CD46 engagement on human CD4+ T cells produces T regulatory type 1-like regulation of antimycobacterial T cell responses

Complement regulatory proteins: Candidate biomarkers in differentiating tuberculosis pleural effusion

Background and aims

Complement activation is essential for tuberculosis pleural effusion. However, little is known about the value of complement regulatory protein (CD46, CD55, and CD59) in the differential diagnosis of tuberculosis.

Materials and methods

Ninety-nine patients with exudative pleural effusion admitted to Xiangya Hospital of Central South University from June 1, 2021to November 14, 2022 were enrolled. The expression levels of soluble CD46 (sCD46), soluble CD55 (sCD55), and soluble CD59 (sCD59) in pleural effusion were quantified by enzyme-linked immunosorbent assay, and the receiver operating characteristic (ROC) curves were plotted to evaluate the diagnostic and co-diagnostic values.

Results

The ADA level is higher in TPE patients than non-TPE patients. It is well-found that TPE patients had lower levels of sCD46, sCD55, and sCD59 compared with non-TPE patients. Moreover, the expression of sCD46, sCD55, and sCD59 in pleural effusion was negatively correlated with ADA. In addition, the diagnostic efficacy of sCD46, sCD55 and sCD59 was comparable to that of ADA, with 0.896, 0.857, 0.858 and 0.893, respectively. Furthermore, combine detection of sCD46, sCD55, sCD59 and ADA could improve the diagnostic accuracy.

Conclusions

Complement regulatory factors (CD46, CD55, and CD59) were validated by this project to be promising candidate biomarkers for the diagnosis of TPE with high accuracy. The combination of the CD46, CD55, and CD59 and ADA assay exist a better diagnostic value in TPE.

Psychological stress creates an immune suppressive environment in the lung that increases susceptibility of aged mice to Mycobacterium tuberculosis infection

Age is a major risk factor for chronic infections, including tuberculosis (TB). Elderly TB patients also suffer from elevated levels of psychological stress. It is not clear how psychological stress impacts immune response to Mycobacterium tuberculosis (M.tb). In this study, we used social disruption stress (SDR) to investigate effects of psychological stress in young and old mice. Unexpectedly, we found that SDR suppresses lung inflammation in old mice as evidenced by lower pro-inflammatory cytokine levels in bronchial lavage fluid and decreased cytokine mRNA expression by alveolar macrophages. To investigate effects of stress on M.tb infection, mice were subjected to SDR and then infected with M.tb. As previously reported, old mice were better at controlling infection at 30 days than young mice. This control was transient as CFUs at 60 days were higher in old control mice compared to young mice. Consistently, SDR significantly increased M.tb growth at 60 days in old mice compared to young mice. In addition, SDR in old mice resulted in accumulation of IL-10 mRNA and decreased IFN-γ mRNA at 60 days. Also, confocal microscopy of lung sections from old SDR mice showed increased number of CD4 T cells which express LAG3 and CD49b, markers of IL-10 secreting regulatory T cells. Further, we also demonstrated that CD4 T cells from old SDR mice express IL-10. Thus, we conclude that psychological stress in old mice prior to infection, increases differentiation of IL-10 secreting T cells, which over time results in loss of control of the infection.

Deep Proteomic Analysis on Biobanked Paraffine-Archived Melanoma with Prognostic/Predictive Biomarker Read-Out

The discovery of novel protein biomarkers in melanoma is crucial. Our introduction of formalin-fixed paraffin-embedded (FFPE) tumor protocol provides new opportunities to understand the progression of melanoma and open the possibility to screen thousands of FFPE samples deposited in tumor biobanks and available at hospital pathology departments. In our retrospective biobank pilot study, 90 FFPE samples from 77 patients were processed. Protein quantitation was performed by high-resolution mass spectrometry and validated by histopathologic analysis. The global protein expression formed six sample clusters. Proteins such as TRAF6 and ARMC10 were upregulated in clusters with enrichment for shorter survival, and proteins such as AIFI1 were upregulated in clusters with enrichment for longer survival. The cohort's heterogeneity was addressed by comparing primary and metastasis samples, as well comparing clinical stages. Within immunotherapy and targeted therapy subgroups, the upregulation of the VEGFA-VEGFR2 pathway, RNA splicing, increased activity of immune cells, extracellular matrix, and metabolic pathways were positively associated with patient outcome. To summarize, we were able to (i) link global protein expression profiles to survival, and they proved to be an independent prognostic indicator, as well as (ii) identify proteins that are potential predictors of a patient's response to immunotherapy and targeted therapy, suggesting new opportunities for precision medicine developments.

