Fc-gamma receptors(FcγR) on dendritic cells(DC) are key regulators of immune responses, linking humoral and innate immunity and modulating antigen presentation, DC maturation, and T-cell polarization. In mice, four FcγR have been described, FcγRI(CD64), FcγRIIb(CD32b), FcγRIII(CD16), and FcγRIV(CD16.2). While FcγR have historically been studied for function, emerging evidence indicates subset-specific expression patterns in immune cells and identifies a DC3 population within the SIRPα⁺ compartment, distinguishable from cDC2 by differential FcγRIII/IIb(CD16/32b) expression.
In this study, we could recapitulate these observations in previously published tissues, such as the spleen and bone marrow, and observed a similar distribution of CD16/32b expressing SIRPα⁺ DC in in-vitro bone marrow derived DC(BMDC) cultured with Flt3L. In contrast, GM-CSF-derived-BMDC displayed uniformly high CD16/32b expression within the SIRPα⁺ compartment, consistent with a monocyte-derived phenotype, which we validated using independent antibody clones and FcγRIV blocking strategies to exclude technical artefacts.
Our analysis of the DC compartment in peripheral tissues of adult C57BL/6 mice, which included skin, lung, liver, testis, and epididymis, similarly revealed uniformly high CD16/32b expression within the SIRPα⁺ compartment, yet was not expressed by DC in the intestine, preventing the identification of discrete cDC2 and DC3 subsets by flow cytometry. Moreover, CD16/32b surface expression was not restricted to SIRPα⁺ DC but also observed in DC1, pDC, macrophages, neutrophils, monocytes, and Langerhans cells to various degrees, limiting its specificity as a phenotypic marker.
Furthermore, CD16/32b surface expression in lymph nodes was consistently lower in migratory vs resident DC, suggesting an DC activation associated regulation. In BMDC, we could confirm that Toll-like receptor and cytokine signalling dynamically regulated CD16/32b surface expression. Type-2 stimuli preferentially upregulates CD32b expression, whereas type-1 signals significantly reduces it. Collectively, our findings highlight the limitations of CD16/32b as a surface marker of DC3 and point toward an active regulation of CD32b expression upon DC activation.