Numbers of CD3+ T cells are significantly higher in the brain of PD-1 KO mice compared to WT control, and there was a trend towards increased numbers during the course of prion disease in WT mice, which did not further increase in the absence of PD-1 (Fig. 3A), demonstrating there is not an increased influx of T cells into the prion brain upon PD-1 deficiency. The gene discussed is PDCD1; the disease is prion disease.