Immunity to fungal infections

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

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.

Killing of Aspergillus fumigatus spores by human lung macrophages: a paradoxical effect of heat-labile serum components

Spores of Aspergillus fumigatus can interfere with certain aspects of phagocytic cell function. In this study we have looked at the ability of human pulmonary macrophages to bind and kill spores of A. fumigatus which have been opsonized in untreated and heat-treated serum. For comparison, the non-pathogenic fungus Penicillium ochrochloron has been used. More than 60% of spores of both fungal species became cell-associated with the macrophages following incubation at 37 degrees C for 1 h. Spores of A. fumigatus opsonized in 5% pooled normal serum were significantly more resistant to killing by pulmonary macrophages than similarly opsonized spores of P. ochrochloron (p less than 0.02). However, serum which had been heated to 56 degrees C for 30 min prior to opsonization significantly increased (by approximately 80%) the ability of pulmonary macrophages to kill spores of A. fumigatus when compared with untreated sera (p less than 0.001). No such difference occurred with spores of P. ochrochloron. These unexpected observations fit with the known propensity of A. fumigatus to colonize the airways of patients with asthma and cystic fibrosis, conditions in which a protein-rich bronchial exudate characteristically occurs. Moreover, the presence of such a protective mechanism in a soil organism strongly suggests that complement-like substances may also play a role in protozoal phagocytosis. This is an area of research that does not appear to have been investigated.

Rapid killing of monocytes in vitro by Candida albicans yeast cells

To study the interaction between Candida albicans blastoconidia and human phagocytes, we incubated peripheral leukocytes with fungi for 1 h at 37 degrees C and stained the cells with fluorescent vital stains ethidium bromide (EB) and fluorescein diacetate. Fungi that had been phagocytosed showed little staining; however, some leukocytes containing blastoconidia exhibited nuclear staining with EB, even though their cell membranes showed no signs of penetration by fungi. The number of EB-positive leukocytes was related to viability of the yeast cells and the temperature at which they were maintained before use. Because efforts to quantitate EB-positive leukocytes microscopically were frustrated by cell aggregation, we labeled the leukocytes with 51Cr and measured isotope release. We determined that leukocytes incubated with viable fungi released significantly more isotope than cells incubated alone or with killed blastoconidia. Furthermore, 51Cr release correlated directly with concentration of fungi in the assay, time of incubation, and temperature at which fungi were maintained before use. Using a number of isolates of C. albicans and several other species of Candida, we found that all exhibited cytotoxic activity against leukocytes, but the level of activity varied among organisms. Finally, we depleted or enriched peripheral leukocytes for specific cell populations and determined that only monocytes released more 51Cr after incubation with viable blastoconidia. Blastoconidia can lyse phagocytic cells through germination and penetration of cell membranes within 1 to 2 h, but the cytotoxic phenomenon we describe occurs within 15 to 30 min after yeast cells have been phagocytosed. Therefore, this capacity may represent a more immediate response by blastoconidia against phagocytosis and killing by monocytes.

Adherence of Candida to corneal surface

In the pathogenesis of mycotic infections, adherence of the microbes to surface structures prior to invasion appears to be the initial and essential step in a susceptible host. Adherence and inhibition of adherence of Candida albicans to rabbit corneal surface was investigated in vitro by light and scanning electron microscopic examinations. The results indicate that blastospores of Candida albicans rarely bind to intact corneal epithelium, but consistently adhere to stroma denuded of epithelium. Such adherence was inhibited by concanavalin A. With its strong affinity for the yeast cell wall carbohydrate mannan, concanavalin A may block the site of attachment of yeast cells to the corneal surface.