Complement component 3 is required for optimal expansion of CD8 T cells during a systemic viral infection

Autoantigen and IL-2 activated CD4+CD25+T regulatory cells are induced to express CD8 and are autoantigen specific in inhibiting experimental autoimmune encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE) induced by immunization with myelin basic protein (MBP) is a self-limiting disease model of multiple sclerosis. CD4+CD25+Foxp3+T cells play a role in limiting autoimmune disease but treatment with antigen naïve CD4+CD25+ cells does not reduce EAE. This study examined if in vitro activation by MBP and rIL-2 induced CD4+CD25+Foxp3+ cells that could inhibit EAE. Culture of CD4+CD8-CD25+cells from naïve rats with MBP and rIL-2 induced activated Treg that reduced the severity of clinical EAE and infiltration of CD8+T cells and macrophage into brain stem. CD4+CD25+T cells activated by an irrelevant autoantigen and rIL-2 did not suppress EAE. Resting CD4+CD25+T cells activated by autoantigen and rIL-2 have mRNA for Infgr, Il12rb2, Il5 but not Tbet, Gata3, Ilr5ra or Ifng. These changes in mRNA expression are the markers of Ts1 cells. A proportion of CD4+CD8-CD25+ cells activated by MBP/rIL-2 were induced to express CD8α, CD8β and CD62L. Depletion of CD4+CD8α+CD25+ cells removed the capacity of MBP and rIL-2 activated CD4+CD25+T cells to suppress EAE. This study demonstrated that in vitro activation of CD4+CD8-CD25+ cells by MBP/rIL-2 induced relevant antigen-specific Treg within days, which expressed CD8α, CD8β and CD62L with a Ts1 phenotype and that had greater potency than freshly isolated antigen naive CD4+CD25+Treg in suppressing clinical severity of EAE and immune inflammation in CNS. These findings may guide development of antigen-specific Treg for therapy.

Complement C3 Deficiency Enhances Renal Leptospiral Load and Inflammation While Impairing T Cell Differentiation During Chronic Leptospira interrogans Infection

Leptospirosis is a neglected zoonotic disease caused by pathogenic Leptospira spp., affecting an estimated of one million people annually and resulting in approximately 60,000 fatalities. The disease can lead to hepatic, renal, and pulmonary dysfunctions and may contribute to the development of chronic kidney disease. The Complement System plays a crucial role in eliminating bacteria by generating opsonins, anaphylotoxins, that degranulate mastocytes and basophils, and attract immune cells to the infection site, among other important functions. This study aimed to investigate the role of C3, the central protein of the Complement System, in vivo during chronic infection of L interrogans serovar Copenhageni strain FIOCRUZ L1-130 (LIC). C57BL/6 wild-type (WT) and C3 knockout (C3KO) mice were infected with 10 8 LIC and monitored at 15, 30, 60, 90 and 180 days' post-infection (d.p.i.). LIC-infected C3KO mice exhibited a significantly higher leptospiral load in the kidneys compared to WT counterparts. Both groups showed local inflammation at 15 and 30 d.p.i., but only C3KO LIC-infected mice had a higher number of Leptospira DNA copies at 30 d.p.i. At the same time point, C3KO LIC-infected mice developed a larger fibrotic area than WT mice. Interestingly, independent of C3, mice pre-treated with a nephrotoxic drug increased the renal inflammatory response; however, this pretreatment did not affect the local leptospiral load in infected mice. Additionally, levels of specific IgG2b and IgG3 antibodies were significantly higher in LIC-infected C3KO mice compared to WT mice. Proteomic analysis showed lower levels of C5/C5a in WT and C3KO LIC-infected mice, as well as in C3KO control mice. M-CSF and SDF-1 cytokine levels were reduced in both LIC-infected groups. Furthermore, naïve T lymphocytes count (both CD4 + and CD8 + ) were higher in LIC-infected C3KO mice, whereas effector CD8 + T lymphocyte numbers declined during LIC infection - a phenomenon more pronounced in C3KO mice. Overall, this study demonstrates that during LIC infection, the absence of C3 does not affect mouse survival but leads to increased renal leptospiral load and fibrosis. Additionally, it highlights the crucial role of C3 in supporting the maturation and differentiation of T lymphocytes into pre-effector cells, underscoring its importance as a key link between the innate and adaptive immune responses in leptospirosis.

Author Summary

Leptospirosis is an infectious disease with approximately one million new cases annually and is responsible for about 5% of deaths, particularly in underdeveloped countries. Our objective is to understand the immune response in leptospirosis, specifically the role of the Complement C3 protein. In this study, we observed that C3 deficiency is associated with a higher renal leptospiral load and an increased incidence of renal fibrosis after one month of chronic infection. Notably, C3 also influences the differentiation of helper and cytotoxic T lymphocytes into effector cells, potentially contributing to the increased severity of chronic leptospirosis observed in C3-deficient animals.

Murine C3 of the complement system affects infection by Leptospira interrogans

Leptospirosis is an infectious neglected disease estimated to affect more than one million people worldwide each year. The Complement System plays a vital role in eliminating infectious agents. However, its precise role in leptospirosis remains to be fully understood. We investigated the importance of C3 in L. interrogans serovar Kennewicki strain Pomona Fromm (LPF) infection. Lack of C3 leads to decreased leukocyte number, impaired inflammatory response and failure to eliminate bacteria during the early stages of infection, which may cause interstitial nephritis later. These findings could be explained, at least in part, by the lower presence of local opsonins. Furthermore, antibody production against Leptospira was compromised in the absence of C3, highlighting the importance of CR2 in B lymphocyte proliferation and the adjuvant role of C3d in humoral immunity. Leptospires can be eliminated through the urine, and according to our study, the lack of C3 delays the elimination of LPF through urine during the early stages of the infection. These results strongly suggest the crucial role of C3 protein in orchestrating an appropriate inflammatory response against LPF infection and in effectively eliminating the bacteria from the body during the acute phase of leptospirosis.

Complement and T cell activation in transplantation

The complement system plays a critical role in modulating adaptive T cell responses. Coordination of the proinflammatory signaling cascade and complement regulators permits efficient T cell priming and survival, while minimizing off-target damage to healthy host cells. In the context of transplantation, anti-donor T cell immunity remains a barrier to long term graft health and complement-targeted therapies have shown the potential to significantly improve patient outcomes. Here we will review our current understanding of complement-mediated T cell function and how these findings may be harnessed in organ transplantation.

Complement system component 3 deficiency modulates the phenotypic profile of murine macrophages

The Complement System is composed of more than 40 proteins that act in innate and adaptive immunity. C3 is the most abundant one and C3-deficient patients are more susceptible to recurrent and severe infections. Several studies have demonstrated the importance of C3 in controlling infections. However, its role in leukocyte biology is still poorly understood. This study aimed to evaluate several cellular parameters in macrophages from C3-deficient mice and compare them to similar cells from wild-type counterparts. We observed that in the absence of C3, the population of F4/80low macrophages in the peritoneal cavity of thioglycolate-treated mice is diminished, probably due to the lack of chemotactic factors like C3a and low levels of C5a. Using fluorescence microscopy analysis, we observed that macrophages from C3-deficient mice exhibited morphological alterations when compared to similar cells from wild-type mice. We observed a significant increase in the expression of CD11c, which is part of CR4 (CD11c/CD18), in macrophages from C3-deficient compared to cells from wild-type mice. Treatment with 12-o-tetradecanoylphorbol-13-acetate, stimulated ROS production and MAPK activation by macrophages. However, these parameters were lower in macrophages from C3-deficient mice when compared to wild-type counterparts. In addition, the phagocytosis of iC3b-opsonized Zymosan particles was diminished in macrophages from C3-deficient mice. Our results suggest that C3 deficiency in C57Black/6 mice may influence specific morphological and functional parameters of macrophages, cells of fundamental importance for both the innate and acquired immune responses.

