Visceral leishmaniasis (VL) or kala-azar, a cosmopolitan vector-borne zoonotic disease, is the most severe and fetal form of leishmaniasis if not diagnosed and untreated in time. The disease is caused by parasitic protozoan species of the Leishmania donovani complex and is transmitted by sandflies (1).
In the recent past, the parasites have developed resistance to the existing drugs and also due to lack of effective human vaccine against VL, lead to increase in the incidence of VL (2–5). Different generations of vaccines based on different antigens of the parasite have been examined to fulfill the hopes for an appropriate concept for leishmaniasis treatment, and prevention or induce protection with long-term immunity (5–8).
Among the various antigens that serve as targets for VL vaccine design, fucose-mannose ligand (FML) has attracted much attention owing to its excellent immunoprotective properties against experimental VL in several animal models (9). The FML is a glycoprotein antigen which is present both in amastigotes and in motile promastigotes of L. donovani complex. Even though the FML is a potent immunogen in rabbits and dogs (e.g., Leishmune® a vaccine for canine VL consists of FML and saponin) (10), it does not have adequate immunogenicity in human (5, 11).
Previous studies have demonstrated that macrophages play a pivotal role in the outcome of Leishmania infection depending on type of macrophages; classically activated (M1) macrophages as efficient type against Leishmania parasites or alternatively activated (M2) macrophages as favoring survival and growth of Leishmania parasites (12, 13). In response to different microbial stimuli and immune status of the microenvironment, naive macrophages (M0) differentiate to either M1 or M2 subpopulation with different patterns of cytokine production and distinct properties. Concerning stimuli that can affect shift macrophages toward M1 or M2 subpopulation, in a recent study we evaluated the immunomodulatory effects of FML on macrophages (14). Our findings showed that although the FML significantly increases nitric oxide (NO), IL-12p70 and IP-10 production in macrophages, but cannot alter TNF-α production in them (14). The most surprising aspect of this study was that FML significantly increases the production of IL-10, an immunosuppressive cytokine, from macrophages (14).
Glycyrrhizin (GL) is a natural triterpenoid saponin derived from the root of licorice (Glycyrrhiza glabra) that has been associated with numerous pharmacologic effects, including anti-bacterial, anti-inflammatory, anti-oxidant, anti-viral, anti-tumor, hepatoprotective, and immunomodulatory activities (15–18). Stimulation of IL-12 and NO production and suppression of IL-10 production from macrophages (19, 20), augmentation of NK cell activity (19), up regulation of costimulatory molecules on dendritic cells (15), increasing of T cells proliferation, reduction of IL-4 production from T cells, direction of immune response toward Th1 (15), are the most important known immunomodulatory activities of GL.
It is well known that the GL plays an important role in immunomodulatory activities such as stimulation of IL-12 and NO production and suppression of IL-10 production from macrophages (19, 20). The GL also plays a central role in the augmentation of NK cell activity, up-regulation of costimulatory molecules on dendritic cells, increasing of T cells proliferation, reduction of IL-4 production from T cells, and direction of the immune response toward Th1, (15).
With regard to new improvements in produce human VL vaccine by using purified FML and considering the importance of macrophages in protection and control against VL, it seems that characterization of the immunomodulatory effects a combination of FML/GL on macrophages can be useful for finding ways to increase the immunogenicity of FML and vaccine development. Therefore, for the first time, we investigated the effects of FML/GL on production of cytokines and NO by macrophages, in vitro.
With regard to the improvements in producing the human VL vaccines by using purified FML and considering the importance of macrophages in protection and control of VL, it is important to characterize the immunomodulatory effects of combination of FML/GL on macrophages which can be useful in finding ways to increase the immunogenicity of FML and vaccine development.