Adhesion G protein-coupled receptors (aGPCRs) are two-subunit molecules, consisting of an adhesive extracellular ? subunit that couples noncovalently to a seven-transmembrane ? subunit. The cooperation between the two subunits and the effect of endogenous ligands on the functioning of aGPCRs is poorly understood. In this study, we investigated the interaction between the pan-leukocyte aGPCR CD97 and its ligand CD55. We found that leukocytes from CD55-deficient mice express significantly increased levels of cell surface CD97 that normalized after transfer into wild-type mice because of contact with CD55 on both leukocytes and stromal cells. Downregulation of both CD97 subunits occurred within minutes after first contact with CD55 in vivo, which correlated with an increase in plasma levels of soluble CD97. In vitro, downregulation of CD97 on CD55-deficient leukocytes cocultured with wild-type blood cells was strictly dependent on shear stress. In vivo, CD55-mediated downregulation of CD97 required an intact circulation and was not observed on cells that lack contact with the blood stream, such as microglia. Notably, de novo ligation of CD97 did not activate signaling molecules constitutively engaged by CD97 in cancer cells, such as ERK and protein kinase B/Akt. We conclude that CD55 downregulates CD97 surface expression on circulating leukocytes by a process that requires physical forces, but based on current evidence does not induce receptor signaling. This regulation can restrict CD97-CD55-mediated cell adhesion to tissue sites.
In addition to its complement-regulating activity, CD55 is a ligand of the adhesion class G protein-coupled receptor CD97; however, the relevance of this interaction has remained elusive. We previously showed that mice lacking a functional CD97 gene have increased numbers of granulocytes.
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.
Echovirus 7 (EV7) belongs to the Enterovirus genus within the family Picornaviridae. Many picornaviruses use IgG-like receptors that bind in the viral canyon and are required to initiate viral uncoating during infection. However, in addition, some of the enteroviruses use an alternative or additional receptor that binds outside the canyon. Decay-accelerating factor (DAF) has been identified as a cellular receptor for EV7. The crystal structure of EV7 has been determined to 3.1-Å resolution and used to interpret the 7.2-Å-resolution cryo-electron microscopy reconstruction of EV7 complexed with DAF. Each DAF binding site on EV7 is near a 2-fold icosahedral symmetry axis, which differs from the binding site of DAF on the surface of coxsackievirus B3, indicating that there are independent evolutionary processes by which DAF was selected as a picornavirus accessory receptor. This suggests that there is an advantage for these viruses to recognize DAF during the initial process of infection.
The complement system contributes to autoimmune injury, but its involvement in promoting the development of autoimmune diabetes is unknown. In this study, our goal was to ascertain the role of complement C3 in autoimmune diabetes.
To investigate whether the presence of decay-accelerating factor (or CD55), an intrinsic complement regulator, protects against the development of vascular disease, given that complement activation can affect leukocytes and platelets.
CD55 (decay-accelerating factor) is best known for its role in the negative regulation of the complement system. Indeed, lack of this molecule leads to disease aggravation in many autoimmune disease models. However, CD55 is abundantly present on fibroblast-like synoviocytes and is also a ligand of the adhesion-class heptahelical receptor CD97, which is expressed by infiltrating macrophages. Treatment with antibodies to CD97 ameliorates the collagen-induced model of rheumatoid arthritis (RA) in DBA/1 mice, but the net contribution of CD55 is unknown. This study was undertaken to investigate the role of CD55 in experimental RA.
Immune-mediated rejection remains a significant obstacle preventing long term survival of transplanted organs. Emerging information derived from multiple groups has recently shown that the complement system, traditionally considered a central arm of innate immunity and a primary effector arm of antibody-mediated immunity, plays an additional key role as a regulator of adaptive alloreactive T cell immunity. Complement components produced by immune cells are activated locally and the resultant activation products guide the development of effector T cell immune responses. In the context of organ transplantation, manipulation of local complement activation influences the strength and effector functions of alloreactive T cells which are central mediators of immune mediated rejection. Further definition of the molecular basis underlying complements effects on cellular alloimmunity has the potential to provide novel targets for the prevention and treatment of injury to solid organ transplants.
