Dendritic Cells Are Dispensable for T Cell Priming and Control of Acute Lymphocytic Choriomeningitis Virus Infection

Efficacy of LCMV-based cancer immunotherapies is unleashed by intratumoral injections of polyI:C

BACKGROUND

Lymphocytic choriomeningitis virus (LCMV) belongs to the Arenavirus family known for inducing strong cytotoxic T-cell responses in both mice and humans. LCMV has been engineered for the development of cancer immunotherapies, currently undergoing evaluation in phase I/II clinical trials. Initial findings have demonstrated safety and an exceptional ability to activate and expand tumor-specific T lymphocytes. Combination strategies to maximize the antitumor effectiveness of LCMV-based immunotherapies are being explored.

METHODS

We assessed the antitumor therapeutic effects of intratumoral administration of polyinosinic:polycytidylic acid (poly(I:C)) and systemic vaccination using an LCMV-vector expressing non-oncogenic versions of the E6 and E7 antigens of human papillomavirus 16 (artLCMV-E7E6) in a bilateral model engrafting TC-1/A9 cells. This cell line, derived from the parental TC-1, exhibits low MHC class I expression and is highly immune-resistant. The mechanisms underlying the combination's efficacy were investigated through bulk RNA-seq, flow cytometry analyses of the tumor microenvironment, selective depletions using antibodies and clodronate liposomes, Batf3 deficient mice, and in vivo bioluminescence experiments. Finally, we assessed the antitumor effectiveness of the combination of artLCMV-E7E6 with BO-112, a GMP-grade poly(I:C) formulated in polyethyleneimine, currently under evaluation in clinical trials.

RESULTS

Intratumoral injection of poly(I:C) enhanced the antitumor efficacy of artLCMV-E7E6 in both injected and non-injected tumor lesions. The combined treatment resulted in a significant delay in tumor growth and often complete eradication of several tumor lesions, leading to significantly improved survival compared with monotherapies. While intratumoral administration of poly(I:C) did not impact LCMV vector biodistribution or transgene expression, it significantly modified leucocyte infiltrates within the tumor microenvironment and amplified systemic efficacy through proinflammatory cytokines/chemokines such as CCL3, CCL5, CXCL10, TNF, IFNα, and IL12p70. Upregulation of MHC on tumor cells and a reconfiguration of the gene expression programs related to tumor vasculature, leucocyte migration, and the activation profile of tumor-infiltrating CD8+ T lymphocytes were observed. Indeed, the antitumor effect relied on the functions of CD8+ T lymphocytes and macrophages. The synergistic efficacy of the combination was further confirmed when BO-112 was included.

CONCLUSION

Intratumoral injection of poly(I:C) sensitizes MHClow tumors to the antitumor effects of artLCMV-E7E6, resulting in a potent therapeutic synergy.

Antigen-dependent multistep differentiation of T follicular helper cells and its role in SARS-CoV-2 infection and vaccination

T follicular helper (Tfh) cells play an essential role in regulating the GC reaction and, consequently, the generation of high‐affinity antibodies and memory B cells. Therefore, Tfh cells are critical for potent humoral immune responses against various pathogens and their dysregulation has been linked to autoimmunity and cancer. Tfh cell differentiation is a multistep process, in which cognate interactions with different APC types, costimulatory and coinhibitory pathways, as well as cytokines are involved. However, it is still not fully understood how a subset of activated CD4⁺ T cells begins to express the Tfh cell‐defining chemokine receptor CXCR5 during the early stage of the immune response, how some CXCR5⁺ pre‐Tfh cells enter the B‐cell follicles and mature further into GC Tfh cells, and how Tfh cells are maintained in the memory compartment. In this review, we discuss recent advances on how antigen and cognate interactions are important for Tfh cell differentiation and long‐term persistence of Tfh cell memory, and how this is relevant to the current understanding of COVID‐19 pathogenesis and the development of potent SARS‐CoV‐2 vaccines.

Viral nucleoprotein antibodies activate TRIM21 and induce T cell immunity

Nucleoprotein (N) is an immunodominant antigen in many enveloped virus infections. While the diagnostic value of anti‐N antibodies is clear, their role in immunity is not. This is because while they are non‐neutralising, they somehow clear infection by coronavirus, influenza and LCMV in vivo. Here, we show that anti‐N immune protection is mediated by the cytosolic Fc receptor and E3 ubiquitin ligase TRIM21. Exploiting LCMV as a model system, we demonstrate that TRIM21 uses anti‐N antibodies to target N for cytosolic degradation and generate cytotoxic T cells (CTLs) against N peptide. These CTLs rapidly eliminate N‐peptide‐displaying cells and drive efficient viral clearance. These results reveal a new mechanism of immune synergy between antibodies and T cells and highlights N as an important vaccine target.

