optimal expansion of parasite-specific effector CD8+ T cells. Analysis of the early events associated with T cell activation reveals that the α chain of LFA1, CD11a, is a target of T-bet, and T-bet is necessary for CD8+ T cell upregulation of this integrin, which influences the initial priming of CD8+ effector T cells. We propose that the early expression of T-bet represents a T cell-intrinsic factor that optimizes T-DC interactions necessary to generate effector responses.

A Lower Dose of Infection Generates a Better Long-Term Immune Response against Toxoplasma gondii

Toxoplasma gondii, an obligate intracellular pathogen, induces a strong immune response in the infected host. In the encephalitis model of infection, long-term protective immunity is mediated by CD8 T cells, with the CD4 T cell population providing important help. Most of the immune studies have used a 10- to 20-cyst dose of T. gondii, which leads to T cell dysfunctionality during the late phase of chronic infection and increases the chances of reactivation. In the current study, we compared the immune response of mice orally infected with either 2 or 10 cysts of T. gondii. During the acute phase, we demonstrate that the lower dose of infection generates a reduced number of CD4 and CD8 T cells, but the frequency of functional CD4 or CD8 T cells is similar in animals infected with two different doses. However, Ag-experienced T cells (both CD4 and CD8) are better maintained in lower dose-infected mice at 8 wk postinfection, with an increase number functional cells that exhibit lower multiple inhibitory receptor expression. In addition to better long-term T cell immunity, animals infected with a lower dose display reduced inflammation manifested by lesser Ag-specific T cell and cytokine responses during the very early stage of the acute infection. Our studies suggest a previously unappreciated role of dose-dependent early programming/imprinting of the long-term CD4/CD8 T cell response during T. gondii infection. These observations point to the need for an in-depth analysis of how early events shape long-term immunity against this pathogen.

The Impact of Helminth Coinfection on Innate and Adaptive Immune Resistance and Disease Tolerance during Toxoplasmosis

More than 2 billion people worldwide are infected with helminths. Thus, it is possible for individuals to experience concomitant infection with helminth and intracellular microbes. Although the helminth-induced type 2 response can suppress type 1 proinflammatory responses required for the immunity against intracellular pathogens in the context of a coinfection, conflicting evidence suggest that helminth infection can enhance antimicrobial immunity. Using a coinfection model with the intestinal helminth Heligmosomoides polygyrus followed by infection with Toxoplasma gondii in Mus Musculus, we showed that the complex and dynamic effect of helminth infection is highly suppressive during the innate phase (days 0-3) of T. gondii infection and less stringent during the acute phase (d10). Helminth coinfection had a strong suppressive effect on the neutrophil, monocytic, and early IFN-γ/IL-12 responses. The IFN-γ response was later restored by compensatory production from T cells despite decreased effector differentiation of T. gondii-specific CD8 T cells. In accordance with the attenuated IFN-γ response, parasite loads were elevated during the acute phase (d10) of T. gondii infection but were transiently controlled by the compensatory T cell response. Unexpectedly, 40% of helminth-coinfected mice exhibited a sustained weight loss phenotype during the postacute phase (d14-18) that was not associated with T. gondii outgrowth, indicating that coinfection led to decreased disease tolerance during T. gondii infection. Our work uncovers the dynamic nature of the helminth immunomodulatory effects on concomitant infections or immune responses and unveils a loss of disease tolerance phenotype triggered by coinfection with intestinal helminth.

Engineered human cytokine/antibody fusion proteins expand regulatory T cells and confer autoimmune disease protection

Low-dose human interleukin-2 (hIL-2) treatment is used clinically to treat autoimmune disorders due to the cytokine's preferential expansion of immunosuppressive regulatory T cells (Tregs). However, off-target immune cell activation and short serum half-life limit the clinical potential of IL-2 treatment. Recent work showed that complexes comprising hIL-2 and the anti-hIL-2 antibody F5111 overcome these limitations by preferentially stimulating Tregs over immune effector cells. Although promising, therapeutic translation of this approach is complicated by the need to optimize dosing ratios and by the instability of the cytokine/antibody complex. We leverage structural insights to engineer a single-chain hIL-2/F5111 antibody fusion protein, termed F5111 immunocytokine (IC), which potently and selectively activates and expands Tregs. F5111 IC confers protection in mouse models of colitis and checkpoint inhibitor-induced diabetes mellitus. These results provide a roadmap for IC design and establish a Treg-biased immunotherapy that could be clinically translated for autoimmune disease treatment.

Synergy between Toxoplasma gondii type I ΔGRA17 immunotherapy and PD-L1 checkpoint inhibition triggers the regression of targeted and distal tumors

Background

In this study, we hypothesize that the ability of the protozoan Toxoplasma gondii to modulate immune response within the tumor might improve the therapeutic effect of immune checkpoint blockade. We examined the synergetic therapeutic activity of attenuated T. gondii RH ΔGRA17 strain and programmed death ligand-1 (PD-L1) treatment on both targeted and distal tumors in mice.

Methods

The effects of administration of T. gondii RH ΔGRA17 strain on the tumor volume and survival rate of mice bearing flank B16-F10, MC38, or LLC tumors were studied. We characterized the effects of ΔGRA17 on tumor biomarkers’ expression, PD-L1 expression, immune cells infiltrating the tumors, and expression of immune-related genes by using immunohistochemistry, immunofluorescence, flow cytometry, NanoString platform, and real-time quantitative PCR, respectively. The role of immune cells in the efficacy of ΔGRA17 plus PD-L1 blockade therapy was determined via depletion of immune cell subtypes.

Results

Treatment with T. gondii ΔGRA17 tachyzoites and anti-PD-L1 therapy significantly extended the survival of mice and suppressed tumor growth in preclinical mouse models of melanoma, Lewis lung carcinoma, and colon adenocarcinoma. Attenuation of the tumor growth was detected in the injected and distant tumors, which was associated with upregulation of innate and adaptive immune pathways. Complete regression of tumors was underpinned by late interferon-gamma-producing CD8⁺ cytotoxic T cells.

Conclusion

The results from these models indicate that intratumoral injection of ΔGRA17 induced a systemic effect, improved mouse immune response, and sensitized immunologically ‘cold’ tumors and rendered them sensitive to immune checkpoint blockade therapy.

The Immunoproteasome Subunits LMP2, LMP7 and MECL-1 Are Crucial Along the Induction of Cerebral Toxoplasmosis

Cell survival and function critically relies on the fine-tuned balance of protein synthesis and degradation. In the steady state, the standard proteasome is sufficient to maintain this proteostasis. However, upon inflammation, the sharp increase in protein production requires additional mechanisms to limit protein-associated cellular stress. Under inflammatory conditions and the release of interferons, the immunoproteasome (IP) is induced to support protein processing and recycling. In antigen-presenting cells constitutively expressing IPs, inflammation-related mechanisms contribute to the formation of MHC class I/II-peptide complexes, which are required for the induction of T cell responses. The control of Toxoplasma gondii infection relies on Interferon-γ (IFNγ)-related T cell responses. Whether and how the IP affects the course of anti-parasitic T cell responses along the infection as well as inflammation of the central nervous system is still unknown. To answer this question we used triple knockout (TKO) mice lacking the 3 catalytic subunits of the immunoproteasome (β1i/LMP2, β2i/MECL-1 and β5i/LMP7). Here we show that the numbers of dendritic cells, monocytes and CD8⁺ T cells were reduced in Toxoplasma gondii-infected TKO mice. Furthermore, impaired IFNγ, TNF and iNOS production was accompanied by dysregulated chemokine expression and altered immune cell recruitment to the brain. T cell differentiation was altered, apoptosis rates of microglia and monocytes were elevated and STAT3 downstream signaling was diminished. Consequently, anti-parasitic immune responses were impaired in TKO mice leading to elevated T. gondii burden and prolonged neuroinflammation. In summary we provide evidence for a critical role of the IP subunits β1i/LMP2, β2i/MECL-1 and β5i/LMP7 for the control of cerebral Toxoplasma gondii infection and subsequent neuroinflammation.

