FcRY, an Fc receptor related gene differentially expressed during B lymphocyte development and activation

FCRLA-A Resident Endoplasmic Reticulum Protein that Associates with Multiple Immunoglobulin Isotypes in B Lineage Cells

FCRLA is homologous to receptors for the Fc portion of IgG (FcγR) and is located in the same region of human chromosome one, but has several unusual and unique features. It is a soluble resident ER protein retained in this organelle by unknown mechanisms involving the N-terminal domain, a disordered domain with three Cys residues in close proximity in the human protein. Unlike the FcγRs, FCRLA is not glycosylated and has no transmembrane region. FCRLA is included in this CTMI volume on IgM-binding proteins because it binds IgM in the ER, but quite surprisingly, given the isotype-restricted ligand specificity of the other FcRs, it also binds all other Ig isotypes so far tested, IgG and IgA. In the case of IgM, there is even preferential binding of the secretory and not the transmembrane form. Among B cells, FCRLA is most highly expressed in the germinal center and shows little expression in plasma cells. Based on these observations, we propose that one human FCRLA function is to stop GC B cells from secreting IgM, which would act as a decoy receptor, thus preventing the B cells from capturing antigen, processing it, and presenting the antigen-derived peptides to T follicular helper cells. Without help from these T cells, there would be limited B cell isotype switching, proliferation, and differentiation. On the other hand, FCRLA is downregulated in plasma cells, where IgM secretion is an essential function. FCRLA may also act as a chaperone involved by unknown mechanisms in the proper assembly of Ig molecules of all isotypes.

Emerging roles for the FCRL family members in lymphocyte biology and disease

Members of the extended Fc receptor-like (FCRL) family in humans and mice are preferentially expressed by B cells and possess tyrosine-based immunoregulatory function. Although the majority of these proteins repress B cell receptor-mediated activation, there is an emerging evidence for their bifunctionality and capacity to counter-regulate adaptive and innate signaling pathways. In light of these findings, the recent discovery of ligands for several of these molecules has begun to reveal exciting potential for them in normal lymphocyte biology and is launching a new phase of FCRL investigation. Importantly, these fundamental developments are also setting the stage for defining their altered roles in the pathogenesis of a growing number of immune-mediated diseases. Here we review recent advances in the FCRL field and highlight the significance of these intriguing receptors in normal and perturbed immunobiology.

Cutting edge: human FcRL4 and FcRL5 are receptors for IgA and IgG

Fc receptor-like (FcRL) proteins are a family of cellular receptors homologous to FcγRI and are predominantly expressed by B cells. They function to costimulate or inhibit BCR signaling through consensus ITAMs and ITIMs; however, the extracellular ligands of these receptors remain unknown or controversial. In this study, we tested the ability of human FcRL proteins to bind Igs and found FcRL4 and FcRL5 to be bona fide Fc receptors. In cellular binding assays, FcRL4 bound efficiently to IgA and FcRL5 binds all IgG isotypes with varied efficiency. Additionally, we generated mAbs capable of specifically blocking these interactions. Given their expression on activated B cells and potential for inhibitory signaling, FcRL4 and FcRL5 are likely to be important for immune complex-dependent human B cell regulation, and they represent novel therapeutic targets for receptor blockade therapies.

Defining the immunological phenotype of Fc receptor-like B (FCRLB) deficient mice: Confounding role of the inhibitory FcγRIIb

Fc receptor-like A (FCRLA) and FCRLB have homology to the transmembrane FCRL family members (FCRL 1-6) and to the conventional receptors for the Fc portion of immunoglobulin, but uniquely are cytosolic proteins expressed in B cells. Here we describe the phenotype of Fcrlb-gene targeted mice. B cell development and in vitro responses are normal; however, antibody responses to a T-dependent antigen are elevated. The gene encoding the inhibitory FcγRIIb is located nearby Fcrlb. Although Fcrlb-gene targeting had no effect on the function or basal expression of FcγRIIb, its expression was reduced following activation. This abnormal regulation was due to co-inheritance of Fcgr2b and the mutant Fcrlb allele from the 129 ES cells. A promoter polymorphism in the 129/Sv Fcgr2b allele results in diminished upregulation of FcγRIIb following B cell activation. Thus, we speculate that the enhanced antibody response seen in the FCRLB-deficient mice may be due to the Fcgr2b promoter.

