Higher immune response induced by vaccination in Houhai acupoint relates to the lymphatic drainage of the injection site.

Houhai acupoint (HA) is a site for acupuncture stimulation, located in the fossa between the anus and tail base in animals. To evaluate HA as a potential immunization site, the immune responses were compared when HA and the conventional site nape were vaccinated in rats. The results showed that injection of a porcine epidemic diarrhea virus (PEDV) vaccine in HA induced significantly higher IgG, IgG1, IgG2, splenocyte proliferation and mRNA expression of IL-2, IL-4 and IFN-γ than in the nape. To search for the underlying mechanisms, the draining lymph nodes for HA and the nape were investigated. When rats were injected in HA with Indian ink, 11 lymph nodes including caudal mesenteric lymph node and bilateral gluteal lymph nodes, posterior inguinal lymph nodes, lumbar lymph nodes, internal iliac lymph nodes and popliteal lymph nodes were visibly stained with the ink and injection of a model antigen ovalbumin (OVA) in HA resulted in detection of OVA by western blotting while in the same lymph nodes only a pair of lymph nodes (central brachial lymph nodes) were observed when Indian ink or OVA was injected in the nape. IL-2 mRNA expression was detected in all the lymph nodes when PEDV vaccine was injected. Therefore, the enhanced immune response elicited by vaccination in HA may be attributed to more lymphocytes activated.