Adoptive T Cell Therapy of Human CMV Infection in Immunocompromised Hosts: In Vitro Generation of CMV-Specific T Cells from Naïve Precursors

Adoptive transfer of virus-specific T cells is a promising therapy for the treatment of infections in immunocompromised patients. Virus-specific T cells can readily be obtained from antigen-experienced, but not naïve donors. In this study we describe a cell culture system for the in vitro generation of CMV-specific T cells from naive T cells derived from CMV-seronegative donors. We isolated naïve T cells by magnetic depletion of non-T cells, CD25⁺ regulatory T cells, and CD45RO⁺ effector and memory T cells from peripheral blood mononuclear cells (PBMC) of CMV-seronegative donors. These naïve T cells were co-cultured with autologous mature monocyte-derived DC (MoDC) loaded with a pool of overlapping peptides from the CMV protein pp65. CD3-depleted autologous PBMC were used as feeder cells and CD28 antibody, IL-2, IL-7, and IL-15 were added to the culture. Already only 9–13 days after starting the priming culture, frequencies of 0.0024% and 0.009% pp65_495–503/A2-tetramer⁺ cells among CD8⁺ T cells were found for 2 HLA-A2⁺ blood donors. In contrast pp65_495–503/A2-tetramer⁺ T cells were not detectable when naive T cells were cultured with unpulsed MoDC. Tetramers are suitable tools for the identification of antigen-specific T cells but are restricted to single epitopes of mainly CD8⁺ T cells. To analyze primed CD4⁺ T cells as well as CD8⁺ T cells having specificities other than for the peptide pp65_495–503, we looked for upregulation of the activation marker CD137 after a second stimulation and found increased frequencies of CD137⁺ CD4⁺ T cells as well as CD137⁺ CD8⁺ T cells in the pp65-primed cell cultures only when restimulated with the peptide pool of pp65. Because IFN-γ is important for the control of CMV infection, we studied the capability of the in vitro primed pp65-specific CD4⁺ and CD8⁺ T cells to produce this cytokine. Restimulation of the T cells with pp65 peptide pool induced IFN-γ secretion in up to 3.9% of the CD8⁺ T cells and up to 3.8% of the CD4⁺ T cells in each of six donors tested. No specific IFN-γ production was detected after restimulation with an irrelevant IE-1 peptide pool. As expected the frequency of pp65-specific T cells in the priming cultures is low. For generation of T cell lines, we magnetically enrich pp65- specific T cells according to their IFN-γ secretion using the cytokine secretion assay technology. After further cultivation for 2 weeks the antigen-specificity of the expanded T cells was again evaluated. Only if restimulated with the pp65 peptide pool 56.6% of the CD4⁺ T cells showed upregulated expression of the activation marker CD154 (CD40L). Cytokine analysis of the cells revealed IFN-γ production in 40.2% of the CD4⁺ T cells, of which 36% co-expressed IL-2, indicating the functionality of the in vitro primed and expanded T cells. In conclusion, we established a cell culture system for in vitro priming of CMV-specific CD4⁺ and CD8⁺ T cells derived from peripheral blood of donors not infected by CMV. This should extend the application of adoptive T cell therapy to patients for whom immune donors are not available.