Soluble Fc Receptor for IgM in Sera From Subsets of Patients With Chronic Lymphocytic Leukemia as Determined by a New Mouse Monoclonal Antibody

Physiological and Pathophysiological Roles of IgM Fc Receptor (FcµR) Isoforms

IgM is the first antibody to emerge during phylogeny, ontogeny, and immune responses and serves as a first line of defense. Effector proteins interacting with the Fc portion of IgM, such as complement and its receptors, have been extensively studied for their functions. IgM Fc receptor (FcµR), identified in 2009, is the newest member of the FcR family and is intriguingly expressed by lymphocytes only, suggesting the existence of distinct functions as compared to the FcRs for switched Ig isotypes, which are expressed by various immune and non-hematopoietic cells as central mediators of antibody-triggered responses by coupling the adaptive and innate immune responses. Results from FcµR-deficient mice suggest a regulatory function of FcµR in B cell tolerance, as evidenced by their propensity to produce autoantibodies of both IgM and IgG isotypes. In this article, we discuss conflicting views about the cellular distribution and potential functions of FcµR. The signaling function of the Ig-tail tyrosine-like motif in the FcµR cytoplasmic domain is now formally shown by substitutional experiments with the IgG2 B cell receptor. The potential adaptor protein associating with FcµR and the potential cleavage of its C-terminal cytoplasmic tail after IgM binding are still enigmatic. Critical amino acid residues in the Ig-like domain of FcµR for interacting with the IgM Cµ4 domain and the mode of interaction are now defined by crystallographic and cryo-electron microscopic analyses. Some discrepancies on these interactions are discussed. Finally, elevated levels of a soluble FcµR isoform in serum samples are described as the consequence of persistent B cell receptor stimulation, as seen in chronic lymphocytic leukemia and probably in antibody-mediated autoimmune disorders.

Functional Characterization of Largemouth Bass (Micropterus salmoides) Soluble FcγR Homolog in Response to Bacterial Infection

Fc receptors (FcRs) are key players in antibody-dependent cellular phagocytosis (ADCP) with their specific recognition of the Fc portion of an immunoglobulin. Despite reports of FcγR-mediated phagocytosis in mammals, little is known about the effects of soluble FcγRs on the immune response. In this study, FcγRIα was cloned from the largemouth bass (Micropterus salmoides) (MsFcγRIα). Without a transmembrane segment or a cytoplasmic tail, MsFcγRIα was identified as a soluble form protein and widely distributed in the spleen, head kidney, and intestine. The native MsFcγRIα was detected in the serum of Nocardia seriolae-infected largemouth bass and the supernatants of transfected HEK293 cells. Additionally, it was verified that the transfected cells' surface secreted MsFcRIα could bind to largemouth bass IgM. Moreover, the expression changes of MsFcγRIα, Syk, and Lyn indicated that MsFcγRIα was engaged in the acute phase response to bacteria, and the FcγR-mediated phagocytosis pathway was activated by Nocardia seriolae stimulation. Furthermore, recombinant MsFcγRIα could enhance both reactive oxygen species (ROS) and phagocytosis to Nocardia seriolae of leukocytes, presumably through the interaction of MsFcγRIα with a complement receptor. In conclusion, these findings provided a better understanding of the function of soluble FcγRs in the immune response and further shed light on the mechanism of phagocytosis in teleosts.