Reactivation of CMV remains a major cause of morbidity and mortality in immunocompromised recipients of allogeneic stem cell transplantation. Antiviral pharmacotherapy may not be sufficient due to significant toxicity and moderate efficacy. It has been shown that adoptive transfer of donor-derived CMV-specific T cells may be an effective strategy to control established CMV infection. For a persistent function in vivo the presence of both virus-specific CD8+ and CD4+ T cells is essential. Therefore, we developed an optimized protocol for the generation of CMV pp65-specific CD8+ and CD4+ T cell lines which is fully compliable with Good Manufacturing Practice (GMP) conditions. Enrichment for CMV-specific T cells followed by only a short culture period is likely to retain maximal in vivo potential. PBMCs from 7 CMV seropositive donors were stimulated with recombinant pp65 protein (7–70 μg/ml) and/or HLA-A*0201/HLA-B*0702 restricted immunodominant pp65 peptides (NLV/TPR). Peptides used were clinical grade, and recombinant protein was gamma-irradiated (50 kGy, −80 C°) to eliminate possible microbiological contamination. High dose gamma-irradiation of pp65 protein resulted in partial degradation, but antigenic presentation was maintained. IFNγ producing cells were enriched using the IFNγ secretion assay (Miltenyi Biotec) at day 1 after stimulation, and cultured with autologous feeders (10x) and IL-2 (10 or 50 IU IL-2/ml) with or without CD3/28 expansion beads. Addition of high concentrations of protein during initial stimulation had a negative effect on enrichment probably due to non-specific stimulation of cells. Addition of immunodominant pp65 peptides promoted isolation efficiency and proliferation of epitope-specific CD8+ T cells in some donors. Cell lines were analyzed at different time points (day 4–15) using peptide-MHC tetramer and phenotypic markers. In addition, pp65-specificity was evaluated by intracellular IFNγ staining after restimulation with a pp65 protein-spanning pool of 15-mer peptides. CMV-specific lysis was tested in a 51-Cr release assay on pp65-transduced target cells. Enrichment of IFNγ producing cells after pp65 protein stimulation resulted in pp65-specific cell lines consisting of both CD8+ and CD4+ T cells. The T cell subset distribution directly after enrichment did not change during culture and was reproducible for each donor. Moreover, the composition of T cell lines reflected the pp65-specific response in donor PBMC starting material. The CD8+ compartment contained the known immunodominant tetramer staining cells (range 5–100%). The majority of both CD8+ and CD4+ T cells produced IFNγ upon restimulation with the pp65 peptide-pool, and showed CMV-specific lysis of target cells. The phenotype of pp65-specific T cells was predominant CD28+/CD45RO+ and CD45RA−/CCR7−/CD62L−, although CCR7 and CD62L were transiently expressed at day 4 and 7 after stimulation. Cryopreservation did not affect the composition or functionality of T cell lines. In conclusion, this procedure yields GMP-grade T cell lines comprising both CD8+ and CD4+ CMV-specific T cells. Processing and presentation of CMV protein by donor antigen-presenting cells enables selection of the full pp65-specific donor repertoire, without restrictions related to HLA or known epitopes. The choice for a moderate or more vigorous expansion after enrichment remains arbitrary and needs to be evaluated in clinical trials.
Disclosure: No relevant conflicts of interest to declare.