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. PGE2 is a lipid mediator with potent immunosuppressive effects against antimicrobial functions. We hypothesize that enhanced PGE2 production post-BMT impairs host defense. We demonstrate that lung homogenates from BMT mice contain 2.8-fold more PGE2 than control mice, and alveolar epithelial cells (2.7-fold), AMs (125-fold), and PMNs (10-fold) from BMT animals all overproduce PGE2. AMs also produce increased prostacyclin (PGI2) post-BMT. Interestingly, the E prostanoid (EP) receptors EP2 and EP4 are elevated on donor-derived phagocytes post-BMT. Blocking PGE2 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 PGE2 reversed the beneficial effects of indomethacin in vitro. Importantly, in vivo treatment with indomethacin reduced PGE2 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 PGE2 post-BMT is a critical factor determining impaired host defense against pathogens.

Prostaglandin E 2 (PGE 2 ) is a potent lipid mediator that effects changes in cell functions through ligation of four distinct G proteincoupled E prostanoid (EP) receptors (EP1-EP4). PGE 2 inhibits bacterial killing and reactive oxygen intermediate (ROI) production by alveolar macrophages (AMs), although little is known about the operative molecular mechanisms. The aims of this study were to evaluate the molecular mechanisms and the specific EP receptors through which PGE 2 inhibits killing of Klebsiella pneumoniae by AMs. The treatment of AMs with PGE 2 suppressed the killing of K. pneumoniae, and this effect was blocked by an adenylyl cyclase inhibitor and mimicked by agonists for the stimulatory G protein (G s )-coupled EP2 and EP4 receptors. Conversely, microbicidal activity was augmented by pretreatment with the cyclooxygenase inhibitor, indomethacin, and antagonists of EP2 and EP4. Similar results were found when ROI production was examined. PGE 2 inhibition of killing and ROI generation was associated with its activation of the cAMP effectors, protein kinase A and exchange protein directly activated by cAMP-1, as well as attenuation of the phosphorylation and translocation of the reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase component, p47phox, to the phagosomal membrane. We conclude that PGE 2 suppresses the microbicidal activity of AMs through the G s -coupled EP2/EP4 receptors, with increased cAMP inhibiting the assembly and activation of p47phox.

Cyclic adenosine monophosphate (cAMP) was the original "second messenger" to be discovered. Its formation is promoted by adenylyl cyclase activation after ligation of G protein-coupled receptors by ligands including hormones, autocoids, prostaglandins, and pharmacologic agents. Increases in intracellular cAMP generally suppress innate immune functions, including inflammatory mediator generation and the phagocytosis and killing of microbes. The importance of the host cAMP axis in regulating antimicrobial defense is underscored by the fact that microbes have evolved virulence-enhancing strategies that exploit it. Many clinical situations that predispose to infection are associated with increases in cAMP, and therapeutic strategies to interrupt cAMP generation or actions have immunostimulatory potential. This article reviews the anatomy of the cAMP axis, the mechanisms by which it controls phagocyte immune function, microbial strategies to dysregulate it, and its clinical relevance.

Bone marrow transplantation (BMT) is an important therapeutic option for a variety of malignant and nonmalignant disorders. Unfortunately, BMT recipients are at increased risk of infection, and in particular, pulmonary complications occur frequently. Although the risk of infection is greatest during the neutropenic period immediately following transplant, patients are still vulnerable to pulmonary infections even after neutrophil engraftment. We evaluated the risk of infection in this postengraftment period by using a well-established mouse BMT model. Seven days after syngeneic BMT, B6D2F1 mice are no longer neutropenic, and by 3 wk, they demonstrate complete reconstitution of the peripheral blood. However, these mice remain more susceptible throughout 8 wk to infection after intratracheal administration of Pseudomonas aeruginosa; increased mortality in the P. aeruginosa-infected BMT mice correlates with increased bacterial burden in the lungs as well as increased systemic dissemination. This heightened susceptibility to infection was not secondary to a defect in inflammatory cell recruitment to the lung. The inability to clear P. aeruginosa in the lung correlated with reduced phagocytosis of the bacteria by alveolar macrophages (AMs), but not neutrophils, decreased production of TNF-α by AMs, and decreased levels of TNF-α and IFN-γ in the bronchoalveolar lavage fluid following infection. Expression of the β2 integrins CD11a and CD11c was reduced on AMs from BMT mice compared with wild-type mice. Thus, despite restoration of peripheral blood count, phagocytic defects in the AMs of BMT mice persist and may contribute to the increased risk of infection seen in the postengraftment period.