Bacterial membrane phosphatidylglycerols activate a population of human CD1a-restricted t cells

Expressed on epidermal Langerhans cells, CD1a presents a range of self-lipid antigens found within the skin. However, the extent to which CD1a presents microbial ligands from skin bacteria is unclear. Here, we identified CD1a-dependent T cell responses to phosphatidylglycerol (PG), a ubiquitous bacterial membrane phospholipid, as well as to lysyl phosphatidylglycerol (lysylPG), a modified PG, present in the many gram positive bacteria, and highly abundant in Staphylococcus aureus. Specifically, PG and lysylPG together constitute the vast majority of the structural membrane lipids in Staphylococcus aureus. The crystal structure of the CD1a-lysylPG complex showed that the acyl chains were buried within the A′- and F′-pockets of CD1a, while the lysine-modified phosphoglycerol headgroup protruded from the F′-portal and was available for T cell receptor contact. Using lysylPG-loaded CD1a tetramers, we identified T cells in peripheral blood and in skin that respond to this lipid in a dose dependent manner. Tetramer+ CD4+ T cell lines secreted Th2 cytokines in response to lysylPG as well as to co-cultures of CD1a+ dendritic cells and Staphylococcus aureus. The expansion of CD4+ CD1a-lysylPG tetramer+ T cells in atopic dermatitis patients, indicates a response to a lipid made by bacteria associated with atopic dermatitis, and provide a link supporting involvement of PG-based lipid-specific T cells in the pathogenesis of dermatitis. We identify bacterial lipids PG and lysylPG as antigens for CD1a-restricted T cells and use RNAsequencing to gain insight in the gene expression profiles and functions of CD1a-(lysyl)PG reactive T cells