Complement-Dependent Enhancement of CD8+T Cell Immunity to Lymphocytic Choriomeningitis Virus Infection in Decay-Accelerating Factor-Deficient Mice

Complement regulators as novel targets for anti-cancer therapy: A comprehensive review

Cancer remains a formidable global health challenge requiring the continued exploration of innovative therapeutic approaches. While traditional treatment strategies including surgery, chemotherapy, and radiation therapy have had some success, primarily in early-stage disease, the quest for more targeted, personalized, safer, and effective therapies remains an ongoing pursuit. Over the past decade, significant advances in the field of tumor immunology have dramatically shifted a focus towards immunotherapy, although the ability to harness and coopt the immune system to treat cancer is still just beginning to be realized. One important area that has yet to be fully explored is the complement system, an integral part of innate immunity that has gathered attention recently as a source of potential targets for anti-cancer therapy. The complement system has a complex and context dependent role in cancer biology in that it not only contributes to immune surveillance but also may promote tumor progression. Complement regulators, including CD46, CD55, CD59, and complement factor H, exercise defined control over complement activation, and have also been acknowledged for their role in the tumor microenvironment. This review explores the intricate role of complement regulators in cancer development and progression, examining their potential as therapeutic targets, current strategies, challenges, and the evolving landscape of clinical research.

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 in ischaemia-reperfusion injury and transplantation

Until recently, the only known condition in which complement could mediate transplant injury was the rare occurrence of antibody-mediated rejection, in which the original concept of antibody immunity against the transplant was supported by complementary proteins present in the serum. This has changed within the last two decades because of evidence that the processes of ischaemia–reperfusion injury followed by T cell–mediated rejection are also critically dependent on components generated by the complement system. We now have a clearer understanding of the complement triggers and effectors that mediate injury, and a detailed map of their local sites of production and activation in the kidney. This is providing helpful guidelines as to how these harmful processes that restrict transplant outcomes can be targeted for therapeutic benefit. Here we review some of the recent advances highlighting relevant therapeutic targets.

Synovial Complement Factors in Patients with Periprosthetic Joint Infection after Undergoing Revision Arthroplasty of the Hip or Knee Joint

The role and diagnostic value of the synovial complement system in patients with low-grade periprosthetic joint infection (PJI) are unclear. We sought to evaluate, for the first time, the usefulness of synovial complement factors in these patients by measuring the individual synovial fluid levels of complement factors (C1q, C3b/iC3b, C4b, C5, C5a, C9, factor B, factor D, factor H, factor I, properdin, and mannose-binding lectin [MBL]). The patients (n = 74) were classified into septic (n = 28) and aseptic (n = 46). Receiver-operator characteristic curves and a multiple regression model to determine the feasibility of a combination of the tested cytokines to determine the infection status were calculated. The synovial fluid levels of C1q, C3b/C3i, C4b, C5, C5a, MBL, and properdin were significantly elevated in the PJI group. The best sensitivity and specificity was found for C1q. The multiple regression models revealed that the combination of C1q, C3b/C3i, C4b, C5, C5a, and MBL was associated with the best sensitivity (83.3%) and specificity (79.2%) for a cutoff value of 0.62 (likelihood ratio: 4.0; area under the curve: 0.853). Nevertheless, only a combined model showed acceptable results. The expression patterns of the complement factors suggested that PJI activates all three pathways of the complement system.

Hassan MS, C. Cork S, Abdul-Careem MF. Infectious Bronchitis Coronavirus Infection in Chickens: Multiple System Disease with Immune Suppression

In the early 1930s, infectious bronchitis (IB) was first characterized as a respiratory disease in young chickens; later, the disease was also described in older chickens. The etiology of IB was confirmed later as being due to a coronavirus: the infectious bronchitis virus (IBV). Being a coronavirus, IBV is subject to constant genome change due to mutation and recombination, with the consequence of changing clinical and pathological manifestations. The potential use of live attenuated vaccines for the control of IBV infection was demonstrated in the early 1950s, but vaccine breaks occurred due to the emergence of new IBV serotypes. Over the years, various IBV genotypes associated with reproductive, renal, gastrointestinal, muscular and immunosuppressive manifestations have emerged. IBV causes considerable economic impacts on global poultry production due to its pathogenesis involving multiple body systems and immune suppression; hence, there is a need to better understand the pathogenesis of infection and the immune response in order to help developing better management strategies. The evolution of new strains of IBV during the last nine decades against vaccine-induced immune response and changing clinical and pathological manifestations emphasize the necessity of the rational development of intervention strategies based on a thorough understanding of IBV interaction with the host.

The Role of Membrane Bound Complement Regulatory Proteins in Tumor Development and Cancer Immunotherapy

It has long been understood that the control and surveillance of tumors within the body involves an intricate dance between the adaptive and innate immune systems. At the center of the interplay between the adaptive and innate immune response sits the complement system—an evolutionarily ancient response that aids in the destruction of microorganisms and damaged cells, including cancer cells. Membrane-bound complement regulatory proteins (mCRPs), such as CD46, CD55, and CD59, are expressed throughout the body in order to prevent over-activation of the complement system. These mCRPs act as a double-edged sword however, as they can also over-regulate the complement system to the extent that it is no longer effective at eliminating cancerous cells. Recent studies are now indicating that mCRPs may function as a biomarker of a malignant transformation in numerous cancer types, and further, are being shown to interfere with anti-tumor treatments. This highlights the critical roles that therapeutic blockade of mCRPs can play in cancer treatment. Furthermore, with the complement system having the ability to both directly and indirectly control adaptive T-cell responses, the use of a combinatorial approach of complement-related therapy along with other T-cell activating therapies becomes a logical approach to treatment. This review will highlight the biomarker-related role that mCRP expression may have in the classification of tumor phenotype and predicted response to different anti-cancer treatments in the context of an emerging understanding that complement activation within the Tumor Microenvironment (TME) is actually harmful for tumor control. We will discuss what is known about complement activation and mCRPs relating to cancer and immunotherapy, and will examine the potential for combinatorial approaches of anti-mCRP therapy with other anti-tumor therapies, especially checkpoint inhibitors such as anti PD-1 and PD-L1 monoclonal antibodies (mAbs). Overall, mCRPs play an essential role in the immune response to tumors, and understanding their role in the immune response, particularly in modulating currently used cancer therapeutics may lead to better clinical outcomes in patients with diverse cancer types.

