The Chinese herbal medicine ox knee (A. bidentata) possesses pro-inflammatory activities, and polysaccharides from this herb have been shown to enhance immunity . This study demonstrated that pretreatment with ABPS could delay the progression of malaria in mice, probably through boosting of host immunity. It was also found that ABPS could effectively control parasitaemia levels and extend host survival. ABPS treatment enhanced the production of IFN-γ, TNF-α and NO, and increased the number of F4/80+CD36+ macrophages. ABPS also promoted the differentiation and activation of DCs with increased expression of maturation markers. This study further verified the immuno-modulatory activity of ABPS during the early stage of P.y17XL infection in this murine malaria model, which provided some insights into the regulatory mechanisms by which ABPS mediate anti-malarial protection.
During malaria infection, effective Th1 immune responses at the early stages of infection critically influence the later development and final outcome of the disease [31–34]. IFN-γ is found to be necessary during the resolution of primary infection and for limiting parasite replication in early phase of the infection [35, 36]. In current study, it was found that pretreatment with ABPS significantly stimulated IFN-γ, TNF-α and NO production in vivo during P.y17XL infection, which might be responsible for the observed improvement in the extended survival in ABPS-treated mice.
DCs provide a critical link between the innate and adaptive immune responses. Increased expression of co-stimulatory molecules, which is characteristic of maturation of DCs, is crucial to the activation of T cells . These molecules have been shown to play an important role in murine malaria models. High expression of MHC-II molecules is crucial for DCs to present antigens to CD4+ T cells. It has been found that blocking the CD80/CD86 signaling pathway disrupts the Th1/Th2 balance in the Plasmodium chabaudi AS malaria model . It is reported that ABPS can induce murine DCs phenotypic maturation in vitro as revealed by increased expression of CD86, CD40 and MHC-II . In this study, the significant up-regulation of co-stimulatory molecules on the surface of CD11c+ cells in ABPS-treated mice was detected, indicating that ABPS treatment could significantly improve the maturation of DCs.
TLRs are expressed on or within innate immune cells including DCs, and recognize pathogen-associated molecular patterns from different microorganisms . Accumulating evidence suggests that TLRs are also involved in both rodent and human malaria [40–42]. It has been reported that TLR9 responds to haemozoin and parasite protein-DNA complex released from the parasitized erythrocytes [41, 43, 44], and TLR9 polymorphisms are associated with disease manifestation in malaria . The significantly elevated expression of TLR9 in DCs in P.y17XL-infected mice pretreated with ABPS suggests that TLR9 might be essential for ABPS-mediated immuno-regulation during malaria infection.
Increasing evidence indicates that innate immune responses contribute to the control of blood-stage malaria infection, reduced parasite burden, and slowing down of the progression to severe disease. Macrophages have a remarkable non-specific ability to phagocytize and kill protozoan parasites. Both human and rodent macrophages effectively phagocytize P.y17XL-parasitized erythrocytes [45, 46]. Furthermore, the class B family of scavenger receptor CD36 on macrophages is also implied in protection against malaria, since CD36-deficient individuals are at a greater risk of developing severe and cerebral malaria . Whereas the total number of macrophages (F4/80+) in ABPS-pretreated mice was relatively unchanged, the number of F4/80+CD36+ macrophages significantly increased at 3 dpi in ABPS pretreated mice. As such, the increase in the number of CD36+ macrophages might be involved in phagocytosis of parasitized erythrocytes, leading to reduced parasitaemia at 5 dpi in ABPS-pretreated mice.
Natural Treg expand during Plasmodium infection [5, 24, 48, 49] and have been shown to inhibit the development of Th1 immune responses [3, 24], leading to persistent rise of parasitaemia [24, 50]. Here it was found that ABPS only improved the activation of DCs, which may improve the initiation of an antigen-specific immune response, whereas no effect was observed on Treg or production of the anti-inflammatory cytokine IL-10. This suggests that the immuno-regulatory effects of ABPS may be selective on boosting different host immune responses.
Taken together, the current study further confirmed the immuno-regulatory effects of ABPS. The results have demonstrated that ABPS could activate immune responses against malaria blood-stage infection in mice and that this effect may be mediated by stimulation of DC maturation and activation of F4/80+CD36+ macrophages, which may phagocytose parasitized erythrocytes and release pre-inflammatory cytokines. These findings laid the foundation for further testing the potential benefits of administering ABPS as an "immuno-prophylaxis" to inhibit preemptively the progression of malaria infection.