Because B cell chronic lymphocytic leukemia (B-CLL) can not be cured with current therapies, but in general has a slow progression and rather long median survival, it is considered an attractive candidate for active T cell mediated immunotherapy. However B-CLL cells have poor antigen presenting capacity because they express low levels of co-stimulatory molecules. Moreover, most immunotherapeutic strategies require knowledge of the eliciting tumor antigen and/or ex vivo manipulation of patient cells. To circumvent these drawbacks we aim to redirect existing viral immunity towards B-CLL. Previously, we have shown that in patients with B-CLL considerably expanded numbers of cytomegalovirus (CMV)-specific CD45RA+CD27 CD8+ cytotoxic T cells are present (
W. Mackus et al. Blood 2003; 102:1057
). These cells are potent cytotoxic effector cells when directed against B-CLL cells loaded with CMV peptide (A.Kater et al. Br.J. Haematol. 2004; 126:512
). In the current study, we apply a novel bridging reagent to redirect CMV-specific CTL to specifically target B-CLL. The targeting complex is composed of a streptavidin fused anti-CD20 single chain variable fragment (scFv) in combination with biotinylated MHC class I molecules containing CMV pp65 peptide (HLA/CMV). We demonstrate that this complex is stable on the cell surface for ≥24 hours, and that B-CLL cells coated with this CD20-HLA/CMV complex can be lysed by autologous CMV-specific CD8+ CTL with similar efficiency as B-CLL cells directly loaded with CMV-peptide. Killing occurs at scFv CD20 concentrations of ≥100 ng ml−1 and HLA/CMV concentrations of ≥20 ng ml−1. Lysis of CD20-HLA/CMV complex coated CLL cells could not be blocked by anti-LFA1 antibodies, in contrast to B-CLL cells directly loaded with CMV-peptide, indicating a different immunological synapse. HLA-A2 positive B-CLL cells coated with HLA-B7 /CMV complexes were only lysed by HLA-B7 positive CMV-specific CTL, whereas HLA-A2/CMV complex targeted HLA-A2 positive B-CLL cells were unaffected by HLA-B7 positive CMV specific CTL, proving HLA restriction of the killing.. Furthermore, CD20-HLA/CMV complex coated B-CLL cells induce both proliferation and cytokine production (interferon γ, tumor necrosis factor α, and macrophage inflammatory protein-1 β) in CMV-specific CD8+ T cells. Thus, CD20-HLA/CMV complexes elicit both immune activation and direct cytotoxicity towards B-CLL cells. The findings of our study constitute a necessary step towards possible application of CD20-HLA/CMV complexes for immunotherapy of B cell malignancies. It is obvious that this recently recognized capacity to redirect existing antiviral immunity towards tumor cells has a utility in cancer immunotherapy far beyond CMV and B-CLL.