Distinct host cell proteins incorporated by SIV replicating in CD4+ T cells from natural disease resistant versus non-natural disease susceptible hosts

BACKGROUND

Enveloped viruses including the simian immunodeficiency virus (SIV) replicating within host cells acquire host proteins upon egress from the host cells. A number of studies have catalogued such host proteins, and a few have documented the potential positive and negative biological functions of such host proteins. The studies conducted herein utilized proteomic analysis to identify differences in the spectrum of host proteins acquired by a single source of SIV replicating within CD4+ T cells from disease resistant sooty mangabeys and disease susceptible rhesus macaques.

RESULTS

While a total of 202 host derived proteins were present in viral preparations from CD4+ T cells from both species, there were 4 host-derived proteins that consistently and uniquely associated with SIV replicating within CD4+ T cells from rhesus macaques but not sooty mangabeys; and, similarly, 28 host-derived proteins that uniquely associated with SIV replicating within CD4+ T cells from sooty mangabeys, but not rhesus macaques. Of interest was the finding that of the 4 proteins uniquely present in SIV preparations from rhesus macaques was a 26 S protease subunit 7 (MSS1) that was shown to enhance HIV-1 'tat' mediated transactivation. Among the 28 proteins found in SIV preparations from sooty mangabeys included several molecules associated with immune function such as CD2, CD3ε, TLR4, TLR9 and TNFR and a bioactive form of IL-13.

CONCLUSIONS

The finding of 4 host proteins that are uniquely associated with SIV replicating within CD4+ T cells from disease susceptible rhesus macaques and 28 host proteins that are uniquely associated with SIV replicating within CD4+ T cells from disease resistant sooty mangabeys provide the foundation for determining the potential role of each of these unique host-derived proteins in contributing to the polarized clinical outcome in these 2 species of nonhuman primates.

FCRLA is a resident endoplasmic reticulum protein that associates with intracellular Igs, IgM, IgG and IgA

Fc receptor-like A (FCRLA) is an unusual member of the extended Fc receptor family. FCRLA has homology to receptors for the Fc portion of Ig (FCR) and to other FCRL proteins. However, unlike these other family representatives, which are typically transmembrane receptors with extracellular ligand-binding domains, FCRLA has no predicted transmembrane domain or N-linked glycosylation sites and is an intracellular protein. We show by confocal microscopy and biochemical assays that FCRLA is a soluble resident endoplasmic reticulum (ER) protein, but it does not possess the amino acid sequence KDEL as an ER retention motif in its C-terminus. Using a series of deletion mutants, we found that its ER retention is most likely mediated by the amino terminal partial Ig-like domain. We have identified ER-localized Ig as the FCRLA ligand. FCRLA is unique among the large family of Fc receptors, in that it is capable of associating with multiple Ig isotypes, IgM, IgG and IgA. Among hemopoietic cells, FCRLA expression is restricted to the B lineage and is most abundant in germinal center B lymphocytes. The studies reported here demonstrate that FCRLA is more broadly expressed among human B lineage cells than originally reported; it is found at significant levels in resting blood B cells and at varying levels in all B-cell subsets in tonsil.

Fc receptor genes and the systemic lupus erythematosus diathesis

Fc receptors represent a distinct group of hematopoeitic cell surface glycoproteins that have a characterized role in affecting the efficiency of the mononuclear phagocyte system to clear IgG immune complexes. Functional genetic variations in this family of receptors have been identified as heritable susceptibility factors for SLE and lupus nephritis across diverse populations. In this review, we describe the roles of the classical Fc receptors for IgG (Fc gamma) and non-classical Fc-like receptors (FCR1-FCRL6L), Fc receptors for IgE (Fc epsilon RI) and IgA and IgM (Fc alpha/mu R) in SLE diathesis. The combined effects of these genes on SLE pathogenesis, either via linkage disequilibrium or epistasis with additional genetic or environmental factors, provide a challenge for future investigations. The pursuit of a polygenic SLE-profile that includes longitudinal evaluations of SLE and markers involved in the protean clinical manifestations associated with SLE will facilitate our understanding of the cascade of inflammatory events associated with the diathesis.

