Human natural killer cells do not inhibit growth of Cryptococcus neoformans in the absence of antibody.

The interaction between human natural killer (NK) cells and yeast cells of Cryptococcus neoformans was investigated because experiments in mice indicated that NK cells inhibited the growth of C. neoformans. Strains of C. neoformans serotype A that differed in both resistance to alveolar macrophages and the size and composition of their capsules were evaluated. Human NK cells, which were isolated from normal peripheral blood, were activated by preincubation with interleukin-2 and alpha interferon to generate lymphokine-activated killer (LAK) cells. Yeast cells of C. neoformans were incubated with effector cells (NK or LAK cells); and inhibition of yeast cell growth was measured at 4, 8, and 24 h by comparing quantitative plate counts with controls consisting of yeasts in the absence of effector cells. The cytolytic activity of effector cells against target cells was confirmed by the release of radiolabel from 51Cr-labeled K-562 tumor cells. Neither NK nor LAK cells inhibited the growth of 13 strains of C. neoformans at effector to target cell ratios of as high as 500:1. Monocytes, which were isolated from the same populations of leukocytes as the NK cells, inhibited the growth of two strains of C. neoformans at effector to target cell ratios of 100:1 (92 and 46% inhibition), 50:1 (87 and 17%), and 1:1 (49 and 0%). NK cells could inhibit the growth of C. neoformans by an antibody-dependent cellular cytotoxicity mechanism in the presence of rabbit anticryptococcal antiserum at dilutions up to 1:4,000. Purified capsular polysaccharide of C. neoformans had no effect on the viability or tumoricidal activity of NK or LAK cells. These data suggest that human NK and LAK cells are not impaired by C. neoformans, and in the absence of antibody, which is rarely detectable in patients, they afford much less protection against C. neoformans than monocytes do.