Immunomodulatory functions of murine CR1/2

Harnessing the potential of red blood cells in immunotherapy

Red blood cell (RBC) transfusion represents one of the earliest and most widespread forms of cellular therapy. While the primary purpose of RBC transfusions is to enhance the oxygen-carrying capacity of the recipient, RBCs also possess unique properties that make them attractive vehicles for inducing antigen-specific immune tolerance. Preclinical studies have demonstrated that RBC transfusion alone, in the absence of inflammatory stimuli, often fails to elicit detectable alloantibody formation against model RBC antigens. Several studies also suggest that RBC transfusion without inflammation may not only fail to generate a detectable alloantibody response but can also induce a state of antigen-specific non-responsiveness, a phenomenon potentially influenced by the density of the corresponding RBC alloantigen. The unique properties of RBCs, including their inability to divide and their stable surface antigen expression, make them attractive platforms for displaying exogenous antigens with the goal of leveraging their ability to induce antigen-specific non-responsiveness. This could facilitate antigen presentation to the host's immune system without triggering innate immune activation, potentially enabling the induction of antigen-specific tolerance for therapeutic applications in autoimmune disorders, preventing immune responses against protein therapeutics, or reducing alloreactivity in the setting of transfusion and transplantation.

Complement-targeted therapies in kidney transplantation-insights from preclinical studies

Aberrant activation of the complement system contributes to solid-organ graft dysfunction and failure. In kidney transplantation, the complement system is implicated in the pathogenesis of antibody- and cell-mediated rejection, ischemia-reperfusion injury, and vascular injury. This has led to the evaluation of select complement inhibitors (e.g., C1 and C5 inhibitors) in clinical trials with mixed results. However, the complement system is highly complex: it is composed of more than 50 fluid-phase and surface-bound elements, including several complement-activated receptors-all potential therapeutic targets in kidney transplantation. Generation of targeted pharmaceuticals and use of gene editing tools have led to an improved understanding of the intricacies of the complement system in allo- and xeno-transplantation. This review summarizes our current knowledge of the role of the complement system as it relates to rejection in kidney transplantation, specifically reviewing evidence gained from pre-clinical models (rodent and nonhuman primate) that may potentially be translated to clinical trials.

Mechanistic Understanding of Cell Recognition and Immune Reaction via CR1/CR3 by HAP- and SiO2-NPs

Nanodrug carrier will eventually enter the blood when intravenously injected or in other ways. Meanwhile, a series of toxic effects were caused to the body with the formation of nanoparticle protein corona. In our studies, we try to reveal the recognition mechanism of nanoparticle protein corona by monocyte and the damage effect on immune cells by activated complement of hydroxyapatite nanoparticles (HAP-NPs) and silicon dioxide nanoparticles (SiO₂-NPs). So expressions of TLR4/CR1/CR were analyzed by flow cytometry (FCM) in order to illuminate the recognition mechanism of nanoparticle protein corona by monocyte. And the expression of ROS, cytokines, adhesion molecules, and arachidonic acid was measured when THP-1 and HUVECs were stimulated by NP-activated complement. The results showed that HAP-NPs can be recognized by the opsonin receptor (iC3b/CR3) model, while plasma protein, opsonin receptor, and Toll-like receptors are all likely launch cell recognition of SiO₂-NPs. And it was considerate that NP-activated complement can damage THP-1 and HUVECs, including oxidative stress, inflammation, and increased vascular permeability. So the surface of nanodrug carrier can be modified to avoid being clear and reduce the efficacy according to the three receptors (TLR4/CR1/CR3).

The versatile functions of complement C3-derived ligands

The complement system is a major component of immune defense. Activation of the complement cascade by foreign substances and altered self-structures may lead to the elimination of the activating agent, and during the enzymatic cascade, several biologically active fragments are generated. Most immune regulatory effects of complement are mediated by the activation products of C3, the central component. The indispensable role of C3 in opsonic phagocytosis as well as in the regulation of humoral immune response is known for long, while the involvement of complement in T-cell biology have been revealed in the past few years. In this review, we discuss the immune modulatory functions of C3-derived fragments focusing on their role in processes which have not been summarized so far. The importance of locally synthesized complement will receive special emphasis, as several immunological processes take place in tissues, where hepatocyte-derived complement components might not be available at high concentrations. We also aim to call the attention to important differences between human and mouse systems regarding C3-mediated processes.

