Pulmonary response to repeated exposure to Micropolyspora faeni

Most human exposure to agents that cause hypersensitivity pneumonitis (HP) result in transient episodes of HP that resolve quickly. We repeatedly injected Micropolyspora faeni, which is responsible for farmer's lung disease, into rabbits in an attempt to elucidate mechanisms for this phenomenom (i.e., resolution of abnormalities). The character and the extent of lung disease, the amount of anti-M. faeni serum antibody, and skin reactivity to M. faeni were evaluated after 3 sensitizing and 2, 4, or 8 challenge injections. We also determined the fate of 125I labeled M. faeni injected intratracheally into both normal and previously exposed rabbits. Increased numbers of lymphocytes, macrophages, and few polymorphonuclear leukocytes were present in interstitial and intraalveolar regions and bronchial walls. Interstitial fibrosis was not observed. The extent of cellular abnormalities was maximal after 2 challenges and regressed thereafter, despite continuing intratracheal injections. Serum anti-M. faeni antibody peaked after 4 intratracheal challenges. Anti-M. faeni antibody level at the time of death appeared to be proportional to the extent of inflammatory reaction within the lung. Previous exposure of rabbits to M. faeni was associated with more rapid appearance of 125I in blood in the first 2 h after intratracheal injection of 125I M. faeni. However, 24 h after injection, there was less 125I in the lungs and more in the urine of immunized rabbits than in normal rabbits. Repeated intratracheal injections of M. faeni into rabbits produces transient interstitial, intraalveolar, and peribronchial inflammatory infiltration that regresses without fibrosis despite continued antigenic challenge. Immunization appears to markedly decrease pulmonary exposure to antigen that results from an intratracheal injection of M. faeni.