Activation of human monocytes via their sIgA receptors

We have studied the interaction of secretory immunoglobulin A (sIgA) derived from human breast milk with human monocytes. The presence of specific sIgA receptors on the monocyte membrane was confirmed by dose-dependent inhibition of E-sIgA rosette formation and by the binding of iodinated sIgA to monocyte monolayers. Binding was dependent on both the number of monocytes, as well as the amount of [¹²⁵I]sIgA, and could be inhibited by unlabelled sIgA. Incubation of monocyte monolayers in the presence of increasing concentrations of secretory IgA and F(ab')₂ anti-IgA resulted in a dose-dependent increase of the oxidative burst, as measured by H₂O₂ production. Neither sIgA of anti-IgA alone, nor incubation of IgG with anti-IgA, had any effect on the oxidative burst. These studies indicate that human monocytes have a receptor for sIgA and that specific activation of the monocytes occurs via these receptors.