Interleukin-2 activation of human bone marrow in long-term cultures: An effective strategy for purging and generation of anti-tumor cytotoxic effectors

Our previous studies have shown that IL-2 activated bone marrow cells develop potent tumoricidal activity in vitro and in vivo. With the dual aim of in vitro purging and generation of effectors which could mediate graft-versus-leukemia effects in vivo, IL-2 activation of human bone marrow in long-term cultures (LTC) was tested. Marked cytotoxicity was seen against A375 (melanoma), K562 (CML) and Daudi (lymphoma) cell lines in IL-2 (1000 U/ml) stimulated cultures. Hematopoietic progenitor cell number in these cultures was assessed by day 14 clonogenic assays. In 1-week-old IL-2 stimulated cultures a higher number of clonogenic cells was seen than control LTCs without IL-2. However, thereafter accelerated decrease in the number of clonogenic cells was seen in IL-2 cultures. In vitro purging efficacy was tested by elimination of A375 and K562 cells mixed with normal marrow at 1:10 and 1:100 ratios and co-cultured for 10 days. In IL-2 stimulated cultures, A375 cells capable of proliferation were not detectable at both mixing ratios. K562 elimination was complete only at 1:100 ratio. After 10 days in culture, no Ph¹-positive metaphases were seen in IL-2 stimulated BM cultures of 4 patients with CML. These results indicate that IL-2 activation of BM in 1-2 week cultures can lead to generation of marked anti-tumor cytotoxicity and effective in vitro purging in a variety of tumor types.