Human alveolar macrophages (AM) are antigen-presenting cells that have an important immune effector function in the lung. We have previously shown that AM produce a specific interleukin-1 (IL-1) inhibitor of 20 to 25 kD that blocks biologic activities of IL-1 alpha and IL-1 beta such as prostaglandin E2 production by fibroblasts. This inhibitor acts as a receptor antagonist (IL-1ra) by binding to the IL-1 receptor. We are now presenting evidence that the natural AM-derived IL-1ra is immunologically identical to IL-1ra cloned from human peripheral blood monocytes and shows a band at 20 kD compatible with the natural glycosylated IL-1ra. No constitutive expression of IL-1 mRNA was detected when analyzed by Northern blot immediately after bronchoalveolar lavage from six control patients. Comparison of in vitro kinetics of IL-1ra, IL-1 alpha, and IL-1 beta analyzed during culture in the presence or absence of phorbol myristate acetate revealed that their mRNA expression was asynchronous. IL-1 alpha and IL-1 beta mRNA were expressed after as little as 15 min, whereas IL-1ra mRNA was detectable only after 3 h in culture. The production of IL-1ra was measured by enzyme-linked immunosorbent assay and compared with that of IL-1 alpha and IL-1 beta. In freshly isolated AM (10(6)/ml), cell-associated IL-1ra was present in an average amount of 2.0 +/- 0.5 ng/ml, i.e., 25 and 100 times more than IL-1 alpha and IL-1 beta, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)