CD8A and neoplasm: Due to relatively low levels of tumor Ag and a weak self-Ag TCR signal (60) of anti-tumor CD8+ T cells, the TCR is not activated strongly enough to activate Blimp-1 and, at the same time, the weak activation of the TCR sets the first phase of the biphasic response of the dose-dependent PD-1 DIFFL motif (Figure 2).