Increased expression of complement receptor type 1 (CR1, CD35) on human peripheral blood T lymphocytes after polyclonal activation in vitro

A novel mouse model expressing human forms for complement receptors CR1 and CR2

BACKGROUND

The complement cascade is increasingly implicated in development of a variety of diseases with strong immune contributions such as Alzheimer's disease and Systemic Lupus Erythematosus. Mouse models have been used to determine function of central components of the complement cascade such as C1q and C3. However, species differences in their gene structures mean that mice do not adequately replicate human complement regulators, including CR1 and CR2. Genetic variation in CR1 and CR2 have been implicated in modifying disease states but the mechanisms are not known.

RESULTS

To decipher the roles of human CR1 and CR2 in health and disease, we engineered C57BL/6J (B6) mice to replace endogenous murine Cr2 with human complement receptors, CR1 and CR2 (B6.CR2CR1). CR1 has an array of allotypes in human populations and using traditional recombination methods (Flp-frt and Cre-loxP) two of the most common alleles (referred to here as CR1long and CR1short) can be replicated within this mouse model, along with a CR1 knockout allele (CR1KO). Transcriptional profiling of spleens and brains identified genes and pathways differentially expressed between mice homozygous for either CR1long, CR1short or CR1KO. Gene set enrichment analysis predicts hematopoietic cell number and cell infiltration are modulated by CR1long, but not CR1short or CR1KO.

CONCLUSION

The B6.CR2CR1 mouse model provides a novel tool for determining the relationship between human-relevant CR1 alleles and disease.

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.

Complement Receptor 1: disease associations and therapeutic implications

Exaggerated complement activation is a key event in the pathogenesis of a range of autoimmune and inflammatory diseases. Complement Receptor 1 (CR1) has emerged as a molecule of immense interest in gaining insight to the susceptibility, pathophysiology, diagnosis, prognosis and therapy of such diseases. This review brings forth a composite view of the current understanding on the structure, functions, genetics, disease associations and therapeutic implications of CR1.

The complement receptor 1, CR1 (CD35), mediates inhibitory signals in human T-lymphocytes

The modulation the specific, adaptive immune response by complement, particularly of by complement C3, is mainly attributed to its interaction with complement receptors on B-lymphocytes. The function of complement receptors on T-lymphocytes, in contrast, is less well understood, although expression of the complement receptor (CR)1 and CR3 on T-cells has been described years ago. In the present study we investigated the effect of antibodies to CR1 on T-cell lines and peripheral T-cells of healthy donors, respectively. Antibodies to CR1 profoundly inhibited the proliferation of the T-cells; of note is, that exogenously added interleukin 2, though enhancing proliferation, did not overcome the inhibitory effect mediated by anti-CR1. While anti-CR1 had no effect on the activation of the immediate early genes c-jun or c-fos nor on the early increase of gamma interferon- or interleukin 2-specific RNA, the protein synthesis of those cytokines was inhibited. Moreover, synthesis of the proliferating cell nuclear antigen (PCNA) was reduced as was the expression of cyclins, particularly of cyclin A and cyclin D3. Taken together, the data indicate that triggering CR1 inhibits proliferation of T-lymphocytes by a mechanism operating downstream of the initial signalling events.

Receptors for complement C3 on T-lymphocytes: Relics of evolution or functional molecules?

Despite the fact that receptors for complement on T-cells have been described many years ago the function remains unclear as is the role of complement in the T-cell response. In this review we will evaluate how the accumulated wisdom concur with the current concepts of the adaptive T-cell response.

Development of complement therapeutics for inhibition of immune-mediated red cell destruction

A major objective of my National Blood Foundation (NBF)-funded proposal was to produce recombinant soluble forms of a complement regulatory protein called complement receptor 1 (CR1) that carries the Knops blood group system antigens to perform antibody neutralization studies. By generating these recombinant proteins, we were able to inhibit several Knops antibodies in patient serum samples, thereby demonstrating their usefulness for clinical use. Interestingly, the recombinant CR1 proteins generated through NBF funding were also found to strongly reduce complement-mediated red cell destruction in a mouse hemolytic transfusion model. In this review, I will outline our NBF-funded studies, give an overview of recent advances from our group and others in the development of complement therapeutics, and highlight their potential use in the transfusion medicine setting.

