Immunogenic fractions of Cryptococcus neoformans

Vaccines and Protective Immune Memory against Cryptococcosis

Cryptococcosis is a potentially lethal disease caused by fungal pathogens including Cryptococcus neoformans and Cryptococcus gattii species complex. These fungal pathogens live in the environment and are associated with certain tree species and bird droppings. This infectious disease is not contagious, and healthy individuals may contract cryptococcal infections by inhaling the airborne pathogens from the environment. Although cleaning a contaminated environment is a feasible approach to control environmental fungal pathogens, prophylactic immunization is also considered a promising method to regulate cryptococcal infections. We review the history of the development of cryptococcal vaccines, vaccine components, and the various forms of immune memory induced by cryptococcal vaccines.

Extracellular proteins of Cryptococcus neoformans and host antibody response

Proteins secreted by the fungal pathogen Cryptococcus neoformans may be involved in invasion and could be useful in vaccine design. Despite the medical importance of this fungus, little is known about its extracellular proteins or the immune response to these antigens. To study C. neoformans extracellular proteins, 12 strains were metabolically radiolabeled and protein supernatants were analyzed. Both strain- and growth condition-dependent differences were observed. Enzymatic assays of filtered culture supernatants revealed butyrate esterase and caprylate esterase lipase activity for 11 of 12 strains, as well as acid phosphatase, naphthol-AS-BI-phosphohydrolase, and beta-glucosidase activities in some strains. Serum from infected rodents immunoprecipitated several secreted proteins, consistent with in vivo expression and development of an antibody response. For strain 24067, two immunodominant species, of approximately 75 and 30 kDa, were recognized. The relative intensity of the autoradiographic bands depended on the route of infection for both rats and mice. In summary, our results indicate that (i) there are multiple proteins in C. neoformans culture supernatants, (ii) there are strain differences in supernatant protein profiles, (iii) there are differences in supernatant protein profile depending on the growth conditions, (iv) there are several new extracellular and/or cell-associated enzymatic activities, and (v) antibodies to several supernatant proteins are made in the course of infection.

An anti-Cryptococcus neoformans monoclonal antibody directed against galactoxylomannan

Six monoclonal antibodies (mAb) were produced by immunizing mice with a Cryptococcus neoformans serotype A spheroplast lysate (CNSL). Two mAb were of the IgG1 isotype; the others were IgM. The results obtained with one IgM mAb (CN6) are reported herein. This mAb recognized the four serotypes of C. neoformans and no cross-reactions were observed with extracts from Cryptococcus melibiosum, Candida albicans, Saccharomyces cerevisiae, Torulopsis glabrata or Trichosporon beigelii. Fractionation of the CNSL by gel filtration revealed that mAb CN6 recognized high molecular weight substances as well as a range of smaller molecules. Indirect ELISA inhibition studies showed that this mAb recognized substances in a cryptococcal culture filtrate. Inhibition studies and agglutination tests using latex beads sensitized with purified CN6 showed that CN6 strongly reacted with the C. neoformans serotype A cell envelope galactoxylomannan-mannoprotein complex (GAlXM-MP) and only weakly with the glucuronoxylomannan (GXM) component. These tests also showed that purified galactoxylomannan (GalXM) from C. neoformans serotype A was more reactive than purified mannoprotein (MP). An anti-GalXM mAb might be a useful tool for monitoring the clinical course of cryptococcal infections.

Vaccines against fungal infections

The state-of-the-art reached in developing protective immunity against fungal infections through vaccination makes a survey of methodologies and results timely. This review describes experimental vaccinations against dermatophytes, pathogenic yeasts, and dimorphic fungi with special attention to the anti-Coccidioides immitis vaccine, which has reached clinical trials, and to the anti-Candida albicans and anti-Histoplasma capsulatum ribosomal vaccines. Also covered are vaccination experiments in compromised hosts aimed at eliciting acquired resistance to opportunistic fungal infections which constitute risk factors for these hosts. Immunization procedures include live, killed, and attenuated organisms, as well as different subcellular fractions such as cytoplasmic extracts, fungal culture filtrates, cell walls, or ribosomal fractions. A variety of experimental animal models and isolated human trials constitute the subjects in these studies. Acquired immunity has been evaluated through assessment of resistance to infection and determination of specific immune responses. It has been demonstrated that fungal vaccines do elicit both humoral and cell-mediated immunity in the immunized host. For some vaccines (e.g., H. capsulatum), a correlation between the induced immunity and protection was observed and the immunity could be adoptively transferred. In view of the potential of vaccines against fungal infections, a perspective on their applicability, significance, and value for human use is discussed.

Cellular immunity in a cutaneous model of cryptococcosis

The temporal development of cellular immune responses in mice inoculated cutaneously with viable Cryptococcus neoformans 145 was determined in vivo and in vitro by comparing several antigen preparations for their efficacy in the assays selected. Three antigens derived from C. neoformans 145, viz., a culture filtrate preparation (CneF-145), a membrane extract (B-HEX), and soluble cytoplasmic substances (SCS), were compared for their ability to detect delayed hypersensitivity (DH) in vivo in a footpad assay or to stimulate lymphocytes in vitro in a thymidine incorporation assay. DH to B-HEX could be demonstrated as early as 1 week after infection, whereas significant responses to SCS and CneF-145 were not regularly detected until 3 weeks after infection. Substantial reactions were observed to all three antigens up to 12 weeks, although they peaked at 2 to 3 weeks. Reactions to B-HEX and SCS were somewhat better than those to CneF. Differences in the efficacies of the three antigens were not obvious after the sixth week of infection, however. In vitro, lymph node cells from infected animals were stimulated significantly with all three antigens beginning at week 1. As with DH, however, responses to CneF-145 were usually less than those to SCS and B-HEX. In vitro lymphocyte responses waned after approximately 6 weeks, whereas DH responses were clearly positive through 12 weeks. In addition to the studies in infected animals, animals immunized with heat-killed cells of C. neoformans 145 or 184 were tested 6 to 8 days later for DH with CneF-145, CneF-184, or B-HEX derived from C. neoformans 145. The CneF-145 and CneF-184 were equally effective for detecting DH, regardless of the cryptococcal strain used for immunization. Likewise, the B-HEX detected equivalent responses in mice sensitized with each cryptococcal strain. Since all three antigens were soluble and easily extracted and since each elicited significant cellular immune responses in infected animals, further studies involving their specificity and the nature of their reactive components seems warranted as they may help evaluate immune responses in humans infected with this fungus.