PDCD1 and infection: Analysis of PD-1 expression by flow cytometry at day 6 and at a later time point (8 months after the initial infection) (Fig. 5A) indicated that WT parasites induced a high proportion of PD-1+CD4+ and PD-1+CD8+ T cells in the spleen presumably representing a pool of T cells devoid of anti-parasite effector function.