Naïve CD8 T cell IFNγ responses to a vacuolar antigen are regulated by an inflammasome-independent NLRP3 pathway and Toxoplasma gondii ROP5

Host resistance to Toxoplasma gondii relies on CD8 T cell IFNγ responses, which if modulated by the host or parasite could influence chronic infection and parasite transmission between hosts. Since host-parasite interactions that govern this response are not fully elucidated, we investigated requirements for eliciting naïve CD8 T cell IFNγ responses to a vacuolar resident antigen of T. gondii, TGD057. Naïve TGD057 antigen-specific CD8 T cells (T57) were isolated from transnuclear mice and responded to parasite-infected bone marrow-derived macrophages (BMDMs) in an antigen-dependent manner, first by producing IL-2 and then IFNγ. T57 IFNγ responses to TGD057 were independent of the parasite’s protein export machinery ASP5 and MYR1. Instead, host immunity pathways downstream of the regulatory Immunity-Related GTPases (IRG), including partial dependence on Guanylate-Binding Proteins, are required. Multiple T. gondii ROP5 isoforms and allele types, including ‘avirulent’ ROP5A from clade A and D parasite strains, were able to suppress CD8 T cell IFNγ responses to parasite-infected BMDMs. Phenotypic variance between clades B, C, D, F, and A strains suggest T57 IFNγ differentiation occurs independently of parasite virulence or any known IRG-ROP5 interaction. Consistent with this, removal of ROP5 is not enough to elicit maximal CD8 T cell IFNγ production to parasite-infected cells. Instead, macrophage expression of the pathogen sensors, NLRP3 and to a large extent NLRP1, were absolute requirements. Other members of the conventional inflammasome cascade are only partially required, as revealed by decreased but not abrogated T57 IFNγ responses to parasite-infected ASC, caspase-1/11, and gasdermin D deficient cells. Moreover, IFNγ production was only partially reduced in the absence of IL-12, IL-18 or IL-1R signaling. In summary, T. gondii effectors and host machinery that modulate parasitophorous vacuolar membranes, as well as NLR-dependent but inflammasome-independent pathways, determine the full commitment of CD8 T cells IFNγ responses to a vacuolar antigen.

Parasites are excellent “students” of our immune system as they can deflect, antagonize and confuse the immune response making it difficult to vaccinate against these pathogens. In this report, we analyzed how a widespread parasite of mammals, Toxoplasma gondii, manipulates an immune cell needed for immunity to many intracellular pathogens, the CD8 T cell. Host pathways that govern CD8 T cell production of the immune protective cytokine, IFNγ, were also explored. We hypothesized the secreted T. gondii virulence factor, ROP5, work to inhibit the MHC 1 antigen presentation pathway therefore making it difficult for CD8 T cells to see T. gondii antigens sequestered inside a parasitophorous vacuole. However, manipulation through T. gondii ROP5 does not fully explain how CD8 T cells commit to making IFNγ in response to infection. Importantly, CD8 T cell IFNγ responses to T. gondii require the pathogen sensor NLRP3 to be expressed in the infected cell. Other proteins associated with NLRP3 activation, including members of the conventional inflammasome activation cascade pathway, are only partially involved. Our results identify a novel pathway by which NLRP3 regulates T cell function and underscore the need for NLRP3-activating adjuvants in vaccines aimed at inducing CD8 T cell IFNγ responses to parasites.

Global status of Toxoplasma gondii infection and associated risk factors in people living with HIV

OBJECTIVE

Toxoplasma infection remains as the most common cause of focal brain lesions among people living with HIV (PLHIV) despite the decline in opportunistic infections with the introduction of antiretroviral treatment. This study was conducted to provide a summary of evidence about the seroprevalence of Toxoplasma gondii and prevalence of active T. gondii infection and associated risk factors among PLHIV.

DESIGN

Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines were followed. Scopus, PubMed, Science Direct and EMBASE were searched from 1997 to July 2018. All peer-reviewed original research articles describing T. gondii infection among PLHIV with different diagnostic methods were included.

METHODS

Incoherence and heterogeneity between studies were quantified by I index and Cochran's Q test. Publication and population bias were assessed with funnel plots and Egger's regression asymmetry test. All statistical analyses were performed using StatsDirect.

RESULTS

In total, 111 studies from 37 countries assessing 66 139 blood samples were included in this study. The pooled prevalence of T. gondii infection among PLHIV was 3.24% by IgM and 26.22% by molecular methods using the random-effects model. Pooled seroprevalence of T. gondii by IgG was 44.22%. There was a relationship between Toxoplasma prevalence and sex, raw meat consumption, contact with cat and knowledge about toxoplasmosis.

CONCLUSION

High Toxoplasma seroprevalence among PLHIV observed in this study emphasizes the need for implementing screening and prophylaxis tailored to the local context. Owing to the serious and significant clinical manifestations of the parasite in case of reactivation, early identification of seropositivity for initiating prophylaxis among those with a CD4 cell count of less than 200 cells/ml is recommended.

The IL-12- and IL-23-Dependent NK Cell Response Is Essential for Protective Immunity against Secondary Toxoplasma gondii Infection

NK cells can develop cell-intrinsic memory-like characteristics. Whether they develop these characteristics during Toxoplasma gondii infection is unknown. We addressed this question and dissected the mechanisms involved in secondary NK cell responses using a vaccine-challenge mouse model of T. gondii infection. NK cells were required for control of and survival after secondary T. gondii infection. NK cells increased in number at the reinfection site and produced IFN-γ. To test if these T. gondii experienced NK cells were intrinsically different from naive NK cells, we performed NK cell adoptive transfer into RAG2/cγ-chain^-/- mice, NK cell fate mapping, and RAG1^-/- mice vaccine-challenge experiments. Although NK cells contributed to immunity after reinfection, they did not develop cell-intrinsic memory-like characteristics after T. gondii vaccination. The mechanisms required for generating these secondary NK cell responses were investigated. Secondary NK cell responses were CD4⁺ or CD8⁺ T cell independent. Although IL-12 alone is required for NK cell IFN-γ production during primary T. gondii infection, in the absence of IL-12 using IL-12p35^-/- mice or anti-IL-12p70, secondary NK cell responses were only partially reduced after reinfection. IL-23 depletion with anti-IL-23p19 in vivo also significantly reduced the secondary NK cell response. IL-12 and IL-23 blockade with anti-IL-12p40 treatment completely eliminated secondary NK cell responses. Importantly, blockade of IL-12, IL-23, or both significantly reduced control of parasite reinfection and increased parasite burden. Our results define a previously unknown protective role for NK cells during secondary T. gondii infection that is dependent on IL-12 and IL-23.

