Involvement of the HCK and FGR src-Family Kinases in FCRL4-Mediated Immune Regulation

Abstract

FCRL4 is an immunoregulatory receptor expressed by a subpopulation of memory B cells. These tissue-based cells express increased levels of the src-family kinases HCK and FGR. In this study, we investigate the roles of these src-family kinases in FCRL4-mediated immunoregulation of B cells in the context of previously unrecognized palmitoylation of the receptor. We observed enhanced phosphorylation of FCRL4 on tyrosine residues in the presence of the HCK p59 or FGR. This phosphorylation was markedly reduced in assays using a palmitoylation-defective mutant of FCRL4. In reporter gene studies, we observe that FCRL4 expression enhances CpG-mediated activation of NF-κB signaling. Surprisingly, using a reporter gene linked to activation of the MAPK substrate Elk-1 in response to Ag receptor ligation, we find that FCRL4 has inhibitory activity in cells coexpressing FGR but an activating function in cells coexpressing HCK p59. We provide evidence that in primary memory B cells, expression of FCRL4 leads to increased expression of IL-10 in the presence of FGR or HCK p59 in response to CpG, but increased levels of IFN-γ only in the context of coexpression of FGR. Our study supports the specific requirement of HCK p59 and FGR src-family kinases for FCRL4-mediated immunomodulatory activity and indicates that palmitoylation serves as an additional level of regulatory control of FCRL4.

FcRL4 Expression Identifies a Pro-inflammatory B Cell Subset in Viremic HIV-Infected Subjects

“…Further, HcK downmodulation resulted in a reduction of TLR-signaling in FcRL4 B-cell transfectants. These data confirm recent reports by Liu et al ( 33 ) and suggest that FcRL4 in human B cells likely recruits the Src-kinase family member HcK, resulting in amplification of TLR-signaling. However, further studies are needed to determine the precise association between FcRL4 and HcK.…”

Single cell resolution landscape of equine peripheral blood mononuclear cells reveals diverse immune cell subtypes including T-bet+B cells

“…The equine T-bet + B cell populations identified in the present study share many features with the atypical memory B cell populations described in humans. In addition to TBX21/T-bet expression, horse T-bet + B cell populations exhibit similar gene expression patterns, including enriched expression of FCRL4, FGR, and HCK, the protein products of which modulate BCR signaling (38). Furthermore, a recent study of T-bet + B cells in HIV demonstrated that they express genes characteristic of germinal center B cells (16).…”

“…In humans, T-bet + B cells have been described as "atypical memory B cells," appearing in the peripheral blood during chronic infection and/or inflammation (17,34). Although specific markers and/or gene expression patterns vary in different datasets, these B cells are often found to express ITGAM (CD11b), ITGAX (CD11c), as well as genes that modulate BCR signaling (including FCRL4, FGR, and HCK) (35)(36)(37)(38)(39). Moreover, a recent study of T-bet + B cell populations in the context of chronic HIV infection demonstrated expression of genes associated with germinal center B cells (16,40).…”

Roles for the FCRL6 Immunoreceptor in Tumor Immunology

“…Molecular dissection of the FCRL4 cytoplasmic tail has demonstrated that its function is altered by the recruitment of at least two different SFKs. FCRL4 wields suppressive activity in B cells co-expressing FGR, but promotes activation in B cells co-expressing HCK p59 ( 26 ). Thus, as opposed to the classical FCR for IgG and IgE, these findings highlight that many FCRLs possess multifaceted regulatory potential.…”