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.

Blockade of the C5a-C5aR axis alleviates lung damage in hDPP4-transgenic mice infected with MERS-CoV

The pathogenesis of highly pathogenic Middle East respiratory syndrome coronavirus (MERS-CoV) remains poorly understood. In a previous study, we established an hDPP4-transgenic (hDPP4-Tg) mouse model in which MERS-CoV infection causes severe acute respiratory failure and high mortality accompanied by an elevated secretion of cytokines and chemokines. Since excessive complement activation is an important factor that contributes to acute lung injury after viral infection, in this study, we investigated the role of complement in MERS-CoV-induced lung damage. Our study showed that complement was excessively activated in MERS-CoV-infected hDPP4-Tg mice through observations of increased concentrations of the C5a and C5b-9 complement activation products in sera and lung tissues, respectively. Interestingly, blocking C5a production by targeting its receptor, C5aR, alleviated lung and spleen tissue damage and reduced inflammatory responses. More importantly, anti-C5aR antibody treatment led to decreased viral replication in lung tissues. Furthermore, compared with the sham treatment control, apoptosis of splenic cells was less pronounced in the splenic white pulp of treated mice, and greater number of proliferating splenic cells, particularly in the red pulp, was observed. These data indicate that (1) dysregulated host immune responses contribute to the severe outcome of MERS; (2) excessive complement activation, triggered by MERS-CoV infection, promote such dysregulation; and (3) blockade of the C5a-C5aR axis lead to the decreased tissue damage induced by MERS-CoV infection, as manifested by reduced apoptosis and T cell regeneration in the spleen. Therefore, the results of this study suggest a new strategy for clinical intervention and adjunctive treatment in MERS-CoV cases.

Expression of CD55, CD59, and CD35 on red blood cells of β-thalassaemia patients

AIM OF THE STUDY

β-thalassaemia (β-Thal) is considered a severe, progressive haemolytic anaemia, which needs regular blood transfusions for life expectancy. Complement-mediated erythrocyte destruction can cause both intravascular and extravascular haemolysis. Complement regulatory proteins protect cells from such effects of the complement system. We aimed to perform quantitative analysis of membrane-bound complement regulators, CD55 (decay accelerating factor - DAF), CD35 (complement receptor type 1 - CR1), and CD59 (membrane attack complex inhibitory factor - MACIF) on peripheral red blood cells by flow cytometry.

MATERIAL AND METHODS

The present study was carried out on 47 β-thalassemia major (β-TM) patients, 20 β-thalassaemia intermedia (β-TI) patients, and 17 healthy volunteers as control subjects.

RESULTS

CD55 levels of β-TM patients (58.64 ±17.06%) were significantly decreased compared to β-TI patients (83.34 ±13.82%) and healthy controls (88.57 ±11.69%) (p < 0.01). CD59 levels of β-TM patients were not significantly different than β-TI patients and controls, but CD35 levels were significantly lower in the β-TM patients (3.56 ±4.87%) and β-TI patients (12.48 ±9.19%) than in the control group (39.98 ±15.01%) (p < 0.01).

CONCLUSIONS

Low levels of CD55 and CD35 in thalassaemia major patients indicates a role for them in the aetiopathogenesis of haemolysis in this disease, and also this defect in a complement system may be responsible for the chronic complications seen in these patients.

Deficiency of CC chemokine ligand 2 and decay-accelerating factor causes retinal degeneration in mice

CC chemokine ligand 2 (CCL2) recruits macrophages to reduce inflammatory responses. Decay-accelerating factor (DAF) is a membrane regulator of the classical and alternative pathways of complement activation. In view of the link between complement genes and retinal diseases, we evaluated the retinal phenotype of C57BL/6J mice and mice lacking Ccl2 and/or Daf1 at 12 months of age, using scanning laser ophthalmoscopic imaging, electroretinography (ERG), histology, immunohistochemistry, and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) analysis. In comparison to C57BL/6J mice, mutant mice had an increased number of autofluorescent foci, with the greatest number in the Ccl2(-/-)/Daf1(-/-) retina. ERG amplitudes in Ccl2(-/-)/Daf1(-/-), Ccl2(-/-) and Daf1(-/-) mice were reduced, with the greatest reduction in Ccl2(-/-)/Daf1(-/-) mice. TUNEL-positive cells were not seen in C57BL/6J retina, but were prevalent in the outer and inner nuclear layers of Ccl2(-/-)Daf1(-/-) mice and were present at reduced density in Ccl2(-/-) or Daf1(-/-) mice. Cell loss was most pronounced in the outer and inner nuclear layers of Ccl2(-/-)/Daf1(-/-) mice. The levels of the endoplasmic reticulum chaperone GPR78 and transcription factor ATF4 were significantly increased in the Ccl2(-/-)/Daf1(-/-) retina. In comparison to the C57BL/6J retina, the phosphorylation of NF-κB p65, p38, ERK and JNK was significantly upregulated while SIRT1 was significantly downregulated in the Ccl2(-/-)/Daf1(-/-) retina. Our results suggest that loss of Ccl2 and Daf1 causes retinal neuronal death and degeneration which is related to increased endoplasmic reticulum stress, oxidative stress and inflammation.

