Expression of Ia by murine alveolar macrophages is upregulated during the evolution of a specific immune response in pulmonary parenchyma.

These studies were performed to test the hypothesis that the evolution of a specific immune response in lung parenchyma upregulates the expression of Ia on surface membranes of murine alveolar macrophages. A secondary antibody-forming cell response to sheep erythrocytes was generated in lung parenchyma by intratracheal antigen challenge of systemically primed mice. During the immune response, alveolar macrophages were retrieved by bronchoalveolar lavage, and the percentages and total numbers of Ia-positive macrophages were measured by indirect immunofluorescence. The expression of Ia on surface membranes of lavaged alveolar macrophages increased in association with the generation of antibody-forming cell responses in lung tissue. This increase in Ia expression was antigen specific; intratracheal challenge with noncrossreacting antigen did not increase Ia expression. Nonspecific inflammation of the lung, induced by intratracheal hydrochloric acid, elicited increases in total numbers of macrophages that were similar in magnitude to those induced by specific immune responses, but increased Ia expression only modestly. In unprimed mice, intratracheal antigen challenge did not increase Ia expression by alveolar macrophages unless the mice had received immune splenocytes by adoptive transfer at the time of challenge. The results show that the generation of a specific immune response in pulmonary parenchyma upregulates the expression of Ia by murine alveolar macrophages in vivo and suggest that the accumulation of antigen-reactive lymphocytes in the lung plays an important role in this upregulation.