Introduction. B cell Chronic Lymphocytic Leukemia (B-CLL) is prototypic for human cancers that escape immune surveillance. These cells not only lack immunogenic tumor-specific epitopes but also express low levels of surface molecules that are necessary for optimal interaction with naive T cells. Different strategies have been tried to generate B-CLL specific T cells that can be used for adoptive immunotherapy. Yet it is uncertain whether these T cell specific responses are strong enough to eliminate the neoplastic clone. Previously, we have demonstrated that CMV seropositive patients with B-CLL have significantly increased relative and absolute numbers of CD8+ T cells exhibiting a CD45RA+CD27 cytotoxic CMV specific phenotype. In the present study we investigated the possibility to direct these in vivo generated CMV specific effector cells to the poorly immunogenic B-CLL cells.

Results. A considerable fraction (6%; range 1.0 to 16.4) of the circulating CD8+ T cell pool of the four B-CLL patients studied consisted of CMV-specific T cells, as visualized with either tetrameric HLA-A2.1/NLVPMVATV complexes or HLA-B7.2/TPRVTGGGAM complexes. Activation of these cells by CMV-peptide in the presence of IL-2 at day 0, and restimulation at day 8 with either CMV peptide loaded EBV transformed cell-lines or autologous peptide-loaded B-CLL cells, resulted in a near 100-fold absolute expansion of tetramer positive CD8+ T cells (on average 1.9 x 106 at day 0, and 2.2 x 108 at week 3). These in vitro restimulated CMV-specific T cells were non-cytotoxic for non-peptide loaded B-CLL cells, but showed a very efficient killing of CMV peptide loaded B-CLL cells, with a 50% specific lysis at an effector:target ratio as low as 4:1. Thus, despite their general anti-apoptotic profile, B-CLL cells are excellent targets and undergo apoptosis after CTL attack. This CTL specific killing was found to be completely dependent on the granzyme and perforin pathway. Finally we showed that the CMV specific effector cells in B-CLL patients do not need restimulation to effectuate a cytotoxic response towards CMV peptide loaded B-CLL cells.

Conclusion. Directing virus-specific T cells to B-CLL tumors may overcome the inadequate immunostimulatory capacity of these cells and could be exploited for T-cell mediated immunotherapy.

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