Affinity threshold conversion: the progressive differentiation checkpoint at which the TCRα/CD3δ signaling axis controls thymic positive selection

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Abstract Upon positive selection of conventional αβ T cells, CD4+ CD8+ double-positive (DP) cells transition to single-positive (SP) and reduce their responsiveness to weak peptide/MHC ligands (affinity threshold conversion, ATC). We report that ATC actually begins prior to the completion of positive selection in the DP stage, and this checkpoint is where the TCRα/CD3δ signaling axis controls positive selection. We dissected ATC in the OT1 TCR transgenic system in conjunction with several different genes whose null mutations block thymic development at DP stage: β2m, CD3δ, and/or TCRα-Connecting Peptide Motif (αCPM). In the absence of MHC class I (β2m0), CD3δ or αCPM mutants were blocked at pre-selection DP stage and displayed immature, high-sensitivity to weak TCR ligands. In contrast, when MHC class I was present (β2m+), CD3δ or αCPM mutants were blocked at DP stage but underwent partial ATC; they began positive selection but did not complete it. Upon exogenous TCR engagement of partial-ATC thymocytes, the proximal signaling protein interaction network showed overall weakened activity, notably decreasing SOS1 participation linked to the downstream Ras/Raf/ERK selection pathway. Provision of high affinity peptide/MHC in FTOC rescued completion of positive selection for partial-ATC mutant thymocytes. Finally, comparing nontransgenic, polyclonal B6 vs. CD3δ0 DP thymocytes, although most cells showed equal maturation state, a subset of cells in B6 progressed in maturation relative to CD3δ0. We propose the existence of an intermediate checkpoint inside DP stage, in which ATC already begins to reduce responsiveness to selection ligands, yet further TCR signaling is required to complete positive selection.