Complement modulation of T cell immune responses during homeostasis and disease

The complement system is an ancient and critical effector mechanism of the innate immune system as it senses, kills, and clears infectious and/or dangerous particles and alerts the immune system to the presence of the infection and/or danger. Interestingly, an increasing number of reports have demonstrated a clear role for complement in the adaptive immune system as well. Of note, a number of recent studies have identified previously unknown roles for complement proteins, receptors, and regulators in T cell function. Here, we will review recent data demonstrating the influence of complement proteins C1q, C3b/iC3b, C3a (and C3aR), and C5a (and C5aR) and complement regulators DAF (CD55) and CD46 (MCP) on T cell function during homeostasis and disease. Although new concepts are beginning to emerge in the field of complement regulation of T cell function, future experiments should focus on whether complement is interacting directly with the T cell or is having an indirect effect on T cell function via APCs, the cytokine milieu, or downstream complement activation products. Importantly, the identification of the pivotal molecular pathways in the human systems will be beneficial in the translation of concepts derived from model systems to therapeutic targeting for treatment of human disorders.

Systemic C3 modulates CD8+ T cell contraction after Listeria monocytogenes infection

Ag-specific CD8(+) T cell contraction (contraction), which occurs after the resolution of infection, is critical for homeostasis of the immune system. Although complement components regulate the primary CD8(+) T cell response, there is insufficient evidence supporting their role in regulating contraction and memory. In this study, we show that C3-deficient (C3(-/-)) mice exhibited significantly less CD8(+) T cell contraction than did wild-type mice postinfection with recombinant Listeria monocytogenes expressing OVA. Kinetic analyses also revealed decreased contraction in mice treated with cobra venom factor to deplete C3, which was consistent with the results in C3(-/-) recipient mice transplanted with bone marrow cells from the same donors as wild-type recipient mice. The phenotypes of memory cells generated by C3(-/-) mice were not altered compared with those of wild-type mice. Further, C5aR signaling downstream of C3 was not involved in the regulation of contraction. Moreover, the regulation of contraction by C3 may be independent of the duration of antigenic stimulation or the functional avidity of effector CD8(+) T cells. However, reduced contraction in C3(-/-) mice was accompanied by a decrease in the proportion of KLRG-1(hi) (killer-cell lectin-like receptor G1) CD127(lo) short-lived effector cells at the peak of the response and correlated with a reduction in the levels of inflammatory cytokines, such as IL-12 and IFN-γ, produced early postinfection. These results provide new insights into the role of systemic C3 in regulating contraction following intracellular bacterial infection and may help to develop vaccines that are more effective.

Decay-accelerating factor 1 deficiency exacerbates leptospiral-induced murine chronic nephritis and renal fibrosis

Leptospirosis is a global zoonosis caused by pathogenic Leptospira, which can colonize the proximal renal tubules and persist for long periods in the kidneys of infected hosts. Here, we characterized the infection of C57BL/6J wild-type and Daf1^−/− mice, which have an enhanced host response, with a virulent Leptospira interrogans strain at 14 days post-infection, its persistence in the kidney, and its link to kidney fibrosis at 90 days post-infection. We found that Leptospira interrogans can induce acute moderate nephritis in wild-type mice and is able to persist in some animals, inducing fibrosis in the absence of mortality. In contrast, Daf1^−/− mice showed acute mortality, with a higher bacterial burden. At the chronic stage, Daf1^−/− mice showed greater inflammation and fibrosis than at 14 days post-infection and higher levels at all times than the wild-type counterpart. Compared with uninfected mice, infected wild-type mice showed higher levels of IL-4, IL-10 and IL-13, with similar levels of α-smooth muscle actin, galectin-3, TGF-β1, IL-17, IFN-γ, and lower IL-12 levels at 90 days post-infection. In contrast, fibrosis in Daf1^−/− mice was accompanied by high expression of α-smooth muscle actin, galectin-3, IL-10, IL-13, and IFN-γ, similar levels of TGF-β1, IL-12, and IL-17 and lower IL-4 levels. This study demonstrates the link between Leptospira-induced murine chronic nephritis with renal fibrosis and shows a protective role of Daf1.

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.

Protection provided by a herpes simplex virus 2 (HSV-2) glycoprotein C and D subunit antigen vaccine against genital HSV-2 infection in HSV-1-seropositive guinea pigs