Rhoptry and Dense Granule Secreted Effectors Regulate CD8+ T Cell Recognition of Toxoplasma gondii Infected Host Cells

Toxoplasma gondii secretes rhoptry (ROP) and dense granule (GRA) effector proteins to evade host immune clearance mediated by interferon gamma (IFN-γ), immunity-related GTPase (IRG) effectors, and CD8⁺ T cells. Here, we investigated the role of parasite-secreted effectors in regulating host access to parasitophorous vacuole (PV) localized parasite antigens and their presentation to CD8⁺ T cells by the major histocompatibility class I (MHC-I) pathway. Antigen presentation of PV localized parasite antigens by MHC-I was significantly increased in macrophages and/or dendritic cells infected with mutant parasites that lacked expression of secreted GRA (GRA2, GRA3, GRA4, GRA5, GRA7, GRA12) or ROP (ROP5, ROP18) effectors. The ability of various secreted GRA or ROP effectors to suppress antigen presentation by MHC-I was dependent on cell type, expression of IFN-γ, or host IRG effectors. The suppression of antigen presentation by ROP5, ROP18, and GRA7 correlated with a role for these molecules in preventing PV disruption by IFN-γ-activated host IRG effectors. However, GRA2 mediated suppression of antigen presentation was not correlated with PV disruption. In addition, the GRA2 antigen presentation phenotypes were strictly co-dependent on the expression of the GRA6 protein. These results show that MHC-I antigen presentation of PV localized parasite antigens was controlled by mechanisms that were dependent or independent of IRG effector mediated PV disruption. Our findings suggest that the GRA6 protein underpins an important mechanism that enhances CD8⁺ T cell recognition of parasite-infected cells with damaged or ruptured PV membranes. However, in intact PVs, parasite secreted effector proteins that associate with the PV membrane or the intravacuolar network membranes play important roles to actively suppress antigen presentation by MHC-I to reduce CD8⁺ T cell recognition and clearance of Toxoplasma gondii infected host cells.

Toxoplasma gondii: CD8 T Cells Cry for CD4 Help

Toxoplasma gondii, an apicomplexan parasite, is a pathogenic protozoan that can infect the central nervous system. In pregnant women, infection can result in congenital problems of the fetus, while in immunocompromised individual it can lead to severe neurological consequences. Although CD8 T cells play an important effector role in controlling the chronic infection, their maintenance is dependent on the critical help provided by CD4 T cells. In a recent study, we demonstrated that reactivation of the infection in chronically infected host is a consequence of CD8 T dysfunction caused by CD4 T cell exhaustion. Furthermore, treatment of chronically infected host with antigen-specific non-exhausted CD4 T cells can restore CD8 T cell functionality and prevent reactivation of the latent infection. The exhaustion status of CD4 T cells is mediated by the increased expression of the transcription factor BLIMP-1, and deletion of this molecule led to the restoration of CD4 T cell function, reversal of CD8 exhaustion and prevention of reactivation of the latent infection. In a recent study from our laboratory, we also observed an increased expression of miR146a levels by CD4 T cells from the chronically infected animals. Recent reports have demonstrated that microRNAs (especially miR146a) has a strong impact on the immune system of T. gondii infected host. Whether these molecules have any role in the BLIMP-1 up-regulation and dysfunctionality of these cells needs to be investigated.

Secretory Microneme Proteins Induce T-Cell Recall Responses in Mice Chronically Infected with Toxoplasma gondii

Current diagnosis of toxoplasmosis relies almost exclusively on antibody detection, and while detection of IgG provides a useful estimate of prior infection, it does not alone indicate immune status. In contrast, detection of IFN-γ responses to T. gondii antigens has been used to monitor immune responsiveness in HIV-infected patients, thus providing valuable predictions about the potential for disease reactivation. However, specific T. gondii antigens that can be used in assays to detect cellular immunity remain largely undefined. In this study, we examined the diagnostic potential of microneme antigens of T. gondii using IFN-γ detection assays. Our findings demonstrate that MIC antigens (MIC1, MIC3, MIC4, and MIC6) elicit IFN-γ responses from memory T cells in chronically infected mice. Monitoring IFN-γ production by T cells stimulated with MIC antigens provided high sensitivity and specificity for detection of T. gondii infection in mice. Taken together, these studies suggest that microneme antigens might be useful as an adjunct to serological testing to monitor immune status during infection.

Microneme (MIC) proteins play important roles in the recognition, adhesion, and invasion of host cells by Toxoplasma gondii. Previous studies have shown that MIC proteins are highly immunogenic in the mouse and recognized by human serum antibodies. Here we report that T. gondii antigens MIC1, MIC3, MIC4, and MIC6 were capable of inducing memory responses leading to production of gamma interferon (IFN-γ) by T cells from T. gondii-infected mice. Production of IFN-γ was demonstrated using enzyme-linked immunosorbent spot (ELISPOT) assay and also intracellular cytokine staining. All four MIC antigens displayed very high sensitivity (100%) and specificity (86 to 100%) for detecting chronic infection. Interestingly, IFN-γ was produced by both CD4⁺ and CD8⁺ T cells in BALB/c mice but primarily by CD4⁺ T cells in C57BL/6 mice. Phenotypic characterization of IFN-γ-producing CD4⁺ and CD8⁺ T cells in BALB/c mice and CD4⁺ T cells in C57BL/6 mice revealed effector memory T cells (CD44^hi CD62L^lo) as the predominant cells that contributed to IFN-γ production in response to MIC antigens. Effector memory responses were seen in mice of different major histocompatibility complex class II (MHC-II) haplotypes, suggesting that MIC antigens contain epitopes that are broadly recognized.

IMPORTANCE Current diagnosis of toxoplasmosis relies almost exclusively on antibody detection, and while detection of IgG provides a useful estimate of prior infection, it does not alone indicate immune status. In contrast, detection of IFN-γ responses to T. gondii antigens has been used to monitor immune responsiveness in HIV-infected patients, thus providing valuable predictions about the potential for disease reactivation. However, specific T. gondii antigens that can be used in assays to detect cellular immunity remain largely undefined. In this study, we examined the diagnostic potential of microneme antigens of T. gondii using IFN-γ detection assays. Our findings demonstrate that MIC antigens (MIC1, MIC3, MIC4, and MIC6) elicit IFN-γ responses from memory T cells in chronically infected mice. Monitoring IFN-γ production by T cells stimulated with MIC antigens provided high sensitivity and specificity for detection of T. gondii infection in mice. Taken together, these studies suggest that microneme antigens might be useful as an adjunct to serological testing to monitor immune status during infection.

Toxoplasma: Immunity and Pathogenesis

Toxoplasma gondii infection induces a strong immunity in the host. Although the response is manifested by innate response during early infection, adaptive immunity is critical for long-term protection. Amongst the adaptive immune response CD4 T cells play an important helper role for CD8 T cells which are the primary effector cells responsible for controlling the infection. Notwithstanding the induction of robust CD8 T immunity during acute infection, the parasite is not eradicated. One of the reasons for this is the functional exhaustion of CD8 T cells during latent infection. Recent studies from our laboratory have reported that primary cause of CD8 T cell exhaustion is compromised CD4 T cell help during latent toxoplasmosis. CD8 T cell dysfunctionality is preceded by CD4 exhaustion and effector immunity is severely compromised.

PD-L1, TIM-3, and CTLA-4 Blockade Fails To Promote Resistance to Secondary Infection with Virulent Strains of Toxoplasma gondii

T cell exhaustion is a state of hyporesponsiveness that develops during many chronic infections and cancer. Neutralization of inhibitory receptors, or "checkpoint blockade," can reverse T cell exhaustion and lead to beneficial prognoses in experimental and clinical settings. Whether checkpoint blockade can resolve lethal acute infections is less understood but may be beneficial in vaccination protocols that fail to elicit sterilizing immunity. Since a fully protective vaccine for any human parasite has yet to be developed, we explored the efficacy of checkpoint inhibitors in a mouse model of Toxoplasma gondii reinfection. Mice chronically infected with an avirulent type III strain survive reinfection with the type I RH strain but not the MAS, GUY-DOS, and GT1 parasite strains. We report here that mouse susceptibility to secondary infection correlates with the initial parasite burden and that protection against the RH strain is dependent on CD8 but not CD4 T cells in this model. When given a lethal secondary infection, CD8 and CD4 T cells upregulate several coinhibitory receptors, including PD-1, TIM-3, 4-1bb, and CTLA-4. Moreover, the gamma interferon (IFN-γ) response of CD8 but not CD4 T cells is significantly reduced during secondary infection with virulent strains, suggesting that checkpoint blockade may reduce disease severity. However, single and combination therapies targeting TIM-3, CTLA-4, and/or PD-L1 failed to reverse susceptibility to secondary infection. These results suggest that additional host responses, which are refractory to checkpoint blockade, are likely required for immunity to this pathogen.

