Despite advances in diagnosis, therapy, and prophylaxis, Pneumocystis carinii pneumonia (PCP) remains a leading cause of morbidity and mortality in patients with HIV infection. The disease is caused by an enigmatic pathogen whose basic biology remains poorly understood because of difficulty in culturing the organism in vitro. Thus, animal models remain a mainstay of investigation into the pathogenesis of PCP. Although progress has been made in the definition of protective immunity in the immediate response to P carinii infection, an extremely important gap remains related to studies of the pathogenesis of this infection which centers around an inability to quantify various steps in pathogenesis in vivo. Surfactant protein A (SP-A) and surfactant protein D (SP-D) are collagen like lectins (collectins) which are highly expressed in the distal airways and alveolus. Both proteins have been shown to bind to and mediate the attachment of P carinii to the alveolar macrophage and to modulate cytokine elaboration by T-cell populations. Recent development of both SP-A and SP-D knockout mice has begun to implicate important roles for these proteins in local lung host defense and in modulation of the pulmonary inflammatory response. We hypothesize that SP-A and SP-D: a) Augment clearance of P. carinii from the alveolar space; b) Moderate a balance between pro- and anti-inflammatory responses which occurs during PCP. We propose to utilize the recently developed SP-A and SP-D knockout mice as new models of P. carinii infection to define the role of the collectins in host defense against PCP. The specific aims are: 1) Using SP-A and SP-D knockout mice, determine the role of the lung collectins (SP-A, SP-D) in the establishment and clearance of P. carinii infection in vivo; 2) Characterize the host immune response to P. carinii in SP-A and SP-D deficient models.; 3) Characterize changes in surfactant composition and function during development and progression of PC pneumonia in surfactant protein knockout mice; 4) Determine the effect of exogenous replacement of SP-A and/or SP-D on the clearance of P. carinii infection. The project will combine elements of several established thematic research programs to yield a comprehensive investigative proposal including: 1) A Principal Investigator with expertise in all major aspects of surfactant biology 2) Consultants recognized as authorities on immunocompromised mouse models of PCP and in generation of surfactant protein knockout mice. Results from these studies will extend our understanding of the pathogenesis of P. carinii infection.