Expression of the autoimmune Fcgr2b NZW allele fails to be upregulated in germinal center B cells and is associated with increased IgG production

The inhibitory receptor FcgammaRIIb regulates B-cell functions. Genetic studies have associated Fcgr2b polymorphisms and lupus susceptibility in both humans and murine models, in which B cells express reduced FcgammaRIIb levels. Furthermore, FcgammaRIIb absence results in lupus on the appropriate genetic background, and lentiviral-mediated FcgammaRIIb overexpression prevents disease in the NZM2410 lupus mouse. The NZM2410/NZW allele Fcgr2b is, however, located in-between Sle1a and Sle1b, two potent susceptibility loci, making it difficult to evaluate Fcr2b(NZW) independent contribution. By using two congenic strains that each carries only Sle1a (B6.Sle1a(15-353)), or Fcr2b(NZW) in the absence of Sle1a or Sle1b (B6.Sle1(111-148)), we show that the Fcr2b(NZW) allele does not upregulate its expression on germinal center B cells and plasma cells, as does the C57BL/6 allele on B6.Sle1a(15-353) B cells. Furthermore, in the absence of the flanking Sle1a and Sle1b, Fcr2b(NZW) does not produce an autoimmune phenotype, but is associated with an increased number of class-switched plasma cells. These results show that while a lower level of FcgammaRIIb does not by itself induce the development of autoreactive B cells, it has the potential to amplify the contribution of autoreactive B cells induced by other lupus-susceptibility loci by enhancing the production of class-switched plasma cells.

Species-specific evolution of the FcR family in endothermic vertebrates

In primates and rodents, the extended FcR family is comprised of three subsets: classical FcRs, structurally diverse cell surface receptors currently designated FCRL1-FCRL6, and intracellular proteins FCRLA and FCRLB. Using bioinformatic analysis, we revealed the FcR-like genes of the same three subsets in the genome of dog, another representative of placental mammals, and in the genome of short-tailed opossum, a representative of marsupials. In contrast, a single FcR-like gene was found in the current version of the chicken genome. This in silico finding was confirmed by the gene cloning and subsequent Southern blot hybridization. The chicken FCRL gene encodes a cell surface receptor with the extracellular region composed of four Ig-like domains of the D1-, D2-, D3-, and D4-subtypes. The gene is expressed in lymphoid and non-lymphoid tissues. Phylogenetic analysis of the mammalian and chicken genes suggested that classical FcRs, FCRLA, and FCRLB emerged after the mammalian-avian split but before the eutherian-marsupial radiation. The data obtained show that the repertoire of the classical FcRs and surface FcR-like proteins in mammalian species was shaped by an extensive recombination process, which resulted in domain shuffling and species-specific gain and loss of distinct exons or entire genes.

Fc receptor-like molecules

Discovery of a large family of Fc receptor-like (FCRL) molecules, homologous to the well-known receptors for the Fc portion of immunoglobulin (FCR), has uncovered an impressive abundance of immunoglobulin superfamily (IgSF) genes in the human 1q21-23 chromosomal region and revealed significant diversity for these genes between humans and mice. The observation that FCRL representatives are members of an ancient multigene family that share a common ancestor with the classical FCR is underscored by their linked genomic locations, gene structure, shared extracellular domain composition, and utilization of common cytoplasmic tyrosine-based signaling elements. In contrast to the conventional FCR, however, FCRL molecules possess diverse extracellular frameworks, autonomous or dual signaling properties, and preferential B lineage expression. Most importantly, there is no strong evidence thus far to support a role for them as Ig-binding receptors. These characteristics, in addition to their identification in malignancies and autoimmune disorders, predict a fundamental role for these receptors as immunomodulatory agents in normal and subverted B lineage cells.