Self-pMHCII complexes are variably expressed in the thymus and periphery independent of mRNA expression but dependent on the activation state of the APCs

• A T cell hybridoma detecting endogenous positive selecting self-pMHCII was generated.
• Positive and negative selecting thymic APCs can present the same self-pMHCII complex.
• Peripheral expression of self-pMHCII is affected by the APC activation state.
• Thymic and peripheral expression of self-pMHCII complex did not reflect mRNA levels.

Self-peptide MHCII ligands are critical for selection of CD4+ T cells in the thymus, and maintenance in the periphery. To date, no investigation as to the exact thymic and peripheral expression of a naturally occurring positive selecting self-peptide MHCII (self-pMHCII) complex has taken place. We have generated a sensitive T cell hybridoma to functionally detect the endogenous presentation of a confirmed positive selecting self-pMHCII complex for a CD4+ transgenic T cell. Using this tool to survey and quantify the expression selecting of self-pMHCII, we have shown unequivocal proof that a known CD4+ selecting ligand can be presented on both positive and negative selecting thymic APCs. We also show that peripheral presentation of this same selecting ligand is affected by the activation state of the APCs. Furthermore, discrepancies between the gene expression and self-pMHCII complex presentation of this bona fide selecting ligand suggest that functional detection self-ligand complexes will be required to establish a complete view of the naturally presented endogenous self-pMHC landscape.