By utilizing the TCR transgenic system specific for the LCMV immunodominant epitope GP33–41 (P14) and adoptive transfer experiments, we previously showed that P14.PTPN22−/− CD8 T cells fail to expand in PTPN22-sufficient hosts following infection with the acute viral strain of LCMV (Arm) due to T cell-intrinsic defects in IFNAR signaling (12). Here, PTPN22 is linked to infection.