A new Fc receptor homolog, FREB2, found in germinal center B cells

The potential of FCRL genes as targets for cancer treatment: insights from bioinformatics and immunology

Cancer is a prevalent and dangerous disease that requires a multifaceted approach to treatment. The FCRL family gene has been linked to immune function and tumor progression. Bioinformatics may help unravel their role in cancer treatment. We conducted a comprehensive analysis of the FCRL family genes in pan-cancer using publicly available databases and online tools. Specifically, we examined gene expression, prognostic significance, mutation profiles, drug resistance, as well as biological and immunomodulatory roles. Our data were sourced from The Cancer Genome Atlas, Genotype-Tissue Expression, cBioPortal, STRING, GSCALite, Cytoscape, and R software. The expression of FCRL genes varies significantly across different tumor types and normal tissues. While high expression of most FCRL genes is associated with a protective effect in many cancers, FCRLB appears to be a risk factor in several types of cancer. Alterations in FCRL family genes, particularly through amplification and mutation, are common in cancers. These genes are closely linked to classical cancer pathways such as apoptosis, epithelial-mesenchymal transition (EMT), estrogen receptor (ER) signaling, and DNA damage response. Enrichment analysis indicates that FCRL family genes are predominantly associated with immune cell activation and differentiation. Immunological assays demonstrate a strong positive correlation between FCRL family genes and tumor-infiltrating lymphocytes (TILs), immunostimulators, and immunoinhibitors. Furthermore, FCRL family genes can enhance the sensitivity of various anticancer drugs. The FCRL family genes are vital in cancer pathogenesis and progression. Targeting these genes in conjunction with immunotherapy could enhance cancer treatment efficacy. Further research is required to determine their potential as therapeutic targets.

Exercise-Induced Extracellular Vesicles Delay the Progression of Prostate Cancer

Increasing evidence suggests that regular physical exercise not only reduces the risk of cancer but also improves functional capacity, treatment efficacy and disease outcome in cancer patients. At least partially, these effects are mediated by the secretome of the tissues responding to exercise. The secreted molecules can be released in a carrier-free form or enclosed into extracellular vesicles (EVs). Several recent studies have shown that EVs are actively released into circulation during physical exercise. Here, we for the first time investigated the effects of exercise-induced EVs on the progression of cancer in an F344 rat model of metastatic prostate cancer. Although we did not observe a consistent increase in the circulating EV levels, RNA sequencing analysis demonstrated substantial changes in the RNA content of EVs collected before and immediately after forced wheel running exercise as well as differences between EVs from runners at resting state and sedentary rats. The major RNA biotype in EVs was mRNA, followed by miRNA and rRNA. Molecular functions of differentially expressed RNAs reflected various physiological processes including protein folding, metabolism and regulation of immune responses triggered by the exercise in the parental cells. Intravenous administration of exercise-induced EVs into F344 rats with orthotopically injected syngeneic prostate cancer cells PLS10, demonstrated reduction of the primary tumor volume by 35% and possibly—attenuation of lung metastases. Hence, our data provide the first evidence that exercise-induced EVs may modulate tumor physiology and delay the progression of cancer.

Identification and expression analysis of Fc receptor-like proteins in Japanese flounder (Paralichthys olivaceus)

Fc receptors (FcRs) are specific to the Fc portion of immunoglobulin (Ig) molecules. Here, four Fc receptor-like proteins, JF-FcR-like protein 1-4, were identified in Japanese flounder. Their open reading frames encoded 358, 255, 519 and 441 amino acid residues, respectively. JF-FcR-like protein mRNAs were mainly detected in kidney and spleen of healthy fish. Injection of formalin-killed cells (FKCs) of Edwardsiella tarda significantly increased the spleen mRNA levels of JF-FcR-like protein 1 but not the other JF-FcR-like proteins. Injection of FKC of Streptococcus iniae did not significantly affect any of the JF-FcR-like protein mRNAs. These findings suggest that the FcR-like proteins have different involvements in pathogen responses.

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.

