Human alveolar macrophages inhibit receptor-mediated increases in intracellular calcium concentration in lymphocytes

Prior studies have demonstrated that human alveolar macrophages (AM) are suppressive of lymphocyte function, through the mechanism of inhibition is unclear. In the current study, human AM inhibited receptor-mediated increases in intracellular calcium concentration ([Ca2+]i) in T cells, natural killer cells, and B cells. This effect was produced by either live or fixed AM, while peripheral blood monocytes caused a minimal reduction in [Ca2+]i. The inhibitory effect of AM was seen following 1 to 2 h of incubation with lymphocytes, was complete at 16 h, and did not affect ionomycin-mediated [Ca2+]i. Inhibition of [Ca2+]i by AM correlated with suppression of T-lymphocyte proliferation and cytotoxic T-lymphocyte activity in response to alloantigen and Staphylococcus A-induced immunoglobulin production. Our findings suggest that a membrane signal on AM is capable of inhibiting receptor-mediated signal transduction in lymphocytes and that this is likely a major mechanism by which immune responses are downregulated in the alveolus.