Urokinase-type plasminogen activator in inflammatory cell recruitment and host defense against Pneumocystis carinii in mice

Abstract

Effective host defense against Pneumocystis carinii depends upon the integrated actions of inflammatory cells and mediators in the lungs. Using immunocompetent and immunosuppressed mice, our laboratory has defined inflammatory changes in the lungs in response to P. carinii. However, the essential molecules and mechanisms required for cellular recruitment and for host defense against P. carinii are undefined. We hypothesized that urokinase-type plasminogen activator (uPA), a protease intimately involved in inflammatory cell migration and activation, is required for clearance of P. carinii. To test this hypothesis in vivo, we compared the intensity of P. carinii infection and inflammation in the lungs of mice lacking the uPA gene (uPA knockout mice) and in the lungs of wild-type mice. After intratracheal inoculation with P. carinii organisms, uPA knockout mice developed uniformly heavy P. carinii pneumonia while wild-type mice cleared the P. carinii inoculum. Bronchoalveolar lavage fluid from uPA knockout mice contained significantly smaller numbers of cells than did lavage fluid from wild-type mice. We conclude that deletion of the uPA gene prevents the clearance of P. carinii and reduces inflammatory cell recruitment. Therefore, uPA is an important participant in the network of inflammatory events required for the clearance of P. carinii, confirming an important role for this molecule in pulmonary host defense against opportunistic pathogens.

FIG. 1

Intensity of P. carinii infection. Wild-type (n = 9) and uPA knockout (n = 10) mice were inoculated intratracheally with P. carinii organisms, and the intensity of infection was scored 4 weeks after inoculation, using a histologic grading scale. Wild-type mice cleared the P. carinii inoculum, with only a single mouse demonstrating several individual, widely scattered P. carinii cysts. Conversely, uPA knockout mice were uniformly heavily infected with P. carinii, with all mice demonstrating grade 3 or grade 4 infections. Data represent medians obtained from two individual experiments. ∗, P < 0.007 by the Mann-Whitney test compared with wild-type mice.

FIG. 2

Histology of P. carinii infection. Lungs of wild-type mice inoculated with P. carinii demonstrated perivascular and peribronchiolar mononuclear cell infiltrates (arrow) (A) but no P. carinii organisms (C). Lungs of uPA knockout mice inoculated with P. carinii demonstrated markedly reduced mononuclear cell inflammation but contained alveolar exudate indicative of P. carinii infection (arrows) (B), confirmed by the presence of P. carinii organisms (arrowheads) (D). (A and B) H&E stain; magnification, ×200. (C and D) Gomori methenamine silver stain; magnification, ×400.

FIG. 3

Inflammatory cell accumulation. Wild-type (n = 5) and uPA knockout (n = 5) mice were inoculated intratracheally with P. carinii organisms, and cells in BAL fluid were enumerated 4 weeks after inoculation. Data represent means ± SEM; ∗, P < 0.02 by t test compared with wild-type mice.

FIG. 4

Cellular constituents of BAL fluid. Wild-type (n = 5) and uPA knockout (n = 5) mice were inoculated intratracheally with P. carinii organisms, and cells in BAL fluid were characterized 4 weeks after inoculation. Total numbers of lymphocytes (A), macrophages (B), and neutrophils (C) are indicated. Data represent means ± SEM; ∗, P < 0.05 by t test compared with wild-type mice.