Emerging patterns of regulatory T cell function in tuberculosis

Tuberculosis (TB) is one of the top 10 causes of mortality worldwide from a single infectious agent and has significant implications for global health. A major hurdle in the development of effective TB vaccines and therapies is the absence of defined immune‐correlates of protection. In this context, the role of regulatory T cells (T_reg), which are essential for maintaining immune homeostasis, is even less understood. This review aims to address this knowledge gap by providing an overview of the emerging patterns of T_reg function in TB. Increasing evidence from studies, both in animal models of infection and TB patients, points to the fact the role of T_regs in TB is dependent on disease stage. While T_regs might expand and delay the appearance of protective responses in the early stages of infection, their role in the chronic phase perhaps is to counter‐regulate excessive inflammation. New data highlight that this important homeostatic role of T_regs in the chronic phase of TB may be compromised by the expansion of activated human leucocyte antigen D‐related (HLA‐DR)⁺CD4⁺ suppression‐resistant effector T cells. This review provides a comprehensive and critical analysis of the key features of T_reg cells in TB; highlights the importance of a balanced immune response as being important in TB and discusses the importance of probing not just T_reg frequency but also qualitative aspects of T_reg function as part of a comprehensive search for novel TB treatments.

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.

Decreased number and impaired function of type 1 regulatory T cells in autoimmune diseases

Type 1 regulatory T (Tr1) cell is a special type of T regulatory cells with surface molecular markers such as lymphocyte-activation gene 3 and CD49b. A key property of Tr1 cells is the capability to produce high-level interleukin 10 (IL-10) upon activation, in a FOXP3-independent manner. The immunosuppressive function of IL-10 producing Tr1 cells has been extensively studied for many years. Autoimmune diseases (AIDs) are conditions in which the immune system breaks down and starts to attack the body. AIDs include inflammatory bowel disease, rheumatoid arthritis, multiple sclerosis (MS), type 1 diabetes mellitus, Greaves' disease, and so forth. In recent years, more and more studies have documented that the number of Tr1 cells is decreased and the function is inhibited in a variety of AIDs, among which MS is the most widely studied. The protocol for engineering Tr1 cell therapy has been established and is gradually being used in clinical practice in recent years. Tr1 cell therapy has been proven to be safe and effective, but it is mainly involved in myeloid leukemia, graft versus host disease currently. Its therapeutic role in AIDs still needs to be further explored. In this study, we will summarize the research advances of Tr1 cells in AIDs, which will provide useful information for treating AIDs through Tr1 cell therapy in the future.

Circulating HLA-DR+CD4+ effector memory T cells resistant to CCR5 and PD-L1 mediated suppression compromise regulatory T cell function in tuberculosis

