Latex protein extracts from Calotropis procera with immunomodulatory properties protect against experimental infections with Listeria monocytogenes

Background The latex from the medicinal plant Calotropis procera is often used in folk medicine against infectious and inflammatory diseases. Purpose In this study, we investigate a protein fraction with immunomodulatory properties, named LPPI, against experimental infections, in vitro and in vivo, with a virulent strain of Listeria monocytogenes. Study design LPPI was exposed to cultured macrophages or Swiss mice and then challenged with L. monocytogenes. Methods Peritoneal macrophages were obtained from Swiss mice, and cultured in 96-well microplates. Soluble latex proteins (LP) were subjected to fractionation by ion-exchange chromatography. The major peak (LPPI) was added into wells at 10 or 100 µg/ml. Albumin (100 µg/ml) was used for comparison between protein treatments. After incubation for 1 h at 5% CO2/ 37°C, the supernatant was discarded and 0.2 ml of L. monocytogenes overnight culture was added in the wells. Following 4 h and 24 h infection, the cytokine mRNA expression was evaluated as well as the number of intracellular colony forming units. Swiss mice (n = 16) were injected intraperitoneally (i.p.) with LPPI (5 and 10 mg/kg) while the control mice received albumin (10 mg/kg) or LP (10 mg/kg). After 24 h, all animal groups were challenged with L. monocytogenes (106 CFU/ ml), also by i.p. route. Results LPPI was not toxic to uninfected macrophages (pMO) and significantly increased mRNA expression of TNF-α, IL-6, IL-1β and iNOS. Following infection, cell viability was reduced by 50% in albumin-treated pMO (control); but only 17% in pMO treated with LPPI at 100 µg/ml. In this case, LPPI increased expression of TNF-α and IL-6 whereas the number of bacterial colony-forming units was reduced 100-fold in comparison to control groups. Swiss mice pretreated with LPPI showed dose-dependent survival rates that reached 80%, while mice that received albumin died 1–3 days after infection. After 24 h infection, leukocyte migration to the infectious foci was high in LPPI-treated mice whereas the number of viable bacteria in the peritoneal fluid, liver and bloodstream were significantly reduced. Conclusion We conclude that LPPI present immunomodulatory properties that are beneficial for prevention of systemic bacterial infections caused by the intracellular bacteria L. monocytogenes.