We compared the interactions of autologous, HLA-identical and mismatched CD56+ cells with chronic myelogenous leukemia (CML) CD34+ cells. In 14 donor-recipient pairs, HLA-matched donor CD56+ cells inhibited CML CFU-GM at an E/T ratio of 10:1 (mean inhibition 42±9%). This was comparable to the 39.5±7% inhibition observed with 14 HLA-mismatched unrelated CD56+ cell donors, indicating that KIR incompatibility was not required for the antileukemic effect. Both CD56+CD3- (NK) and CD56+CD3+(NK-T) subsets inhibited colony formation of CD34+ CML cells but not normal CD34+ cells from the stem cell donor, suggesting that CD56+ cells recognized surface molecules specific to CML cells. Using indirect immunofluorescence to detect NKG2D ligands, 11/12 CML CD34+ samples were found to be MICA/B+ positive, while normal CD34 cells were uniformly negative. Five of six MICA/B+ CML samples bound a NKG2D/FC+ chimeric molecule indicating the presence of functional ligands. CML CFU-GM inhibition was partially blocked by anti-NKG2D suggesting that NKG2D-MICA/B interactions contributed to the antileukemic activity of NK and NK-T cells. Colony formation of MICA/B+ CML cells was also inhibited by caspase-blocked HLA matched donor CD56+ NK cells indicating that the effect was not FAS-dependent. However, soluble MICA/B was detected in patient serum by immunoprecipitation and CML patient serum blocked anti-MICA/B binding to CML cells, suggesting that in vivo anti-leukemic activity of CD56+ cells is diminished. Furthermore NKG2D expression on CD3+ cells was found to be low in 3 of 4 CML patients, which was accompanied by a reduced IL-10 production upon NKG2D ligation by an anti-NKG2D moAb. These studies confirm early observations of KIR-independent antileukemic cytotoxicity by autologous and HLA matched NK cells, implicating pro-cytotoxic signaling via NKG2D and its ligand MICA/B. They also reveal mechanisms whereby CML cells may escape from NK regulation in vivo.

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