Abnormal alveolar macrophage populations in bone marrow transplant recipients with pneumonitis

The purpose of this investigation was to determine whether there is any relationship between different subsets of alveolar macrophages and type of infection or survival from interstitial pneumonitis following bone marrow transplantation (BMT). The population of alveolar macrophages found in bronchoalveolar lavage fluid (BALF) from 16 BMT recipients with 19 episodes of interstitial pneumonitis was investigated, using immunocytochemical methods. Results were compared with those from seven normal volunteers. The results showed that patients with pneumonitis had significantly higher numbers of total cells in BALF than normals but reduced proportions of macrophages, although the absolute numbers were unchanged. Of the cells present which were morphologically macrophages, there were raised proportions of both RFD1+ cells (interdigitating cells) and RFD7+ cells (mature macrophages) in patients compared with normals, but expansion of these two subsets could be explained, in part, by a significant increase in cells positive for both markers (42% in patients compared with 9% in normals). Proportions of cells with the monocyte phenotype (CD14+, UCHM1) were also significantly raised in patients with pneumonitis (17% compared with 6% in normals). These patients, however, had significantly reduced proportions of macrophage-like cells which were positive for the DR antigen (Class II major histocompatibility complex (MHC) antigen) (47% compared with 88% in normals), and this abnormality was greater still in patients who died from pneumonitis (40%) compared with those who survived (52%). The results of this study indicate a breakdown of local immunoregulation, thus contributing to the high incidence of, and mortality from, opportunistic pulmonary infections in this group.