PDCD1 and infection: We then employed a flow cytometry-based sorting strategy where, on day 9 post infection, CD4 T cells were isolated from the draining LN of infected mice, enriched based on negative paramagnetic bead isolation and then sorted by flow cytometry based on expression of the cell-surface markers CD4, CD44 (to enrich for antigen experienced cells), CXCR5 and PD-1 (Fig. 1a).