Natural killer (NK) cell activity during lymphatic regeneration: early appearance of Thy-1+ NK cells and highly interleukin 2-(IL 2) receptive, Thy-1- cells.

The appearance of natural killer (NK) cells, i.e., cells that can lyse tumor-derived target cells without intentional immunization, was studied during regeneration after a sublethal dose (300 mg/kg) of cyclophosphamide (Cy). It was found that NK activity started to increase in parallel with the number of cells in the spleen, reaching normal levels 8 to 9 days after Cy injection. The early appearing NK cells differed, however, from the NK cells of the normal spleen; all the activity was abolished by treatment with a monoclonal anti-Thy-1 antibody plus complement (C). The early regenerating population contained cells that could given rise in vitro, in the presence of interleukin 2 (IL 2), to higher amounts of NK-active cells than cells of a normal spleen. Pretreatment with anti-Thy-1 (plus C) did not inhibit the formation of these in vitro NK-active cells, indicating that they were not derived from these Thy-1+ NK cells. The NK cells formed in vitro expressed both the asialo GM1 and the Thy-1 antigens. These data suggest that i) the expression of the Thy-1 antigen on NK cells is connected to their stage of maturation or to the rate of proliferation, and ii) the precursors for the in vitro activated NK cells are not NK-active cells themselves.