Infection frequencies of Th17 cells in PB. PB lymphocytes from a cohort of HIV-infected patients were stimulated with anti-CD3 overnight in the presence of brefeldin A and stained as in “Intracellular cytokine assay.” CD27⁺ memory CD4 T cells that produced IFN-γ, IL-17, or neither were then sorted by flow cytometry, and the infection frequency was determined by quantitative PCR for viral DNA as described in “Quantitative PCR.”