HIF-1α hampers dendritic cell function and Th1 generation during chronic visceral leishmaniasis

Inflammation, although responsible for controlling infection, is often associated with the pathogenesis of chronic diseases. Leishmania donovani, the causative agent of visceral leishmaniasis, induces a strong inflammatory response that leads to splenomegaly and ultimately immune suppression. Inflamed tissues are typically characterized by low levels of oxygen, a microenvironment that triggers the hypoxia-inducible transcription factor 1α (HIF-1α). Although HIF-1α plays an integral role in dendritic cell function, its involvement in the generation of protective Th1 responses against Leishmania has not yet been studied. Here we demonstrate that HIF-1α inhibits IL-12 production in dendritic cells, limiting therefore Th1 cell development. Indeed, depletion of HIF-1α in CD11c⁺ cells resulted in higher and sustained expression of IL-12 and complete abrogation of IL-10. Moreover, CD11c-specific HIF-1α-deficient mice showed higher frequencies of IFN-γ-producing CD4 T cells in the spleen and bone marrow and, consequently, a significantly reduced parasite burden in both organs. Taken together, our results suggest that HIF-1α expression in dendritic cells largely contributes to the establishment of persistent Leishmania infection and may therefore represent a possible therapeutic target.

Immunocompetent host develops mild intestinal inflammation in acute infection with Toxoplasma gondii

Toxoplasma gondii (T. gondii) is the causative agent of toxoplasmosis, common zoonosis among vertebrates and high incidence worldwide. During the infection, the parasite needs to transpose the intestinal barrier to spread throughout the body, which may be a trigger for an inflammatory reaction. This work evaluated the inflammatory alterations of early T. gondii infection in peripheral blood cells, in the mesenteric microcirculation, and small intestinal tissue by measurement of MPO (myeloperoxidase) activity and NO (nitric oxide) level in rats. Animals were randomly assigned into control group (CG) that received saline orally and groups infected with 5,000 oocysts for 6 (G6), 12 (G12), 24 (G24), 48 (G48) and 72 hours (G72). Blood samples were collected for total and differential leukocyte count. Intravital microscopy was performed in the mesentery to evaluate rolling and adhesion of leukocytes. After euthanasia, 0.5cm of the duodenum, jejunum and ileum were collected for the determination of MPO activity, NO level and PCR to identify the parasite DNA and also the mesentery were collected to perform immunohistochemistry on frozen sections to quantify adhesion molecules ICAM-1, PECAM-1 and P-Selectin. The parasite DNA was identified in all infected groups and there was an increase in leukocytes in the peripheral blood and in expression of ICAM-1 and PECAM-1 in G6 and G12, however, the expression of P-selectin was reduced in G12. Leukocytes are in rolling process during the first 12 hours and they are adhered at 24 hours post-infection. The activity of MPO increased in the duodenum at 12 hours, and NO increased in the jejunum in G72 and ileum in G24, G48 and G72. Our study demonstrated that T. gondii initiates the infection precociously (at 6 hours) leading to a systemic activation of innate immune response resulting in mild inflammation in a less susceptible experimental model.

MHC I presentation of Toxoplasma gondii immunodominant antigen does not require Sec22b and is regulated by antigen orientation at the vacuole membrane

The intracellular Toxoplasma gondii parasite replicates within a parasitophorous vacuole (PV). T. gondii secretes proteins that remain soluble in the PV space, are inserted into PV membranes or are exported beyond the PV boundary. In addition to supporting T. gondii growth, these proteins can be processed and presented by MHC I for CD8⁺ T-cell recognition. Yet it is unclear whether membrane binding influences the processing pathways employed and if topology of membrane antigens impacts their MHC I presentation. Here we report that the MHC I pathways of soluble and membrane-bound antigens differ in their requirement for host ER recruitment. In contrast to the soluble SAG1-OVA model antigen, we find that presentation of the membrane-bound GRA6 is independent from the SNARE Sec22b, a key molecule for transfer of host endoplasmic reticulum components onto the PV. Using parasites modified to secrete a transmembrane antigen with opposite orientations, we further show that MHC I presentation is highly favored when the C-terminal epitope is exposed to the host cell cytosol, which corresponds to GRA6 natural orientation. Our data suggest that the biochemical properties of antigens released by intracellular pathogens critically guide their processing pathway and are valuable parameters to consider for vaccination strategies.

CD103+ CD8 T Cells in the Toxoplasma-Infected Brain Exhibit a Tissue-Resident Memory Transcriptional Profile

During chronic infection, memory T cells acquire a unique phenotype and become dependent on different survival signals than those needed for memory T cells generated during an acute infection. The distinction between the role of effector and memory T cells in an environment of persistent antigen remains unclear. Here, in the context of chronic Toxoplasma gondii infection, we demonstrate that a population of CD8 T cells exhibiting a tissue-resident memory (T_RM) phenotype accumulates within the brain. We show that this population is distributed throughout the brain in both parenchymal and extraparenchymal spaces. Furthermore, this population is transcriptionally distinct and exhibits a transcriptional signature consistent with the T_RM observed in acute viral infections. Finally, we establish that the CD103⁺ T_RM population has an intrinsic capacity to produce both IFN-γ and TNF-α, cytokines critical for parasite control within the central nervous system (CNS). The contribution of this population to pro-inflammatory cytokine production suggests an important role for T_RM in protective and ongoing immune responses in the infected CNS.

Accession number: GSE95105

Cutting Edge: Helminth Coinfection Blocks Effector Differentiation of CD8 T Cells through Alternate Host Th2- and IL-10-Mediated Responses

Concurrent helminth infection potently inhibits T cell immunity; however, whether helminthes prevent T cell priming or skew clonal recruitment and effector differentiation is not known. Using coinfection with two natural mouse pathogens, Heligmosomoides polygyrus and Toxoplasma gondii, to investigate the negative impact of helminthes on the CD8 T cell response, we demonstrate helminth-induced suppression of IL-12-dependent differentiation of killer-like receptor G1+ effector CD8 T cells and IFN-γ production. Nevertheless, reversal of helminth suppression of the innate IL-12 response of CD8α+ dendritic cells, which occurred in STAT6-deficient mice, was not sufficient to normalize CD8 T cell differentiation. Instead, a combined deficiency in IL-4 and IL-10 was required to reverse the negative effects of helminth coinfection on the CD8 T cell response. Monoclonal T. gondii-specific CD8 T cells adoptively transferred into coinfected mice recapitulated the spectrum of helminth-induced effects on the polyclonal CD8 T response, indicating the lack of requirement for clonal skewing.

