Universal CD137 Expression upon Activation Allows Efficient Isolation of a Broad Repertoire of Virus-Specific CD8+ and CD4+ T Cells for Adoptive Immunotherapy.

Reactivation of adenovirus (ADV), cytomegalovirus (CMV) and Epstein-Barr virus (EBV) can cause serious morbidity and mortality during the prolonged period of immune deficiency following allogeneic stem cell transplantation. It has been shown that adoptive transfer of donor-derived virus-specific T cells can be a successful strategy to control viral reactivation. To provide safe and effective anti-viral immunotherapy, we aimed to generate combined CD8+ and CD4+ T cell lines with high specificity for a broad range of viral epitopes. Isolation by the IFNg capture assay of virus-specific T cells that produce IFNg upon activation allows the generation of highly specific T cell lines without the need for extensive culture. However, it has been recently shown that specific upregulation of the co-stimulatory molecule CD137 upon antigen-specific activation of CD8+ and CD4+ T cells can also be used for isolation. We therefore analyzed IFNg production and CD137 expression by CD8+ and CD4+ T cells upon incubation of peripheral blood mononuclear cells (PBMC) from seropositive donors with peptides corresponding to 17 defined MHC class I restricted minimal epitopes from 10 different ADV, CMV, EBV and influenza (FLU) proteins, and 15-mer or 30-mer peptides containing MHC class II restricted epitopes from CMV pp65 or ADV hexon. Using tetramer and intracellular IFNg staining we first determined the fraction of CD8+ T cells that produced IFNg upon activation with the minimal epitopes. Specific IFNg production was observed for 58–100% of tetramer+ CD8+ T cells specific for CMV pp65 (n=6), and 83% for FLU (n=1), but only 18–58% for CMV pp50 (n=3) or IE-1 (n=3), 4–91% for EBV latent (n=3) and lytic (n=3) epitopes, and 41–63% for ADV hexon (n=2). In contrast to the variation in the fraction of IFNg-producing cells, we observed homogeneous upregulation of CD137 by the virus-specific tetramer+ T cell populations upon activation. In 2 cases where no CD137 expression by tetramer+ T cells could be detected, no IFNg production was observed either. These data suggest that the majority of CD8+ T cells specific for CMV pp65 or FLU can be isolated on basis of IFNg production, but only part of CD8+ T cell populations specific for other viral proteins, while complete virus-specific CD8+ T cell populations may be isolated on basis of CD137 expression. Activation of CD4+ T cells specific for CMV pp65 or ADV hexon with 15-mer or 30-mer peptides induced both specific IFNg production and CD137 expression. To investigate whether multiple virus-specific T cell populations could be isolated simultaneously, we next determined the kinetics of IFNg production after activation with defined MHC class I epitopes or peptides containing MHC class II epitopes. CMV- and EBV-specific CD8+ T cells and CMV-specific CD4+ T cells showed a rapid induction of IFNg production, which peaked after 4 hours and decreased thereafter. In contrast, ADV- and FLU-specific CD8+ T cells and ADV-specific CD4+ T cells, predominantly having a more early differentiation phenotype (CD27+CD28+) compared to CMV- and EBV-specific T cells, showed peak IFNg production after 8 hours that continued for more than 48 hours. This difference in phenotype and IFNg kinetics may suggest that the persistent and frequent presentation of CMV and EBV epitopes in vivo, in contrast to an intermittent exposure to ADV and FLU epitopes, drives differentiation and shapes the kinetics of the IFNg response of specific T cells. Kinetic analysis of CD137 expression showed uniform upregulation by virus-specific CD8+ T cell populations from day 1 to day 4 after activation, which peaked at day 2, suggesting that this may be the optimal time point for CD137-based isolation. In a limited number of experiments, virus-specific CD8+ and CD4+ T cells could be isolated based on CD137 expression within the same timeframe. These data indicate that virus-specific T cell populations can be more efficiently isolated at one time point on basis of CD137 expression than on basis of IFNg production, due to differences in IFNg kinetics. In conclusion, this study shows that T cell lines generated by CD137 isolation may comprise a significant number of virus-specific T cells which do not produce IFNg, but may have other effector functions. Furthermore, CD137-based enrichment may be more robust and allows the efficient simultaneous isolation of multiple virus-specific T cell populations due to uniform kinetics of CD137 expression.