Exploration of Human Lung CD4+ T cell Heterogeneity
Anna E. Oja1, Berber Piet2, Martijn A. Nolte1, René A.W. van Lier1, Pleun Hombrink1
1Department of Hematopoiesis, Sanquin Research and Landsteiner Laboratory, Amsterdam, 2Department of Respiratory Medicine, OLVG, Amsterdam
T cells mediate adaptive immune responses. While circulating T cells protect against systemic infections, non-circulating resident memory T cells (TRM) are critical for protection in barrier tissues such as the lungs, skin and intestine. While the mechanisms by which TRM are recruited have become increasingly appreciated, major question remains regarding the heterogeneity and spatial organisation of these cells. While CD8+ TRM are extensively investigated, the role of the more abundant CD4+ TRM remains elusive. Single-cell analysis based on flow cytometry revealed three subsets of lung CD4+ TRM: CD69+CD103+, CD69+CD103- and CD69-CD103- cells. Using bulk-cell transcriptome analysis, we identified the transcriptional profile of human lung CD4+CD103+ TRM and compared this to blood CD4+ T cell subsets. We demonstrated that lung CD4+CD103+ TRM represent an unique subset of CD4+ T cells that is very different from circulating CD4+ T cells, but closely related to CD8+ TRM. CD4+ TRM expressed unique sets of chemokine receptors and adhesion molecules necessary for tissue homing. We revealed that resting CD4+CD103+TRM constitutively express high mRNA levels of cytotoxic mediators, but no protein. Functionally this was reflected by a fast recall response, magnitude of cytokine production, and a high degree of polyfunctionality. Phenotypic validation of target genes revealed a previously unrecognized complexity of the CD4+ TRM compartment in the lungs, as the expression of VLA-1, CXCR6 and the inhibitory 2B4 did not strictly associate to any of the conventional TRM phenotypes. Future single-cell approaches may deal with this caveat of population-averaged measurements in conventional transcriptome analysis experiments. Better understanding of the molecular and functional characteristics of heterogeneous CD4+ TRM populations may improve rational vaccine design and the efficacy of adoptive T cell therapy.