Viral infection is a major cause of disease and mortality after allogeneic hematopoietic stem cell transplantation (AHSCT) in hematologic malignancies. It has been shown that adoptive transfer of expanded virus-specific T cells (VSTs) was capable of treating infections that are resistant to conventional therapies. Recently, it has been shown that PD-L1 expression by activated T cells plays a major role on their survival and activity. Here, we investigated PD-L1 expression during VSTs expansion and the effect of PD-L1 blockade on the phenotype and functional activity of these VSTs.
VSTs were generated from healthy donors PBMCs with or without the addition of PD-L1 antibody after stimulation in G-Rex-10 flasks with a pool of 11 overlapping peptides libraries spanning the 5 most immunodominant viral antigens. All VSTs were collected at day 14. We used ELISpot assay to measure IFN-γ production by VSTs and flow cytometry analysis for phenotyping.
Blockade of PD-L1 has induced 1.8 folds higher proliferation rate and 2 folds increase in IFN-γ production against the viral antigens. After 6 days of expansion, 78% of CD4+ and 60% of CD8+ VST subsets expressed the PD-L1 molecule. At the end of expansion, CD4+ PD-L1+ VSTs were reduced to 6.8% whereas the CD8+ PD-L1+ VSTs were increased to 87.6%. Interestingly, PD-L1 blockade resulted in a further reduction in the CD4+ PD-L1+ population (2.2%), without affecting the CD8+ PD-L1+ VSTs (89.5%). After PD-L1 treatment, the cytotoxicity marker CD107 was slightly increased (1.2 folds) in the CD4+ cells but decreased by 2 folds in the CD8+ VSTs and CD4+ CD45RO+ memory T cells were decreased from 46.1% to 33%.
We have standardized an ex vivo protocol for rapid expansion of VSTs against major viruses causing infection after AHSCT. These VSTs were shown to highly express PD-L1. Blocking of PD-L1 improved the expansion, the anti-viral specificity and the cytotoxicity of these VSTs. This should promote a long-lasting antiviral activity of VSTs in patients undergoing AHSCT. Further investigations are being carried out to confirm these results.
Medical Research Center, Hamad Medical Corporation, Doha, Qatar.
Medical Research Center, Hamad Medical Corporation, Doha, Qatar.
All authors have declared no conflicts of interest.