Low frequency of NK-cell progenitors and development of suppressor cells in IL-2-dependent cultures of spleen cells from low NK-reactive SJL/J mice.

Studies were performed to determine the basis for low NK activity in the spleens of SJL/J mice. In contrast to substantial boosting by IL-2 or IFN of NK activity of spleen cells from high NK-reactive strains of mice, no detectable effect of these cytokines on SJL spleen cells was seen. When SJL spleen cells were cultured for 7 days in the presence of IL-2, the frequency of proliferating cells was comparable to that of other strains of mice. However, the SJL spleen cells showed a frequency of NK-cell progenitors which was at least 50 times lower than that of the CBA/J spleen cells. In addition to having no detectable effector-cell activity, the cultures of SJL spleen cells contained suppressor cells which were able to inhibit the NK activity of spleen cells from other strains of mice. These suppressor cells did not adhere to plastic or nylon wool and were found in low-density fractions after Percoll density gradient centrifugation. The cultured SJL spleen cells had a high percentage of cells capable of binding to NK-susceptible target cells, which was similar to that seen in lytic cultures from other strains of mice. Thus, the suppressor activity may be attributable to competitive inhibition of the interaction of effector cells with target cells. Although several of the characteristics of suppressor cells were similar to those of cultured effector cells, they may not represent inactive or pre-NK cells since their progenitors were Thy1+ and AsialoGM1- whereas the progenitors of cultured effector cells in high NK strains were Thy1- and GM1+. The ability to eliminate the progenitors of the suppressor cells by pretreatment of spleen cells with anti-Thy1 plus C' also suggested that the low or undetectable NK activity of the cultured SJL cells is not attributable to suppression but may be due to an inherent deficit of NK cells.