Polysaccharides from the Chinese medicinal herb Achyranthes bidentata enhance anti-malarial immunity during Plasmodium yoelii 17XL infection in mice

Background: Clinical immunity to malaria in human populations is developed after repeated exposure to malaria. Regulation and balance of host immune responses may lead to optimal immunity against malaria parasite infection. Polysaccharides (ABPS) derived from the Chinese herb ox knee Achyranthes bidentata possess immunomodulatory functions. The aim of this study is to use the rodent malaria model Plasmodium yoelii 17XL (P. y17XL) to examine whether pretreatment with ABPS will modulate host immunity against malaria infection and improve the outcome of the disease. Methods: To determine whether ABPS could modulate immunity against malaria, mice were pretreated with ABPS prior to blood-stage infection by P. y17XL. Host survival and parasitaemia were monitored daily. The effect of pretreatment on host immune responses was studied through the quantitation of cytokines, dendritic cell populations, and natural regulatory T cells (Treg). Results: Pretreatment with ABPS prior to infection significantly extended the survival time of mice after P. y17XL infection. At three and five days post-infection, ABPS pretreated mice developed stronger Th1 immune responses against malaria infection with the number of F4/80(+)CD36(+) macrophages and levels of IFN-gamma, TNF-alpha and nitric oxide being significantly higher than in the control group. More importantly, ABPS-treated mice developed more myeloid (CD11c(+)CD11b(+)) and plasmacytoid dendritic cells (CD11c(+)CD45R(+)/B220(+)) than control mice. ABPS pretreatment also resulted in modulated expression of MHC-II, CD86, and especially Toll-like receptor 9 by CD11c(+) dendritic cells. In comparison, pretreatment with ABPS did not alter the number of natural Treg or the production of the anti-inflammatory cytokine IL-10. Conclusion: Pretreatment with the immuno-modulatory ABPS selectively enhanced Th1 immune responses to control the proliferation of malaria parasites, and prolonged the survival of mice during subsequent malaria infection.