Precursors of human CD4+ cytotoxic T lymphocytes identified by single-cell transcriptome analysis

CD4+ cytotoxic T lymphocytes (CD4-CTLs) have been reported to play a protective role in several viral infections. However, little is known in humans about the biology of CD4-CTL generation, their functional properties, heterogeneity and clonal diversity, especially in relation to other well-described CD4+ memory T cell subsets. We performed single-cell RNA-seq in over 9000 cells to unravel CD4-CTL heterogeneity, transcriptional profile and clonality in humans. The single-cell differential gene expression analysis, revealed a spectrum of known transcripts, including several linked to cytotoxic and co-stimulatory function, and transcripts of unknown cytotoxicity-related function that are expressed at higher levels in the TEMRA subset, which is highly enriched for CD4-CTLs, compared to cells in the central and effector memory subsets (TCM, TEM). Simultaneous T cells antigen receptor (TCR) analysis in single-cells and bulk subsets revealed that CD4-TEMRA cells show marked clonal expansion compared to TCM and TEM cells and that the majority of CD4-TEMRA were dengue virus (DENV)-specific in subjects with previous DENV infection. The profile of CD4-TEMRA was highly heterogeneous across subjects, with four distinct clusters identified by the single-cell analysis. Most importantly, we identified distinct clusters of CD4-CTL effector and precursor cells in the TEMRA subset; the precursor cells shared TCR clonotypes with CD4-CTL effectors and were distinguished by high expression of the interleukin-7 receptor. Our identification of a CD4-CTL precursor population may allow further investigation of how CD4-CTLs arise in humans and thus could provide insights into the mechanisms that may be utilized to generate durable and effective CD4-CTL immunity.

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