Complementation of a capsule deficient Cryptococcus neoformans with CAP64 restores virulence in a murine lung infection.

Cryptococcosis is a systemic infection in humans caused by the opportunistic fungal pathogen, Cryptococcus neoformans. The infection usually presents as chronic meningoencephalitis, but infects via the respiratory tract. A polysaccharide capsule is a major virulence factor, which allows the yeast to resist host defenses. However, the essential role of the capsule in allowing it to resist host defenses during the initial lung infection has not been clearly shown. A mutant acapsular C. neoformans strain 602 was complemented with the CAP64 gene to obtain an encapsulated strain, TYCC38-602. TYCC38-602 persisted in the lungs of C.B-17 mice after intratracheal inoculation and disseminated to the brain, whereas the mutant acapsular 602 and the plasmid control transformant CIP3-602 strains grew less readily in the lung and were infrequently detected in the brain. T cell-mediated immunity, developed to the encapsulated organism, was required to control growth within the lungs and had a significant impact on numbers of yeasts detected in the brain. The parent acapsular strain, but not the transformant control, also required T cells for optimal inhibition of growth within the lung, but not for maintaining control of the colony-forming units (cfu) in the brain. In summary, the cryptococcal capsule plays an important role in lung virulence and dissemination to the brain, and intact immunity is required to control lung growth of the encapsulated yeast.