Functional regulation of human neutrophil Fc γ receptors

Interaction between Fc receptors expressed on phagocytic cells and antibodies play a critical role in innate immune response. Interestingly, immune cells such as neutrophils, monocytes, and dendritic cells (DCs) express multiple Fc receptors for IgG (FcγR) with overlapping ligand specificity. These receptors compete for the same ligand on the target and are known to transduce positive and negative signals to the same cell, depending on presence of type of signaling motif in their cytoplasmic domain. Neutrophils, the first line of defense against bacterial infection and the major phagocytic cell in the blood, express two types of FcγRs depending on the species. In humans, the neutrophils co-express immunoreceptor tyrosine-based activation motif (ITAM) containing CD32A and glycosyl-phosphatidyl inositol (GPI)-anchored CD16B, which is in contrast to co-expression of ITAM containing CD16A and ITIM containing CD32B in mouse neutrophils. Recent studies in gene knockout mice have demonstrated that the negative signaling by CD32B plays a critical role in preventing immune complex (IC)-mediated autoimmune diseases by regulating the activation signal delivered by CD16A. However, it is not known how the function of ITAM signaling CD32A is regulated in human neutrophils. Recent observations from our laboratory suggest that in human neutrophils, the CD32A receptor is regulated at the ligand-binding stage. Using a CD16B-deficient donor, we found that the CD32A expressed on resting neutrophils is unable to bind ligand; however, once neutrophils are activated with fMLP, a bacterial chemotactic peptide, the CD32A is functionally active in binding ligand. We also observed that this regulation is neutrophil-specific phenomenon. These observations suggest that FcγR can be regulated by distinct mechanisms and factors such as membrane-anchoring, cell-specific signaling, and avidity modulation that may be coordinately involved in regulating the function of human FcγR. Because neutrophils may be activated during infectious and inflammatory diseases, the knowledge of functional regulation of FcγR will be useful in designing therapies for many autoimmune diseases.