The safety and efficacy profile of eculizumab in myasthenic crisis: a prospective small case series

Eculizumab has improved recovery from ventilatory support in myasthenic crisis (MC) cases. However, the safety and efficacy profiles from prospective studies are still lacking. This study aimed to explore eculizumab's safety and efficacy in a prospective case series of patients with refractory MC. We followed a series of anti-acetylcholine receptor (AChR) antibody-positive myasthenia gravis (MG) patients who received eculizumab as an add-on therapy for 12 weeks during MC to facilitate the weaning process and reduced disease activity. Serum anti-AChR antibodies and peripheral immune molecules associated with the complement pathway were evaluated before and after eculizumab administration. Compared to the baseline Myasthenia Gravis Foundation of America (MGFA)-quantitative MG test (QMG) scores (22.25 ± 4.92) and MG-activities of daily living (MG-ADL; 18.25 ± 2.5) scores at crisis, improvements were observed from 4 weeks (14.5 ± 10.47 and 7.5 ± 7.59, respectively) through 12 weeks (7.5 ± 5.74 and 2.25 ± 3.86, respectively) post-treatment. Muscle strength consistently improved across ocular, bulbar, respiratory, and limb/gross domain groups. One patient died of cardiac failure at 16 weeks. Three cases remained in remission at 24 weeks, with a mean QMG score of 2.67 ± 2.89 and ADL score of 0.33 ± 0.58. No significant side effects were reported. Serum CH50 and soluble C5b-9 levels significantly declined, while there were no significant changes in serum anti-AChR antibody levels, C1q, C5a levels, or peripheral lymphocyte proportions. Eculizumab was well tolerated and showed efficacy in this case series. Large prospective cohort studies with extended follow-up periods are needed to further explore the safety and efficacy profile in real-world practice.

Complementing Testicular Immune Regulation: The Relationship between Sertoli Cells, Complement, and the Immune Response

Sertoli cells within the testis are instrumental in providing an environment for spermatogenesis and protecting the developing germ cells from detrimental immune responses which could affect fertility. Though these immune responses consist of many immune processes, this review focuses on the understudied complement system. Complement consists of 50+ proteins including regulatory proteins, immune receptors, and a cascade of proteolytic cleavages resulting in target cell destruction. In the testis, Sertoli cells protect the germ cells from autoimmune destruction by creating an immunoregulatory environment. Most studies on Sertoli cells and complement have been conducted in transplantation models, which are effective in studying immune regulation during robust rejection responses. In grafts, Sertoli cells survive activated complement, have decreased deposition of complement fragments, and express many complement inhibitors. Moreover, the grafts have delayed infiltration of immune cells and contain increased infiltration of immunosuppressive regulatory T cells as compared to rejecting grafts. Additionally, anti-sperm antibodies and lymphocyte infiltration have been detected in up to 50% and 30% of infertile testes, respectively. This review seeks to provide an updated overview of the complement system, describe its relationship with immune cells, and explain how Sertoli cells may regulate complement in immunoprotection. Identifying the mechanism Sertoli cells use to protect themselves and germ cells against complement and immune destruction is relevant for male reproduction, autoimmunity, and transplantation.

Cellular changes in eculizumab early responders with generalized myasthenia gravis

Eculizumab (ECU), a C5 complement inhibitor, is approved to treat acetylcholine receptor autoantibody positive generalized myasthenia gravis (AChR MG). The clinical effect of ECU relies on inhibition of the terminal complement complex; however, the effect of ECU on lymphocytes is largely unknown. We evaluated innate and adaptive immunity among AChR MG patients (N = 3) before ECU and ≥3 months later while on stable therapy, and found reduced activation markers in memory CD4⁺ T cell subsets, increased regulatory T cell populations, and reduced frequencies of CXCR5⁺HLA-DR⁺CCR7⁺ Tfh subsets and CD11b⁺ migratory memory B cells. We observed increases within CD8⁺ T cell subsets that were terminally differentiated and senescent. Our data suggest complement inhibition with ECU modulates the adaptive immunity in patients with MG, consistent with preclinical data showing changes in complement-mediated signaling by T- and antigen-presenting cells. These findings extend our understanding of ECU's mechanism of action when treating patients with MG.

The dual role of complement in cancers, from destroying tumors to promoting tumor development

Complement is an important branch of innate immunity; however, its biological significance goes far beyond the scope of simple nonspecific defense and involves a variety of physiological functions, including the adaptive immune response. In this review, to unravel the complex relationship between complement and tumors, we reviewed the high diversity of complement components in cancer and the heterogeneity of their production and activation pathways. In the tumor microenvironment, complement plays a dual regulatory role in the occurrence and development of tumors, affecting the outcomes of the immune response. We explored the differential expression levels of various complement components in human cancers via the Oncomine database. The gene expression profiling interactive analysis (GEPIA) tool and Kaplan-Meier plotter (K-M plotter) confirmed the correlation between differentially expressed complement genes and tumor prognosis. The tumor immune estimation resource (TIMER) database was used to statistically analyze the effect of complement on tumor immune infiltration. Finally, with a view to the role of complement in regulating T cell metabolism, complement could be a potential target for immunotherapies. Targeting complement to regulate the antitumor immune response seems to have potential for future treatment strategies. However, there are still many complex problems, such as who will benefit from this therapy and how to select the right therapeutic target and determine the appropriate drug concentration. The solutions to these problems depend on a deeper understanding of complement generation, activation, and regulatory and control mechanisms.

Complement and Chlamydia psittaci: Early Complement-Dependent Events Are Important for DC Migration and Protection During Mouse Lung Infection

The zoonotic intracellular bacterium Chlamydia psittaci causes life-threatening pneumonia in humans. During mouse lung infection, complement factor C3 and the anaphylatoxin C3a augment protection against C. psittaci by a so far unknown mechanism. To clarify how complement contributes to the early, innate and the late, specific immune response and resulting protection, this study addresses the amount of C3, the timing when its presence is required as well as the anaphylatoxin receptor(s) mediating its effects and the complement-dependent migration of dendritic cells. Challenge experiments with C. psittaci on various complement KO mice were combined with transient decomplementation by pharmacological treatment, as well as the analysis of in vivo dendritic cells migration. Our findings reveal that a plasma concentration of C3 close to wildtype levels was required to achieve full protection. The diminished levels of C3 of heterozygote C3+/- mice permitted already relative effective protection and improved survival as compared to C3-/- mice, but overall recovery of these animals was delayed. Complement was in particular required during the first days of infection. However, additionally, it seems to support protection at later stages. Migration of CD103+ dendritic cells from the infected lung to the draining lymph node-as prerequisite of antigen presentation-depended on C3 and C3aR and/or C5aR. Our results provide unique mechanistic insight in various aspects of complement-dependent immune responses under almost identical, rather physiological experimental conditions. Our study contributes to an improved understanding of the role of complement, and C3a in particular, in infections by intracellular bacteria.

