Probing the sensitivity of negative selection using two HEL TCR transgenic mice

Abstract

Thymocytes that recognize self peptide‐MHC (pMHC) complexes with high avidity are eliminated by the process of negative selection. Using in vitro analysis, it has been shown that as few as three pMHC complexes can mediate the deletion of double positive thymocytes. Here, we examine negative selection in two TCR transgenic mice whose T‐cells recognize differentially presented epitopes of HEL (hen egg white lysozyme) on Ak. We show that 3A9 thymocytes which recognize the major HEL epitope, undergo efficient negative selection when mice are injected intravenously with soluble HEL. As little as 5 μg (350 pmol) intravenous HEL results in 3A9 thymocyte deletion. In contrast, LB11.3 thymocytes which recognize a minor epitope of HEL, require 50 μg (3.5 nmol) intravenous HEL to be deleted. Using 125I‐HEL, we show that only 0.25% of the injected HEL enters the thymus. This corresponds to 0.87 pmol or 5.2 x1011 molecules of HEL after a 5 μg intravenous injection. Together, with published data on HEL uptake, processing efficiency and Ak expression, we estimate 5.2 x106– 5.2 x107 pMHC complexes in the thymus are sufficient to delete 3A9 T‐cells. Assuming 106 APCs in the thymus, this suggest that negative selection can occur in response to single digit numbers of pMHC complexes, supporting previous in vitro findings and highlighting the remarkable sensitivity of this process. This work is supported by NIH 5T32AI007163.