Encapsulation and melanin formation as indicators of virulence in Cryptococccus neoformans

Abstract

Acapsular (Cap-) mutants of Cryptococcus neoformans var. neoformans that produce melanin (Mel+) on diphenol media at 30°C but not at 37°C were found to be avirulent for mice. Compared with wild-type isolates, the mutants had a lower rate of L-3,4-dihydroxyphenylalanine uptake at 37°C and showed an insignificant level of phenoloxidase activity at both temperatures. To study the relationship of Cap and Mel phenotypes to virulence in mice, we crossed one of the mutants (Cap- Mel-) with a wild type (Cap+ Mel+) to obtain four classes of progeny (Cap+ Mel+, Cap+ Mel-, Cap- Mel+, and Cap- Mel-). The progeny with the Cap+ Mel+ phenotype and the wild-type parent (Cap+ Mel+) were inoculated into mice (106 cells per mouse) and, within 40 days, produced fatal infection in 90 to 100% of the animals. None of the other three phenotypes produced fatal infection within the same period. While progeny with the Cap+ Mel- phenotype did produce fatal infection after 40 days, 70 to 90% of the mice survived at least until day 70. However, in the isolates recovered from the brain tissue of a mouse that died on day 68, nearly 40% of the CFU had reverted to the Cap+ Mel+ type. The virulence of one of these revertant Cap+ Mel+ isolates was compared with that of a Cap+ Mel- isolate recovered from the same tissue. One hundred percent of the mice inoculated with the revertant died within 35 days, while no fatal infection was produced in the mice inoculated with the Cap+ Mel- isolate within the same period. The isolates with the Cap- Mel+ or Cap- Mel- phenotype not only failed to produce fatal infection but failed to revert to the Cap+ Mel+ type in the mouse brain during the experimental period. These results indicate that both the Cap+ phenotype and the Mel+ phenotype are important indicators of virulence in C. neoformans.