Identification of Differentially Expressed Genes in Mouse Spermatogenesis

Complementary DNA microarray and quantitative polymerase chain reaction were used as tools for discovering genes that are differentially expressed in the mouse under normal physiological conditions at distinctive stages of male germ cell development, that is, type A spermatogonia, pachytene spermatocytes, and round spermatids. By using this strategy, we identified a set of genes exhibiting differential expression patterns in spermatogenesis, suggesting that specific functions of the encoded products occurred during the developmental process. Among them were several genes previously not known to be active in testis, which signified undiscovered functional roles of these genes during spermatogenesis. Many of the genes identified were not previously characterized. This study highlights new targets for manipulation to unravel the molecular mechanism of spermatogenesis.

Review: Complement and its regulatory proteins in kidney diseases

Complement is a part of the body's innate immune system that helps defend the host from microbial infection. It is tightly controlled by a number of cell surface and fluid-phase proteins so that under normal circumstances injury to autologous tissues is avoided. In many pathological settings, such as when the complement regulatory mechanisms are dysfunctional or overwhelmed, complement attack of autologous tissues can occur with severe, sometimes life-threatening consequences. The kidney appears to be particularly vulnerable to complement-mediated inflammatory injury and many kidney pathologies have been linked to abnormal complement activation. Clinical and experimental studies have shown that complement attack can be a primary cause in rare, genetically predisposed kidney diseases or a significant contributor to kidney injury caused by other etiological factors. Here we provide a brief review of recent advances on the activation and regulation of the complement system in kidney disease, with a particular emphasis on the relevance of complement regulatory proteins.

The structure, genetic polymorphisms, expression and biological functions of complement receptor type 1 (CR1/CD35)

The complement system is comprised of soluble and cell surface associated proteins that recognize exogenous, altered, or potentially harmful endogenous ligands. In recent years, the complement system--particularly component C3 and its receptors--have been demonstrated to be a key link between innate and adaptive immunity. Complement receptor type 1 (CR1), the receptor for C3b/C4b complement peptides, has emerged as a molecule of immense interest in gaining insight to the susceptibility, pathophysiology, diagnosis, prognosis and therapy of such diseases. In this review, we wish to briefly bring forth the structure, genetic polymorphisms, expression and biological functions of CR1.

C3 promotes expansion of CD8+ and CD4+ T cells in a Listeria monocytogenes infection

It is known that C3 is required for optimal expansion of T cells during acute viral infections. However, it is not yet determined whether T cell responses to intracellular bacterial infections require C3. Therefore, we have investigated the requirement for C3 to elicit potent T cell responses to Listeria monocytogenes (LM). We show that expansion of Ag-specific CD8 and CD4 T cells during a primary response to LM was markedly reduced in the absence of C3 activity. Further studies indicated that, unlike in an influenza virus infection, the regulation of LM-specific T cell responses by C3 might not involve the downstream effector C5a. Moreover, reduced T cell responses to LM was not linked to defective maturation of dendritic cells or developmental anomalies in the peripheral T cell compartment of C3-deficient mice. Experiments involving adoptive transfer of C3-deficient CD8 T cells into the C3-sufficient environment of wild-type mice showed that these T cells do not have intrinsic proliferative defects, and a paracrine source of C3 will suffice for clonal expansion of CD8 T cells in vivo. However, stimulation of purified C3-deficient CD8 T cells by plastic-immobilized anti-CD3 showed that C3 promotes T cell proliferation directly, independent of its effects on APC. On the basis of these findings, we propose that diminished T cell responses to LM in C3-deficient mice might be at least in part due to lack of direct effects of C3 on T cells. These studies have furthered our understanding of C3-mediated regulation of T cell immunity to intracellular pathogens.

CR1/2 is an important suppressor of Adenovirus-induced innate immune responses and is required for induction of neutralizing antibodies

Human complement receptors 1 and 2 are well described as important regulators of innate and adaptive immune responses, having pivotal roles in regulating complement activation (CR1) and B cell maturation/survival. In contrast, the role of the murine homologues of CR1 and CR2 (mCR1/2) have been primarily defined as modulating activation of the adaptive immune system, with very little evidence available about the role of mCR1/2 in regulating the innate immune responses to pathogens. In this manuscript, we confirm that mCR1/2 plays an important role in regulating both the innate and adaptive immune responses noted after Adenovirus (Ad) mediated gene transfer. Our results uncovered a novel role of mCR1/2 in down-regulating several, complement dependent innate immune responses. We also unveiled the mechanism underlying the complement dependent induction of neutralizing antibodies to Ad capsids as a CR1/2 dependent phenomenon that correlates with B-cell activation. These results confirm that Ad interactions with the complement system are pivotal in understanding how to maximize the safety or potency of Ad mediated gene transfer for both gene therapy and vaccine applications.

