In vitro binding of natural killer cells to Cryptococcus neoformans targets

Nylon wool-nonadherent splenic cells from 7- to 8-week-old CBA mice were further fractionated on discontinuous Percoll gradients. Enrichment of natural killer (NK) cells in Percoll fractions 1 and 2 was confirmed by morphological examination, by immunofluorescent staining, and by assessing the cytolytic activity of each Percoll cell fraction against YAC-1 targets in the 4-h51Cr release assay. Cells isolated from each Percoll fraction were tested for growth-inhibitory activity against Cryptococcus neoformans, a pathogenic yeastlike organism, by using an in vitro 18-h growth inhibition assay. The results showed that NK cell enrichment was concomitant with enrichment of anti-Cryptococcus activity in Percoll fractions 1 and 2. Cells from NK cell-rich fractions formed conjugates with the mycotic targets similar to the conjugates reported in NK cell-tumor systems. In addition, the percentage of effector cell-Cryptococcus conjugates was directly proportional to the level of the C. neoformans growth-inhibitory activity of the effector cells used. Scanning electron microscopy of the effector cell-Cryptococcus conjugates showed direct contact between the effector cells and the cryptococcal targets. An immunolabeling method combined with scanning electron microscopy was used to demonstrate that the effector cells attached to C. neoformans were asialo GM1 positive and, therefore, had NK cell characteristics.