Chronic T cell activation is a hallmark of pulmonary tuberculosis (PTB). The mechanisms underpinning this important phenomenon are however, poorly elucidated, though known to rely on control of T effector cells (Teff) by regulatory T cells (Treg). Our studies show that circulating natural Treg cells in adults with PTB preserve their suppressive potential but Teff cells from such subjects are resistant to Treg-mediated suppression. We found this to be due to expansion of an activated Teff subset identified by Human Leukocyte Antigen (HLA)-DR expression. Sensitivity to suppression was restored to control levels by depletion of this subset. Comparative transcriptome analysis of Teff cells that contain HLA-DR⁺ cells versus the fraction depleted of this population identified putative resistance mechanisms linked to IFNG, IL17A, IL22, PD-L1 and β-chemokines CCL3L3, CCL4 expression. Antibody blocking experiments confirmed HLA-DR⁺ Teff cells, but not the fraction depleted of HLA-DR⁺ effectors, to be resistant to Treg suppression mediated via CCR5 and PD-L1 associated pathways. In the presence of HLA-DR⁺ Teff cells, activation of NFκB downstream of CCR5 and PD-L1 was perturbed. In addition, HLA-DR⁺ Teff cells expressed significantly higher levels of Th1/Th17 cytokines that may regulate Treg function through a reciprocal counter-balancing relationship. Taken together, our study provides novel insight on how activated HLA-DR⁺CD4⁺ T cells may contribute to disease associated inflammation by compromising Treg-mediated suppression in PTB.

An important marker of progression to PTB following Mycobacterium tuberculosis (Mtb) infection in humans is elevated frequencies of HLA-DR⁺CD4⁺ T cells, reflecting chronic T cell activation. However, the mechanisms by which activated HLA-DR⁺CD4⁺ T cells contribute to disease process is not known. We show that CD25^- HLA-DR⁺CD4⁺ memory Teff from PTB patients are resistant to suppression mediated by Treg cells. An unbiased transcriptome analysis identified several key pathways that contribute to this resistance. Specifically, presence of HLA-DR⁺CD4⁺ T cells renders the effector population resistant to CCR5 and PD-L1 mediated suppression by Treg cells. In addition, the HLA-DR⁺CD4⁺ memory Teff cells express elevated levels of Th1/Th17 cytokines known to counter-regulate and dampen Treg suppression. These findings provide fresh insight to disease process in TB and identify HLA-DR⁺ Teff resistant to Treg suppression as a potential functional marker of disease.

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.

Anaphylatoxin C5a Regulates 6-Sulfo-LacNAc Dendritic Cell Function in Human through Crosstalk with Toll-Like Receptor-Induced CREB Signaling

Activation of antigen-presenting dendritic cells (DCs) and the complement system are essential early events in the immune defense against invading pathogens. Recently, we and others demonstrated immunological crosstalk between signaling from receptors recognizing complement activation products and PAMPs on DCs. This affects DC effector function, as demonstrated by the finding that C5a prevents induction of pro-inflammatory cytokines by toll-like receptor (TLR) ligands in human monocyte-derived DCs (moDCs). Here, we demonstrate that this regulatory crosstalk is specifically important in 6-sulfo LacNAc dendritic cells (slanDCs), the most pro-inflammatory DC subset found in human. C5aR and TLR signaling show profound interference in the ERK/p38/CREB1 signaling pathways. C5aR signaling accelerates TLR-induced CREB1 phosphorylation both in moDC and slanDC. This is key in the regulatory effect of C5a on pro-inflammatory DC maturation by mediating induction of IL-10, which subsequently inhibits pro-inflammatory cytokine production via negative feedback signaling. Importantly, the regulatory effect of C5a affects T-cell immunity by decreasing Th1 and cytotoxic CD8 T-cell responses. The finding that the pro-inflammatory effector function of slanDC can be down modulated by activation products of the complement system highlights the existence of intricate regulatory interactions between various arms of the immune system. Intensive immune monitoring of patients suffering from complement-mediated diseases or patients receiving complement modulating compounds can give more inside in the contribution of complement receptor and TLR crosstalk in APCs in disease.

Differential mucosal IL-10-induced immunoregulation of innate immune responses occurs in influenza infected infants/toddlers and adults