The innate immune system has been implicated in the pathogenesis of alcoholic liver disease. Although innate immunity is usually considered an early response to injury, previous work implicating innate immunity in ethanol-induced liver injury focuses primarily on long-term ethanol exposure. We investigated the early period of ethanol exposure to determine whether there were temporal associations between activation of innate immune responses and known correlates of liver injury. Female C57BL/6 mice were allowed free access to an ethanol-containing Lieber-DeCarli diet or were pair-fed a control diet. Within 4 days of ethanol exposure, we observed a striking spike in expression of hepatic proinflammatory cytokines-including tumor necrosis factor alpha (TNF-alpha), interleukin-6, and interferon-gamma-prior to hepatic triglyceride accumulation or increased plasma alanine aminotransferase activities, as well as before the induction of cytochrome P450 2E1 or oxidative stress. This early spike in inflammatory cytokines coincided with deposition of C3b-iC3b/C3c (C3b) in the liver. This deposition, resulting from the cleavage of the third component of the complement system (C3), is evidence for activation of complement in response to ethanol. C3(-/-) mice were protected from the early, ethanol-induced increase in hepatic TNF-alpha expression. Ethanol increased C3b deposition in mice deficient in C3a receptor or C5a receptor, as well as in wild-type mice depleted of hepatic macrophages; however, there was no increase in hepatic TNF-alpha in the absence of C3a receptor, C5a receptor, or hepatic macrophages. In contrast, the absence of Toll-like receptor 4 (TLR-4) had no effect on the early, ethanol-induced increase in either C3b or TNF-alpha.
Signaling through the G protein-coupled receptors for the complement fragments C3a and C5a (C3aR and C5aR, respectively) by dendritic cells and CD4(+) cells provides costimulatory and survival signals to effector T cells. Here we found that when signals from C3aR and C5aR were not transduced into CD4(+) cells, signaling via the kinases PI(3)K?, Akt and mTOR ceased, activation of the kinase PKA increased, autoinductive signaling by transforming growth factor-?1 (TGF-?1) initiated and CD4(+) T cells became Foxp3(+) induced regulatory T cells (iT(reg) cells). Endogenous TGF-?1 suppressed signaling through C3aR and C5aR by preventing the production of C3a and C5a and upregulating C5L2, an alternative receptor for C5a. The absence of signaling via C3aR and C5aR resulted in lower expression of costimulatory molecules and interleukin 6 (IL-6) and more production of IL-10. The resulting iT(reg) cells exerted robust suppression, had enhanced stability and suppressed ongoing autoimmune disease. Antagonism of C3aR and C5aR can also induce functional human iT(reg) cells.
Acute graft-versus-host disease (GvHD) is a serious complication of allogeneic hematopoietic cell transplantation (allo-HCT) that results from donor allogeneic T cell attack on host tissues. Based on previous work implicating immune cell-derived C3a and C5a as regulators of T cell immunity, we examined the effects of locally produced C3a and C5a on murine T cell-mediated GvHD. We found that total body irradiation, a conditioning regimen required to permit engraftment of allo-HCT, caused upregulation and activation of alternative pathway complement components by recipient APCs. Allo-HCT with decay accelerating factor-null (Daf1(-/-)) host BM and Daf1(-/-) donor lymphocytes led to exacerbated GvHD outcome and resulted in splenic and organ-infiltrating T cell expansion. T cells deficient in C3a receptor (C3aR) and/or C5a receptor (C5aR) responded weakly in allogeneic hosts and exhibited limited ability to induce GvHD. Using a clinically relevant treatment strategy, we showed that pharmacological C5aR blockade reduced GvHD morbidity. Our data mechanistically link APC-derived complement to T cell-mediated GvHD and support complement inhibition as a therapeutic strategy for GvHD in humans.
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