Dual function of human IgA antibodies: Inhibition of phagocytosis in circulating neutrophils and enhancement of responses in IL-8-stimulated cells

We have sought to elucidate the responses of human peripheral blood neutrophils to antigenic surfaces complexed with human specific IgA antibodies obtained either as myeloma proteins that recognize staphylococcal-α-toxin, or from the serum of patients with subacute bacterial endocarditis due to Streptococcus mutants or from colostrum. In contrast to IgG, IgA antibodies bound to antigen-coated fluorescent microspheres, and subsequently exposed to complement (or not), did not promote phagocytosis, as measured by flow cytometric enumeration of cell-associated microspheres. Instead, IgA antibodies interfered with complement-dependent phagocytosis mediated by IgG antibodies. These properties were shown by different forms of IgA antibodies including serum and secretory IgA, as well as by monoclonal or polyclonal antibodies. Neutrophils did not respond to the production of superoxide to IgA antibodies complexed with antigen-coated microspheres or with antigen deposited on a solid surface and IgA antibodies suppressed IgG antibody- and complement-mediated superoxide release. However, neutrophils pretreated with interleukin-8 ingested IgA opsonized microspheres and released superoxide when exposed to IgA antibody anticomplexes. IgG antibody-antigen complexes did not stimulate increased superoxide release in interleukin-8-treated neutrophils. These findings were consistent with a selective increase in the surface expression of FcαR by interleukin-8-treated neutrophils. We conclude that IgA antibodies interfere with the phagocytic activities of normal circulating human neutrophils and may promote these activities in inflammatory neutrophils activated by interleukin-8 in which FcαR is up-regulated.