Modulation of Host Signaling Mechanisms by Polysaccharide Immunomodulator as a Novel Therapeutic Approach Against Visceral Leishmaniasis

Visceral leishmaniasis (VL) or Kala azar is caused by the parasite Leishmania donovani. VL is associated with high fever, splenomegaly and hepatomegaly and if untreated, it can be fatal Current first-line chemotherapy against VL rather depends on a limited number of drugs like pentavalent antimonial, amphotericin B, and miltefosine, but these drugs have serious problems associated with them as emerging resistance, high toxicity, serious side effects, or high costs. Inactivation of cell-mediated immune system results in parasite-specific clinical symptoms like fever, cachexia, hepatosplenomegaly, anemia and blood cytopenia during VL. Immune response against microbes including, Leishmania is directed via pro-inflammatory and modulatory cytokines. Th1/Th2 paradigm is shifts in favour of Leishmania, and thus parasite survival, during infection resulting in impaired microbicidal machinery of macrophages as evidenced by modification.. IL-12 driven increase in Th1 cells, macrophage activation, through production of IFN-γ and generation of ROS and NO contributes to the control of L. donovani infection. Th1 cytokine generation and production of ROS is mediated by fucoidan, so we tested its therapeutic efficacy in both in vitro and in vivo model of visceral leishmaniasis. Our data demonstrated that fucoidan could confer complete protection to both antimonysusceptible and –resistant L. donovani strains by switching the functional differentiation of Th2-type CD4+ cells to Th1-type as well as up regulation of NO and ROS. NO and ROS generation is a protective strategy of macrophages to clear the Leishmania parasites, which survives within the hostile environment of macrophages by hindering their microbicidal mechanisms of production of ROS and NO. Numerous pathways are intimately involved in strictly regulating the macrophage effector response and protein kinase C (PKC) signaling pathway is 80 one of the most ancient and conserved pathways, which plays an important role in generation of ROS and NO. Leishmania parasite promotes its survival within the macrophage by dysregulating PKC pathway. PKC is activated via its membrane insertion and, lipophosphoglycan (LPG), a leishmanial surface molecule is known to negatively regulate typical PKC isoforms by inhibiting their membrane insertion. Leishmania is known to increase the expression of atypical PKC isoforms (PKC-ε and ζ), while Ca2+ dependent PKC-β expression decreases during infection. IL-10 production is a characterstic Th2 cytokine which increases during infection and Leishmania parasite are known to increase IL-10 production via deactivation of PKC-β. Moreover NF-κB, a major transcription factor known to assist host immune system in clearing the parasite burden is also dysregulated by Leishmania, via modulation of PKC signaling. MAPK pathway, which is upstream to NF-κB is also known to be inhibited by Leishmania parasites for its survival with the host. Activation of MAPK signaling pathways by various stimuli induced NF-κB activation either through the phosphorylation of its inhibitor, IκB or by direct post-transcriptional modification of its p65 subunit. Thus, Leishmania successfully evades the macrophage microbicidal machinery and thrives in, through deregulating production of NO and ROS. Establishment of an effective immune response therefore may depend on stimulation of PKC signaling as well as MAPK-mediated activation of NF-κB, which may prove a potential advancement in therapy of leishmaniasis. Previously we have already established that fucoidan can completely cure both antimony-susceptible and-resistant strains of L. donovani through NO and ROS upregulation. Shifting of Th1/Th2 paradigm in favour of host was the main effect of fucoidan, which ultimately resulted in increased production of NO and ROS and clearance of parasetimia. Our present study clearly demonstrated that antileishmanial response of fucoidan was mediated via induction of iNOS and pro-inflammatory cytokines as well as activation of p38 and ERK1/2 MAPK, which in turn activated NF-κB. Interestingly, further investigations illuminated the functional involvement of distinct PKC isoforms in fucoidan-mediated MAPK activation. Moreover, the present investigation provides the first in vivo evidence 81 for the fact that activation of NF-κB by fucoidan may be a prerequisite for shifting the cytokine balance toward disease-resolving Th1 mode for complete elimination of organ parasite burden. Chapter III Natural compounds are gaining importance since it has filled the void in the search of the alternative compounds with anti-parasitic activity. Many dietary compounds have assessed as potential chemopreventive agents. Fucoidan, a polyanionic, sulphated polysaccharide mainly consisting of L-fucose, is known to maintain the cell wall integrity of brown algae, and has many important biological functions including anti-angiogenic, anti-tumour, antimalarial, contraceptive and immuno-modulatory activities. Immunomodulatory and antitumor effects of Fucoidan may be mediated by the release of NO and ROS from macrophages. Phosphorylation of kinase plays an important role in activation of macrophages, but uncontrolled activation of anything is harmful which could be detrimental for the cell. It is necessary for the cells that both the protein kinases and phosphatases maintain their physiological balance to sustain a normal regulation of events. The present investigation was aimed towards exploring whether ROS has any effect on the kinase phosphatase balance maintained by fucoidan mediated antileishmanial effect in macrophage culture as well as in animal model of visceral leishmaniasis. Moreover, we found that ROS quencher NAC can revert the beneficial effects mediated by fucoidan as Th1 cytokine production, activation of p38 and ERK MAPK