A prophylactic vaccine for genital herpes disease remains an elusive goal. We report the results of two studies performed collaboratively in different laboratories that assessed immunogenicity and vaccine efficacy in herpes simplex virus 1 (HSV-1)-seropositive guinea pigs immunized and subsequently challenged intravaginally with HSV-2. In study 1, HSV-2 glycoproteins C (gC2) and D (gD2) were produced in baculovirus and administered intramuscularly as monovalent or bivalent vaccines with CpG and alum. In study 2, gD2 was produced in CHO cells and given intramuscularly with monophosphoryl lipid A (MPL) and alum, or gC2 and gD2 were produced in glycoengineered Pichia pastoris and administered intramuscularly as a bivalent vaccine with Iscomatrix and alum to HSV-1-naive or -seropositive guinea pigs. In both studies, immunization boosted neutralizing antibody responses to HSV-1 and HSV-2. In study 1, immunization with gC2, gD2, or both immunogens significantly reduced the frequency of genital lesions, with the bivalent vaccine showing the greatest protection. In study 2, both vaccines were highly protective against genital disease in naive and HSV-1-seropositive animals. Comparisons between gD2 and gC2/gD2 in study 2 must be interpreted cautiously, because different adjuvants, gD2 doses, and antigen production methods were used; however, significant differences invariably favored the bivalent vaccine. Immunization of naive animals with gC2/gD2 significantly reduced the number of days of vaginal shedding of HSV-2 DNA compared with that for mock-immunized animals. Surprisingly, in both studies, immunization of HSV-1-seropositive animals had little effect on recurrent vaginal shedding of HSV-2 DNA, despite significantly reducing genital disease.

Immune cell-derived C3a and C5a costimulate human T cell alloimmunity

Emerging evidence indicates that complement provides costimulatory signals for murine T cells but whether complement impacts human T cells remains unclear. We observed production of complement activation products C3a and C5a during in vitro cultures of human T cells responding to allogeneic dendritic cells (DC). Both partners expressed the receptors for C3a (C3aR) and C5a (C5aR) and C3aR- and C5aR-antagonists inhibited T cell proliferation. Recombinant C3a/C5a promoted CD4(+) T cell expansion, bypassed the inhibitory effects of CTLA4-Ig, and induced AKT phosphorylation, the latter biochemically linking C3aR/C5aR to known T cell signaling pathways. Lowering DC C3a/C5a production by siRNA knockdown of DC C3 reduced T cell alloresponses. Conversely downregulating DC expression of the complement regulatory protein decay-accelerating factor increased immune cell C3a/C5a and augmented T cell proliferation, identifying antigen presenting cells as the dominant complement source. Pharmacological C5aR blockade reduced graft versus host disease (GVHD) scores, prolonged survival, and inhibited T cell responses in NOD scid γc(null) mouse recipients of human peripheral blood mononuclear cells, verifying that the mechanisms apply in vivo. Together our findings unequivocally document that immune cell-derived complement impacts human T cell immunity and provide the foundation for future studies targeting C3aR/C5aR as treatments of GVHD and organ transplant rejection in humans.

Anaphylatoxins in organ transplantation

C3a and C5a (also called anaphylatoxins) are inflammatory peptides generated during complement activation. They do not only play important roles in innate immunity through the initiation and regulation of inflammatory responses, but also significantly influence adaptive immune responses. Organ transplantation triggers an initial inflammatory response and subsequent to the specific immune response (also called the alloimmune response), both of which contribute to graft rejection. Emerging evidence suggests that anaphylatoxins, particularly C5a, are significantly involved in both inflammatory and alloimmune responses following organ transplantation, thus influencing graft outcome. This review will provide the information on our current understanding of the roles for anaphylatoxins in ischemia-reperfusion injury, graft rejection, and transplant tolerance, and the therapeutic potential of targeting anaphylatoxin receptors in organ transplantation.

Complement inhibition in cancer therapy

For decades, complement has been recognized as an effector arm of the immune system that contributes to the destruction of tumor cells. In fact, many therapeutic strategies have been proposed that are based on the intensification of complement-mediated responses against tumors. However, recent studies have challenged this paradigm by demonstrating a tumor-promoting role for complement. Cancer cells seem to be able to establish a convenient balance between complement activation and inhibition, taking advantage of complement initiation without suffering its deleterious effects. Complement activation may support chronic inflammation, promote an immunosuppressive microenvironment, induce angiogenesis, and activate cancer-related signaling pathways. In this context, inhibition of complement activation would be a therapeutic option for treating cancer. This concept is relatively new and deserves closer attention. In this article, we summarize the mechanisms of complement activation on cancer cells, the cancer-promoting effect of complement initiation, and the rationale behind the use of complement inhibition as a therapeutic strategy against cancer.

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.

Opposing roles for complement component C5a in tumor progression and the tumor microenvironment

Promoting complement (C) activation may enhance immunological mechanisms of anti-tumor Abs for tumor destruction. However, C activation components, such as C5a, trigger inflammation, which can promote tumor growth. We addressed the role of C5a on tumor growth by transfecting both human carcinoma and murine lymphoma with mouse C5a. In vitro growth kinetics of C5a, control vector, or parental cells revealed no significant differences. Tumor-bearing mice with C5a-transfected xenografted tumor cells had significantly less tumor burden as compared with control vector tumors. NK cells and macrophages infiltrated C5a-expressing tumors with significantly greater frequency, whereas vascular endothelial growth factor, arginase, and TNF-α production were significantly less. Tumor-bearing mice with high C5a-producing syngeneic lymphoma cells had significantly accelerated tumor progression with more Gr-1+CD11b+ myeloid cells in the spleen and overall decreased CD4+ and CD8+ T cells in the tumor, tumor-draining lymph nodes, and the spleen. In contrast, tumor-bearing mice with low C5a-producing lymphoma cells had a significantly reduced tumor burden with increased IFN-γ-producing CD4+ and CD8+ T cells in the spleen and tumor-draining lymph nodes. These studies suggest concentration of local C5a within the tumor microenvironment is critical in determining its role in tumor progression.

The role of complement regulatory proteins in peripheral blood cells of patients with systemic lupus erythematosus: review

CD55, CD59, CD35 and CD46 are cell membrane proteins that have regulatory properties on the activation of the complement cascade. Deficiency in the expression of these proteins may be associated with lower protection of healthy cells against complement mediated lysis and also with the accumulation of immune complexes in tissues. Few studies assess the expression of these proteins in patients with SLE and the mechanisms that regulate reduction in cellular expression, whereas its impact on manifestations of systemic lupus erythematosus is still unknown.

