The cells responsible for murine natural cytotoxic (NC) activity: a multi-lineage system

Previous studies on the surface phenotype of natural cytotoxic (NC) cells defined by negative selection with antibodies and complement showed that most if not all NC activity is the property of "null" cells that did not express a variety of lymphoid markers, including some expressed by natural killer (NK) cells. In the present study we show that when murine C57BL/6 spleen cells were sorted by flow cytometry into fractions positive or negative for Qa-5, Ly-2.2, Thy-1.2, L3T4, or surface immunoglobulin (sIg) and for high or low expression of H-2Kb, the pattern of NC activities was quite different from the negative selection experiments with antibody and complement. Enrichment of NC activity tested against WEHI-164 targets was observed in the H-2Kb high, Qa-5+, Thy-1.2+, and Ly-2.2- fractions, and to a lesser extent in the L3T4+ and sIg- fractions. However, significant NC activity, although lower than in the unseparated cells, was also found in the H-2Kb low, Qa-5-, Thy-1.2-, L3T4-, Ly-2.2+, and sIg+. With the exception of the anti-Ig, all the reagents were monoclonal antibodies. By comparison, NK activity tested against YAC-1 targets was clearly enriched in the H-2Kb high, Ly-2.2-, sIg-, and to a lesser extent, Thy-1.2+ sorted fractions, whereas most of the NK activity was in the L3T4- fractions. These results indicate that NC activity against WEHI-164 targets is mediated by an heterogeneous population of effector cells, which includes cells with markers of both the T and the B lineages, as well as of NK cells. These studies also show that negative selection with antibodies and complement is not always a reliable method for defining the surface phenotype of effector cells.