Oligoclonal CD4+CXCR5+ T cells with a cytotoxic phenotype in tonsils and blood

In human studies, mononuclear cells from peripheral blood (PBMC) often serve as a source of clinical study markers. Therefore, it is essential to understand to what extent phenotypes of PBMCs reflect their tissue-resident equivalents. In order to determine the heterogeneity of T-follicular helper (TFH) cells in peripheral blood versus tonsils, CD3+CD4+CD45RA–CXCR5+ cells of both origins were sorted and transcriptomes, TCR repertoires and cell-surface protein expression were analysed by single-cell RNA sequencing, flow cytometry and immunohistochemistry. Four subsets of TFH cells with classical T-follicular gene expression patterns were mostly found in tonsils. Interestingly, however, all circulating CD3+CD4+CXCR5+ T-cell subpopulations also appeared in tonsils. Still, when circulating, cells of the same cluster rather displayed markers of proliferation and migration, whereas their tonsillar counterparts exhibited known TFH characteristics resembling bona fide germinal center-typical TFH cell subtypes. Surprisingly, one distinct and oligoclonal CD4+CXCR5+ subpopulation displayed pronounced cytotoxic properties. Those ‘killer TFH (TFK) cells’ were found among PBMCs as well as tonsillar cells but were located outside of germinal centers. Accordingly, they were the only CD4+CXCR5+ T-cell subtype in tonsils featuring transcripts for CXCR3 and PSLG1. They appeared terminally differentiated and could be distinguished from all other TFH subsets by expression of NKG7 (TIA-1), granzymes, perforin, CCL5, CCR5, EOMES, CRTAM and CX3CR1. This hitherto undescribed subpopulation is indicative for chronic infection and inflammation and therefore is a valuable candidate to be included in any CD4+CXCR5+ TFH cell assessment.