Inhibition of formylmethionyl-leucyl-phenylalanine-stimulated neutrophil chemiluminescence by human immunoglobulin A paraproteins.

Immunoglobulin A (IgA) paraproteins from patients with myeloma have been shown to inhibit human neutrophil chemotaxis to C5a, casein, and chemotactic factors produced by Escherichia coli. This study demonstrates that these paraproteins also inhibit neutrophil chemotaxis in response to the synthetic peptide formylmethionyl-leucyl-phenylalanine (f-MLP). Furthermore, the neutrophil chemiluminescence response stimulated by f-MLP was markedly suppressed by the presence of IgA paraprotein. Maximal inhibition of chemiluminescence was observed when the paraprotein was present during the chemiluminescence response. The inhibitory activity was substantially reduced by removal of the Fc region of IgA or by conversion of polymeric IgA to monomeric IgA by limited reduction and alkylation. Additional experiments showed that these IgA paraproteins inhibited C5a but not phorbol myristate acetate-stimulated chemiluminescence. These observations are constant with the hypothesis that polymeric forms of IgA bind to human neutrophils and interfere with the binding of chemotactic factor to its receptor or the consequent receptor-mediated oxidative burst or both.