CD4 and infection: Following Lm-2W infection, three subsets of 2W1S-specific CD4+ T cells have been elegantly described 19: CXCR5−PD-1−T-bet+ effector T cells (where PD-1 is programmed death-1), CXCR5+PD-1−Bcl-6+ cells that give rise to central memory cells and CXCR5+PD-1+Bcl-6+ TFH cells.