PDCD1 and cystic kidney disease: However, in the strains that present with more rapidly progressive PKD (129S6/SvEvTac or BALB/cJ), not only did PD-1–positive CD8+ T cells increase in number with more severe cystic kidney disease when Pkd1RC/RC mice were compared with WT mice, but PD-1–positive CD8+ T cells became more prominent among the CD8+ T cell population (Figure 1, A and B, and Supplemental Table 1).