C-Reactive Protein Triggers Calcium Signalling in Human Neutrophilic Granulocytes via FcγRIIa in an Allele-Specific Way

Immune and Inflammation in Acute Coronary Syndrome: Molecular Mechanisms and Therapeutic Implications

Acute coronary syndrome (ACS) is a major cause of acute death worldwide. Both innate and adaptive immunity regulate atherosclerosis progression, plaque stability, and thrombus formation. Immune and inflammation dysfunction have been indicated in the pathogenesis of ACS. The imbalance in the proatherogenic and antiatherogenic immune networks promotes the transition of plaques from a stable to unstable state and results in the occurrence of acute coronary events. The residual inflammatory risk (RIR) has received increasing attention in recent years, and lowering RIR has been expected to improve the outcomes of ACS patients. The CANTOS, COLCOT, and LoDoCo trials verified the benefits of reducing cardiovascular events using anti-inflammation therapies; however, most of the other studies focusing on lowering RIR produced negative or contradicting results. Therefore, restoring the balance in autoimmune regulation is essential because proatherogenic and antiatherogenic immunomodulatory effects are equally important in the complex human immune network. In this review, we summarized the recent evidence of the roles of proatherogenic and antiatherogenic immune networks in the pathogenesis of ACS and discussed how immune and inflammation contribute to atherosclerosis progression, plaque instability, and adverse cardiovascular events. We also provide a “from bench to bedside” perspective of a novel and promising personalized strategy in RIR intervention and therapeutic approaches for the treatment of ACS.

Fc Gamma Receptor IIA (CD32A) R131 Polymorphism as a Marker of Genetic Susceptibility to Sepsis

Sepsis is a devastating disease that can affect humans at any time between neonates and the elderly and is associated with mortality rates that range from 30 to 80%. Despite intensive efforts, its treatment has remained the same over the last few decades. Fc receptors regulate multiple immune responses and have been investigated in diverse complex diseases. FcγRIIA (CD32A) is an immunoreceptor, tyrosine-based activation motif-bearing receptor that binds immunoglobulin G and C-reactive protein, important opsonins in host defense. We conducted a study of 702 patients (184 healthy individuals, 171 non-infected critically ill patients, and 347 sepsis patients) to investigate if genetic polymorphisms in the CD32A coding region affect the risk of septic shock. All individuals were genotyped for a variant at position 131 of the FcγRIIA gene. We found that allele G, associated with the R131 genotype, was significantly more frequent in septic patients than in the other groups (p = 0.05). Our data indicate that FcγRIIA genotyping can be used as a marker of genetic susceptibility to sepsis.

FcγRIIB and autoimmunity

Autoimmune diseases are characterized by adaptive immune responses against self-antigens, including humoral responses resulting in the production of autoantibodies. Autoantibodies generate inflammation by activating complement and engaging Fcγ receptors (FcγRs). The inhibitory receptor FcγRIIB plays a central role in regulating the generation of autoantibodies and their effector functions, which include activation of innate immune cells and the cellular arm of the adaptive immune system, via effects on antigen presentation to CD4 T cells. Polymorphisms in FcγRIIB have been associated with susceptibility to autoimmunity but protection against infections in humans and mice. In the last few years, new mechanisms by which FcγRIIB controls the adaptive immune response have been described. Notably, FcγRIIB has been shown to regulate germinal center B cells and dendritic cell migration, with potential impact on the development of autoimmune diseases. Recent work has also highlighted the implication of FcγRIIB on the regulation of the innate immune system, via inhibition of Toll-like receptor- and complement receptor-mediated activation. This review will provide an update on the role of FcγRIIB in adaptive immune responses in autoimmunity, and then focus on their emerging function in innate immunity.

Human platelet IgG Fc receptor FcγRIIA in immunity and thrombosis

Beyond their prominent role in hemostasis and thrombosis, platelets are increasingly recognized as having immunologic functions. Supporting this, human platelets express FcγRIIA (CD32a), a low-affinity Fc receptor (FcR) for the constant region of IgG that recognizes immune complexes (ICs) and IgG-opsonized cells with high avidity. In leukocytes, FcγRIIA engagement initiates strong effector functions that are key for immune and inflammatory responses, including cytokine release, antibody-dependent cell-mediated killing of pathogens, and internalization of ICs. However, the physiologic relevance of platelet-expressed FcγRIIA has received little attention in previous reviews on FcRs. This article summarizes and discusses the available information on human platelet FcγRIIA. The importance of this receptor in heparin-induced thrombocytopenia, a prothrombotic adverse drug effect, is well documented. However, studies demonstrating platelet activation by IgG-opsonized bacteria point to the physiologic relevance of platelet FcγRIIA in immunity. In this context, platelet activation and secretion may facilitate both a direct antimicrobial function of platelets and crosstalk with other immune cells. Additionally, a role for platelet FcγRIIA in IgG-independent hemostasis and physiologic thrombosis, by means of amplifying integrin αII b β3 outside-in signaling, has also been proposed. Nonetheless, the thrombotic complications found in some infective and autoimmune diseases may result from unbalanced FcγRIIA-mediated platelet aggregation. Moreover, FcγRIIA is not expressed in mice, and thrombocytopenia and/or thrombotic events found after drug administration can only be recapitulated by the use of human FcγRIIA-transgenic mice. Altogether, the available data support a functional role for platelet FcγRIIA in health and disease, and emphasize the need for further investigation of this receptor.

Pentraxins: structure, function, and role in inflammation

The pentraxins are an ancient family of proteins with a unique architecture found as far back in evolution as the Horseshoe crab. In humans the two members of this family are C-reactive protein and serum amyloid P. Pentraxins are defined by their sequence homology, their pentameric structure and their calcium-dependent binding to their ligands. Pentraxins function as soluble pattern recognition molecules and one of the earliest and most important roles for these proteins is host defense primarily against pathogenic bacteria. They function as opsonins for pathogens through activation of the complement pathway and through binding to Fc gamma receptors. Pentraxins also recognize membrane phospholipids and nuclear components exposed on or released by damaged cells. CRP has a specific interaction with small nuclear ribonucleoproteins whereas SAP is a major recognition molecule for DNA, two nuclear autoantigens. Studies in autoimmune and inflammatory disease models suggest that pentraxins interact with macrophage Fc receptors to regulate the inflammatory response. Because CRP is a strong acute phase reactant it is widely used as a marker of inflammation and infection.