Antibody-dependent natural killer cell-mediated growth inhibition of Cryptococcus neoformans

Previous data from this laboratory indicate that normal murine nylon wool nonadherent splenic cells with characteristics of natural killer (NK) cells effectively inhibit in vitro growth of Cryptococcus neoformans, a yeastlike pathogen. Since NK cells have been shown to be involved in antibody-dependent, cell-mediated cytotoxicity against immunoglobulin G (IgG)-coated tumor cells and xenogenic erythrocytes, we were interested in assessing the effects of the IgG fraction of rabbit anticryptococcal serum on NK cell-mediated inhibition of C. neoformans growth. Early in the study it became apparent that the conventional method of determining the numbers of CFU that was used previously for assessment of viable cryptococci at the end of the growth inhibition assay was not reliable for these studies, owing to minor clumping of the organisms in the presence of anticryptococcal antibody. Therefore, the BACTEC radiometric system was evaluated and determined to be a reliable replacement for the CFU count method. Using the BACTEC methodology, we showed that the anticryptococcal antibody significantly augmented the in vitro ability of NK cells to inhibit the growth of C. neoformans compared with normal rabbit serum or tissue culture medium. Furthermore, the antibody alone did not have an adverse effect on the organism, confirming that reduced growth indices obtained from test wells containing antibody, NK cells, and cryptococci were due to the effects of the NK cells. Maximum anticryptococcal activity of the NK cells was observed in the presence of 16 micrograms of IgG per ml; however, significant augmentation of anticryptococcal activity was seen with antibody concentrations as low as 3 micrograms/ml. Using different populations of murine splenic cells which had varying degrees of NK cell activity, we were able to show that NK cell activities, as determined by 51Cr release from YAC-1 targets, directly correlated with antibody-dependent, cell-mediated growth inhibition against cryptococci, suggesting that NK cells were effector cells in the antibody-dependent assays. Furthermore, in every case, the antibody-dependent activity of NK cells against C. neoformans was higher than the spontaneous activity of NK cells against the organism, emphasizing that NK cell activity against cryptococci can be augmented by specific antibody. When NK cell numbers were enriched by Percoll fractionation of nylon wool nonadherent splenic cells, antibody-dependent and spontaneous growth inhibitory activities of the effector cells were concomitantly augmented, confirming that NK cells were the effector cells in antibody-dependent growth inhibition of cryptococci.(ABSTRACT TRUNCATED AT 400 WORDS)