Intrinsic B cell hypo-responsiveness in mice prematurely expressing human CR2/CD21 during B cell development

We previously reported that human CR2 (hCR2) prematurely expressed under a murine Vlambda2 promoter/Vlambda2-4 enhancer minigene during the CD43+ CD25- late pro-B cell stage of development results in peripheral B cells with impaired responses to immunization with T-dependent antigens. Herein, we show that hCR2 transgenic (Tg) mice also demonstrate a severe defect in T-independent antigen responses and are substantially protected from clinical arthritis, synovitis and cartilage/bone destruction in a collagen-induced arthritis model. This outcome is found despite the apparently normal development of autoreactive T cells with equivalent cytokine and proliferative responses to antigen when compared to non-Tg control mice. These data suggest the presence of an intrinsic B cell defect in the hCR2 Tg mice. We also show that an hCR2-dependent Ca2+ influx can be generated in both developing and mature Tg B cells, but with different rates of decay as compared to control wild-type (WT) mice. In addition, although analysis of tyrosine-phosphorylated proteins in WT and Tg B cells following B cell receptor (BCR)-induced activation revealed the presence of distinctly different phosphorylation patterns, no differences were identified in several candidate protein targets. Overall, these data suggest that premature hCR2 expression and the consequences thereof during B cell development intrinsically alters the way mature B cells develop and subsequently respond to antigen through the BCR signaling complex.

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.

Expression of CR1/2 receptor on alloantigen-stimulated mouse T cells

Antibodies can mediate injury of organ transplants by several mechanisms, including complement activation and interaction with Fc receptors on cells. We tested the hypothesis that antibodies could also cause up-regulation of complement receptors on cells to increase the responses to complement activation by interaction with split products of C3. In our experimental model, B10.A (H-2(a)) cardiac transplants survive significantly longer in C57BL/6 (H-2(b)) immunoglobulin knockout recipients (IgKO) than in their wild-type counterparts. Passive transfer of specific antibodies to donor MHC class I to IgKO recipients of cardiac allografts at the time coinciding with a vigorous cellular infiltration reconstituted acute rejection. We tested the effects of alloantibodies on CR1/2 expression by alloantigen-stimulated T cells. Both CD4(+)/CR1/2(+) and CD8(+)/CR1/2(+) populations of T cells were expanded in C57BL/6 splenocytes stimulated by B10.A alloantigen in 7-day MLR after coculture with endothelial cells sensitized with IgG1 and IgG2b mAb specific to MHC. Endothelial cells sensitized with antibodies also caused an expansion of CD8(+) T cells expressing CR1/2 in lymph node lymphocytes harvested from a C57BL/6 recipient of a B10.A cardiac allograft. These data suggest that antibodies can augment the cellular rejection process through expanding the population of T cells interacting with complement split products.

Amino acid patterns within short consensus repeats define conserved duplicons shared by genes of the RCA complex

Complement control proteins (CCPs) contain repeated protein domains, short consensus repeats (SCRs), which must be relevant to diverse functions such as complement activation, coagulation, viral binding, fetal implantation, and self-nonself recognition. Although SCRs share some discontinuous and imperfect motifs, there are many variable positions and indels making classification in subfamilies extremely difficult. Using domain-by-domain phylogenetic analysis, we have found that most domains can be classified into only 11 subfamilies, designated a, b, c, d, e, f, g, h, i, j, or k and identified by critical residues. Each particular CCP is characterized by the order of representatives of the subfamilies. Human complement receptor 1 (CR1) has ajefbkd repeated four times and followed by ch. The classification crosses CCPs and indicates that a particular CCP is a function of the mix of SCRs. The aje set is a feature of several CCPs including human CR1 and DAF and murine Crry and appears to be associated with the success or failure of implantation inter alia. This approach facilitates genomic analysis of available sequences and suggests a framework for the evolution of CCPs. Units of duplication range from single SCRs, to septamers such as efbkdaj, to extensive segments such as MCP-CR1L. Imperfections of duplication with subsequent deletion have contributed to diversification.

