South Asian Journal of Parasitology

Background:  Visceral leishmaniasis (VL) is a severe disease with high morbidity and mortality in tropical regions. Current treatments are limited, highlighting the need for alternatives like vaccine development and adjunct therapy, where plant-derived adjuvants like Carissa edulis show promise.

Aims: The study aims to compare the adjuvant (vaccination) capacity of Carissa edulis extract combined with soluble Leishmania donovani antigen (SLA) in BALB/c mice.

Methodology: This in vivo experimental study was conducted in BALB/c mice. Eighty BALB/c inbred mice of both sexes of aged between 8-10 weeks were purchased from the animal house, Department of Zoological Sciences, Kenyatta University. The mice were allocated into different groups of 12 mice each. Three mice in each group were sacrificed, spleen and blood taken to measure parasite load and immune parameters. Eighty BALB/c mice received immunisations with SLA alone, C. edulis extract alone, SLA + C. edulis or phosphate-buffered saline (PBS) and then challenged with 1 × 10⁶ L. donovani promastigotes. parasite burden, IFN-γ and IL-10, total IgG, IgG1, and IgG2a were being observed for eight weeks.

Results: The most extensive decline in the splenic parasite burden was observed in the SLA + C. edulis group and maintained the parasites at the levels of about 6 to 8 per cent at week 8, which was significantly lower when compared to PBS (15-30%; p = 0.0031), SLA alone (10-12%; p = 0.01) and C. edulis alone (10-12%; p = 0.0011). The analysis of area-under-the-curve revealed that the cumulative parasite burden of the combination group (−61.39 ± 18.27) was much less than that of the SLA only (150.15 ± 16.01) and PBS only (519.28 ± 22.93; p < 0.0001). The highest IFN-γ level (mean 217.4 pg/mL; peak of approximately 300 pg/mL at week 8) was induced by combination immunisation, which was far higher than SLA alone (113.2 pg/mL), C. edulis alone (67.4 pg/mL), and PBS (p = 0.0013). The level of IL-10 was relatively lower in the combination group (3540 pg/mL) when compared to SLA alone (approximately 50 pg/mL) or C. edulis alone (approximately 60 pg/mL; p = 0.0209).

Conclusion: The results have shown that the extract of C. edulis is synergistically antigenic, stimulating the Th1- biased response with increased IFN-γ and IgG2a, and decreased IL-10, as well as enhanced parasite clearance.