Peiyuan xuanhua jiedu decoction potentiates SARS-CoV-2 vaccine-induced immunity via STAT3-dependent Tfh cell differentiation

Background Respiratory infectious diseases are highly contagious and prevalent, posing severe global public health threats. Vaccination is the primary effective intervention against life-threatening respiratory viral infections. However, current vaccines confer limited and short-lived immune protection, compromising their clinical utility. Safe and potent novel immune adjuvants are therefore critical to enhance and sustain vaccine-triggered immune responses. Peiyuan xuanhua jiedu (PY) decoction has been clinically validated to alleviate COVID‑19 symptoms, with activity associated with immune enhancement, as well as anti‑inflammatory, antioxidant, and immunomodulatory effects. Objective This study aimed to characterize the immunoadjuvant activity of PY decoction in SARS‑CoV‑2 vaccination and clarify its underlying cellular and molecular mechanisms. Methods A SARS‑CoV‑2 vaccination mouse model was established to evaluate the in vivo adjuvant efficacy of PY decoction. Vaccine-induced humoral immune responses were assessed by antibody titers. Flow cytometry was used to profile key immune cell subsets, including B cells, plasma cells, T follicular helper (Tfh) cells, germinal center B (GCB) cells, and memory T cells. Integrated network pharmacology and transcriptomic analysis were performed to identify the core regulatory signaling pathway. Results PY decoction significantly increased RBD-binding IgG titers compared to the Vaccine-only group at both early and late time points. At the peak immune response on day 28, the geometric mean titer (GMT) in the PY group reached 1:6,400, 3.2-fold higher than that in the Vaccine-only group (GMT: 1:2,000; P < 0.01). At day 120, the PY group maintained a GMT of 1:3,200, whereas the Vaccine-only group declined to near-baseline levels (GMT: 1:400; P < 0.001), demonstrating prolonged antibody persistence. For neutralizing antibodies at day 120, the PY group exhibited lower VSV-SARS-CoV-2 infection rates than the Vaccine-only group at both 1:80 dilution (mean: 9.6% vs. 28.6%) and 1:320 dilution (23.3% vs. 45.1%), with lg(pVNT 50 ) values of ∼2.0 vs. ∼1.7, suggesting enhanced neutralizing capacity. PY decoction also induced a transient elevation of Th1/Th2-related cytokines, driving early immune activation. Mechanistically, PY decoction upregulated B cell co-stimulatory molecules CD40 and CD80, promoted plasma cell and Tfh cell generation without affecting GCB cell formation, and potentiated T cell activation and memory T cell development. Further experiments verified that PY decoction facilitates Tfh cell differentiation via activation of the IL-6-STAT3 signaling pathway. Notably, our findings extend current understanding by demonstrating that a multi-herb traditional Chinese medicine formula can sustain vaccine-induced humoral immunity through STAT3-dependent Tfh differentiation, in contrast to previous TCM-based adjuvant studies that focused mainly on short-term antibody responses. Conclusion Oral PY decoction augments and stabilizes vaccine-induced immune responses by regulating the differentiation and functional maturation of B and T cells. IL-6-STAT3-dependent promotion of Tfh cell differentiation is the core mechanism mediating its immunoadjuvant effect.