Complement and Chlamydia psittaci: Non-Myeloid-Derived C3 Predominantly Induces Protective Adaptive Immune Responses in Mouse Lung Infection

Recent advances in complement research have revolutionized our understanding of its role in immune responses. The immunomodulatory features of complement in infections by intracellular pathogens, e.g., viruses, are attracting increasing attention. Thereby, local production and activation of complement by myeloid-derived cells seem to be crucial. We could recently show that C3, a key player of the complement cascade, is required for effective defense against the intracellular bacterium Chlamydia psittaci. Avian zoonotic strains of this pathogen cause life-threatening pneumonia with systemic spread in humans; closely related non-avian strains are responsible for less severe diseases of domestic animals with economic loss. To clarify how far myeloid- and non-myeloid cell-derived complement contributes to immune response and resulting protection against C. psittaci, adoptive bone marrow transfer experiments focusing on C3 were combined with challenge experiments using a non-avian (BSL 2) strain of this intracellular bacterium. Surprisingly, our data prove that for C. psittaci-induced pneumonia in mice, non-myeloid-derived, circulating/systemic C3 has a leading role in protection, in particular on the development of pathogen-specific T- and B- cell responses. In contrast, myeloid-derived and most likely locally produced C3 plays only a minor, mainly fine-tuning role. The work we present here describes authentic, although less pronounced, antigen directed immune responses.

Viral Evasion of the Complement System and Its Importance for Vaccines and Therapeutics

The complement system is a key component of innate immunity which readily responds to invading microorganisms. Activation of the complement system typically occurs via three main pathways and can induce various antimicrobial effects, including: neutralization of pathogens, regulation of inflammatory responses, promotion of chemotaxis, and enhancement of the adaptive immune response. These can be vital host responses to protect against acute, chronic, and recurrent viral infections. Consequently, many viruses (including dengue virus, West Nile virus and Nipah virus) have evolved mechanisms for evasion or dysregulation of the complement system to enhance viral infectivity and even exacerbate disease symptoms. The complement system has multifaceted roles in both innate and adaptive immunity, with both intracellular and extracellular functions, that can be relevant to all stages of viral infection. A better understanding of this virus-host interplay and its contribution to pathogenesis has previously led to: the identification of genetic factors which influence viral infection and disease outcome, the development of novel antivirals, and the production of safer, more effective vaccines. This review will discuss the antiviral effects of the complement system against numerous viruses, the mechanisms employed by these viruses to then evade or manipulate this system, and how these interactions have informed vaccine/therapeutic development. Where relevant, conflicting findings and current research gaps are highlighted to aid future developments in virology and immunology, with potential applications to the current COVID-19 pandemic.

A Factor I-Like Activity Associated with Chikungunya Virus Contributes to Its Resistance to the Human Complement System

Chikungunya virus (CHIKV) is an emerging pathogen capable of causing explosive outbreaks. Prior studies showed that exacerbation in arthritogenic alphavirus-induced pathogenesis is attributed to its interaction with multiple immune components, including the complement system. Viremia concomitant to CHIKV infection makes exposure of the virus to complement unavoidable, yet very little is known about CHIKV-complement interactions. Here, we show that CHIKV activated serum complement to modest levels in a concentration- and time-dependent manner, but the virus effectively resisted complement-mediated neutralization. Heat-inactivated serum from seropositive donors could actively neutralize CHIKV due to the presence of potent anti-CHIKV antibodies. Deposition of key complement components C3 and C4 did not alter the resistance of CHIKV to complement. Further, we identified a factor I-like activity in CHIKV that limited complement by inactivating C3b into inactive C3b (iC3b), the complement component known to significantly contribute to disease severity in vivo, but this activity had no effect on C4b. Inactivation of C3b by CHIKV was largely dependent on the concentration of the soluble host cofactor factor H and the virus concentration. A factor I function-blocking antibody had only a negligible effect on the factor I-like activity associated with CHIKV, suggesting that this activity is independent of host factor I and could be of viral origin. Thus, our findings suggest a complement modulatory action of CHIKV which not only helps the virus to evade human complement but may also have implications in alphavirus-induced arthritogenic symptoms.IMPORTANCE Chikungunya virus is a vector-borne pathogen of global significance. The morbidity associated with chikungunya virus (CHIKV) infection, neurovirulence and adaptability to Aedes albopictus, necessitates a deeper understanding of the interaction of CHIKV with the host immune system. Here, we demonstrate that CHIKV is resistant to neutralization by one of the potent barriers of the innate immune arm, the complement system. Chikungunya virus showed marked resistance to complement despite activation and deposition of complement proteins. Interestingly the C3 component associated with the virion was found to be inactive C3b (iC3b), a key factor implicated in the pathogenesis and disease severity in the mouse model of Ross River virus infection. CHIKV also had an associated unique factor I-like activity that mediated the inactivation of C3b into iC3b. We have unraveled a smart strategy adopted by CHIKV to limit complement which has serious implications in viral dissemination, pathogenesis, and disease.

Activation of a nerve injury transcriptional signature in airway-innervating sensory neurons after lipopolysaccharide-induced lung inflammation

The lungs and the immune and nervous systems functionally interact to respond to respiratory environmental exposures and infections. The lungs are innervated by vagal sensory neurons of the jugular and nodose ganglia, fused together in smaller mammals as the jugular-nodose complex (JNC). Whereas the JNC shares properties with the other sensory ganglia, the trigeminal (TG) and dorsal root ganglia (DRG), these sensory structures express differential sets of genes that reflect their unique functionalities. Here, we used RNA sequencing (RNA-seq) in mice to identify the differential transcriptomes of the three sensory ganglia types. Using a fluorescent retrograde tracer and fluorescence-activated cell sorting, we isolated a defined population of airway-innervating JNC neurons and determined their differential transcriptional map after pulmonary exposure to lipopolysaccharide (LPS), a major mediator of acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) after infection with gram-negative bacteria or inhalation of organic dust. JNC neurons activated an injury response program, leading to increased expression of gene products such as the G protein-coupled receptor Cckbr, inducing functional changes in neuronal sensitivity to peptides, and Gpr151, also rapidly induced upon neuropathic nerve injury in pain models. Unique JNC-specific transcripts, present at only minimal levels in TG, DRG, and other organs, were identified. These included TMC3, encoding for a putative mechanosensor, and urotensin 2B, a hypertensive peptide. These findings highlight the unique properties of the JNC and reveal that ALI/ARDS rapidly induces a nerve injury-related state, changing vagal excitability.

Dietary resveratrol supplementation inhibits heat stress-induced high-activated innate immunity and inflammatory response in spleen of yellow-feather broilers