Young children (<5 years of age but especially those <2-year old) exhibit high rates of morbidity and frequently require hospitalizations due to complications from respiratory viral infections. This is also a population for which we understand less about how their unique level of immunological maturation affects their antiviral immune responses. However, we do know from prior studies that their T cells appear to apoptose in the lungs owing to limited interferon (IFN)γ autocrine signaling during infection. To begin to further understand additional limits, we utilized an infant/toddler murine model infected with influenza virus with an adult comparator. In our model, young mice exhibited lower interleukin (IL)-10⁺IFNγ⁺ co-producing CD4 T cells infiltrating the lungs that paralleled with a failed switch from an innate to adaptive immune response at the mid infection stage. Specifically, limited co-IL-10 production correlated with a lack of influenza-specific antibodies and subsequent complement receptor signaling (complement receptor type-1 related gene Y (CCRY)/p65) to the lung infiltrating CD4 T cells therefore limiting their IKAROs upregulation. Thus, limited IL-10 production appeared to diminish signaling to lung macrophages to stop accumulating late into infection. Taken together, our results suggest a novel role for complement mediated signaling in CD4 T cells with respect to IL-10 co-production. Furthermore, a subsequent failure to shift from the unfocused innate immune response to the specific adaptive responses may be a principle cause in the enhanced morbidity common in respiratory viral infection of young children.

CD46 Activation Regulates miR-150-Mediated Control of GLUT1 Expression and Cytokine Secretion in Human CD4+ T Cells

CD46 is a cell surface complement inhibitor widely expressed in human tissues, in contrast to mice, where expression is limited to the testes. In humans, it has been identified as an important T cell costimulatory receptor, and patients deficient in CD46 or its endogenous ligands are unable to mount effective Th1 T cell responses. Stimulation of human CD4(+) T cells with CD3 and CD46 also leads to the differentiation of a "switched" Th1 population, which shuts down IFN-γ secretion and upregulates IL-10 and is thought to be important for negative feedback regulation of the Th1 response. In the present study, we show that CD46 costimulation leads to amplified microRNA (miR) expression changes in human CD4(+) T cells, with associated increases in activation more potent than those mediated by the "classic" costimulator CD28. Blockade of cell surface CD46 inhibited CD28-mediated costimulation, identifying autocrine CD46 signaling as downstream of CD28. We also identify a downregulation of miR-150 in CD46-costimulated T cells and identify the glucose transporter 1 encoding transcript SLC2A1 as a target of miR-150 regulation, connecting miR-150 with modulation of glucose uptake. We also investigated microRNA expression profiles of CD46-induced switched IL-10-secreting Th1 T cells and found increased expression of miR-150, compared with IFN-γ-secreting Th1 cells. Knockdown of miR-150 led to a reduction in IL-10 but not IFN-γ. CD46 therefore controls both Th1 activation and regulation via a miR-150-dependent mechanism.

Complementing T Cells' Functions: Bringing in Metabolism Matters

Components of the complement system act directly on T cells to alter conventional and regulatory T cell subsets. In this issue of Immunity, Kolev, Dimeloe, Le Friec et al. (2015) provide evidence of a mechanism by which the complement stimulates sustained mTORC1 activation and regulates cellular metabolism.

The role of decay accelerating factor in environmentally induced and idiopathic systemic autoimmune disease

Decay accelerating factor (DAF) plays a complex role in the immune system through complement-dependent and -independent regulation of innate and adaptive immunity. Over the past five years there has been accumulating evidence for a significant role of DAF in negatively regulating adaptive T-cell responses and autoimmunity in both humans and experimental models. This review discusses the relationship between DAF and the complement system and highlights major advances in our understanding of the biology of DAF in human disease, particularly systemic lupus erythematosus. The role of DAF in regulation of idiopathic and environmentally induced systemic autoimmunity is discussed including studies showing that reduction or absence of DAF is associated with autoimmunity. In contrast, DAF-mediated T cell activation leads to cytokine expression consistent with T regulatory cells. This is supported by studies showing that interaction between DAF and its molecular partner, CD97, modifies expression of autoimmunity promoting cytokines. These observations are used to develop a hypothetical model to explain how DAF expression may impact T cell differentiation via interaction with CD97 leading to T regulatory cells, increased production of IL-10, and immune tolerance.

