To verify whether alveolar macrophages (AM) of patients with hypersensitivity pneumonitis (HP) increase their antigen-presenting capacity by upregulating the expression of B7 costimulatory molecules (CD80, CD86), and whether a viral infection enhances this expression whereas cigarette smoking abrogates it, we performed bronchoalveolar lavage (BAL) on 18 patients with HP; 10 asymptomatic, virus-exposed subjects (AS); 18 nonsmokers; and 12 smokers. Influenza virus infection of AM from nonsmokers and smokers was induced in vitro. Expression of CD80 and CD86 on AM, and of CD28 and CTLA4 on T cells, was evaluated. The percentage of CD80(+) AM was greater in HP patients (34.6 +/- 7.7) and in AS (23.9 +/- 7.6) than in nonsmokers (6.7 +/- 1.6) or smokers (2.5 +/- 0.3). An increase in CD86(+) cells (62.3 +/- 5.9) was found in HP patients as compared with nonsmokers (24.2 +/- 3.8) and smokers (4.5 +/- 1.0). CD28 and CTLA4 molecules were highly expressed on all T cells. In vitro virus infection upregulated CD80 and CD86 expression in AM of normal nonsmoking subjects but not on those of smokers. These results suggest that: (1) an upregulation of B7 molecule expression is involved in the lymphocytic alveolitis of HP; (2) a viral infection could enhance HP by increasing B7 expression; and (3) the protective effect of cigarette smoking in HP may be due to the low level of expression of costimulatory molecules on AM from smokers, and to their resistance to further upregulation.