The aim of this study was to investigate the effect of dietary resveratrol supplementation on innate immunity and inflammatory responses in the spleen of yellow-feather broilers under heat stress. A total of 288 yellow-feather broilers of 28-day-old were randomly assigned to 3 treatment groups with 6 replicates. A thermo-neutral group (TN) (24 ± 2°C) received a basal diet and another 2 heat-stressed groups (37 ± 2°C for 8 h/D and 24 ± 2°C for the remaining time) were fed the basal diet (HT) or basal diet with 500 mg/kg resveratrol (HT+Res) for 14 consecutive days. The results showed that heat stress decreased (P < 0.05) the growth index of thymus, spleen, and bursa of Fabricius, reduced (P < 0.05) the levels of complement C3 and C4 in serum. Heat stress also caused activation of inflammatory immune responses evidenced by increased (P < 0.05) the mRNA abundance of HSP (heat shock protein) 70, toll-like receptor (TLR)1, TLR4, TLR5, myeloid differentiation factor-88 (MyD88), nucleotide-binding oligomerization domain 1 (NOD1), Dectin-1, transforming growth factor-β-activated kinase 1 (TAK1), interleukin (IL)-1, IL-4, IL-6, and tumor necrosis factor (TNF)-α, but decreased the mRNA abundance of interferon (IFN)-γ, activated nuclear factor kappa B (NF-κB), mitogen-activated protein kinases (MAPK), and phosphoinositide-3 kinases-protein kinase B (PI3K/AKT) signaling pathways. Dietary supplementation with resveratrol improved (P < 0.05) the growth index of thymus, spleen and bursa Fabricius, and increased (P < 0.05) the serum level of complement C3 under heat stress. In addition, resveratrol reduced (P < 0.05) the mRNA abundance of HSP70, TLR4, TLR5, NOD1, Dectin-1, and TAK1, and inhibited the NF-κB, MAPK and PI3K/AKT signaling pathway via down-regulated the phosphorylation of p65, extracellular signal-regulated kinases 1/2, c-Jun N-terminal protein kinase and AKT, as well as decreased the inflammatory cytokines expression, including IL-1, IL-4, IL-6, and TNF-α in the spleen under heat stress. Collectively, dietary resveratrol could have beneficial effects to regulate innate immunity and inflammatory response, via inhibiting the activation of NF-κB, MAPK, and PI3K/AKT signaling pathways induced by heat stress in the spleen.

Complementing the Cancer-Immunity Cycle

Reactivation of cytotoxic CD8+ T-cell responses has set a new direction for cancer immunotherapy. Neutralizing antibodies targeting immune checkpoint programmed cell death protein 1 (PD-1) or its ligand (PD-L1) have been particularly successful for tumor types with limited therapeutic options such as melanoma and lung cancer. However, reactivation of T cells is only one step toward tumor elimination, and a substantial fraction of patients fails to respond to these therapies. In this context, combination therapies targeting more than one of the steps of the cancer-immune cycle may provide significant benefits. To find the best combinations, it is of upmost importance to understand the interplay between cancer cells and all the components of the immune response. This review focuses on the elements of the complement system that come into play in the cancer-immunity cycle. The complement system, an essential part of innate immunity, has emerged as a major regulator of cancer immunity. Complement effectors such as C1q, anaphylatoxins C3a and C5a, and their receptors C3aR and C5aR1, have been associated with tolerogenic cell death and inhibition of antitumor T-cell responses through the recruitment and/or activation of immunosuppressive cell subpopulations such as myeloid-derived suppressor cells (MDSCs), regulatory T cells (Tregs), or M2 tumor-associated macrophages (TAMs). Evidence is provided to support the idea that complement blocks many of the effector routes associated with the cancer-immunity cycle, providing the rationale for new therapeutic combinations aimed to enhance the antitumor efficacy of anti-PD-1/PD-L1 checkpoint inhibitors.

Expression of complement C3, C5, C3aR and C5aR1 genes in resting and activated CD4+ T cells

Complement activation is traditionally thought to occur in the extracellular space. However, it has been suggested that complement proteins are activated and function at additional locations. T cells contain intracellular stores of C3 and C5 that can be cleaved into C3a and C5a and bind to intracellular receptors, which have been shown to be of vital importance for the differentiation and function of these cells. However, whether the origin of the complement proteins located within T cells is derived from endogenous produced complement or from an uptake dependent mechanism is unknown. The presence of intracellular C3 in T cells from normal donors was investigated by fluorescence microscopy and flow cytometry. Moreover, mRNA expression levels of several genes encoding for complement proteins with primary focus on C3, C3aR, C5 and C5aR1 during resting state and upon activation of CD4⁺ T cells were investigated by a quantitative PCR technique. Furthermore, the gene expression level was evaluated at different time points. We confirmed the presence of intracellular C3 protein in normal T-cells. However, we could not see any increase in mRNA levels using any activation strategy tested. On the contrary, we observed a slight increase in C3 and C5aR1 mRNA only in the non-activated T-cells compared to the activated T cells, and a decrease in the activated T-cells at different incubation time points. Our results show that there is a baseline intracellular expression of the complement C3, C5, C3aR and C5aR1 genes in normal CD4⁺ T cells, but that expression is not increased during T-cell activation, but rather down regulated. Thus, the pool of intracellular complement in CD4⁺ T cells may either be due to accumulated complement due low-grade expression or arise from the circulation from an uptake dependent mechanism, but these possibilities are not mutually exclusive.

Complement receptor CD46 co-stimulates optimal human CD8+ T cell effector function via fatty acid metabolism

The induction of human CD4+ Th1 cells requires autocrine stimulation of the complement receptor CD46 in direct crosstalk with a CD4+ T cell-intrinsic NLRP3 inflammasome. However, it is unclear whether human cytotoxic CD8+ T cell (CTL) responses also rely on an intrinsic complement-inflammasome axis. Here we show, using CTLs from patients with CD46 deficiency or with constitutively-active NLRP3, that CD46 delivers co-stimulatory signals for optimal CTL activity by augmenting nutrient-influx and fatty acid synthesis. Surprisingly, although CTLs express NLRP3, a canonical NLRP3 inflammasome is not required for normal human CTL activity, as CTLs from patients with hyperactive NLRP3 activity function normally. These findings establish autocrine complement and CD46 activity as integral components of normal human CTL biology, and, since CD46 is only present in humans, emphasize the divergent roles of innate immune sensors between mice and men.

Complement and the Regulation of T Cell Responses

The complement system is an evolutionarily ancient key component of innate immunity required for the detection and removal of invading pathogens. It was discovered more than 100 years ago and was originally defined as a liver-derived, blood-circulating sentinel system that classically mediates the opsonization and lytic killing of dangerous microbes and the initiation of the general inflammatory reaction. More recently, complement has also emerged as a critical player in adaptive immunity via its ability to instruct both B and T cell responses. In particular, work on the impact of complement on T cell responses led to the surprising discoveries that the complement system also functions within cells and is involved in regulating basic cellular processes, predominantly those of metabolic nature. Here, we review current knowledge about complement's role in T cell biology, with a focus on the novel intracellular and noncanonical activities of this ancient system.

Unexpected Roles for Intracellular Complement in the Regulation of Th1 Responses

The complement system is generally recognized as an evolutionarily ancient and critical part of innate immunity required for the removal of pathogens that have breached the protective host barriers. It was originally defined as a liver-derived serum surveillance system that induces the opsonization and killing of invading microbes and amplifies the general inflammatory reactions. However, studies spanning the last four decades have established complement also as a vital bridge between innate and adaptive immunity. Furthermore, recent work on complement, and in particular its impact on human T helper 1 (Th1) responses, has led to the unexpected findings that the complement system also functions within cells and that it participates in the regulation of basic processes of the cell, including metabolism. These recent new insights into the unanticipated noncanonical activities of this ancient system suggest that the functions of complement extend well beyond mere host protection and into cellular physiology.

Lymphocyte integration of complement cues

We address current data, views and puzzles on the emerging topic of regulation of lymphocytes by complement proteins or fragments. Such regulation is believed to take place through complement receptors (CR) and membrane complement regulators (CReg) involved in cell function or protection, respectively, including intracellular signalling. Original observations in B cells clearly support that complement cues through CR improve their performance. Other lymphocytes likely integrate complement-derived signals, as most lymphoid cells constitutively express or regulate CR and CReg upon activation. CR-induced signals, particularly by anaphylatoxins, clearly regulate lymphoid cell function. In contrast, data obtained by CReg crosslinking using antibodies are not always confirmed in human congenital deficiencies or knock-out mice, casting doubts on their physiological relevance. Unsurprisingly, human and mouse complement systems are not completely homologous, adding further complexity to our still fragmentary understanding of complement-lymphocyte interactions.