Intratumoral regulatory T cells with higher prevalence and more suppressive activity in hepatocellular carcinoma patients

Regulatory T cells (Treg) play a vital role in immunosuppressive crosstalk; however, Tregs from different locations lead to different clinical outcomes. Our aim was, therefore, to compare the prevalences and suppressive phenotypes of Tregs in the peripheral blood, peritumor, and intratumor of patients with hepatocellular carcinoma (HCC). METHODS : The frequencies and phenotypes of CD4(+) CD25(+) CD127(low/-) CD49d(-) Tregs in the periphery, peritumor, and intratumor of 78 HCC patients and 12 healthy controls were evaluated by flow cytometry. Treg-cell suppressive activity was determined using an in vitro CD154 expression assay. Tregs from tumor and paired peritumor were then hybridized using an Agilent whole genome oligo microarray, and selected genes were validated by real-time polymerase chain reaction. Functional analysis of the microarray data was performed using Kyoto Encyclopedia of Genes and Genomes and Gene Ontology analyses. RESULTS : Intratumoral Tregs exhibited higher frequencies and more suppressive phenotypic functions than those in peritumor and periphery, whereas there was no difference between the latter two. Functional analysis showed that complement cascades, p53, and glycosylphosphatidylinositol-anchor biosynthesis pathways were significantly upregulated in intratumoral Tregs; the salivary secretion pathway was significantly downregulated in intratumoral Tregs, and immune cells and tumor-immuno-related Gene Ontology terms were significantly affected. CONCLUSIONS : Tregs in different locations exhibited different functional statuses. A higher prevalence and more suppressive phenotype suggested a critical role for intratumoral Tregs in the formation of multicellular immunosuppressive networks. HCC immunotherapy may be improved, therefore, by specific locational Tregs elimination or suppression.

CD4+ T Lymphocytes, especially Th2 cells, contribute to the progress of renal fibrosis

BACKGROUND

Renal tubulointerstitial fibrosis is the final common stage of renal failure. CD4+ T lymphocyte recruitment and activation after injury could be the very important early event that mediates the onset of renal fibrogenesis. But the role of CD4+ T lymphocytes in renal fibrosis is controversial and its cellular mechanism needs to be further investigated.

METHODS

Biopsy specimens were from patients with minimal-change or IgA nephropathy. Mouse renal fibrosis was induced by unilateral ureteral obstruction (UUO). CD4+ T lymphocytes of wild BALB/c mice were depleted with anti-CD4 antibody. BALB/c Nu/Nu mice were reconstituted with polarized Th1 or Th2 cells by tail vein injection.

RESULTS

Our study demonstrated that massive CD4+ T lymphocytes infiltrated fibrotic kidneys of patients. The depletion of CD4+ T lymphocytes inhibited UUO-induced mouse renal fibrosis. In the process of UUO-induced renal fibrosis, the ratios of Th2/Th1 increased with time. Results have also shown that Th2-reconstituted mice developed renal fibrosis more easily than Th1-reconstituted mice, which manifested by interstitial expansion and collagen deposition, higher expression of α-SMA and vimentin and increased expression of fibronectin, TGF-β and collagen I. We also found that CD4+ T cells from Th1-reconstitued mice tended to secrete IL-4 and IL-13 Th2-like cytokines.

CONCLUSION

In conclusion, our study demonstrated the importance of CD4+ T lymphocytes in renal fibrosis and gave the first direct evidence that Th2 cells play a pivotal role in UUO-induced renal fibrosis. Inhibition of CD4+ T lymphocyte differentiation to Th2 would be a potential therapeutic intervention to prevent renal fibrosis.

Adipose-derived mesenchymal stromal cells from genetically modified pigs: immunogenicity and immune modulatory properties

BACKGROUND AIMS

The immunomodulatory and anti-inflammatory effects of mesenchymal stromal cells (MSC) could prove to be a potential therapeutic approach for prolongation of survival of cell xenotransplantation. Adipose (Ad) MSC from genetically modified pigs could be an abundant source of pig donor-specific MSC.