Transnuclear CD8 T cells specific for the immunodominant epitope Gra6 lower acute-phase Toxoplasma gondii burden

We generated a CD8 T‐cell receptor (TCR) transnuclear (TN) mouse specific to the L^d‐restricted immunodominant epitope of GRA6 from Toxoplasma gondii as a source of cells to facilitate further investigation into the CD8 T‐cell‐mediated response against this pathogen. The TN T cells bound L^d‐Gra6 tetramer and proliferated upon unspecific and peptide‐specific stimulation. The TCR beta sequence of the Gra6‐specific TN CD8 T cells is identical in its V‐ and J‐region to the TCR‐β harboured by a hybridoma line generated in response to Gra6 peptide. Adoptively transferred Gra6 TN CD8 T cells proliferated upon Toxoplasma infection in vivo and exhibited an activated phenotype similar to host CD8 T cells specific to Gra6. The brain of Toxoplasma‐infected mice carried Gra6 TN cells already at day 8 post‐infection. Both Gra6 TN mice as well as adoptively transferred Gra6 TN cells were able to significantly reduce the parasite burden in the acute phase of Toxoplasma infection. Overall, the Gra6 TN mouse represents a functional tool to study the protective and immunodominant specific CD8 T‐cell response to Toxoplasma in both the acute and the chronic phases of infection.

Secretion of Rhoptry and Dense Granule Effector Proteins by Nonreplicating Toxoplasma gondii Uracil Auxotrophs Controls the Development of Antitumor Immunity

Nonreplicating type I uracil auxotrophic mutants of Toxoplasma gondii possess a potent ability to activate therapeutic immunity to established solid tumors by reversing immune suppression in the tumor microenvironment. Here we engineered targeted deletions of parasite secreted effector proteins using a genetically tractable Δku80 vaccine strain to show that the secretion of specific rhoptry (ROP) and dense granule (GRA) proteins by uracil auxotrophic mutants of T. gondii in conjunction with host cell invasion activates antitumor immunity through host responses involving CD8α+ dendritic cells, the IL-12/interferon-gamma (IFN-γ) TH1 axis, as well as CD4+ and CD8+ T cells. Deletion of parasitophorous vacuole membrane (PVM) associated proteins ROP5, ROP17, ROP18, ROP35 or ROP38, intravacuolar network associated dense granule proteins GRA2 or GRA12, and GRA24 which traffics past the PVM to the host cell nucleus severely abrogated the antitumor response. In contrast, deletion of other secreted effector molecules such as GRA15, GRA16, or ROP16 that manipulate host cell signaling and transcriptional pathways, or deletion of PVM associated ROP21 or GRA3 molecules did not affect the antitumor activity. Association of ROP18 with the PVM was found to be essential for the development of the antitumor responses. Surprisingly, the ROP18 kinase activity required for resistance to IFN-γ activated host innate immunity related GTPases and virulence was not essential for the antitumor response. These data show that PVM functions of parasite secreted effector molecules, including ROP18, manipulate host cell responses through ROP18 kinase virulence independent mechanisms to activate potent antitumor responses. Our results demonstrate that PVM associated rhoptry effector proteins secreted prior to host cell invasion and dense granule effector proteins localized to the intravacuolar network and host nucleus that are secreted after host cell invasion coordinately control the development of host immune responses that provide effective antitumor immunity against established ovarian cancer.

Beta-catenin signaling drives differentiation and proinflammatory function of IRF8-dependent dendritic cells

Beta-catenin signaling has recently been tied to the emergence of tolerogenic dendritic cells (DCs). In this article, we demonstrate a novel role for beta-catenin in directing DC subset development through IFN regulatory factor 8 (IRF8) activation. We found that splenic DC precursors express beta-catenin, and DCs from mice with CD11c-specific constitutive beta-catenin activation upregulated IRF8 through targeting of the Irf8 promoter, leading to in vivo expansion of IRF8-dependent CD8a+, plasmacytoid, and CD103+ CD11b2 DCs. beta-catenin–stabilized CD8a+ DCs secreted elevated IL-12 upon in vitro microbial stimulation, and pharmacological beta-catenin inhibition blocked this response in wild-type cells. Upon infections with Toxoplasma gondii and vaccinia virus, mice with stabilized DC beta-catenin displayed abnormally high Th1 and CD8+ T lymphocyte responses, respectively. Collectively, these results reveal a novel and unexpected function for beta-catenin in programming DC differentiation toward subsets that orchestrate proinflammatory immunity to infection.

Vaccine-mediated immunity to experimental Mycobacterium tuberculosis is not impaired in the absence of Toll-like receptor 9

Accumulating evidence indicates that inflammatory signals required for maximizing effector T cell generation have opposing effects on the development of memory T cell precursors. Toll-like receptor (TLR)2, and TLR9 significantly contribute to the inflammatory milieu and therefore in this study we examined whether the absence of TLR9 alone or the combined absence of TLR2 and TLR9 would affect vaccine-mediated immunity to Mtb. We found that TLR9KO and TLR2/9DKO mice vaccinated with a live Mtb auxotroph, akin to vaccinated WT mice, exhibited early control of Mtb growth in the lungs compared to their naïve counterparts. The granulomatous response, IFNγ production and cellular recruitment to the lungs were also similar in all the vaccinated groups of mice. These findings indicate that there is minimal contribution from TLR2 and TLR9 in generating memory immunity to Mtb with live vaccines. Defining the innate milieu that can drive maximal memory T cell generation with a tuberculosis vaccine needs further inquiry.

Intravacuolar Membranes Regulate CD8 T Cell Recognition of Membrane-Bound Toxoplasma gondii Protective Antigen

Apicomplexa parasites such as Toxoplasma gondii target effectors to and across the boundary of their parasitophorous vacuole (PV), resulting in host cell subversion and potential presentation by MHC class I molecules for CD8 T cell recognition. The host-parasite interface comprises the PV limiting membrane and a highly curved, membranous intravacuolar network (IVN) of uncertain function. Here, using a cell-free minimal system, we dissect how membrane tubules are shaped by the parasite effectors GRA2 and GRA6. We show that membrane association regulates access of the GRA6 protective antigen to the MHC I pathway in infected cells. Although insertion of GRA6 in the PV membrane is key for immunogenicity, association of GRA6 with the IVN limits presentation and curtails GRA6-specific CD8 responses in mice. Thus, membrane deformations of the PV regulate access of antigens to the MHC class I pathway, and the IVN may play a role in immune modulation.

Review on the identification and role of Toxoplasma gondii antigenic epitopes

Toxoplasma gondii is an obligate intracellular protozoan parasite with a broad range of hosts, and it causes severe toxoplasmasis in both humans and animals. It is well known that the progression and severity of a disease depend on the immunological status of the host. Immunological studies on antigens indicate that antigens do not exert their functions through the entire protein molecule, but instead, specific epitopes are responsible for the immune response. Protein antigens not only contain epitope structures used by B, T, cytotoxic T lymphocyte (CTL), and NK cells to mediate immunological responses but can also contain structures that are unfavorable for protective immunity. Therefore, the study of antigenic epitopes from T. gondii has not only enhanced our understanding of the structure and function of antigens, the reactions between antigens and antibodies, and many other aspects of immunology but it also plays a significant role in the development of new diagnostic reagents and vaccines. In this review, we summarized the immune mechanisms induced by antigen epitopes and the latest advances in identifying T. gondii antigen epitopes. Particular attention was paid to the potential clinical usefulness of epitopes in this context. Through a critical analysis of the current state of knowledge, we elucidated the latest data concerning the biological effects of epitopes and the immune results aimed at the development of future epitope-based applications, such as vaccines and diagnostic reagents.

