Parenchymal Cell TNF Receptors Contribute to Inflammatory Cell Recruitment and Respiratory Failure in Pneumocystis carinii-Induced Pneumonia

The opportunistic organism Pneumocystis carinii ( Pc ) produces a life-threatening pneumonia (PcP) in patients with low CD4 + T cell counts. Animal models of HIV-AIDS-related PcP indicate that development of severe disease is dependent on the presence of CD8 + T cells and the TNF receptors (TNFR) TNFRsf1a and TNFRsf1b. To distinguish roles of parenchymal and hematopoietic cell TNF signaling in PcP-related lung injury, murine bone marrow transplant chimeras of wild-type, C57BL6/J, and TNFRsf1a/1b double-null origin were generated, CD4 + T cell depleted, and inoculated with Pc . As expected, C57 → C57 chimeras (donor marrow → recipient) developed significant disease as assessed by weight loss, impaired pulmonary function (lung resistance and dynamic lung compliance), and inflammatory cell infiltration. In contrast, TNFRsf1a/1b −/− → TNFRsf1a/1b −/− mice were relatively mildly affected despite carrying the greatest organism burden. Mice solely lacking parenchymal TNFRs (C57 → TNFRsf1a/1b −/− ) had milder disease than did C57 → C57 mice. Both groups of mice with TNFR-deficient parenchymal cells had low bronchoalveolar lavage fluid total cell counts and fewer lavageable CD8 + T cells than did C57 → C57 mice, suggesting that parenchymal TNFR signaling contributes to PcP-related immunopathology through the recruitment of damaging immune cells. Interestingly, mice with wild-type parenchymal cells but TNFRsf1a/1b −/− hematopoietic cells (TNFRsf1a/1b −/− → C57) displayed exacerbated disease characterized by increased MCP-1 and KC production in the lung and increased macrophage and lymphocyte numbers in the lavage, indicating a dysregulated immune response. This study supports a key role of parenchymal cell TNFRs in lung injury induced by Pc and a potential protective effect of receptors on radiosensitive, bone marrow-derived cells.