TLR-Induced Murine Dendritic Cell (DC) Activation Requires DC-Intrinsic Complement

Induction of proinflammatory T cell immunity is augmented by innate dendritic cell (DC) maturation commonly initiated by TLR signaling. We demonstrate that ligation of TLR3, TLR4, and TLR9 induces murine DC production of complement components and local production of the anaphylatoxin C5a. In vitro, ex vivo, and in vivo analyses show that TLR-induced DC maturation, as assessed by surface phenotype, expression profiling by gene array, and functional ability to stimulate T cell responses, requires autocrine C3a receptor and C5a receptor (C3ar1/C5ar1) signaling. Studies using bone marrow chimeric animals and Foxp3-GFP/ERT2-Cre/dTomato fate-mapping mice show that TLR-initiated DC autocrine C3ar1/C5ar1 signaling causes expansion of effector T cells and instability of regulatory T cells and contributes to T cell-dependent transplant rejection. Together, our data position immune cell-derived complement production and autocrine/paracrine C3ar1/C5ar1 signaling as crucial intermediary processes that link TLR stimulation to DC maturation and the subsequent development of effector T cell responses.

Understanding human thiol dioxygenase enzymes: structure to function, and biology to pathology

Amino acid metabolism is a significant metabolic activity in humans, especially of sulphur-containing amino acids, methionine and cysteine (Cys). Cys is cytotoxic and neurotoxic in nature; hence, mammalian cells maintain a constant intracellular level of Cys. Metabolism of Cys is mainly regulated by two thiol dioxygenases: cysteine dioxygenase (CDO) and 2-aminoethanethiol dioxygenase (ADO). CDO and ADO are the only human thiol dioxygenases reported with a role in Cys metabolism and localized to mitochondria. This metabolic pathway is important in various human disorders, as it is responsible for the synthesis of antioxidant glutathione and is also for the synthesis of hypotaurine and taurine. CDO is the most extensively studied protein, whose high-resolution crystallographic structures have been solved. As compared to CDO, ADO is less studied, even though it has a key role in cysteamine metabolism. To further understand ADO's structure and function, the three-dimensional structures have been predicted from I-TASSER and SWISS-MODEL servers and validated with PROCHECK software. Structural superimposition approach using iPBA web server further confirmed near-identical structures (including active sites) for the predicted protein models of ADO as compared to CDO. In addition, protein-protein interaction and their association in patho-physiology are crucial in understanding protein functions. Both ADO and CDO interacting partner profiles have been presented using STRING database. In this study, we have predicted a 3D model structure for ADO and summarized the biological roles and the pathological consequences which are associated with the altered expression and functioning of ADO and CDO in case of cancer, neurodegenerative disorders and other human diseases.

Keeping It All Going-Complement Meets Metabolism

The complement system is an evolutionary old and crucial component of innate immunity, which is key to the detection and removal of invading pathogens. It was initially discovered as a liver-derived sentinel system circulating in serum, the lymph, and interstitial fluids that mediate the opsonization and lytic killing of bacteria, fungi, and viruses and the initiation of the general inflammatory responses. Although work performed specifically in the last five decades identified complement also as a critical instructor of adaptive immunity—indicating that complement’s function is likely broader than initially anticipated—the dominant opinion among researchers and clinicians was that the key complement functions were in principle defined. However, there is now a growing realization that complement activity goes well beyond “classic” immune functions and that this system is also required for normal (neuronal) development and activity and general cell and tissue integrity and homeostasis. Furthermore, the recent discovery that complement activation is not confined to the extracellular space but occurs within cells led to the surprising understanding that complement is involved in the regulation of basic processes of the cell, particularly those of metabolic nature—mostly via novel crosstalks between complement and intracellular sensor, and effector, pathways that had been overlooked because of their spatial separation. These paradigm shifts in the field led to a renaissance in complement research and provide new platforms to now better understand the molecular pathways underlying the wide-reaching effects of complement functions in immunity and beyond. In this review, we will cover the current knowledge about complement’s emerging relationship with the cellular metabolism machinery with a focus on the functional differences between serum-circulating versus intracellularly active complement during normal cell survival and induction of effector functions. We will also discuss how taking a closer look into the evolution of key complement components not only made the functional connection between complement and metabolism rather “predictable” but how it may also give clues for the discovery of additional roles for complement in basic cellular processes.

Complement C3F allotype synthesized by liver recipient modifies transplantation outcome independently from donor hepatic C3

Complement component 3 (C3) presents both slow (C3S) and fast (C3F) variants, which can be locally produced and activated by immune system cells. We studied C3 recipient variants in 483 liver transplant patients by RT-PCR-HRM to determine their effect on graft outcome during the first year post-transplantation. Allograft survival was significantly decreased in C3FF recipients (C3SS 95% vs C3FS 91% vs C3FF 83%; P=.01) or C3F allele carriers (C3F absence 95% vs C3F presence 90%, P=.02). C3FF genotype or presence of C3F allele independently increased risk for allograft loss (OR: 2.38, P=.005 and OR: 2.66, P=.02, respectively). C3FF genotype was more frequent among patients whose first infection was of viral etiology (C3SS 13% vs C3FS 18% vs C3FF 32%; P=.04) and independently increased risk for post-transplant viral infections (OR: 3.60, P=.008). On the other hand, C3FF and C3F protected from rejection events (OR: 0.54, P=.03 and OR: 0.63, P=.047, respectively). Differences were not observed in hepatitis C virus recurrence or patient survival. In conclusion, we show that, independently from C3 variants produced by donor liver, C3F variant from recipient diminishes allograft survival, increases susceptibility to viral infections, and protects from rejection after transplantation. C3 genotyping of liver recipients may be useful to stratify risk.

Innate and adaptive immunologic functions of complement in the host response to Listeria monocytogenes infection

Listeria monocytogenes is a leading cause of foodborne-illness associated mortality that has attracted considerable attention in recent years due to several significant outbreaks. It has also served as a model organism for the study of intracellular pathogens. For these reasons the host response to L. monocytogenes has long been the subject of investigation. A potent innate and adaptive immune response is required for containment and clearance of L. monocytogenes. However, some elements of this response, such as type 1 interferons, can be detrimental to the host. Recent studies have revealed novel functions for the complement system, an ancient arm of innate immunity, in this process. Here we review the role of complement in the host response to L. monocytogenes.

Complement-Opsonized HIV-1 Overcomes Restriction in Dendritic Cells

DCs express intrinsic cellular defense mechanisms to specifically inhibit HIV-1 replication. Thus, DCs are productively infected only at very low levels with HIV-1, and this non-permissiveness of DCs is suggested to go along with viral evasion. We now illustrate that complement-opsonized HIV-1 (HIV-C) efficiently bypasses SAMHD1 restriction and productively infects DCs including BDCA-1 DCs. Efficient DC infection by HIV-C was also observed using single-cycle HIV-C, and correlated with a remarkable elevated SAMHD1 T592 phosphorylation but not SAMHD1 degradation. If SAMHD1 phosphorylation was blocked using a CDK2-inhibitor HIV-C-induced DC infection was also significantly abrogated. Additionally, we found a higher maturation and co-stimulatory potential, aberrant type I interferon expression and signaling as well as a stronger induction of cellular immune responses in HIV-C-treated DCs. Collectively, our data highlight a novel protective mechanism mediated by complement opsonization of HIV to effectively promote DC immune functions, which might be in the future exploited to tackle HIV infection.

