Importance of endogenous tumor necrosis factor alpha and gamma interferon in host resistance against Pneumocystis carinii infection.

C.B-17 scid/scid (SCID) mice that have acquired natural pulmonary infection with Pneumocystis carinii clear these organisms by 19 days after reconstitution with spleen cells from immunocompetent mice and therefore serve as a model for studying the pathogenesis of and immunity to P. carinii pneumonia. The present study examined the importance of endogenous tumor necrosis factor alpha (TNF-alpha) and gamma interferon (IFN-gamma) in the clearance of P. carinii by treatment of reconstituted SCID mice with anti-TNF-alpha and anti-IFN-gamma immunoglobulin G (IgG). Treatment of reconstituted mice with monospecific rabbit anti-TNF-alpha IgG almost completely inhibited the clearance of P. carinii from the lungs. In contrast, treatment with either anti-IFN-gamma antibody (polyclonal or monoclonal) or control IgG had no detectable effect on the clearance of P. carinii. The importance of endogenous TNF-alpha in the clearance of P. carinii was further supported by the finding of TNF-alpha but not IFN-gamma in lung homogenate supernatants from reconstituted SCID mice. Further study revealed that for the complete clearance of P. carinii, TNF-alpha must be present at the early stage of reconstitution, since clearance could be blocked by a single injection of anti-TNF-alpha IgG into SCID mice at day 0 but not at day 6 and/or day 12 after reconstitution. These results strongly suggest that, in reconstituted SCID mice, endogenous TNF-alpha is important in host resistance against P. carinii infection, whereas IFN-gamma appears not to play a significant role.