Critical role of prostaglandin E2 overproduction in impaired pulmonary host response following bone marrow transplantation

The success of bone marrow transplantation (BMT) as a therapy for malignant and inherited disorders is limited by infectious complications. We previously demonstrated syngeneic BMT mice are more susceptible to Pseudomonas aeruginosa pneumonia due to defects in the ability of donor-derived alveolar macrophages (AMs), but not polymorphonuclear leukocytes (PMNs), to phagocytose bacteria. We now demonstrate that both donor-derived AMs and PMNs display bacterial killing defects post-BMT. PGE(2) is a lipid mediator with potent immunosuppressive effects against antimicrobial functions. We hypothesize that enhanced PGE(2) production post-BMT impairs host defense. We demonstrate that lung homogenates from BMT mice contain 2.8-fold more PGE(2) than control mice, and alveolar epithelial cells (2.7-fold), AMs (125-fold), and PMNs (10-fold) from BMT animals all overproduce PGE(2). AMs also produce increased prostacyclin (PGI(2)) post-BMT. Interestingly, the E prostanoid (EP) receptors EP2 and EN are elevated on donor-derived phagocytes post-BMT. Blocking PGE(2) synthesis with indomethacin overcame the phagocytic and killing defects of BMT AMs and the killing defects of BMT PMNs in vitro. The effect of indomethacin on AM phagocytosis could be mimicked by an EP2 antagonist, AH-6809, and exogenous addition of PGE(2) reversed the beneficial effects of indomethacin in vitro. Importantly, in vivo treatment with indomethacin reduced PGE(2) levels in lung homogenates and restored in vivo bacterial clearance from the lung and blood in BMT mice. Genetic reduction of cyclooxygenase-2 in BMT mice also had similar effects. These data clearly demonstrate that overproduction of PGE(2) post-BMT is a critical factor determining impaired host defense against pathogens.