C5aR expression in a novel GFP reporter gene knockin mouse: implications for the mechanism of action of C5aR signaling in T cell immunity

C5aR is a G protein-coupled receptor for the anaphylatoxin C5a and mediates many proinflammatory reactions. C5aR signaling also has been shown to regulate T cell immunity, but its sites and mechanism of action in this process remain uncertain. In this study, we created a GFP knockin mouse and used GFP as a surrogate marker to examine C5aR expression. GFP was knocked into the 3'-untranslated region of C5ar1 by gene targeting. We show that GFP is expressed highly on Gr-1(+)CD11b(+) cells in the blood, spleen, and bone marrow and moderately on CD11b(+)F4/80(+) circulating leukocytes and elicited peritoneal macrophages. No GFP is detected on resting or activated T lymphocytes or on splenic myeloid or plasmacytoid dendritic cells. In contrast, 5-25% cultured bone marrow-derived dendritic cells expressed GFP. Interestingly, GFP knockin prevented cell surface but not intracellular C5aR expression. We conclude that C5aR is unlikely to play an intrinsic role on murine T cells and primary dendritic cells. Instead, its effect on T cell immunity in vivo may involve CD11b(+)F4/80(+) or other C5aR-expressing leukocytes. Further, our data reveal a surprising role for the 3'-untranslated region of C5aR mRNA in regulating C5aR protein targeting to the plasma membrane.

Immunization with a vaccine combining herpes simplex virus 2 (HSV-2) glycoprotein C (gC) and gD subunits improves the protection of dorsal root ganglia in mice and reduces the frequency of recurrent vaginal shedding of HSV-2 DNA in guinea pigs compared to immunization with gD alone

Attempts to develop a vaccine to prevent genital herpes simplex virus 2 (HSV-2) disease have been only marginally successful, suggesting that novel strategies are needed. Immunization with HSV-2 glycoprotein C (gC-2) and gD-2 was evaluated in mice and guinea pigs to determine whether adding gC-2 to a gD-2 subunit vaccine would improve protection by producing antibodies that block gC-2 immune evasion from complement. Antibodies produced by gC-2 immunization blocked the interaction between gC-2 and complement C3b, and passive transfer of gC-2 antibody protected complement-intact mice but not C3 knockout mice against HSV-2 challenge, indicating that gC-2 antibody is effective, at least in part, because it prevents HSV-2 evasion from complement. Immunization with gC-2 also produced neutralizing antibodies that were active in the absence of complement; however, the neutralizing titers were higher when complement was present, with the highest titers in animals immunized with both antigens. Animals immunized with the gC-2-plus-gD-2 combination had robust CD4+ T-cell responses to each immunogen. Multiple disease parameters were evaluated in mice and guinea pigs immunized with gC-2 alone, gD-2 alone, or both antigens. In general, gD-2 outperformed gC-2; however, the gC-2-plus-gD-2 combination outperformed gD-2 alone, particularly in protecting dorsal root ganglia in mice and reducing recurrent vaginal shedding of HSV-2 DNA in guinea pigs. Therefore, the gC-2 subunit antigen enhances a gD-2 subunit vaccine by stimulating a CD4+ T-cell response, by producing neutralizing antibodies that are effective in the absence and presence of complement, and by blocking immune evasion domains that inhibit complement activation.

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.

Decay-accelerating factor regulates T-cell immunity in the context of inflammation by influencing costimulatory molecule expression on antigen-presenting cells

Recent studies have indicated a role of complement in regulating T-cell immunity but the mechanism of action of complement in this process remains to be clarified. Here we studied mice deficient in decay-accelerating factor (DAF), a key membrane complement regulator whose deficiency led to increased complement-dependent T-cell immune responses in vivo. By crossing OT-II and OT-I T-cell receptor transgenic mice with DAF-knockout mice, we found that lack of DAF on T cells did not affect their responses to antigen stimulation. Similarly, lack of DAF on antigen-presenting cells (APCs) of naive mice did not alter their T-cell stimulating activity. In contrast, APCs from DAF-knockout mice treated with inflammatory stimuli were found to be more potent T-cell stimulators than cells from similarly treated wild-type mice. Acquisition of higher T-cell stimulating activity by APCs in challenged DAF-knockout mice required C3 and C5aR and was correlated with decreased surface PD-L1 and/or increased CD40 expression. These findings implied that DAF suppressed T-cell immunity as a complement regulator in the context of inflammation but did not play an intrinsic role on T cells or APCs. Collectively, our data suggest a systemic and indirect role of complement in T-cell immunity.

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.

Complement regulation of T-cell alloimmunity

PURPOSE OF REVIEW

The purpose of this review is to summarize recent findings implicating complement as an important regulator of T-cell immune responses. We then provide perspective for how these newly described mechanisms apply to allograft injury and how they could ultimately influence therapy.

RECENT FINDINGS

In addition to known effects of serum complement as an effector arm of antibody-initiated injury, T cells and antigen-presenting cells produce complement proteins and up-regulate complement receptors following cognate interactions. The locally released and activated, immune cell-derived complement signals predominantly through C3a and C5a binding to their receptors expressed on both partners to induce immune cell activation and differentiation. Complement deficiency or blockade limits T-cell-mediated autoimmunity and transplant rejection, whereas removal of the complement regulatory protein decay accelerating factor can enhance T-cell immunity and accelerate graft rejection.