The role of rat Crry, a complement regulatory protein, in proliferation of thymocytes

In our previous work we showed that 3F10 monoclonal antibody (mAb), which recognizes the rat complement receptor 1-related/gene protein y (Crry), induces homotipic aggregation of thymocytes. In this work we studied the effect of 3F10 mAb on proliferation of rat thymocytes stimulated with concanavalin A (ConA) or by cross-linking the T cell receptor (TCR) by anti-alphabetaTCR mAb (R73), in vitro, and the mechanisms involved in the process. Our results show that 3F10 mAb stimulates proliferation of total thymocytes triggered by suboptimal concentrations of ConA or TCR cross-linking, in a dose-dependent manner. Maximal stimulation was observed using 10 microg/ml and 20 microg/ml of 3F10 mAb, respectively. The 3F10-induced stimulation of thymocytes proliferation in the presence of ConA, that was followed by increased production of interleukin-2 (IL-2), up-regulation of the expression of IL-2 receptor alpha (IL-2Ralpha) and was inhibited by anti-CD11a and anti-CD18 mAbs. Purified thymocytes did not respond by proliferation to 3F10 mAb, either alone or in combination with R73 mAb or ConA. Proliferation of these cells was achieved only in the presence of OX-6+ antigen-presenting cells (APC) and additional signals transmitted by TCR or ConA. These results suggest that Crry is involved in the LFA-1 dependent proliferation of thymocytes, a phenomenon that has not been recognized so far.

The role of B cells and alloantibody in the host response to human organ allografts

Some human organ transplants deteriorate slowly over a period of years, often developing characteristic syndromes: transplant glomerulopathy (TG) in kidneys, bronchiolitis obliterans in lungs, and coronary artery disease in hearts. In the past, we attributed late graft deterioration to "chronic rejection", a distinct but mysterious immunologic process different from conventional rejection. However, it is likely that much of chronic rejection is explained by conventional T-cell-mediated rejection (TMR), antibody-mediated rejection (AMR), and other insults. Recently, criteria have emerged to now permit us to diagnose AMR in kidney transplants, particularly C4d deposition in peritubular capillaries and circulating antibody against donor human leukocyte antigens (HLA). Some cases with AMR develop TG, although the relationship of TG to AMR is complex. Thus, a specific diagnosis of AMR in kidney can now be made, based on graft damage, C4d deposition, and donor-specific alloantibodies. Criteria for AMR in other organs must be defined. Not all late rejections are AMR; some deteriorating organs probably have smoldering TMR. The diagnosis of late ongoing AMR raises the possibility of treatment to suppress the alloantibody, but efficacy of the available treatments requires further study.

Locally produced complement and its role in renal allograft rejection

The role of innate immunity in allograft injury is just beginning to become clear, and complement is probably one of a number of factors that are activated very early in the course of transplantation. Kidney transplantation into complement-inhibited rats reduces subsequent inflammation of the graft, probably as a result of reduction of ischemia reperfusion damage as well as diminution of immune mediated damage. Closer analysis of the role of locally synthesised components in mice has suggested that regional synthesis of complement proteins, in particular by the renal tubule, may play a more important role than circulating components. A marked effect on the antidonor T cell response may be explained by the triggering of complement receptors present on antigen presenting cells or T cells infiltrating the graft, or by a more direct effect of complement on the liaison between proximal tubule cells and T cells. Therapeutic control is likely to require a shift to a more targeted approach, directed at complement components produced in the extravascular tissue compartment.

Was it there all the time?

The complement system is old, yet it may still have something new to teach us. For many years, research has existed which shows that C3d, in addition to its established role as an adjuvant, could have an immunosuppressive activity. Being true, it suggests that a common mechanism may be used both by organisms and by their pathogens to prevent unwanted immune responses.

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

In addition to its established role in innate immune mechanisms, complement component C3 is also of critical importance in B cell activation and T cell-dependent Ab responses. In this study, we have examined the requirement for C3 in the generation of primary CD8 T cell responses to an acute systemic viral infection. We compared Ag-specific CD8 T cell responses to lymphocytic choriomeningitis virus (LCMV) between wild-type (+/+) and C3-deficient (C3(-/-)) mice on both 129/B6 and B6 backgrounds. These studies revealed that C3 activity is required for optimal expansion of LCMV-specific effector CD8 T cells in an epitope-dependent fashion, which is influenced by the genetic background of the mice. Studies in complement receptor 1/2 (CR1/CR2)-deficient mice showed that regulation of LCMV-specific CD8 T cell responses by C3 is not dependent upon CR1/CR2. These findings may have implications in vaccine development, therapy of autoimmune diseases, and prevention of graft rejection.

