Degree of ERK activation influences both positive and negative thymocyte selection

Considerable evidence suggests that the ERK pathway is required for positiv e but not negative thymocyte selection. Here, we report that ERK is highly activated in double-positive (DP) thymocytes expressing an MHC class I-rest ricted TCR (P14) in response to negatively selecting conditions, whereas li gands that trigger positive selection induced weaker ERK activation. Bioche mical evidence also shows that death by neglect is associated with a furthe r reduction in ERK activation. These findings are consistent with the affin ity/avidity model of thymocyte selection. To further examine the role of ER K in negative selection we used the MEK-1 inhitibor, PD98059, a specific ph armacological inhibitor of the ERK pathway. Biochemical data demonstrated a reduction of ERK activity by PD98059 in the presence of the negatively sel ecting ligand. Analysis of P14 TCR-transgenic fetal thymic robes cultured w ith PD98059 under negatively selecting conditions showed impaired clonal de letion of DP thymocytes and a concomitant increase in positive selection of functional mature, TCRmi transgenic T cells. This demonstrates that alteri ng ERK activity switched negative to positive selection. Contrary to previo us reports that show an exclusive role for ERK signaling in positive select ion, our data demonstrate that negative selection is also sensitive to the degree of ERK activation.