METHODS

Pig (p) MSC were isolated from adipose tissue of α1,3-galactosyltransferase gene knock-out pigs transgenic for human (h) CD46 (GTKO/hCD46), a potential source of islets. After characterization with differentiation and flow cytometry (FCM), AdMSC were compared with bone marrow (BM) MSC of the same pig and human adipose-derived (hAd) MSC. The modulation of human peripheral blood mononuclear cell (hPBMC) responses to GTKO pig aortic endothelial cells (pAEC) by different MSC was compared by measuring 3H-thymidine uptake. The supernatants from the AdMSC cultures were used to determine the role of soluble factors.

RESULTS

GTKO/hCD46 pAdMSC (i) did not express galactose-α1,3-galactose (Gal) but expressed hCD46, (ii) differentiated into chondroblasts, osteocytes and adipocytes, (iii) expressed stem cell markers, (iv) expressed lower levels of Swine Leucocyte Antigen I (SLAI), Swine Leucocyte Antigen II DR (SLAIIDR) and CD80 than pAEC before and after pig interferon (IFN)-γ stimulation. The proliferative responses of hPBMC to GTKO/hCD46 pAdMSC and hAdMSC stimulators were similar, and both were significantly lower than to GTKO pAEC (P < 0.05). The proliferation of hPBMC to GTKO pAEC was equally suppressed by GTKO/hCD46 pAdMSC and hAdMSC (P > 0.05). The supernatant from GTKO/hCD46 pAdMSC did not suppress the human xenoresponse to GTKO pAEC, which was cell-cell contact-dependent.

CONCLUSIONS

Initial evidence suggests that genetically modified pAdMSC function across the xenogeneic barrier and may have a role in cellular xenotransplantation.

Distinct regulatory CD4+T cell subsets; differences between naïve and antigen specific T regulatory cells

Effector T cells have functional subpopulations with distinct cytokine, cytokine receptor, chemokine receptor and transcription factors. We review how activation of antigen specific Treg induces expression of cytokines, cytokine receptors and chemokine receptors depending upon the effector lineage they are activated by. Activated Treg express receptors that are directly related to the effector T cell lineage. Other classes of Treg are induced in the periphery from effector lineage CD4(+)CD25(-)FOXP3(-)CD127(high)T cells, either by IL-10 or TGF-β or by association with activated CD4(+)CD25(+)FOXP3(+)Treg. Thus Treg are produced and adapt to the specific immune inflammatory environment they are activated within. Activated Treg produce different molecules to mediate suppression, which are tailored to the immune response they are activated by and control.

Optimizing vaccine development

Optimizing the development of modern molecular vaccines requires a complex series of interdisciplinary efforts involving basic scientists, immunologists, molecular biologists, clinical vaccinologists, bioinformaticians and epidemiologists. This review summarizes some of the major issues that must be carefully considered. The intent of the authors is to briefly describe key components of the development process to give the reader an overview of the challenges faced from vaccine concept to vaccine delivery. Every vaccine requires unique features based on the biology of the pathogen, the nature of the disease and the target population for vaccination. This review presents general concepts relevant for the design and development of ideal vaccines protective against diverse pathogens.

CD46 in innate and adaptive immunity: an update

CD46 was discovered in 1986 during a search for novel C3b-binding proteins. CD46 is expressed ubiquitously and functions as a co-factor in the factor I-mediated proteolytic cleavage of C3b and C4b. Its vital role in preventing complement deposition on host tissue is underpinned by the fact that deficiency of CD46 is a predisposing factor for numerous disease conditions arising from complement-mediated 'self-attack'. However, in the last 10 years, it has become apparent that CD46 is also heavily involved in a new and somewhat surprising functional aspect of the complement system: the down-modulation of adaptive T helper type 1 (Th1) immune responses by regulating the production of interferon (IFN)-γ versus interleukin (IL)-10 within these cells. Specifically, this latter function of CD46 is a tantalizing discovery - it may not only have delivered the explanation as to why so many pathogens use and abuse CD46 as cell entry receptor but clearly has important clinical implications for the better understanding of Th1-mediated disease states and novel therapeutic approaches for their amelioration. Here, we summarize and discuss the current knowledge about CD46 and its expanding roles in the immune system.