CXCR5 and infection: Although the expression of PD-1 and CXCR5 significantly increased amongst both NK1.1 positive and negative CD4+ T cells between days 5 and 8 p.i., NK1.1+CD4+ T cells more frequently expressed PD-1 and CXCR5 relative to NK1.1−CD4+ T cells throughout the first two weeks of infection (Figures 4D,E).