This project will study autoimmune-lymphoproliferative syndrome (ALPS) as a model of uncontrolled lymphoproliferation, caused by FAS deficiency. The goal is to delineate the ontogeny, specificity, induction and differentiation of FAS controlled (FC) T cells. Current hypothesis is that any post-thymic T cell can acquire the specific FC T cell differentiation program, provided it receives the appropriate signals in the right environment. Human CyTOF based developmental modelling will be combined with scRNAseq and TCR sequence analysis for a more precise delineation of cell transitions from naïve towards the FC T cell differentiation program. Based on the hypothesis that FC T cell fate is induced in secondary lymphoid organs, the microanatomical localization, cellular interactions partners and ligand/receptor pair expression of FC T cells and interacting immune cells in lymph node tissue will be determined using immunohistochemistry, imaging mass cytometry and spatial multiomics. The generated hypothesis on FC T cell ontogeny and fate induction will be tested by competitive adoptive transfer and infection experiments in Fas-deficient lprcg mice.