Fc Receptor-Like Proteins in Pathophysiology of B-cell Disorder

Members of the family of Fc receptor-like (FcRL) proteins, homologous to FcγRI, have been identified by multiple research groups. Consequently, they have been described using multiple nomenclatures including Fc receptor homologs (FcRH), immunoglobulin superfamily receptor translocation-associated genes (IRTA), immunoglobulin-Fc-gp42-related genes (IFGP), Src homology 2 domain-containing phosphatase anchor proteins (SPAP), and B cell cross-linked by anti-immunoglobulin M-activating sequences (BXMAS). They are now referred to under a unified nomenclature as FCRL. Eight different human FCRL genes have been identified, all of which appear to be related to the genes of the immunoglobulin superfamily (IgSF) of cellular adhesion molecules. These type 1 transmembrane glycoproteins are composed of different combinations of 5 types of immunoglobulin-like domains, with each protein consisting of 3 to 9 domains, and no individual domain type conserved throughout all of the FCRL proteins. Ligands for the majority of the FCRLs remain unknown. In general, FCRL expression is restricted to lymphocytes and is primarily expressed in B-lymphocytes, supporting FCRL’s involvement in a variety of immune disorders. Most FCRLs functionally repress B-cell activation; however, they might have dual roles in lymphocyte functions as these proteins often possess immunoreceptor tyrosine activation (ITAM) and inhibitory (ITIM) motif elements. The biological functions of these newly recognized FCRL proteins are just beginning to emerge, and might provide the insight necessary for understanding pathophysiology of lymphocyte disorders and treating different immune diseases.

Proteomics and Transcriptomics of BJAB Cells Expressing the Epstein-Barr Virus Noncoding RNAs EBER1 and EBER2

In Epstein-Barr virus (EBV) latent infection, the EBV-encoded RNAs EBER1 and EBER2 accumulate in the host cell nucleus to ~10⁶ copies. While the expression of EBERs in cell lines is associated with transformation, a mechanistic explanation of their roles in EBV latency remains elusive. To identify EBER-specific gene expression features, we compared the proteome and mRNA transcriptome from BJAB cells (an EBV-negative B lymphoma cell line) stably transfected with an empty plasmid or with one carrying both EBER genes. We identified ~1800 proteins with at least 2 SILAC pair measurements, of which only 8 and 12 were up- and downregulated ≥ 2-fold, respectively. One upregulated protein was PIK3AP1, a B-cell specific protein adapter known to activate the PI3K-AKT signaling pathway, which regulates alternative splicing and translation in addition to its pro-survival effects. In the mRNA-seq data, the mRNA levels for some of the proteins changing in the SILAC data did not change. We instead observed isoform switch events. We validated the most relevant findings with biochemical assays. These corroborated the upregulation of PIK3AP1 and AKT activation in BJAB cells expressing high levels of both EBERs and EBNA1 (a surrogate of Burkitt’s lymphoma EBV latency I) relative to those expressing only EBNA1. The mRNA-seq data in these cells showed multiple upregulated oncogenes whose mRNAs are enriched for 3´-UTR AU-rich elements (AREs), such as ccl3, ccr7, il10, vegfa and zeb1. The CCL3, CCR7, IL10 and VEGFA proteins promote cell proliferation and are associated with EBV-mediated lymphomas. In EBV latency, ZEB1 represses the transcription of ZEBRA, an EBV lytic phase activation factor. We previously found that EBER1 interacts with AUF1 in vivo and proposed stabilization of ARE-containing mRNAs. Thus, the ~10⁶ copies of EBER1 may promote not only cell proliferation due to an increase in the levels of ARE-containing genes like ccl3, ccr7, il10, and vegfa, but also the maintenance of latency, through higher levels of zeb1.

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.

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.

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.

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

A bioinformatics approach has lead to the identification of FcRY, a new Fc receptor related gene. FcRY is predicted to encode a protein with three immunoglobulin (Ig) domains followed by a mucin-like domain containing a proline-rich stalk and a C-terminal leucine rich region. The predicted protein lacks a hydrophobic domain for insertion into the plasma membrane, suggesting that FcRY is an intracellular or secreted protein. This feature is shared with the product of the FcRX/FCRL/FREB gene that is closely linked to FcRY in both human and mouse genomes. Fcry transcripts are first detectable among mouse B lineage cells at the pre-B cell stage. Splenic B cells of the newly formed, follicular, and marginal zone subsets express Fcry, as do germinal center B cells to a lesser extent. FcRY is also expressed in subpopulations of human B cells. A consistent characteristic of FcRY in both species is low level gene expression, which can be further downregulated in normal mouse B cells by signaling through the B cell receptor (BCR) or CD40, thereby suggesting a correlation between cell cycle entrance and diminished FcRY expression. Fcry is upregulated by short-term treatment with BAFF/BLyS, which promotes B cell survival rather than proliferation. LPS induces very rapid but transient enhancement. We observed a pronounced upregulation of Fcry expression in WEHI 231 cells induced by BCR crosslinking to undergo cell cycle arrest prior to apoptosis, consistent with the possible regulation of Fcry expression by cell cycle status.