Flt3 Ligand Is Essential for Survival and Protective Immune Responses during Toxoplasmosis

Dendritic cells (DCs) are critical for resistance to Toxoplasma gondii, and infection with this pathogen leads to increased numbers of DCs at local sites of parasite replication and in secondary lymphoid organs, but the factors that regulate this expansion are poorly understood. The cytokine Flt3 ligand (Flt3L) is critical for the generation and maintenance of DCs, and Flt3L(-/-) mice were found to be highly susceptible to acute toxoplasmosis. This phenotype correlated with decreased production of IL-12 and IFN-γ, as well as impaired NK cell responses. Surprisingly, despite low basal numbers of DCs, Flt3L(-/-) mice infected with T. gondii displayed an expansion of CD8α(+) and CD11b(lo)CD8α(-) DCs. Infection also induced an expansion of parasite-specific CD4(+) and CD8(+) T cells in Flt3L(-/-) mice; however, these cells were reduced in number and displayed impaired ability to produce IFN-γ relative to wild-type controls. Exogenous IL-12 treatment partially restored NK and T cell responses in Flt3L(-/-) mice, as well as acute resistance; however, these mice eventually succumbed to toxoplasmic encephalitis, despite the presence of large numbers of DCs and T cells in the brain. These results highlight the importance of Flt3L for resistance to toxoplasmosis and demonstrate the existence of Flt3L-independent pathways that can mediate infection-induced expansion of DCs and T cell priming.

An extrafollicular pathway for the generation of effector CD8(+) T cells driven by the proinflammatory cytokine, IL-12

The proinflammatory cytokine IL-12 drives the generation of terminally differentiated KLRG1⁺ effector CD8⁺ T cells. Using a Toxoplasma vaccination model, we delineate the sequence of events that naïve CD8⁺ T cells undergo to become terminal effectors and the differentiation steps controlled by IL-12. We demonstrate that direct IL-12 signaling on CD8⁺ T cells is essential for the induction of KLRG1 and IFN-γ, but the subsequent downregulation of CXCR3 is controlled by IL-12 indirectly through the actions of IFN-γ and IFN-γ-inducible chemokines. Differentiation of nascent effectors occurs in an extrafollicular splenic compartment and is driven by late IL-12 production by DCs distinct from the classical CD8α⁺ DC. Unexpectedly, we also found extensive proliferation of both KLRG1⁻ and KLRG1⁺ CD8⁺ T cells in the marginal zone and red pulp, which ceases prior to the final KLRG1^Hi CXCR3^Lo stage. Our findings highlight the notion of an extrafollicular pathway for effector T cell generation.

DOI:http://dx.doi.org/10.7554/eLife.09017.001

The immune system helps to protect us from cancer, infection by microbes and other diseases. There are several different types of immune cells that each have particular roles. For example, cytotoxic T cells can kill other cells in the body that are damaged or infected. These cells are found in various locations around the body—including a region of the spleen known as the white pulp—where they wait in an inactive state until they detect signals from a damaged or infected cell. These T cells divide and mature to produce populations of active T cells known as effector cytotoxic lymphoid cells (or CTLs for short), a process which is thought to occur within the white pulp.

A small protein called cytokine IL-12 is involved in the production of CTLs. The cytokine is released from other immune cells and causes the activated T cells to divide and mature. It has long been believed that IL-12 produced in the white pulp early on in the process is sufficient to drive this process, but more recent work suggests that sustained production of IL-12 in other areas of the spleen that are accessible to the bloodstream may be needed.

Here, Shah et al. studied the generation of cytotoxic T cells in mice that had been exposed to a vaccine against a disease called Toxoplasmosis. Their experiments show that IL-12 drives both the early and late stages of CTL production. In the early stages, the T cells respond to IL-12 that is secreted by a group of ‘lymphoid dendritic’ cells in the white pulp. However, in the later stages, the T cells move away from the white pulp to other parts of the spleen known as the marginal zone and red pulp, where a distinct group of ‘myeloid dendritic’ cells also produce IL-12 and direct the final maturation of the CTLs.

Shah et al.'s findings also show that the process in which cytotoxic T cells divide and later mature to produce CTLs involves a series of tightly controlled events that mostly occur outside of the white pulp. These observations provide a new perspective on how to develop vaccines and other treatments that more efficiently generate the CTLs needed to protect against infections and cancer.

DOI:http://dx.doi.org/10.7554/eLife.09017.002

IRF-5-Mediated Inflammation Limits CD8+ T Cell Expansion by Inducing HIF-1α and Impairing Dendritic Cell Functions during Leishmania Infection

Inflammation is known to be necessary for promoting, sustaining, and tuning CD8⁺ T cell responses. Following experimental Leishmania donovani infection, the inflammatory response is mainly induced by the transcription factor IRF-5. IRF-5 is responsible for the activation of several genes encoding key pro-inflammatory cytokines, such as IL-6 and TNF. Here, we investigate the role of IRF-5-mediated inflammation in regulating antigen-specific CD8⁺ T cell responses during L. donovani infection. Our data demonstrate that the inflammatory response induced by IRF-5 limits CD8⁺ T cell expansion and induces HIF-1α in dendritic cells. Ablation of HIF-1α in CD11c⁺ cells resulted into a higher frequency of short-lived effector cells (SLEC), enhanced CD8⁺ T cell expansion, and increased IL-12 expression by splenic DCs. Moreover, mice with a targeted depletion of HIF-1α in CD11c⁺ cells had a significantly lower splenic parasite burden, suggesting that induction of HIF-1α may represent an immune evasive mechanism adopted by Leishmania parasites to establish persistent infections.

Inflammation is essential for inducing, sustaining, and regulating CD8⁺ T cell responses. The transcription factor IRF-5 is mainly responsible for initiating the inflammatory response following experimental Leishmani donovani infection. IRF-5 activates several genes encoding key pro-inflammatory cytokines, such as IL-6 and TNF. In this study, we investigate the role of IRF-5-mediated inflammation in regulating antigen-specific CD8⁺ T cell responses during L. donovani infection. Our data demonstrate that the inflammatory response induced by IRF-5 limits the expansion CD8⁺ T cell. This negative effect is mediated by the induction of HIF-1α in dendritic cells. Indeed, we observed a significant increase in CD8⁺ T cell expansion in mice lacking HIF-1α expression in dendritic cells. Moreover, these mice had a significantly lower parasite burden in the spleen, suggesting that induction of HIF-1α may represent an immune evasive mechanism adopted by Leishmania parasites to establish persistent infections.

CD8+ T cell immunity in an encephalitis model of Toxoplasma gondii infection

Toxoplasma gondii infection induces a robust CD8 T cell immunity in the infected host, which is critical for keeping chronic infection under control. IFNγ production and cytolytic activity exhibited by CD8 T cells are critical functions needed to prevent the reactivation of latent infection. Paradoxically, the susceptible mice infected with the parasite develop encephalitis irrespective of the presence of vigorous CD8 T cell immunity. Recent studies from our laboratory have demonstrated that these animals have defect in the memory CD8 T cell population, which become dysfunctional due to exhibition of inhibitory receptors like PD-1. Although the blockade of PD-1-PDL-1 pathway rescues the CD8 response, PD-1(hi) expressing cells are refractory to the treatment. In this review, we discuss the development of CD8 memory response during chronic infection, mechanism responsible for their dysfunctionality, and possible therapeutic measures that can be taken to reverse the process.

Persistence of Toxoplasma gondii in the central nervous system: a fine-tuned balance between the parasite, the brain and the immune system

Upon infection of humans and animals with Toxoplasma gondii, the parasites persist as intraneuronal cysts that are controlled, but not eliminated by the immune system. In particular, intracerebral T cells are crucial in the control of T. gondii infection and are supported by essential functions from other leukocyte populations. Additionally, brain-resident cells including astrocytes, microglia and neurons contribute to the intracerebral immune response by the production of cytokines, chemokines and expression of immunoregulatory cell surface molecules, such as major histocompatibility (MHC) antigens. However, the in vivo behaviour of these individual cell populations, specifically their interaction during cerebral toxoplasmosis, remains to be elucidated. We discuss here what is known about the function of T cells, recruited myeloid cells and brain-resident cells, with particular emphasis on the potential cross-regulation of these cell populations, in governing cerebral toxoplasmosis.