We here give insight into a substantial novel way of dendritic cell modulation at least during acute HIV-1 infection by triggering integrin receptor signaling. We found that complement-opsonization of the virus is able to relieve SAMHD1 restriction in DCs, thereby initiating strong maturation and co-stimulatory capacity of the cells and stimulating efficient cellular and humoral antiviral immune responses. This newly described way of DC modulation by complement might be exploited to find novel therapeutic targets promoting DC immune functions against HIV.

The paradoxical roles of C1q and C3 in autoimmunity

In this review we will focus on the links between complement and autoimmune diseases and will highlight how animal models have provided insights into the manner by which C1q and C3 act to modulate both adaptive and innate immune responses. In particular we will highlight how C1q may not only act as initiator of the classical complement pathway, but can also mediate multiple immune responses in a complement activation independent manner.

Complement: an overview for the clinician

The complement system is an essential component of the immune system. It is a highly integrative system and has a number of functions, including host defense, removal of injured cells and debris, modulation of metabolic and regenerative processes, and regulation of adaptive immunity. Complement is activated via different pathways and it is regulated tightly by several mechanisms to prevent host injury. Imbalance between complement activation and regulation can manifest in disease and injury to self. This article provides an outline of complement activation pathways, regulatory mechanisms, and normal physiologic functions of the system.

Complement receptor type 1 (CR1/CD35) expressed on activated human CD4+ T cells contributes to generation of regulatory T cells

The role of complement in the regulation of T cell immunity has been highlighted recently by several groups. We were prompted to reinvestigate the role of complement receptor type 1 (CR1, CD35) [corrected] in human T cells based on our earlier data showing that activated human T cells produce C3 (Torok et al. (2012) [48]) and also by results demonstrating that engagement of Membrane Cofactor Protein (MCP, CD46) induces a switch of anti-CD35-activated [corrected] helper T cells into regulatory T cells (Kemper et al. (2003) [17]). We demonstrate here that co-ligation of CD46 and CD35, [corrected] the two C3b-binding structures present on activated CD4+ human T cells significantly enhances CD25 expression, elevates granzyme B production and synergistically augments cell proliferation. The role of CR1 in the development of the Treg phenotype was further confirmed by demonstrating that its engagement enhances IL-10 production and reduces IFNγ release by the activated CD4+ T cells in the presence of excess IL-2. The functional in vivo relevance of our findings was highlighted by the immunohistochemical staining of tonsils, revealing the presence of CD4/CD35 [corrected] double positive lymphocytes mainly in the inter-follicular regions where direct contact between CD4+ T cells and B lymphocytes occurs. Regarding the in vivo relevance of the complement-dependent generation of regulatory T cells in secondary lymphoid organs we propose a scenario shown in the figure. The depicted process involves the sequential binding of locally produced C3 fragments to CD46 and CD35 [corrected] expressed on activated T cells, which - in the presence of excess IL-2 - leads to the development of Treg cells.

Intranasal peptide-induced tolerance and linked suppression: consequences of complement deficiency

A role for complement, particularly the classical pathway, in the regulation of immune responses is well documented. Deficiencies in C1q or C4 predispose to autoimmunity, while deficiency in C3 affects the suppression of contact sensitization and generation of oral tolerance. Complement components including C3 have been shown to be required for both B-cell and T-cell priming. The mechanisms whereby complement can mediate these diverse regulatory effects are poorly understood. Our previous work, using the mouse minor histocompatibility (HY) model of skin graft rejection, showed that both C1q and C3 were required for the induction of tolerance following intranasal peptide administration. By comparing tolerance induction in wild-type C57BL/6 and C1q-, C3-, C4- and C5-deficient C57BL/6 female mice, we show here that the classical pathway components including C3 are required for tolerance induction, whereas C5 plays no role. C3-deficient mice failed to generate a functional regulatory T (Treg) -dendritic cell (DC) tolerogenic loop required for tolerance induction. This was related to the inability of C3-deficient DC to up-regulate the arginine-consuming enzyme, inducible nitric oxide synthase (Nos-2), in the presence of antigen-specific Treg cells and peptide, leading to reduced Treg cell generation. Our findings demonstrate that the classical pathway and C3 play a critical role in the peptide-mediated induction of tolerance to HY by modulating DC function.

Complement regulation of T-cell alloimmunity

Complement proteins are generated both by the liver (systemic compartment) and by peripheral tissue-resident cells and migratory immune cells (local compartment). The immune cell-derived, alternative pathway complement components activate spontaneously, yielding local, but not systemic, production of C3a and C5a. These anaphylatoxins bind to their respective G-protein-coupled receptors, the C3a receptor and the C5a receptor, expressed on T cells and antigen-presenting cells, leading to their reciprocal activation and driving T-cell differentiation, expansion, and survival. Complement deficiency or blockade attenuates T-cell-mediated autoimmunity and delays allograft rejection in mice. Increasing complement activation, achieved by genetic removal of the complement regulatory protein decay accelerating factor, enhances murine T-cell immunity and accelerates allograft rejection. Signaling through the C3a receptor and the C5a receptor reduces suppressive activity of natural regulatory T cells and the generation and stability of induced regulatory T cells. The concepts, initially generated in mice, recently were confirmed in human immune cells, supporting the need for testing of complement targeting therapies in organ transplants patients.

C3 opsonization regulates endocytic handling of apoptotic cells resulting in enhanced T-cell responses to cargo-derived antigens

Apoptotic cells are a source of autoantigens and impairment of their removal contributes to the development of autoimmunity in C1q deficiency. However, the lack of complement component 3 (C3), the predominant complement opsonin, does not predispose to autoimmunity, suggesting a modifying role of C3 in disease pathogenesis. To explore this hypothesis, here we investigated the role of C3 in the T-cell response to apoptotic cell-associated antigens. By comparing the phagosome maturation and the subsequent MHC class II presentation of a peptide derived from the internalized cargo between C3-deficient or C3-sufficient dendritic cells, we found that C3 deficiency accelerated the fusion of the apoptotic cargo with lysosomes. As a result, C3 deficiency led to impaired antigen-specific T-cell proliferation in vitro and in vivo. Notably, preopsonization of the apoptotic cells with C3 activation fragments rectified the trafficking and T-cell stimulation defects. These data indicate that activated C3 may act as a "chaperone" in the intracellular processing of an apoptotic cargo and, thus, may modulate the T-cell response to self-antigens displayed on dying cells.

Human complement C3 deficiency: Th1 induction requires T cell-derived complement C3a and CD46 activation

Human T helper type 1 (Th1) responses are essential in defense. Although T cell receptor (TCR) and co-stimulator engagement are indispensable for T cell activation, stimulation of additional receptor pathways are also necessary for effector induction. For example, engagement of the complement regulator CD46 by its ligand C3b generated upon TCR activation is required for IFN-γ production as CD46-deficient patients lack Th1 responses. Utilizing T cells from two C3-deficient patients we demonstrate here that normal Th1 responses also depend on signals mediated by the anaphylatoxin C3a receptor (C3aR). Importantly, and like in CD46-deficient patients, whilst Th1 induction are impaired in C3-deficient patients in vitro, their Th2 responses are unaffected. Furthermore, C3-deficient CD4(+) T cells present with reduced expression of CD25 and CD122, further substantiating the growing notion that complement fragments regulate interleukin-2 receptor (IL-2R) assembly and that disturbance of complement-guided IL-2R assembly contributes to aberrant Th1 effector responses. Lastly, sustained intrinsic production of complement fragments may participate in the Th1 contraction phase as both C3a and CD46 engagement regulate IL-10 co-expression in Th1 cells. These data suggest that C3aR and CD46 activation via intrinsic generation of their respective ligands is an integral part of human Th1 (but not Th2) immunity.

