FcRL4 acts as an adaptive to innate molecular switch dampening BCR signaling and enhancing TLR signaling

Abstract

Ehrhardt et al first described FcRL4 to mark a unique population of memory B cells (MBCs) in human lymphoid tissues near epithelial surfaces that had distinctive functional capabilities. 3 In humans, FcRL4 ϩ MBCs lack CD27, the classic marker for MBCs, and, compared with FcRL4 Ϫ MBCs, express more CD20 and less CD21 and have strongly up-regulated CCR1 and CCR5 that may play a role in their tissue localization. FcRL4 ϩ and FcRL4 Ϫ MBCs have undergone isotype switching and somatic hypermutation to similar levels, but neither expresses transcription factors associated with plasma-cell differentiation. FcRL4 ϩ MBCs do not proliferate in response to BCR cross-linking, but differentiate into plasma cells in response to IL-2, IL-10, and CD40 ligand. These results suggest that FcRL4 ϩ MBCs have dampened signaling through the BCR while maintaining their sensitivity to cytokines and T-cell help. Subsequent comparative transcriptome and proteome analyses revealed that FcRL4 ϩ and FcRL4 Ϫ MBCs had distinctive expression of a variety of genes providing a signature for FcRL4 ϩ MBCs, 4 including genes encoding: (1) cell-surface markers, including CD11c, CCR1, CCR5, RANKL, and DLL1; (2) regulators of the cell cycle; (3) signal-transduction molecules such as FgR and Hck; and (4) transcription factors, including RUNX2 and SOX5. The investigators of that study concluded that these distinctive MBCs residing in close proximity to epithelial tissues in mucosal lymphoid tissues may play an important role in healthy individuals in humoral immune responses at epithelial boundaries in the body in close proximity to the natural microflora and sites of invading pathogens.Moir et al described a similar population of FcRL4 ϩ MBCs in the peripheral blood of HIV-infected viremic individuals. 5 Compared with FcRL4 Ϫ MBCs, these FcRL4 ϩ MBCs expressed high levels of inhibitory receptors, including CD22 and CD85j. The profile of trafficking receptors was similar to that described for FcRL4 ϩ MBCs in tissues by Ehrhardt et al,3,4 suggesting that these FcRL4 ϩ MBCs in the blood of HIV-viremic individuals were migrating from lymphoid tissues to chronically inflamed tissues. Compared with FcRL4 Ϫ MBCs, FcRL4 ϩ MBCs from HIV-viremic individuals showed reduced proliferation and differentiation into plasma cells in response to the cytokines IL-2 and IL-10, T-cell help in the form of CD40L, and BCR cross-linking. 5 Because the pattern of expression of inhibitory and homing receptors by FcRL4 ϩ MBCs in HIV-viremic individuals was similar to that described for virus-specific CD8 ϩ T-cell exhaustion in chronic lymphocytic choriomeningitis virus infections in mice and in HIV-viremic individuals, [6][7][8] Moir et al called these FcRL4 ϩ MBCs "exhausted MBCs." 5,9 HIV-specific B cells were enriched in the FcRL4 ϩ MBC population, in contrast to influenza-specific B cells, which were concentrated in the FcRL4 Ϫ MBC population, 5 suggesting that exhaustion of MBCs was antigen driven and may The online version of this article contains a data supplement.The public...

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

“…Our observations of enhanced cytokine expression levels in FCRL4 wt transfected Bmem cells after CpG treatment are in agreement with observations of increased CD23 expression in R2G6 cells and primary human B cells in response to TLR9 engagement (24). Investigations into cytokine secretion by B cells have increased over the last decade and led to the recognition of regulatory B cell functions rooted in the modulation of proinflammatory and antiinflammatory cytokine levels.…”

“…However, our initial analysis using fusion proteins did not allow us to monitor any effects of the transmembrane region of FCRL4 or of the membrane proximal cysteine 410. Similarly, Sohn et al (24) reported inhibitory activity of FCRL4 in response to Ag receptor ligation in the R2G6 cell line, a subline of Ramos B cells. Expression of FCRL4 in this model system also resulted in significant constitutive tyrosine phosphorylation of FCRL4 and its constitutive association with SHP-1 and SHP-2 phosphatases, and these characteristics might contribute to the observed discrepancy.…”

“…As expected, no tyrosine phosphorylation signal could be detected with the FCRL4 FFF mutant, indicating specificity of the assays. A recent study by Sohn et al (24) implicated FCRL4 as an immunoregulatory factor positioned at the interface of innate and adaptive immune signaling. Similar to our observation of FCRL4 phosphorylation after Ag receptor ligation, we observed increased phosphorylation of the intracellular domain of FCRL4 in response to CpG stimulation of HCK p59-expressing cells (Fig.…”

“…Given the key role of src-family kinases in phosphorylating tyrosine residues of ITIM-and ITAM-bearing receptors (23), we investigated in this study the role of the HCK and FGR kinases in FCRL4-mediated modulation of BCR signaling. Recent evidence shows FCRL4-mediated enhancement of CD23 expression induced by TLR9 stimulation (24), suggesting a function for FCRL4 at the interface of adaptive and innate immunity. Therefore, we extended our study to the potential regulation of signaling by innate receptors.…”

Pregnancy and Malaria Exposure Are Associated with Changes in the B Cell Pool and in Plasma Eotaxin Levels

“…Malaria exposure induces atypical MBC to express the inhibitory receptor Fc receptor-like protein (FcRL) 4 (21,26). FcRL4 dampens BCR activation, but enhances TLR9 signaling, favoring a switch from adaptive to innate B cell signaling (51). Moreover, a recent study has shown that tissue-FcRL4 + B cells produce proinflammatory cytokines like IL-6, TNF, and receptor activator for NF-kB ligand in rheumatoid arthritis (52).…”