Attenuated Toxoplasma gondii Stimulates Immunity to Pancreatic Cancer by Manipulation of Myeloid Cell Populations

Suppressive myeloid cells represent a significant barrier to the generation of productive antitumor immune responses to many solid tumors. Eliminating or reprogramming suppressive myeloid cells to abrogate tumor-associated immune suppression is a promising therapeutic approach. We asked whether treatment of established aggressive disseminated pancreatic cancer with the immunotherapeutic attenuated Toxoplasma gondii vaccine strain CPS would trigger tumor-associated myeloid cells to generate therapeutic antitumor immune responses. CPS treatment significantly decreased tumor-associated macrophages and markedly increased dendritic cell infiltration of the pancreatic tumor microenvironment. Tumor-resident macrophages and dendritic cells, particularly cells actively invaded by CPS, increased expression of costimulatory molecules CD80 and CD86 and concomitantly boosted their production of IL12. CPS treatment increased CD4(+) and CD8(+) T-cell infiltration into the tumor microenvironment, activated tumor-resident T cells, and increased IFNγ production by T-cell populations. CPS treatment provided a significant therapeutic benefit in pancreatic tumor-bearing mice. This therapeutic benefit depended on IL12 and IFNγ production, MyD88 signaling, and CD8(+) T-cell populations. Although CD4(+) T cells exhibited activated effector phenotypes and produced IFNγ, CD4(+) T cells as well as natural killer cells were not required for the therapeutic benefit. In addition, CD8(+) T cells isolated from CPS-treated tumor-bearing mice produced IFNγ after re-exposure to pancreatic tumor antigen, suggesting this immunotherapeutic treatment stimulated tumor cell antigen-specific CD8(+) T-cell responses. This work highlights the potency and immunotherapeutic efficacy of CPS treatment and demonstrates the significance of targeting tumor-associated myeloid cells as a mechanism to stimulate more effective immunity to pancreatic cancer.

Nonreplicating, cyst-defective type II Toxoplasma gondii vaccine strains stimulate protective immunity against acute and chronic infection

Live attenuated vaccine strains, such as type I nonreplicating uracil auxotroph mutants, are highly effective in eliciting lifelong immunity to virulent acute infection by Toxoplasma gondii. However, it is currently unknown whether vaccine-elicited immunity can provide protection against acute infection and also prevent chronic infection. To address this problem, we developed nonreverting, nonreplicating, live attenuated uracil auxotroph vaccine strains in the type II Δku80 genetic background by targeting the deletion of the orotidine 5'-monophosphate decarboxylase (OMPDC) and uridine phosphorylase (UP) genes. Deletion of OMPDC induced a severe uracil auxotrophy with loss of replication, loss of virulence in mice, and loss of the ability to develop cysts and chronic infection. Vaccination of mice using type II Δku80 Δompdc mutants stimulated a fully protective CD8(+) T cell-dependent immunity that prevented acute infection by type I and type II strains of T. gondii, and this vaccination also severely reduced or prevented cyst formation after type II challenge infection. Nonreverting, nonreplicating, and non-cyst-forming Δompdc mutants provide new tools to examine protective immune responses elicited by vaccination with a live attenuated type II vaccine.

Toxoplasma gondii superinfection and virulence during secondary infection correlate with the exact ROP5/ROP18 allelic combination

The intracellular parasite Toxoplasma gondii infects a wide variety of vertebrate species globally. Infection in most hosts causes a lifelong chronic infection and generates immunological memory responses that protect the host against new infections. In regions where the organism is endemic, multiple exposures to T. gondii likely occur with great frequency, yet little is known about the interaction between a chronically infected host and the parasite strains from these areas. A widely used model to explore secondary infection entails challenge of chronically infected or vaccinated mice with the highly virulent type I RH strain. Here, we show that although vaccinated or chronically infected C57BL/6 mice are protected against the type I RH strain, they are not protected against challenge with most strains prevalent in South America or another type I strain, GT1. Genetic and genomic analyses implicated the parasite-secreted rhoptry effectors ROP5 and ROP18, which antagonize the host’s gamma interferon-induced immunity-regulated GTPases (IRGs), as primary requirements for virulence during secondary infection. ROP5 and ROP18 promoted parasite superinfection in the brains of challenged survivors. We hypothesize that superinfection may be an important mechanism to generate T. gondii strain diversity, simply because two parasite strains would be present in a single meal consumed by the feline definitive host. Superinfection may drive the genetic diversity of Toxoplasma strains in South America, where most isolates are IRG resistant, compared to North America, where most strains are IRG susceptible and are derived from a few clonal lineages. In summary, ROP5 and ROP18 promote Toxoplasma virulence during reinfection.

Toxoplasma gondii is a widespread parasite of warm-blooded animals and currently infects one-third of the human population. A long-standing assumption in the field is that prior exposure to this parasite protects the host from subsequent reexposure, due to the generation of protective immunological memory. However, this assumption is based on clinical data and mouse models that analyze infections with strains common to Europe and North America. In contrast, we found that the majority of strains sampled from around the world, in particular those from South America, were able to kill or reinfect the brains of hosts previously exposed to T. gondii. The T. gondii virulence factors ROP5 and ROP18, which inhibit key host effectors that mediate parasite killing, were required for these phenotypes. We speculate that these results underpin clinical observations that pregnant women previously exposed to Toxoplasma can develop congenital infection upon reexposure to South American strains.

CD40 is required for protective immunity against liver stage Plasmodium infection

The costimulatory molecule CD40 enhances immunity through several distinct roles in T cell activation and T cell interaction with other immune cells. In a mouse model of immunity to liver stage Plasmodium infection, CD40 was critical for the full maturation of liver dendritic cells, accumulation of CD8(+) T cells in the liver, and protective immunity induced by immunization with the Plasmodium yoelii fabb/f(-) genetically attenuated parasite. Using mixed adoptive transfers of polyclonal wild-type and CD40-deficient CD8(+) T cells into wild-type and CD40-deficient hosts, we evaluated the contributions to CD8(+) T cell immunity of CD40 expressed on host tissues including APC, compared with CD40 expressed on the CD8(+) T cells themselves. Most of the effects of CD40 could be accounted for by expression in the T cells' environment, including the accumulation of large numbers of CD8(+) T cells in the livers of immunized mice. Thus, protective immunity generated during immunization with fabb/f(-) was largely dependent on effective APC licensing via CD40 signaling.

Diverse roles for T-bet in the effector responses required for resistance to infection

The transcription factor T-bet has been most prominently linked to NK and T cell production of IFN-γ, a cytokine required for the control of a diverse array of intracellular pathogens. Indeed, in mice challenged with the parasite Toxoplasma gondii, NK and T cell responses are characterized by marked increases of T-bet expression. Unexpectedly, T-bet(-/-) mice infected with T. gondii develop a strong NK cell IFN-γ response that controls parasite replication at the challenge site, but display high parasite burdens at secondary sites colonized by T. gondii and succumb to infection. The loss of T-bet had a modest effect on T cell production of IFN-γ but did not impact on the generation of parasite-specific T cells. However, the absence of T-bet resulted in lower T cell expression of CD11a, Ly6C, KLRG-1, and CXCR3 and fewer parasite-specific T cells at secondary sites of infection, associated with a defect in parasite control at these sites. Together, these data highlight T-bet-independent pathways to IFN-γ production and reveal a novel role for this transcription factor in coordinating the T cell responses necessary to control this infection in peripheral tissues.