SUMMARY

Emerging data indicate that immune cell-derived complement physiologically regulates immune cell survival and proliferation, modulating the strength and phenotype of adaptive T-cell immune responses involved in transplant rejection. The recognition of the diversity through which complement participates in allograft injury supports the need for continued design and testing of complement inhibitors in human transplant recipients.

The Vaccinia virus complement control protein modulates adaptive immune responses during infection

Complement activation is an important component of the innate immune response against viral infection and also shapes adaptive immune responses. Despite compelling evidence that complement activation enhances T cell and antibody (Ab) responses during viral infection, it is unknown whether inhibition of complement by pathogens alters these responses. Vaccinia virus (VACV) modulates complement activation by encoding a complement regulatory protein called the vaccinia virus complement control protein (VCP). Although VCP has been described as a virulence factor, the mechanisms by which VCP enhances VACV pathogenesis have not been fully defined. Since complement is necessary for optimal adaptive immune responses to several viruses, we hypothesized that VCP contributes to pathogenesis by modulating anti-VACV T cell and Ab responses. In this study, we used an intradermal model of VACV infection to compare pathogenesis of wild-type virus (vv-VCPwt) and a virus lacking VCP (vv-VCPko). vv-VCPko formed smaller lesions in wild-type mice but not in complement-deficient mice. Attenuation of vv-VCPko correlated with increased accumulation of T cells at the site of infection, enhanced neutralizing antibody responses, and reduced viral titers. Importantly, depleting CD8(+) T cells together with CD4(+) T cells, which also eliminated T helper cell-dependent Ab responses, restored vv-VCPko to wild-type levels of virulence. These results suggest that VCP contributes to virulence by dampening both antibody and T cell responses. This work provides insight into how modulation of complement by poxviruses contributes to virulence and demonstrates that a pathogen-encoded complement regulatory protein can modulate adaptive immunity.

The role of decay accelerating factor in the immunopathogenesis of cytomegalovirus infection

A wide variety of the host immune elements play an influential role in the defence against cytomegalovirus (CMV) infection. However, the role of complement in the clearance of CMV infection is less well studied. Decay accelerating factor (DAF/CD55) is a membrane-bound complement regulatory protein that inhibits the formation and accelerates the decay of C3-convertase. Here we hypothesize that murine CMV (MCMV) utilizes DAF as an immunoevasive strategy through down-regulation of host adaptive responses against the virus. To test our hypothesis, DAF knock-out (DAF KO) C57BL/6 mice and wild-type (WT) littermates were infected with a sublethal dose of MCMV, and their immune responses were compared. WT mice lost 7·8% of their initial weight within the first 4 days after infection and quickly began to recover. This is in contrast to the DAF KO mice, that lost a total of 19·4% of their initial weight and did not start recovery until 6 days post-infection. Flow cytometric analysis of lung digests revealed that infected DAF KO mice had a significantly increased infiltration of inflammatory cells, the majority being CD8(+) T lymphocytes. Serum levels of tumour necrosis factor (TNF)-α and interferon (IFN)-γ were also increased markedly in the DAF KO mice compared to the infected WT mice. More interestingly, increased viral genome copies (DNA) in the splenocytes of DAF KO mice was accompanied with mRNA transcripts in the DAF KO mice, an indication of active viral replication. These data suggest an intriguing effect of reduced DAF expression on host responses following in vivo MCMV infection.

A functional complement system is required for normal T helper cell differentiation

Complement is a fundamental part of the innate immune system, and also modulates B cell responses. Its effects on T cells, however, are less well studied. Here we have studied antigen-specific T cell responses in C3-knockout (C3-KO) C57BL/6 mice. The animals were immunized with ovalbumin (OVA) in complete Freund's adjuvant, which favors T helper 1 (Th1)-type responses. Splenic lymphocytes from C3-KO mice proliferated less in response to OVA stimulation than splenocytes from control wild type (WT) mice. The response in the C3-KO mice was also qualitatively different. The expression of Th1 lineage determining transcription factor T-bet was decreased in OVA-stimulated splenocytes, and the induction of Th1-associated IgG subclasses impaired. In WT mice T cell proliferation in response to OVA was positively correlated with antigen-specific IgG2a and IgG3 levels. In C3-KO mice the proliferative response correlated with antigen-specific IgE levels, consistent with Th2 deviation. The expression of Th1-inducing cytokines IL-12 and IFN-γ was also decreased in the collecting lymph nodes in the C3-KO mice after immunization. Our results show that the complement system and its component C3 participate in the regulation of T cell responses, and that complement function is required for normal T helper cell differentiation.

Deficiency of C5aR prolongs renal allograft survival

Interaction between C5a, a product of complement activation, and its receptor (C5aR) upregulates antigen-specific T cell responses by modulating the activation of antigen-presenting cells and T cells. Whether this C5a-C5aR interaction contributes to the immune responses that promote renal allograft rejection is unknown. Here, we found that deficiency of C5aR in both graft and recipient reduced allospecific T cell responses and prolonged renal allograft survival. In addition, lack of C5aR impaired the function of donor and recipient antigen-presenting cells and inhibited the response of recipient T cells to allostimulation. Furthermore, deficiency of C5aR in both graft and recipient reduced early inflammation in the grafts, with less cellular infiltration around the vessels and fewer F4/80 positive cells in the peritubular interstitium. These data demonstrate that C5aR is critical for a full adaptive immune response and mediates renal allograft rejection. Engagement of C5aR on dendritic cells and T cells modulates their function, enhancing allospecific T cell responses that lead to allograft rejection. Targeting C5a signaling may have therapeutic potential for T cell-mediated graft rejection.