Cord blood and adult T cells show different responses to C1q-bearing immune complexes

We have previously shown that T cells can be activated through cell-surface C1q receptors, resulting in secretion of interferon-gamma (IFN-gamma) and tumor necrosis factor alpha (TNFalpha), further demonstrating the intimate linkage between innate and adaptive immunity. In this current report, we sought to determine whether: (1) T cell responses to C1q-bearing immune complexes are dependent on the maturational status of the T cells and (2) whether signaling through the C1q receptor on T cells modulates conventional activation mediated through the conventional T cell receptor (TCR)/CD3 signaling complex. We first examined the capacity of neonatal T cells to respond to C1q-bearing immune complexes using IFNgamma, IL-2, and MIF secretion as measures of activation (MIF was chosen because of its crucial role in coordinating innate and adaptive immunity). Neonatal T cells produced significantly less IFNgamma but not IL-2, when stimulated by C1q immune complexes compared with adult T cells. MIF levels did not exceed background levels in these experiments. Next, we examined the capacity of C1q-bearing immune complexes to regulate signaling through the conventional TCR/CD3 signaling complex. Pre-incubating adult T cells with C1q-bearing immune complexes significantly reduced IFNgamma secretion when those same cells were subsequently stimulated with anti-CD3 and anti-CD28 monoclonal antibodies. Pre-incubation of neonatal T-cells with C1q-bearing immune complexes had no effect on IFNgamma secretion, although IFNgamma secretion was lower than that found in adult T cells for each experimental condition. We speculate that reduced IFNgamma secretion after pre-incubation with C1q immune complexes may be due to IL-10 secretion, which was observed in C1q-stimulated adult (but not neonatal) T cells.

CONCLUSIONS

C1q-bearing immune complexes exert complex effects on mature T cells that include both pro- and anti-inflammatory responses. Immunologic maturation is required for these effects, as cord blood T cells are relatively hyporesponsive to C1q-bearing immune complexes compared with adult T cells.

T cell activation by soluble C1q-bearing immune complexes: implications for the pathogenesis of rheumatoid arthritis

Both innate and adaptive immune systems are thought to participate in the pathogenesis of rheumatoid arthritis in adults and children. The experiments reported here were undertaken to examine how immune complexes, potent stimulators of inflammation, may regulate cells of the adaptive immune system. Human T cells were prepared from peripheral blood by negative selection and incubated with bovine serum albumin (BSA)-anti-BSA immune complexes that were formed in the presence or absence of human C1q. C1q-bearing immune complexes, but not unopsonized complexes, elicited both TNF-alpha and IFN-gamma secretion from human T cells. Secretion of both cytokines was time- and dose-dependent. Cross-linking C1q on the cell surface of T cells produced the same results. Cytokine secretion was not inhibited by blocking the C3b receptor (CR1, CD35) on T cells prior to incubation with immune complexes. Reverse transcriptase polymerase chain reaction (RT-PCR) of immune complex-stimulated cells revealed accumulation of both TNF-alpha and IFN-gamma mRNA within 2 h post-stimulation. IL-2 was not detected in cell culture supernatants, but IL-2 receptor alpha chain (CD25) was detected in low density on a small proportion of T cells activated by C1q-bearing immune complexes. Secretion of both cytokines was inhibited partially, but not completely, by IL-10. These experiments show that immune complexes, potent inflammatory mediators, may activate T cells through a novel mechanism. These findings have implications for chronic inflammatory diseases in humans.

B-cell antigens within normal and activated human T cells

In this study we compared cell surface staining for human peripheral blood lymphocyte (PBL) CD antigens by flow cytometry, with staining obtained following permeabilization of PBL using the Cytoperm method (Serotec). Six CD antigens (CD20, CD21, CD22, CD32, CD35 and major histocompatibility complex class II antigen) normally found on the surface of B cells, were also found to be expressed within T cells. We also showed, by immunoelectron microscopy, that these inappropriately expressed ('occult') CD antigens are located within cytoplasmic vesicles or within the rough endoplasmic reticulum. Following in vitro activation of T cells a distinct increase in expression of all of these cytoplasmic antigens was observed but staining at the cell surface was, by comparison, weak. We therefore propose that up-regulation of various B-cell CD antigens occurs within the cytoplasm of T cells following activation and that these antigens may be synthesized and released into the fluid-phase as soluble immunoregulatory molecules.