CD4 and polycystic ovary syndrome: Our results showed that the PD-1 expression on CD4+ or CD8+ T cells, which reflects T cell exhaust, positively correlated with the serum E2 level in the infertile patients with PCOS (r2 = 0.424, P < 0.05 and r2 = 0.431, P < 0.05, respectively, Fig. 6a,b).