Fc Receptor-Like A Associates with Intracellular IgG and IgM but Is Dispensable for Antigen-Specific Immune Responses

Abstract

FcR-like (FcRL) proteins comprise a family of lymphocyte receptors with homology to FcγRI. Among these receptors, FcRLA is uniquely interesting due to its intracellular localization, unusual structural features, and high expression within human germinal center and marginal zone B cells. Our analysis of human cell lines has confirmed that this receptor is not secreted but is maintained as an intracellular protein in B cells where it interacts with Igs, consistent with a possible role in Ab assembly. By generating FcRLA-specific antisera as well as knockout mice, we were able to unequivocally demonstrate that FcRLA protein is expressed exclusively in all mouse B cells. We also found that FcRLA is not required for the generation of Ag-specific humoral immune responses to T-dependent or T-independent Ags. However, given its highly conserved structure and universal expression within B cells, it is probable that FcRLA functions similarly in humans and mice. Cumulatively, our data suggest that FcRLA plays a role in Ig assembly that can be compensated for by other proteins.

Emerging Roles for the FCRL Family Members in Lymphocyte Biology and Disease

“…Human FCRLA is predominantly found in subsets of GC B cells, mainly the proliferating centroblasts, but is expressed at some level by all B cell subsets in the tonsil, with the notable exception of PCs where it is very low/absent, (Davis et al, 2002b, Facchetti et al, 2002, Mechetina et al, 2002, Masir et al, 2004) and in freshly isolated blood B cells (Santiago et al, 2011). In mice, FCRLA is broadly expressed among peripheral B cells, being highest in PCs, but is downregulated by GC B cells (Wilson et al, 2010, Reshetnikova et al, 2012). FCRLB has been difficult to study because its low level transcripts are undetectable by Northern blot and even difficult to resolve by RT-PCR; with a single round of amplification, human FCRLB is found in placenta, kidney and spleen (Wilson and Colonna, 2005).…”

“…Early studies using chimeric proteins artificially expressed on the cell surface failed to demonstrate interactions with Igs (Facchetti et al, 2002); however, immunoprecipitation of endogenous FCRLA disclosed its co-association with intracellular IgM, IgG, and IgA in cell lines as well as in primary B cells (Wilson et al, 2010, Santiago et al, 2011). The elevated expression of FCRLA in GC B cells, together with its ability to bind multiple isotypes of intracellular Ig, suggests a possible role for FCRLA in Ig retention during affinity maturation.…”

“…Indeed, FCRLA has been shown to preferentially associate in the ER with the secretory versus membrane form of IgM in the GC-like human B cell line Ramos (Santiago et al, 2011). Although FCRLB has even greater sequence identity with CD64, no evidence of Ig binding has been established for it yet and, like the Fcrla knockout mouse (Wilson et al, 2010), the Fcrlb knockout mouse had no discernible phenotype (Masuda et al, 2010). …”

“…To this point, two transgenic models have now been published describing mice with targeted disruption of Fcrla and Fcrlb . The Colonna group developed Fcrla −/− mice by cre-mediated ablation of the third and fourth exons that encode the two Ig-like domains in 129 ES cells (Wilson et al, 2010). Lymphocyte development was grossly normal including CD4 and CD8 T cell populations in the thymus and B cells in the bone marrow and spleen.…”

Highly parallel characterization of IgG Fc binding interactions

“…[12][13][14][15] Collectively, these natural mechanisms offer a path for similar rational induction of antibody responses with specific functional profiles via vaccination. 16 Furthermore, beyond relatively well-characterized FcγR and complement proteins, a growing number of diverse and structurally unrelated Fc-binding proteins have been identified, ranging from the pH-sensitive neonatal Fc receptor 17 to C-type lectins such as dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN), 18 FcR-Like receptors, 19,20 mannose-binding lectin 2 (MBL2), 21 TRIM21, 22 macrophage mannose receptor (MMR), 23 and Dectin-1. 24 Probing the recognition properties of these and other FcR for engineered and and Margaret e ackerman naturally-produced IgG represents an important avenue to enhance our understanding of their potential role in antibody activity in vivo.…”