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The parasites of the genus Leishmania survive and proliferate in the host phagocytic cells by taking control over their microbicidal functions. The parasite also promotes differentiation of antigen-specific anti-inflammatory cytokines producing effector T cells, which eventually results in disease pathogenesis. The mechanisms that parasites employ to dominate host adaptive immunity are largely unknown. For the first time, we report that L. donovani, which causes visceral leishmaniasis in the Indian subcontinent, upregulates the expression of an immune inhibitory receptor i.e., CD300a on antigen presenting and phagocytic cells to dampen their effector functions. The blocking of CD300a signals in leishmania antigens activated macrophages and dendritic cells enhanced the production of nitric oxide, pro-inflammatory cytokines along with MHCI/II genes expression, and reduced parasitic uptake. Further, the abrogation of CD300a signals in Leishmania infected mice benefited antigen-experienced, i.e., CD4+CD44+ and CD8+CD44+ T cells to acquire more pro-inflammatory cytokines producing phenotypes and helped in the early clearance of parasites from their visceral organs. The CD300a receptor blocking also enhanced the conversion of CD4+ T effectors cells to their memory phenotypes i.e., CCR7high CD62Lhigh up to 1.6 and 1.9 fold after 14 and 21 days post-infection, respectively. These findings implicate that CD300a is an important determinant of host phagocytic cells functions and T cells differentiation against Leishmania antigens.

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In leishmaniasis, the protective immunity is largely mediated by proinflammatory cytokine producing abilities of T cells and an efficient parasite killing by phagocytic cells. Notwithstanding a substantial progress that has been made during last decades, the mechanisms or factors involved in establishing protective immunity against Leishmania are not identified. In ancient Indian literature, metallic “bhasma,” particularly that of “swarna” or gold (fine gold particles), is indicated as one of the most prominent metal-based therapeutic medicine, which is known to impart protective and curative properties in various health issues. In this work, we elucidated the potential of swarna bhasma (SB) on the effector properties of phagocytes and antigen-activated CD4 + T cells in augmenting the immunogenicity of L. donovani antigens. The characterization of SB revealing its shape, size, composition, and measurement of cytotoxicity established the physiochemical potential for its utilization as an immunomodulator. The activation of macrophages with SB enhanced their capacity to produce nitric oxide and proinflammatory cytokines, which eventually resulted in reduced uptake of parasites and their proliferation in infected cells. Further, in Leishmania -infected animals, SB administration reduced the generation of IL-10, an anti-inflammatory cytokine, and enhanced pro-inflammatory cytokine generation by antigen activated CD4 + T cells with increased frequency of double (IFNγ + /TNFα + ) and triple (IFNγ + TNFα + IL-2 + ) positive cells and abrogated disease pathogeneses at the early days of infection. Our results also suggested that cow-ghee (A2) emulsified preparation of SB, either alone or with yashtimadhu, a known natural immune modulator which enhances the SB’s potential in enhancing the immunogenicity of parasitic antigens. These findings suggested a definite potential of SB in enhancing the effector functions of phagocytes and CD4 + T cells against L. donovani antigens. Therefore, more studies are needed to elucidate the mechanistic details of SB and its potential in enhancing vaccine-induced immunity. Supplementary Information The online version contains supplementary material available at 10.1007/s12011-023-03659-3.

The poor abilities of parasitic antigens to induce cell-mediated immunity is one of the major reasons for the limited success of vaccine candidates against all forms of leishmaniasis. Here we find, for the first time, that the adjuvantation of whole-killed Leishmania vaccine with anti-CD200 and anti-CD300a antibodies significantly enhances CD4+ T cells mediated immunity in vaccinated animals and provides strong protection against virulent parasites. The antibody adjuvantation, either alone or with a TLR4 agonist monophosphoryl A (MPL-A), significantly induced the production of pro-inflammatory cytokines viz., IFN-γ, TNF-α, IL-12, and IL-2 by antigen experienced CD4+ T cells, and also enhanced their rate of conversion into their memory phenotypes. The antibody adjuvantation also promoted the establishment of IgG2a-mediated protective humoral immunity against parasitic antigens. The vaccinated animals showed strong resilience against virulent L. donovani parasites as we observed reduced clinical features such as splenomegaly, hepatomegaly, granulomatous tissues in the liver, and significantly less parasitic load in their spleen. The findings of this study demonstrate that the anti-CD200 and anti-CD300a antibodies adjuvantation significantly increases the protective efficacy of the whole-killed Leishmania vaccine. This study opens up a new gateway to diversify the roles of immune inhibitory molecules in vaccine development against leishmaniasis.