H1N1 Ag promotes IL-21 production by PBMCs from vaccine responders on in vitro stimulation. (A) Cryopreserved PBMCs obtained at T0 and 4 weeks after H1N1/09 vaccination (T2) were thawed, rested overnight, and cultured at 1 × 10⁶ cells/mL for 5 days at 37°C in the presence of 5 μg/mL of H1N1 vaccine Ag. On day 5, supernatants were harvested and IL-21 levels were determined by ELISA. (B) Graph shows the linear correlation between the frequency of peripheral TFH cells at T2 with IL-21 levels in H1N1-stimulated PBMC culture supernatants. Bars represent the means and error bars indicate the SD of 5 HCs, 5 HIV⁺ responders (R), and 4 HIV⁺ nonresponders (NR). *P < .05.