High-Affinity Fc Receptor Expression Indicates Relative Immaturity in Human Monocytes

Within monocyte heterogeneity, subsets represent discrete, well-characterized phenotypes. Although many studies have highlighted differences between subsets, there is evidence that subpopulations represent contiguous stages in a maturational series. As CD14(hi)CD64(hi) monocytes have higher proliferative potential than CD14(hi)CD64(lo) monocytes, the surface marker profile on 4 subsets defined by CD14 and CD64 was measured. The profiles were compared to that of subsets defined by the high-affinity IgE receptor (FcɛRIα), CD16, and CD14; further differences in size, granularity, and buoyancy were measured in subsets delineated by these markers. There was a positive correlation between proliferative monocyte (PM) prevalence and CD64 expression on the classical monocyte subset, and also between PM prevalence and circulating FcɛRIα(+) monocytes. The expression of CD64, the high-affinity IgG receptor, on canonical human monocyte subsets was determined before and after short-term culture, and in response to interleukin (IL)-6, IL-10, macrophage colony-stimulating factor, granulocyte/macrophage colony-stimulating factor and interferon-γ; the influence of these cytokines on monocyte subset transition was also measured. The loss of FcɛRIα expression preceded an increase in CD16 expression in whole blood cultures. These data indicate that high-affinity Fc receptors are expressed on less mature monocytes and that FcɛRIα(+) monocytes are developmentally antecedent to the canonical classical and intermediate monocyte subsets.