Single-cell transcriptome and TCR analysis of human Cytomegalovirus (hCMV)-specific T cell memory reveals effector and pre-effectors of CD8+ and CD4+ cytotoxic T cells

The latent human Cytomegalovirus (hCMV) infection can pose a serious threat of reactivation and disease occurrence in immune-compromised individuals, as well as burdens the immune system in immune-competent individuals. Though, T cells are at the core of the protective immune response to hCMV infection, a detailed characterization of different T cell subsets involved in protection against the hCMV infection is lacking. Here we analyzed the single-cell transcriptomes and the single-cell T cell antigen receptor (TCR) repertoires of over 8000 hCMV-reactive peripheral T cells isolated from different memory compartments. The hCMV-reactive T cells were highly heterogeneous and consisted of different developmental memory and functional T cell subsets such as, the long-term memory precursors and effectors, T helper-17, T regulatory cells (TREGs) and cytotoxic T lymphocytes (CTLs). The hCMV-antigen specific TREGs were enriched for molecules linked to their suppressive function and interferon response genes. The CTLs were of two types, the pre-effector and effector like. Of particular interest was the mixture of both CD4-CTLs and CD8-CTLs in both the pre-effector and effector cytotoxic clusters, suggesting that both CD4-CTLs and CD8-CTLs share transcriptomic signatures. The huge TCR clonal expansion of both the cytotoxic clusters imply their predominant role in protective immune response to CMV. Further the clonotype sharing between the CTL clusters and the long-term memory clusters, indicate potential progenitors of CD4-CTLs. Together our study has identified many subsets of hCMV-specific memory T cells that may have implication in better understanding the hCMV-specific T cell immunity to design vaccination strategies and therapeutics.