pTFH cell expansion in vaccine responders is correlated with Ab production and B-cell differentiation. (A) Frequency of pTFH (CD4⁺CXCR5⁺) cells indicated as a percentage of total CD4 T cells in HCs and HIV-infected patients who were responders (R) or nonresponders (NR) to H1N1/09 influenza vaccine. Peripheral TFH cell frequencies were measured prevaccination (T0) and 4 weeks after vaccination (T2). (B-C) Frequency of Ki67⁺ pTFH (B) and non-pTFH (CD4⁺CXCR5⁻; C) cells at T0 and at T2. Box plots represent median with 25th and 75th percentile borders. Error bars represent the 10th and 90th percentile means from 8 HCs, 8 HIV⁺ responders, and 8 HIV⁺ nonresponders. (D-E) Linear correlation between frequency of pTFH cells with frequency of memory B cells (CD20⁺ CD21^high CD10⁻ CD27⁺) at T2 (D) and with H1N1 serum Ab titer at T2 (E).