Role and mechanism of action of complement in regulating T cell immunity

Complement is a part of the innate immune system that contributes to first-line host defense. It is also implicated in a number of human inflammatory conditions and has attracted interest as a potential therapeutic target. Understanding the basic biology of complement and its mechanism(s) of action is imperative for developing complement-based treatments for infectious and autoimmune diseases. One of the exciting new developments in this regard is the revelation that complement plays an important role in T cell immunity. In this review, we highlight recent published studies implicating complement in models of CD4+ and CD8+ T cell immune responses, and discuss its potential mechanism(s) action in these processes. We also comment on issues that may impact data interpretation and draw attention to their consideration in future studies.

Properdin: emerging roles of a pattern-recognition molecule

Complement is an innate immune system that is a first line of defense against pathogens and facilitates elimination of apoptotic and injured cells. During complement activation, the complement convertases are assembled on target surfaces and initiate their proteolytic activities, a process that marks targets for phagocytosis and/or lysis. The complement alternative activation pathway has been implicated in a number of autoimmune conditions including arthritis and age-related macular degeneration. Properdin, a plasma component that is also released by activated neutrophils, is critical in the stabilization of alternative pathway convertases. Recently, it has been shown that properdin is also a pattern-recognition molecule that binds to certain microbial surfaces, apoptotic cells, and necrotic cells. Once bound to a surface, properdin can direct convertase formation and target uptake. New studies are now focusing on a role for properdin in inflammatory and autoimmune diseases. This review examines the new properdin findings and their implications.

Complement and its role in innate and adaptive immune responses

The complement system plays a crucial role in the innate defense against common pathogens. Activation of complement leads to robust and efficient proteolytic cascades, which terminate in opsonization and lysis of the pathogen as well as in the generation of the classical inflammatory response through the production of potent proinflammatory molecules. More recently, however, the role of complement in the immune response has been expanded due to observations that link complement activation to adaptive immune responses. It is now appreciated that complement is a functional bridge between innate and adaptive immune responses that allows an integrated host defense to pathogenic challenges. As such, a study of its functions allows insight into the molecular underpinnings of host-pathogen interactions as well as the organization and orchestration of the host immune response. This review attempts to summarize the roles that complement plays in both innate and adaptive immune responses and the consequences of these interactions on host defense.

Complement: coming full circle

The complement system has long been known to be a major element of innate immunity. Traditionally, it was regarded as the first line of defense against invading pathogens, leading to opsonization and phagocytosis or the direct lysis of microbes. However, from the second half of the twentieth century on, it became clear that complement is also intimately involved in the induction and "fine tuning" of adaptive B- and T-cell responses as well as lineage commitment. This growing recognition of the complement system's multifunctional role in immunity is consistent with the recent paradigm that complement is also necessary for the successful contraction of an adaptive immune response. This review aims at giving a condensed overview of complement's rise from a simple innate stop-and-go system to an essential and efficient participant in general immune homeostasis and acquired immunity.

Immunization with HSV-1 glycoprotein C prevents immune evasion from complement and enhances the efficacy of an HSV-1 glycoprotein D subunit vaccine

Herpes simplex virus type 1 (HSV-1) glycoprotein C (gC-1) binds complement component C3b and inhibits complement-mediated immunity. HSV-1 glycoprotein D (gD-1) is a potent immunogen and a candidate antigen for a subunit vaccine. We evaluated whether combined immunization with gD-1 and gC-1 provides better protection against challenge than gD-1 alone based on antibodies to gC-1 preventing HSV-1-mediated immune evasion. IgG purified from mice immunized with gC-1 blocked C3b binding to gC-1 and greatly increased neutralization by gD-1 IgG in the presence of complement. Passive transfer of gC-1 IgG protected complement intact mice against HSV-1 challenge but not C3 knockout mice, indicating that gC-1 antibody activity in vivo is complement-dependent. Immunizing mice with gD-1 and gC-1 provided better protection than gD-1 alone in preventing zosteriform disease and infection of dorsal root ganglia. Therefore, gC-1 immunization prevents HSV-1 evasion from complement and enhances the protection provided by gD-1 immunization.

Complement promotes the development of inflammatory T-helper 17 cells through synergistic interaction with Toll-like receptor signaling and interleukin-6 production

Toll-like receptors (TLRs) and complement are 2 major components of innate immunity that provide a first-line host defense and shape the adaptive immune responses. We show here that coincidental activation of complement and several TLRs in mice led to the synergistic production of serum factors that promoted T-helper cell 17 (Th17) differentiation from anti-CD3/CD28 or antigen-stimulated T cells. Although multiple TLR-triggered cytokines were regulated by complement, Th17 cell-promoting activity in the serum was correlated with interleukin (IL)-6 induction, and antibody neutralization of IL-6 abrogated the complement effect. By using both in vitro and in vivo approaches, we examined in more detail the mechanism and physiologic implication of complement/TLR4 interaction on Th17-cell differentiation. We found that the complement effect required C5a receptor, was evident at physiologically relevant levels of C5a, and could be demonstrated in cultured peritoneal macrophages as well as in the setting of antigen immunization. Importantly, despite an inhibitory effect of complement on IL-23 production, complement-promoted Th17 cells were functionally competent in causing autoimmunity in an adoptive transfer model of experimental autoimmune encephalomyelitis. Collectively, these data establish a link between complement/TLR interaction and Th17-cell differentiation and provide new insight into the mechanism of action of complement in autoimmunity.

