Evidence for cross-regulation of Fc gamma RIIIB (CD16) receptor-mediated signaling by Fc gamma RII (CD32) expressed on polymorphonuclear neutrophils.
Human polymorphonuclear neutrophils (PMN) express the low affinity receptors for the Fc domain of IgG (Fc gamma R), Fc gamma RII (CD32), and the glycosyl phosphatidylinositol-linked isoform of Fc gamma RIII (Fc gamma RIIIB, CD16) on their cell surface. Both of these receptors have been shown to be signal-transducing molecules. However, the mechanisms involved in such signaling are not clearly understood. In this report, we investigated intracellular Ca2+ ([Ca2+]i) signals triggered in PMN by both the receptors using aggregated human IgG (AggIgG) and specific mAb to Fc gamma RII (KuFc79) and Fc gamma RIII (3G8) as ligands. Addition of AggIgG as well as cross-linking of mAb KuFc79 and 3G8 bound to PMN induced [Ca2+]i flux. However, preincubation of PMN with mAb KuFc79 (whole Ig or Fab fragments) in the absence of cross-linking abrogated the [Ca2+]i flux induced by AggIgG and mAb 3G8, indicating that Fc gamma RII receptor occupancy by mAb KuFc79 can block signals mediated by Fc gamma RIIIB. KuFc79-isotype-matched control mAb (MOPC 195) did not abolish the signals generated by AggIgG and mAb 3G8. In addition, mAb KuFc79 did not abrogate [Ca2+]i responses elicited by the receptor for the chemotactic peptide FMLP indicating that modulation of signal transduction by Fc gamma RII-bound KuFc79 is selective for certain receptors. Immunofluorescence analysis of PMN initially treated with mAb KuFc79 followed by AggIgG showed that KuFc79 did not block the binding of AggIgG to PMN. Similarly, competitive binding studies revealed no stearic hindrance between mAb KuFc79 bound to Fc gamma RII and mAb 3G8 bound to Fc gamma RIIIB. Thus, the ability of mAb KuFc79 to modulate signals induced by AggIgG and 3G8 strongly suggests that Fc gamma RII may regulate Fc gamma RIIIB signaling. While previous studies on Fc gamma RII revealed a requirement for cross-linking of the receptor to induce its effector functions, the present study shows that binding of mAb KuFc79 to Fc gamma RII itself, even in a univalent form, results in cross-regulation of Fc gamma RIIIB-triggered signals. Treatment of PMN with protein tyrosine kinase inhibitors, genistein and herbimycin A, abrogated the [Ca2+]i signals elicited by both mAb KuFc79 and 3G8. These results suggest that tyrosine kinase enzyme(s) associated with these receptors may be crucial for positive/negative signals triggered by Fc gamma RII and Fc gamma RIIIB.