The Innate Immune Response to Infection Induces Erythropoietin-Dependent Replenishment of the Dendritic Cell Compartment

Dendritic cells (DC) play a key role in the adaptive immune response due to their ability to present antigens and stimulate naïve T cells. Many bacteria and viruses can efficiently target DC, resulting in impairment of their immunostimulatory function or elimination. Hence, the DC compartment requires replenishment following infection to ensure continued operational readiness of the adaptive immune system. Here, we investigated the molecular and cellular mechanisms of inflammation-induced DC generation. We found that infection with viral and bacterial pathogens as well as Toll-like receptor 9 (TLR9) ligation with CpG-oligodeoxynucleotide (CpG-ODN) expanded an erythropoietin (EPO)-dependent TER119⁺CD11a⁺ cell population in the spleen that had the capacity to differentiate into TER119⁺CD11c^high and TER119^-CD11c^high cells both in vitro and in vivo. TER119⁺CD11c^high cells contributed to the conventional DC pool in the spleen and specifically increased in lymph nodes draining the site of local inflammation. Our results reveal a so far undescribed inflammatory EPO-dependent pathway of DC differentiation and establish a mechanistic link between innate immune recognition of potential immunosuppressive pathogens and the maintenance of the DC pool during and after infection.

Cyclophilin D Regulates Antiviral CD8+ T Cell Survival in a Cell-Extrinsic Manner

CD8+ T cell-mediated immunity is critical for host defense against viruses and requires mitochondria-mediated type I IFN (IFN-I) signaling for optimal protection. Cyclophilin D (CypD) is a mitochondrial matrix protein that modulates the mitochondrial permeability transition pore, but its role in IFN-I signaling and CD8+ T cell responses to viral infection has not been previously explored. In this study, we demonstrate that CypD plays a critical extrinsic role in the survival of Ag-specific CD8+ T cell following acute viral infection with lymphocytic choriomeningitis virus in mice. CypD deficiency resulted in reduced IFN-I and increased CD8+ T cell death, resulting in a reduced antiviral CD8+ T cell response. In addition, CypD deficiency was associated with an increase in pathogen burden at an early time-point following infection. Furthermore, our data demonstrate that transfer of wild-type macrophages (expressing CypD) to CypD-deficient mice can partially restore CD8+ T cell responses. These results establish that CypD plays an extrinsic role in regulating optimal effector CD8+ T cell responses to viral infection. Furthermore, this suggests that, under certain circumstances, inhibition of CypD function may have a detrimental impact on the host's ability to respond to viral infection.

Granulocyte/Macrophage Colony-Stimulating Factor-Derived Macrophages Exhibit Distinctive Early Immune Response to Lymphocytic Choriomeningitis Virus Infection

Granulocyte/macrophage colony-stimulating factor (GM-CSF) and macrophage CSF (M-CSF) modulate differentiation and immune functions of macrophages (MΦ). Our aim was to evaluate how different MΦ differentiation conditions influence the MΦ response to virus infection. To address this, we differentiated bone marrow-derived MΦ in either GM-CSF or M-CSF and measured the cytokine responses to two different strains of lymphocytic choriomeningitis virus (LCMV) (clone 13; Cl13 or Armstrong; ARM). GM-CSF MΦ infected with either LCMV-ARM or -Cl13 produced more IL-6 than M-CSF MΦ, whereas M-CSF MΦ generated more IL-10 than GM-CSF MΦ. Interestingly, in M-CSF MΦ, LCMV-ARM induced more IL-10 production than Cl13. However, we could not detect any IL-12p70 or IL-23 after infection from either cell types. We also observed that GM-CSF MΦ was more efficient than M-CSF MΦ in supporting antigen-specific CD8⁺ T cell proliferation. Taken together, our data demonstrate that GM-CSF and M-CSF MΦ differ in how they respond to viral infection by their production of different cytokines, and their support for CD8⁺ T cell proliferation.

Dendritic Cells Control Regulatory T Cell Function Required for Maintenance of Intestinal Tissue Homeostasis

Dendritic cells (DCs) together with regulatory T cells (Tregs) are essential mediators of immune homeostasis. Disruption of function or frequency of either cell type can lead to fatal autoimmunity. We previously described that mice constitutively lacking DCs (∆DC) develop autoimmunity characterized by reduced body weight, autoantibodies, and pronounced intestinal inflammation. In this study, we show that lack of DCs leads to an altered gene expression profile in peripheral but not thymic Tregs with increased expression of inhibitory receptors. The suppressive function of Tregs from ΔDC mice was impaired in T cell cocultures. In a model of transfer colitis, Tregs from ∆DC mice were only functional in the presence of DCs in recipient mice. Lack of MHC class II on DCs also resulted in upregulation of inhibitory receptors on Tregs, reduced body weight, and elevated serum IgA levels. Further analysis of the IgA response revealed an expansion of IgA⁺ germinal center B cells and plasma cells in mesenteric lymph nodes and more IgA-coated commensal bacteria in feces of ∆DC mice. Thus, we show a critical role for DCs to establish intestinal homeostasis by regulating Treg function for prevention of spontaneous inflammation.