Mucosal type mast cells express complement receptor type 2 (CD21)

Fragments of complement component C3 generated upon activation of the cascade play an important role in the induction and regulation of immune responses. Receptors interacting with various fragments of this versatile complement protein are expressed on a wide variety of cell types, including lymphocytes, macrophages, dendritic cells, follicular dendritic cells, granulocytes, erythrocytes and consequently, C3-products may influence several biological functions at different sites of the body, where complement activation occurs. Regarding the expression of various C3-receptors on mast cells, mainly rodent serosal type mastocytes have been investigated so far. It has been known for a long time that C3a triggers the release of mediators of immediate type hypersensitivity via binding to serosal-type cells. Complement receptor type 1 (CR1/CD35) and type 2 (CR2/CD21) interacting with the larger activation products, such as C3b and C3d, have so far been shown on serosal type mast cells only. In this study, the expression of CR1/2 on mucosal type mast cells is demonstrated. Using mouse CR1/2 specific single chain antibodies and the natural ligand C3d in cytofluorimetric measurements, we show that the rat mucosal mast cell line RBL-2H3 and mouse bone marrow-derived mast cells (BMMC) express CD21. RT-PCR experiments carried out with mouse CR1 and CR2 specific primers show CD21, but not CD35 specific products in BMMC. It is also demonstrated that, in contrast to serosal type mast cells, mucosal mastocytes do not express CD19. In an attempt to reveal the possible function of CR2 on mucosal type mast cells, the effect of receptor-clustering was tested regarding degranulation, Ca-response and IL-6 production, but no CR2-mediated change was detected in any of these processes.

Local synthesis of complement component C3 regulates acute renal transplant rejection

Accumulating evidence suggests that innate immunity interacts with the adaptive immune system to identify potentially harmful antigens and eliminate them from the host. A central facet of innate immunity is complement, which for some time has been recognized as a contributor to inflammation in transplant rejection but without detailed analysis of its role in what is principally a T cell mediated process. Moreover, epithelial and vascular tissues at local sites of inflammation secrete complement components; however, the role of such local synthesis remains unclear. Here we show that the absence of locally synthesized complement component C3 is capable of modulating the rejection of renal allografts in vivo and regulating T-cell responses in vivo and in vitro. The results indicate that improved success in kidney transplantation could come from therapeutic manipulation of innate immunity in concert with T cell directed immunosuppression.

Membrane complement regulatory proteins: insight from animal studies and relevance to human diseases

The complement system plays an important role in host defense. However, if not properly regulated, activated complement can also cause significant damage to host tissues. To prevent complement-mediated autologous tissue damage, host cells express a number of membrane-bound complement regulatory proteins. These include decay-accelerating factor (DAF, CD55), membrane cofactor protein (MCP, CD46) and CD59. Recent studies of membrane complement regulatory proteins from various animal species have revealed similarities as well as significant differences from the corresponding human proteins. In this review, we summarize recent advances in this area and contrast the structure, function and tissue distribution of membrane complement regulatory proteins in human and nonprimate mammalian species. We also discuss how the characterization of the animal proteins has provided important clues and might continue to show relevance to the pathogenesis and therapeutics of a number of human diseases.

Activation of the EBV/C3d receptor (CR2, CD21) on human B lymphocyte surface triggers tyrosine phosphorylation of the 95-kDa nucleolin and its interaction with phosphatidylinositol 3 kinase

We previously demonstrated that CR2 activation on human B lymphocyte surface triggered tyrosine phosphorylation of a p95 component and its interaction with p85 subunit of phosphatidylinositol 3' (PI 3) kinase. Despite identical molecular mass of 95 kDa, this tyrosine phosphorylated p95 molecule was not CD19, the proto-oncogene Vav, or the adaptator Gab1. To identify this tyrosine phosphorylated p95 component, we first purified it by affinity chromatography on anti-phosphotyrosine mAb covalently linked to Sepharose 4B, followed by polyacrylamide gel electrophoresis. Then, the isolated 95-kDa tyrosine phosphorylated band was submitted to amino acid analysis by mass spectrometry; the two different isolated peptides were characterized by amino acid sequences 100% identical with two different domains of nucleolin, localized between aa 411--420 and 611--624. Anti-nucleolin mAb was used to confirm the antigenic properties of this p95 component. Functional studies demonstrated that CR2 activation induced, within a brief span of 2 min, tyrosine phosphorylation of nucleolin and its interaction with Src homology 2 domains of the p85 subunit of PI 3 kinase and of 3BP2 and Grb2, but not with Src homology 2 domains of Fyn and Gap. These properties of nucleolin were identical with those of the p95 previously described and induced by CR2 activation. Furthermore, tyrosine phosphorylation of nucleolin was also induced in normal B lymphocytes by CR2 activation but neither by CD19 nor BCR activation. These data support that tyrosine phosphorylation of nucleolin and its interaction with PI 3 kinase p85 subunit constitute one of the earlier steps in the specific intracellular signaling pathway of CR2.