B-lymphoma cells escape rituximab-triggered elimination by NK cells through increased HLA class I expression

Objective. Antibody-dependent cellular cytotoxicity (ADCC) by natural killer (NK) cells is a major effector mechanism of the monoclonal anti-CD20 antibody rituximab in eliminating B-cell lymphomas. Resistance to this treatment occurs, although CD20 antigen is expressed oil the tumor cells. Materials and Methods. A model of ADCC was established by stimulating human hulk NK cells and inhibitory killer immunoglobulin receptor (KIR) - defined NK cells from human leukocyte antigen (HLA)-typed donors. NK-cell activation was triggered via stimulation of the Fc receptor with immunoglobulin G aggregates, rituximab-labeled HLA-defined CD20-positive B-lymphohlast cell lines or CD20-positive B-lymphoma cell lines. The effect of KIR ligation by anti-KIR antibodies anti HLA, the HLA expression density and rituximab concentrations on the efficacy of ADCC were analyzed in granzyme B ELISPOT measuring NK-cell activation anti fluorescein-activated cell sorting cytotoxicity assay. Results. HLA, but not CD20 expression density correlated with NK-cell activity against rituximab-labeled targets. ADCC was increased or decreased following HLA shielding or Kill activation by anti-KIR antibodies, respectively. Herein we show that rituximab-induced ADCC is attenuated upon ligation of Kill by HLA molecules expressed on human B-lymphoma target cells. Moreover, anti-KIR antibodies do not only block KIR/HLA interactions, but display agonistic effects at the KIR, which has to be considered for therapeutical applications. Conclusion. KIR activation and HLA expression density are critical determinants for the efficacy of rituximab treatment. An explanation for the failure of rituximab treatment may be the protection of the tumor cells from ADCC by inhibiting NK-cell function with their surface HLA. (C) 2010 ISEH - Society for Hematology and Stem Cells. Published by Elsevier Inc.