An essential role for C5aR signaling in the optimal induction of a malaria-specific CD4+ T cell response by a whole-killed blood-stage vaccine

The protective immunity induced by the whole-killed parasite vaccine against malarial blood-stage infection is dependent on the CD4(+) T cell response. However, the mechanism underlying this robust CD4(+) T cell response elicited by the whole-killed parasite vaccine is still largely unknown. In this study, we observe that immunization with Plasmodium yoelii-parasitized RBC lysate activates complement C5 and generates C5a. However, the protective efficacy against P. yoelii 17XL challenge is considerably reduced, and the malaria-specific CD4(+) T cell activation and memory T cell differentiation are largely suppressed in the C5aR-deficient (C5aR(-/-)) mice. An adoptive transfer assay demonstrates that the reduced protection of C5aR(-/-) mice is closely associated with the severely impaired CD4(+) T cell response. This is further confirmed by the fact that administration of C5aR antagonist significantly reduces the protective efficacy of the immunized B cell-deficient mice. Further study indicates that the defective CD4(+) T cell response in C5aR(-/-) mice is unlikely involved in the expansion of CD4(+)CD25(+)Foxp3(+) T cells, but strongly linked to a defect in dendritic cell (DC) maturation and the ability to allostimulate CD4(+) T cells. These results demonstrate that C5aR signaling is essential for the optimal induction of the malaria-specific CD4(+) T cell response by the whole-killed parasite vaccine through modulation of DCs function, which provides us with new clues to design an effective blood-stage subunit vaccine and helps us to understand the mechanism by which the T cell response is regulated by the complement system.

Complement mediated signaling on pulmonary CD103(+) dendritic cells is critical for their migratory function in response to influenza infection

Trafficking of lung dendritic cells (DCs) to the draining lymph node (dLN) is a crucial step for the initiation of T cell responses upon pathogen challenge. However, little is known about the factors that regulate lung DC migration to the dLN. In this study, using a model of influenza infection, we demonstrate that complement component C3 is critically required for efficient emigration of DCs from the lung to the dLN. C3 deficiency affect lung DC-mediated viral antigen transport to the dLN, resulting in severely compromised priming of virus-specific T cell responses. Consequently, C3-deficient mice lack effector T cell response in the lungs that affected viral clearance and survival. We further show that direct signaling by C3a and C5a through C3aR and C5aR respectively expressed on lung DCs is required for their efficient trafficking. However, among lung DCs, only CD103⁺ DCs make a significant contribution to lung C5a levels and exclusively produce high levels of C3 and C5 during influenza infection. Collectively, our findings show that complement has a profound impact on immune regulation by controlling tissue DC trafficking and highlights a potential utility for complement as an adjuvant in novel vaccine strategies.

Influenza is a global health problem frequented by epidemics and pandemics. Current vaccines against influenza offer limited protection hence the need for reformulation and repeated vaccination. There is a pressing need to develop newer vaccines that are able to generate T cell response. In order to develop such vaccines, there is a need to understand how T cell responses are generated during influenza infection. Influenza specific T cell responses are generated by the dendritic cells (DCs) in the lung. Upon influenza infection, DCs in the lung carry viral peptides to the draining lymph node (dLN) to initiate an immune response. Thus, migration of DCs from the lung to the dLN is an important step in the initiation of influenza specific T cell response. We now show that activation products of the complement system interact with their receptors on the DCs, which signals for the DCs to migrate from the lung to the dLN. Thus, our results reveal a previously unknown function for complement in mediating lung DC migration during influenza infection and highlight its potential as an adjuvant in novel vaccine strategies.

Complement regulation of T cell immunity

Results of studies published since 2002 reveal that T cells and antigen-presenting cells (APCs) produce complement proteins. The immune cell-derived, alternative pathway complement components activate spontaneously, yielding local, but not systemic, production of C3a and C5a. These anaphylatoxins bind to their respective G-protein-coupled receptors, C3aR and C5aR, expressed on both partners. The resultant complement-induced T cell activation and APC activation drive T cell differentiation, expansion and survival. Complement deficiency or blockade attenuates T cell-mediated autoimmunity and delays allograft rejection in mice. Increasing complement activation, achieved by genetic removal of the complement regulatory protein decay-accelerating factor, enhances murine T cell immunity and accelerates allograft rejection. The findings support the need for design and testing of complement inhibitors in humans.

Targeting viral antigens to CD11c on dendritic cells induces retrovirus-specific T cell responses

Dendritic cells (DC) represent the most potent antigen presenting cells and induce efficient cytotoxic T lymphocyte (CTL) responses against viral infections. Targeting antigens (Ag) to receptors on DCs is a promising strategy to enhance antitumor and antiviral immune responses induced by DCs. Here, we investigated the potential of CD11c-specific single-chain fragments (scFv) fused to an immunodominant peptide of Friend retrovirus for induction of virus-specific T cell responses by DCs. In vitro CD11c-specific scFv selectively targeted viral antigens to DCs and thereby significantly improved the activation of virus-specific T cells. In vaccination experiments DCs loaded with viral Ag targeted to CD11c provided improved rejection of FV-derived tumors and efficiently primed virus-specific CTL responses after virus challenge. Since the induction of strong virus-specific T cell responses is critical in viral infections, CD11c targeted protein vaccines might provide means to enhance the cellular immune response to prophylactic or therapeutic levels.

Complement factor C7 contributes to lung immunopathology caused by Mycobacterium tuberculosis

Mycobacterium tuberculosis (MTB) remains a significant global health burden despite the availability of antimicrobial chemotherapy. Increasing evidence indicates a critical role of the complement system in the development of host protection against the bacillus, but few studies have specifically explored the function of the terminal complement factors. Mice deficient in complement C7 and wild-type C57BL/6 mice were aerosol challenged with MTB Erdman and assessed for bacterial burden, histopathology, and lung cytokine responses at days 30 and 60 post-infection. Macrophages isolated from C7 −/− and wild-type mice were evaluated for MTB proliferation and cytokine production. C7 −/− mice had significantly less liver colony forming units (CFUs) at day 30; no differences were noted in lung CFUs. The C7 deficient mice had markedly reduced lung occlusion with significantly increased total lymphocytes, decreased macrophages, and increased numbers of CD4+ cells 60 days post-infection. Expression of lung IFN-γ and TNF-α was increased at day 60 compared to wild-type mice. There were no differences in MTB-proliferation in macrophages isolated from wild-type and knock-out mice. These results indicate a role for complement C7 in the development of MTB induced immunopathology which warrants further investigation.

Innate immune induction and influenza protection elicited by a response-selective agonist of human C5a

The anaphylatoxin C5a is an especially potent mediator of both local and systemic inflammation. However, C5a also plays an essential role in mucosal host defense against bacterial, viral, and fungal infection. We have developed a response-selective agonist of human C5a, termed EP67, which retains the immunoenhancing activity of C5a at the expense of its inflammatory, anaphylagenic properties. EP67 insufflation results in the rapid induction of pulmonary cytokines and chemokines. This is followed by an influx of innate immune effector cells, including neutrophils, NK cells, and dendritic cells. EP67 exhibits both prophylactic and therapeutic protection when tested in a murine model of influenza A infection. Mice treated with EP67 within a twenty-four hour window of non-lethal infection were significantly protected from influenza-induced weight loss. Furthermore, EP67 delivered twenty-four hours after lethal infection completely blocked influenza-induced mortality (0% vs. 100% survival). Since protection based on innate immune induction is not restricted to any specific pathogen, EP67 may well prove equally efficacious against a wide variety of possible viral, bacterial, and fungal pathogens. Such a strategy could be used to stop the worldwide spread of emergent respiratory diseases, including but not limited to novel strains of influenza.