Donor deficiency of decay-accelerating factor accelerates murine T cell-mediated cardiac allograft rejection

Decay-accelerating factor (DAF) is a cell surface regulator that accelerates the dissociation of C3/C5 convertases and thereby prevents the amplification of complement activation on self cells. In the context of transplantation, DAF has been thought to primarily regulate antibody-mediated allograft injury, which is in part serum complement-dependent. Based on our previously delineated link between DAF and CD4 T cell responses, we evaluated the effects of donor Daf1 (the murine homolog of human DAF) deficiency on CD8 T cell-mediated cardiac allograft rejection. MHC-disparate Daf1(-/-) allografts were rejected with accelerated kinetics compared with wild-type grafts. The accelerated rejection predominantly tracked with DAF's absence on bone marrow-derived cells in the graft and required allograft production of C3. Transplantation of Daf1(-/-) hearts into wild-type allogeneic hosts augmented the strength of the anti-donor (direct pathway) T cell response, in part through complement-dependent proliferative and pro-survival effects on alloreactive CD8 T cells. The accelerated allograft rejection of Daf1(-/-) hearts occurred in recipients lacking anti-donor Abs. The results reveal that donor DAF expression, by controlling local complement activation on interacting T cell APC partners, regulates the strength of the direct alloreactive CD8(+) T cell response. The findings provide new insights into links between innate and adaptive immunity that could be exploited to limit T cell-mediated injury to an allograft following transplantation.

Upregulated expression of complement inhibitory proteins on bladder cancer cells and anti-MUC1 antibody immune selection

Membrane complement inhibitors (CD46, CD55 and CD59) are upregulated in some human cancers indicating that they play a role in immune evasion. We investigated complement inhibitor expression in bladder cancer and examined the hypothesis that selective pressure of an antibody response (anti-MUC1) results in the upregulated expression of complement inhibitors on tumor cells. Paired samples of tumor and normal tissue from 22 bladder cancer patients were analyzed for expression of MUC1, CD46, CD55 and CD59, and matched serum samples analyzed for anti-MUC1 IgM and IgG levels. Relationships between anti-MUC1 antibody levels and complement inhibitor expression were investigated. MUC1 mRNA was upregulated in 86% of tumor samples. CD46 was upregulated in 77%, CD55 in 55% and CD59 in 59% of tumors. Low titer anti-MUC1 IgM was detected in normal human sera, but was elevated in 41% of the bladder cancer patients. Anti-MUC1 IgG was virtually absent from normal sera, but present in 32% of the cancer patients. There was a direct relationship between anti-MUC1 antibody titer and expression level of complement inhibitors. Analysis of the correlation of each antibody with the expression of each complement inhibitor by Spearman's rank test revealed a strong correlation between both anti-MUC1 IgM and IgG levels and increased expression of CD46 and CD55, and combined anti-MUC1 IgM/IgG levels correlated with increased expression of all 3 complement inhibitors. In conclusion, the data demonstrate upregulated complement inhibitor expression and the presence of an anti-MUC1 antibody response in bladder cancer patients and support the hypothesis of antibody-mediated immune selection.

Modulation of protective T cell immunity by complement inhibitor expression on tumor cells

Complement-inhibitory proteins expressed on cancer cells can provide protection from antitumor antibodies and may potentially modulate the induction of an immune response to tumor-associated antigens. In the current study, we investigated the consequences of complement inhibitor down-regulation on the effector and inductive phases of an immune response. Stable small interfering RNA-mediated down-regulation of the complement inhibitor Crry on MB49 murine bladder cancer cells increased their susceptibility to monoclonal antibody and complement in vitro. In a syngeneic model of metastatic cancer, the down-regulation of Crry on i.v.-injected MB49 cells was associated with a significant decrease in tumor burden and an increase in the survival of challenged mice. However, monoclonal antibody therapy had no additional benefit. There was an antitumor IgG response, but the response was not effected by Crry down-regulation on inoculated tumor cells. Down-regulation of Crry on MB49 cells resulted in an enhanced antitumor T-cell response in challenged mice (measured by lymphocyte IFN-gamma secretion), and CD8+ T cell depletion of mice prior to injection of MB49 cells completely abrogated the effect of Crry down-regulation on tumor burden and survival. Deficiency of C3 also abrogated the effect of Crry down-regulation on the survival of MB49-challenged mice, indicating a complement-dependent mechanism. These data indicate that complement inhibitors expressed on a tumor cell can suppress a T cell response and that enhancing complement activation on a tumor cell surface can promote protective T cell immunity.

Locally produced C5a binds to T cell-expressed C5aR to enhance effector T-cell expansion by limiting antigen-induced apoptosis

Our recent studies have shown that immune cell-produced complement provides costimulatory and survival signals to naive CD4(+) T cells. Whether these signals are similarly required during effector cell expansion and what molecular pathways link locally produced complement to T-cell survival were not clarified. To address this, we stimulated monoclonal and polyclonal T cells in vitro and in vivo with antigen-presenting cells (APCs) deficient in the complement regulatory protein, decay accelerating factor (DAF), and/or the complement component C3. We found that T-cell expansion induced by DAF-deficient APCs was augmented with diminished T-cell apoptosis, whereas T-cell expansion induced by C3(-/-) APCs was reduced because of enhanced T-cell apoptosis. These effects were traced to locally produced C5a, which through binding to T cell-expressed C5aR, enhanced expression of Bcl-2 and prevented Fas up-regulation. The results show that C5aR signal transduction in T cells is important to allow optimal T-cell expansion, as well as to maintain naive cell viability, and does so by suppressing programmed cell death.