Linking Cell Dynamics With Gene Coexpression Networks to Characterize Key Events in Chronic Virus Infections

The host immune response against infection requires the coordinated action of many diverse cell subsets that dynamically adapt to a pathogen threat. Due to the complexity of such a response, most immunological studies have focused on a few genes, proteins, or cell types. With the development of “omic”-technologies and computational analysis methods, attempts to analyze and understand complex system dynamics are now feasible. However, the decomposition of transcriptomic data sets generated from complete organs remains a major challenge. Here, we combined Weighted Gene Coexpression Network Analysis (WGCNA) and Digital Cell Quantifier (DCQ) to analyze time-resolved mouse splenic transcriptomes in acute and chronic Lymphocytic Choriomeningitis Virus (LCMV) infections. This enabled us to generate hypotheses about complex immune functioning after a virus-induced perturbation. This strategy was validated by successfully predicting several known immune phenomena, such as effector cytotoxic T lymphocyte (CTL) expansion and exhaustion. Furthermore, we predicted and subsequently verified experimentally macrophage-CD8 T cell cooperativity and the participation of virus-specific CD8⁺ T cells with an early effector transcriptome profile in the host adaptation to chronic infection. Thus, the linking of gene expression changes with immune cell kinetics provides novel insights into the complex immune processes within infected tissues.

Hemorrhagic Fever-Causing Arenaviruses: Lethal Pathogens and Potent Immune Suppressors

Hemorrhagic fevers (HF) resulting from pathogenic arenaviral infections have traditionally been neglected as tropical diseases primarily affecting African and South American regions. There are currently no FDA-approved vaccines for arenaviruses, and treatments have been limited to supportive therapy and use of non-specific nucleoside analogs, such as Ribavirin. Outbreaks of arenaviral infections have been limited to certain geographic areas that are endemic but known cases of exportation of arenaviruses from endemic regions and socioeconomic challenges for local control of rodent reservoirs raise serious concerns about the potential for larger outbreaks in the future. This review synthesizes current knowledge about arenaviral evolution, ecology, transmission patterns, life cycle, modulation of host immunity, disease pathogenesis, as well as discusses recent development of preventative and therapeutic pursuits against this group of deadly viral pathogens.

IL-33 Augments Virus-Specific Memory T Cell Inflation and Potentiates the Efficacy of an Attenuated Cytomegalovirus-Based Vaccine

Candidate vaccines designed to generate T cell-based immunity are typically vectored by nonpersistent viruses, which largely fail to elicit durable effector memory T cell responses. This limitation can be overcome using recombinant strains of CMV. Proof-of-principle studies have demonstrated the potential benefits of this approach, most notably in the SIV model, but safety concerns require the development of nonreplicating alternatives with comparable immunogenicity. In this study, we show that IL-33 promotes the accumulation and recall kinetics of circulating and tissue-resident memory T cells in mice infected with murine CMV. Using a replication-deficient murine CMV vector, we further show that exogenous IL-33 boosts vaccine-induced memory T cell responses, which protect against subsequent heterologous viral challenge. These data suggest that IL-33 could serve as a useful adjuvant to improve the efficacy of vaccines based on attenuated derivatives of CMV.

DNA-binding of the Tet-transactivator curtails antigen-induced lymphocyte activation in mice

The Tet-On/Off system for conditional transgene expression constitutes state-of-the-art technology to study gene function by facilitating inducible expression in a timed and reversible manner. Several studies documented the suitability and versatility of this system to trace lymphocyte fate and to conditionally express oncogenes or silence tumour suppressor genes in vivo. Here, we show that expression of the tetracycline/doxycycline-controlled Tet-transactivator, while tolerated well during development and in immunologically unchallenged animals, impairs the expansion of antigen-stimulated T and B cells and thereby curtails adaptive immune responses in vivo. Transactivator-mediated cytotoxicity depends on DNA binding, but can be overcome by BCL2 overexpression, suggesting that apoptosis induction upon lymphocyte activation limits cellular and humoral immune responses. Our findings suggest a possible system-intrinsic biological bias of the Tet-On/Off system in vivo that will favour the outgrowth of apoptosis resistant clones, thus possibly confounding data published using such systems.

Tet-transactivators are used for direct regulation of gene expression, RNA interference and for CRISPR/Cas9-based systems. Here the authors show that DNA-bound Tet-transactivators can induce cell death in antigen-activated lymphocytes in vivo, putting into question the use of, and in vivo data generated with, these molecular tools.