Activation of human CD1a-restricted T cells by bacterial and mitochondrial lipids - PROJECT SUMMARY T cells recognizing peptide antigens have been widely studied in the context of infection, autoimmunity, and cancer. However, the human T cell repertoire also harbors T cells that respond to lipid antigens rather than peptides. These lipid-specific T cells are understudied and, as a result, their functions within the human immune system remain incompletely understood. In this proposal, we will investigate a newly identified population of CD1a-restricted T cells that respond to phosphatidylglycerol (PG), a lipid antigen that is shared between bacteria and mammalian cells. PG is a major structural phospholipid in many bacterial membranes, yet in mammalian cells PG is present in trace amounts, in the inner membrane of mitochondria, where it functions as a building block for cardiolipin. Therefore, PG appears to be a sequestered self-antigen as well as an abundant bacterial antigen. CD1a-PG specific T cells are present in healthy individuals at frequencies similar to CD1d-restricted invariant NKT cells, supporting a normal physiological function for these T cells. Currently, the exact role of PG- specific T cells within the human immune system is unclear. However, because they recognize purified PG both from mammalian and bacterial origin, we postulate that they are unlikely to have strong pro-inflammatory properties, but instead may exert regulatory functions. Furthermore, it remains to be determined under which conditions PG-specific T cells respond to endogenous mitochondrial PG. In this proposal, we will investigate these two aspects of this newly identified T cell population. We will determine (patho)physiological conditions in which mitochondrial PG becomes visible to T cells, including mitophagy, induction of mitochondrial-derived vesicles, or uptake and processing of exogenous mitochondria, as well as uptake of cardiolipin (which harbors PG). We will compare mammalian versus bacterial PG, which differ in acyl chain structure, and determine their relative antigenicity. Furthermore, using single cell RNA and TCR sequencing, in combination with an algorithm integrating gene expression profiles and TCR sequences of T cell clonotypes, we will determine if CD1a-PG specific T cells constitute a distinct T cell subset, similar to iNKT cells or MAIT cells, and gain insight in their functions. Last, we will investigate whether CD1a-PG specific T cells require bacterial exposure for their development and expansion or if they are present at birth. In summary, this proposal aims to understand the origins and immune implications of a shared bacterial and mitochondrial T cell antigen and define conditions in which PG-specific T cells are activated. Results from this proposal will lay the foundation for further studies aimed at determining functions of these lipid-specific T cells in autoimmune or inflammatory conditions in which mitochondrial lipids or other phospholipids are targeted, in particular systemic lupus erythematosus (SLE).
