Phagocytosis of Staphylococcus aureus and Haemophilus influenzae type B opsonized with polyclonal human IgG1 and IgG2 antibodies: Functional hFcγRIIa polymorphism to IgG2

To assess the function of IgG subclass antibodies we studied the opsonization of Staphylococcus aureus (STAW) and Haemophilus influenzae type b (Hib) by natural IgG1 and lgG2 antibodies from five healthy individuals. Phagocytosis by polymorphonuclear leukocytes (PMN) was analyzed using FITC-labeled bacteria and flow cytometry. All PMN donors were typed for the High- and Low-Responder phenotype of the human Fcγ receptor (hFcγ) type IIa (CD32) and the NA1/NA2 allotype of the hFcγRIIIb (CD16). When PMN were used that were heterozygous for hFcγRlla and hFcγRIIIb, phagocytosis of STAW opsonized with IgG1 antibodies was similar to that with IgG2, both in the presence and absence of a source of complement (agammaglobulinemic serum). Phagocytosis of Hib opsonized with IgG2 anti-Hib proved significantly lower (p < 0.05) than with IgG1 anti-Hib using preparations from 4 of the 5 individuals tested. IgG2 anti-Hib from one donor, however, proved more effective than IgG1 anti-Hib. The properties of PMN with hFcγRIIaLR,LR and hFcγRIIaHR,HR were compared, using hFcγRIIIb NA1/NA2-matched PMN. hFcγRIIaHR,HR PMN were virtually incapable of phagocytosing STAW and Hib opsonized with IgG2 antibodies without complement, in contrast to hFcγRIIaLR,LR PMN. Phagocytosis of IgG2-opsonized bacteria by hFcγRIIaLR,LR PMN was effectively inhibited by mAb against hFcγRIIa (IV.3). IgG1-mediated phagocytosis was blocked by mAb against hFcγRIIIbf(CLB/FcRGran 1) to a greater extent than by anti-hFcγRIIa mAb. Inhibition studies with mAb against CR3 (B2.12) and hFcγRIIa showed that both receptors cooperated in phagocytosis. We conclude that PMN phagocytosis can be effectively mediated by antibacterial IgG2 anti-STAW and anti-Hib. Thus, IgG2 may have a critical role in the immune defense against these bacteria. However, the role of antibacterial IgG2 in opsonization and phagocytosis may be limited in individuals homozygous for hFcγRIIaHR.