Functional fate of cytotoxic T lymphocytes: The relationship between functional avidity and antigen density

The encounter of the TCR with its cognate peptide-MHC complex (pMHC) is a crucial step in T-cell activation. One of the most critical parameter, among many factors that influence antigen specific T-cell activation, is the biophysical nature of the TCR-pMHC interaction defined by TCR affinity and avidity. Recent observations demonstrate that T-cell function resulting from specific TCRpMHC interactions display a non-monotonic bell-shaped response, where either increased avidity, increased affinity or increased antigen presentation induce an unresponsive state of CD8 T-cells. Current understanding of how these factors and the interrelationships between them affect functional avidity remains undefined. To answer this question, we generated an experimental system in which all these main parameters of T cell stimulation are controllable. We Engineered T-cells with a transgenic T-cell receptor (TCR) with distinct affinities which target the tumor-associated antigen Tyr-A2. We sorted each transgenic cell population by the level of receptor expression for divide by avidity. Those cells were encountered with target cells that present the antigen at different levels. This unique system allowed understanding the interplay between TCR affinity, avidity and antigen density and their effect on T cell functionality (as measured by cytokine production, T cell degranulation and cytotoxicity). We found that TCRs with medium affinity and avidity are more functionality. The molecular mechanisms underlying these functional outcomes will be further characterized by detailed pathway analysis using phospho-flow cytometry for transcriptional changes. The knowledge obtained by those could have significant impact in understanding basic T-cell biology and decision making in response to antigen.