Systemic delivery of oncolytic viruses: hopes and hurdles

Despite recent advances in both surgery and chemoradiotherapy, mortality rates for advanced cancer remain high. There is a pressing need for novel therapeutic strategies; one option is systemic oncolytic viral therapy. Intravenous administration affords the opportunity to treat both the primary tumour and any metastatic deposits simultaneously. Data from clinical trials have shown that oncolytic viruses can be systemically delivered safely with limited toxicity but the results are equivocal in terms of efficacy, particularly when delivered with adjuvant chemotherapy. A key reason for this is the rapid clearance of the viruses from the circulation before they reach their targets. This phenomenon is mainly mediated through neutralising antibodies, complement activation, antiviral cytokines, and tissue-resident macrophages, as well as nonspecific uptake by other tissues such as the lung, liver and spleen, and suboptimal viral escape from the vascular compartment. A range of methods have been reported in the literature, which are designed to overcome these hurdles in preclinical models. In this paper, the potential advantages of, and obstacles to, successful systemic delivery of oncolytic viruses are discussed. The next stage of development will be the commencement of clinical trials combining these novel approaches for overcoming the barriers with systemically delivered oncolytic viruses.

The role of humoral innate immunity in hepatitis C virus infection

Infection with Hepatitis C Virus (HCV) causes chronic disease in approximately 80% of cases, resulting in chronic inflammation and cirrhosis. Current treatments are not completely effective, and a vaccine has yet to be developed. Spontaneous resolution of infection is associated with effective host adaptive immunity to HCV, including production of both HCV-specific T cells and neutralizing antibodies. However, the supporting role of soluble innate factors in protection against HCV is less well understood. The innate immune system provides an immediate line of defense against infections, triggering inflammation and playing a critical role in activating adaptive immunity. Innate immunity comprises both cellular and humoral components, the humoral arm consisting of pattern recognition molecules such as complement C1q, collectins and ficolins. These molecules activate the complement cascade, neutralize pathogens, and recruit antigen presenting cells. Here we review the current understanding of anti-viral components of the humoral innate immune system that play a similar role to antibodies, describing their role in immunity to HCV and their potential contribution to HCV pathogenesis.

Novel roles of complement in T effector cell regulation

Our understanding of the complement system has markedly evolved from its early beginnings as a protein system merely detecting and tagging a pathogen for further clearance. For example, the repertoire of danger that complement recognizes covers currently a wide range of distinct self and non-self danger signals. Further, complement is now firmly established as instructor of adaptive B and T cell immunity. This review focuses on two the recent emerging paradigms in the field. Firstly, that complement is not only vitally required for the induction of Th1 immunity but also for the timely contraction of this protective response and therefore for prevention of autoimmunity and immune homeostasis. Secondly, that local rather than systemic complement is impacting on immune modulation during a T cell response.

Complement regulates CD4 T-cell help to CD8 T cells required for murine allograft rejection

Although induction of CD8 T-cell responses to transplants requires CD4-cell help, how this help is transmitted remains incompletely characterized. In vitro, cognate interactions between CD4 T cells and dendritic cells (DCs) induce C3a and C5a production. CD8(+) T cells lacking C3a receptor (C3aR) and C5a receptor (C5aR) proliferate weakly to allogeneic DCs despite CD4 help, indicating that CD4-cell help is mediated, in part, through DC-derived C3a/C5a acting on CD8(+) T cell-expressed C3aR/C5aR. In support of this concept, augmenting DC C5a/C3a production bypasses the requirement for CD4- and CD40-dependent help to wild-type CD8(+) T cells. CD4-deficient recipients of allogeneic heart transplants prime weak CD8 responses and do not acutely reject their grafts. In contrast, CD4-deficient chimeric mice possessing decay accelerating factor deficient (Daf1(-/-)) bone marrow, in which DC C3a/C5a production is potentiated, acutely reject transplants through a CD8 cell-dependent mechanism. Furthermore, hearts transplanted into CD40(-/-) mice prime weak CD8-cell responses and survive indefinitely, but hearts transplanted into Daf1(-/-)CD40(-/-) recipients undergo CD8 cell-dependent rejection. Together, the data indicate that heightened production and activation of immune cell-derived complement bypasses the need for CD40/CD154 interactions and implicate antigen-presenting cell-produced C5a and C3a as molecular bridges linking CD4 help to CD8(+) T cells.

A protective role of complement component 3 in T cell-mediated skin inflammation

Keratinocytes synthesize complement component 3 (C3) constitutively, and increased expression of C3 has been described during skin inflammation. In this study, we investigated the role of C3 in T cell-mediated allergic contact dermatitis, which is a clinical manifestation of contact sensitivity (CS). C3-deficient mice (C3KO) showed substantial higher CS responses to haptens, inducing a Th1 cytokine-mediated skin inflammation (2,4-dinitrofluorobenzene and dinitrochlorobenzene), and to haptens known to induce a Th2-polarized inflammatory response (fluoro-isothiocynate and toluene-2,4-diisocyanate) as compared to their wild-type (WT) controls. There was a higher influx of GR-1(+) , CD4(+) , and CD8(+) cells into the skin of hapten-treated C3KO mice compared with WT mice. Activated splenocytes from C3KO mice immunized with DNCB secreted higher amounts of IFN-γ compared with WT controls but not of Th2 (IL-4, IL-5, and IL-10) cytokines or IL-17. A higher secretion of IL-12 from splenocytes of C3KO mice as compared with WT mice was observed after TLR-4 ligand (LPS) or TLR-2 ligand (peptidoglycan) stimulation. Thus, an increased expression of IL-12 and of IFN-γ may be responsible for the increased hapten-induced inflammation in C3 deficiency. Finally, we demonstrated that C3KO mice developed oral tolerance to haptens to a lower degree than WT mice. Our findings provide a new insight into a novel anti-inflammatory role of C3 in skin inflammation.

Use of DAF-displaying adenovirus vectors reduces induction of transgene- and vector-specific adaptive immune responses in mice

Adenovirus (Ad)-based vectors are attractive candidates for a variety of gene-transfer applications. In this study, we found that decay-accelerating factor (DAF)-displaying Ads induce significantly decreased cellular immune responses to transgenes expressed from the vectors in both Ad5-naive and Ad5-immune mice. Specifically, we found a diminished ability of splenocytes to secrete interferon-γ after recall exposure to multiple peptides derived from antigens expressed by DAF-displaying Ads. We also confirmed that DAF-displaying Ads induce decreased numbers of antigen-specific, CD8(+) effector memory and central memory CD8(+) T cells, thereby uncovering a unique role of complement in modulating the induction of robust memory T-cell responses. We also confirmed that DAF-displaying Ads generate significantly reduced titers of Ad capsid-specific neutralizing antibodies after gene transfer in vivo. In conclusion, DAF-displaying Ad5-based vectors exhibit decreased induction of complement-dependent, innate immune responses, resulting in both an improved safety profile and a decreased propensity to induce humoral and cellular adaptive immune responses to Ad capsid proteins and Ad vector-expressed transgene products. This attractive combination of features will be beneficial in a variety of clinically relevant gene-transfer applications.