CYTOTOXICITY OF LAK CELLS AND EBV-SPECIFIC T CELLS AGAINST LYMPHOBLASTOID EBV-POSITIVE B CELLS: A MODEL FOR CELLULAR THERAPY OF PTLD

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696 Posttransplant lymphoproliferative disorder (PTLD) is a significant cause of morbidity and mortality in transplant patients. The presence of the Epstein-Barr virus (EBV) in many of these lymphomas has prompted the use of cellular immunotherapy using either EBV-directed T cells or autologous lymphokine-activated killer (LAK) cells. EBV-specific T cells recognize viral epitopes in association with HLA Class I molecules, whereas LAK cell cytotoxicity is enhanced by their depletion. The purpose of this study was to examine the relative levels of in vitro cytotoxicity of these effector cells against either untreated or HLA Class I-depleted EBV-immortalized B lymphocytes. LAK cells, EBV-specific T cell lines and EBV-immortalized lymphoblastoid cell lines were prepared from healthy donors. The degree of cell-mediated lympholysis (CML) was determined by using a standard 4 hour 51Cr release and the results were expressed as lytic units (LU-number of effectors required for 20% lysis of 5×103 targets). In some cases, HLA Class I expression in the target cells was depleted by a brief exposure to pH 3.3 buffer. Cell viability and Class I depletion were confirmed by trypan blue exclusion and flow cytometry, respectively prior to execution of CML assays. LAK cell-mediated cytotoxicity against autologous EBV target B cells was 14.4 LU which increased to 69.6 LU when target cells were HLA-depleted(p<0.05, Wilcoxon test). On the contrary, EBV-specific T cell-mediated cytotoxicity against untreated B cell targets was 765 LU, and this decreased to 316 LU when HLA-Class I depleted cells were used as targets (p<0.05). Additionally, as compared to that of LAK cells, T cell-mediated cytotoxicity against untreated targets was significantly greater (Friedman's test p=0.03, Tukey multiple comparison p<0.05). We conclude that EBV-specific T cells show superior in vitro cytotoxicity against autologous EBV-positive B cell targets, regardless of the level of HLA Class I expression. The results may explain in part the uniform response of PTLD to T cell therapy, and the less frequent responses to that of LAK cell. The level of cytotoxicity for both effector cell types is influenced by the degree of expression of HLA Class I in this model system. This approach may be useful in examining other factors which may enhance or decrease the cytotoxic response against EBV-positive B cells.