Combination of interleukin-12 gene therapy, metronomic cyclophosphamide and DNA cancer vaccination directs all arms of the immune system towards tumor eradication

In this work a combination therapy that acts upon the immune suppressive, the innate and specific arms of the immune system is proposed. This combination therapy, which consists of intratumoral interleukin-12 (IL-12) gene therapy, human tyrosinase (hTyr) DNA vaccination and metronomic cyclophosphamide (CPX), was evaluated in a B16-F10 mouse model. The following groups were compared: (1) no treatment, (2) control vector, (3) intratumoral IL-12 gene therapy, (4) intratumoral IL-12 gene therapy+metronomic CPX, (5) intratumoral IL-12 gene therapy+metronomic CPX+hTyr DNA vaccination. Next to clinical efficacy and safety, we characterized acute effects of IL-12 and anti-tumor immune response after a second tumor challenge. All treatment groups showed increased survival and higher cure rates than control groups. Survival of non-cured mice was increased when metronomic CPX was combined with IL-12 gene therapy. Furthermore, mice that received metronomic CPX had significantly lower percentages of regulatory T cells. Addition of the hTyr DNA vaccine increased cure rate and resulted in increased survival compared to other treatment groups. We also demonstrated that the manifest necrosis within days after IL-12 gene therapy is at least partly due to IL-12 mediated activation of NK cells. All cured mice were resistant to a second challenge. A humoral memory response against the tumor cells was observed in all groups that received IL-12 gene therapy, while a cellular memory response was observed only in the vaccinated mice. In conclusion, every component of this combination treatment contributed a unique immunologic trait with associated clinical benefits.

Complex immune cell interplay in the gamma interferon response during Toxoplasma gondii infection

Toxoplasma gondii is an obligate intracellular parasite of clinical importance, especially in immunocompromised patients. Investigations into the immune response to the parasite found that T cells are the primary effector cells regulating gamma interferon (IFN-γ)-mediated host resistance. However, recent studies have revealed a critical role for the innate immune system in mediating host defense independently of the T cell responses to the parasite. This body of knowledge is put into perspective by the unifying theme that immunity to the protozoan parasite requires a strong IFN-γ host response. In the following review, we discuss the role of IFN-γ-producing cells and the signals that regulate IFN-γ production during T. gondii infection.

Impact of regulated secretion on antiparasitic CD8 T cell responses

CD8 T cells play a key role in defense against the intracellular parasite Toxoplasma, but why certain CD8 responses are more potent than others is not well understood. Here, we describe a parasite antigen, ROP5, that elicits a CD8 T cell response in genetically susceptible mice. ROP5 is secreted via parasite organelles termed rhoptries that are injected directly into host cells during invasion, whereas the protective, dense-granule antigen GRA6 is constitutively secreted into the parasitophorous vacuole. Transgenic parasites in which the ROP5 antigenic epitope was targeted for secretion through dense granules led to enhanced CD8 T cell responses, whereas targeting the GRA6 epitope to rhoptries led to reduced CD8 responses. CD8 T cell responses to the dense-granule-targeted ROP5 epitope resulted in reduced parasite load in the brain. These data suggest that the mode of secretion affects the efficacy of parasite-specific CD8 T cell responses.

Parasite fate and involvement of infected cells in the induction of CD4+ and CD8+ T cell responses to Toxoplasma gondii

During infection with the intracellular parasite Toxoplasma gondii, the presentation of parasite-derived antigens to CD4⁺ and CD8⁺ T cells is essential for long-term resistance to this pathogen. Fundamental questions remain regarding the roles of phagocytosis and active invasion in the events that lead to the processing and presentation of parasite antigens. To understand the most proximal events in this process, an attenuated non-replicating strain of T. gondii (the cpsII strain) was combined with a cytometry-based approach to distinguish active invasion from phagocytic uptake. In vivo studies revealed that T. gondii disproportionately infected dendritic cells and macrophages, and that infected dendritic cells and macrophages displayed an activated phenotype characterized by enhanced levels of CD86 compared to cells that had phagocytosed the parasite, thus suggesting a role for these cells in priming naïve T cells. Indeed, dendritic cells were required for optimal CD4⁺ and CD8⁺ T cell responses, and the phagocytosis of heat-killed or invasion-blocked parasites was not sufficient to induce T cell responses. Rather, the selective transfer of cpsII-infected dendritic cells or macrophages (but not those that had phagocytosed the parasite) to naïve mice potently induced CD4⁺ and CD8⁺ T cell responses, and conferred protection against challenge with virulent T. gondii. Collectively, these results point toward a critical role for actively infected host cells in initiating T. gondii-specific CD4⁺ and CD8⁺ T cell responses.

CD4⁺ and CD8⁺ T cells are critical for controlling many infections. To generate a T cell response during infection, T cells must encounter the microbial peptides that they recognize bound to MHC molecules on the surfaces of other cells, such as dendritic cells. It is currently unclear how dendritic cells acquire the antigens they present to T cells during infection with many intracellular pathogens. It is possible that these antigens are phagocytosed and processed by dendritic cells, or antigens may be presented by cells that are infected by pathogens such as Toxoplasma gondii, which invades host cells independently of phagocytosis. To differentiate these pathways, we developed a novel technique to track the fate of T. gondii in vivo that distinguishes actively infected cells from those that phagocytosed parasites. This technique was used to examine each of these cell populations. We also used pharmacological inhibitors of parasite invasion, and the transfer of sort-purified infected or uninfected dendritic cells and macrophages to determine what roles phagocytosis and active invasion have in the initiation of T cell responses. Our results demonstrate that phagocytosis of parasites is not sufficient to induce CD4⁺ or CD8⁺ T cell responses, whereas infected cells are critical for this process.

Distinct from its canonical effects, deletion of IL-12p40 induces cholangitis and fibrosis in interleukin-2Rα(-/-) mice

The IL-12 family modulates T cell mediated autoimmune diseases and GWAS in PBC have suggested a critical role of IL-12 and its subunits in modulating portal inflammation. We have taken advantage of an aggressive model of portal inflammation and colitis in IL-2Rα(-/-) mice to study the specific role of IL-12 and, in particular, the immunobiology of p40(-/-)IL-2Rα(-/-) mice. Colonies of IL-2Rα(+/-), IL-2Rα(-/-) and p40(-/-)IL-2Rα(-/-) mice were studied for the natural history of immunopathology in liver and colon using histology and immunohistochemistry. Further, to focus on mechanisms, liver, spleen and mesenteric lymph node flow cytometry was employed to identify specific phenotypes; cytokine analysis on inflammatory cell populations was compared between groups. Finally, Real-Time PCR was used to focus on the genes involved in hepatic fibrosis. Surprisingly, p40(-/-)IL-2Rα(-/-) mice manifest more severe portal inflammation and bile duct damage, including signs of portal hypertension and liver fibrosis, but a significant reduction in colitis. Indeed, p40(-/-)IL-2Rα(-/-) mice reveal a profound hepatic CD8(+) T cell infiltrate, whose major component are effector memory cells as well as enhanced hepatic Th1 but reduced Th17 responses. These observations were confirmed by Real-Time PCR analysis of fibrosis-related genes in the liver. Distinct from its canonical effects, IL-12p40 plays a critical role in autoimmune cholangitis, including hepatic fibrosis. These data take on striking significance for any proposed human trials that modulate the IL-12p40 pathway in human PBC.