Antigen-induced coreceptor down-regulation on thymocytes is not a result of apoptosis

The various stages of T cell development are typically characterized by the expression level of the two coreceptors, CD4 and CD8. During the CD4⁺CD8⁺ (double-positive, DP) stage of development, thymocytes that perceive a low avidity signal through the TCR go on to differentiate (positive selection), and ultimately down-regulate one coreceptor to express either CD4 or CD8. Alternatively, thymocytes that perceive a high avidity signal down-regulate both coreceptors and are induced to die via apoptosis (negative selection). However, it has recently been suggested that positively selected thymocytes may also partially down-regulate both coreceptors before up-regulating the one coreceptor that is ultimately expressed. This would imply that coreceptor down-regulation (dulling) is not a consequence of commitment to the death pathway. To explore this possibility, we have utilized an in vitro assay to demonstrate that dulling occurred in response to both positive and negative selecting ligands in vitro, was not a result of nonspecific membrane perturbation, was not dependent on the type of APC, and occurred before death in vitro. Furthermore, when thymocyte apoptosis was blocked, CD4 and CD8 were down-regulated in response to TCR stimulation. These data suggest that dulling in response to TCR ligation is distinct from death, and support a model in which DP dulling occurs during both positive and negative selection. The biological implications of this phenomenon are discussed.