Immunomodulatory and intracellular antimycobacterial activity of Oxyanthus speciosus investigated using human (U937) and mouse (RAW264.7) macrophage cell lines

Abstract

Tuberculosis (TB), a disease caused by Mycobacterium tuberculosis, remains a significant cause of death due to challenges associated with present chemotherapy. Coinfection with HIV also greatly increases the risk of latent TB infection (LTBI) progressing to active disease due to the fact that HIV suppresses the immune system, thereby allowing infected individuals to become more susceptible to TB infection. Medicinal plants are used in many parts of southern Africa to treat TB-related symptoms including chest pain and coughing. The acetone extract of Oxyanthus speciosus was screened for its immunomodulatory effect against LPS-stimulated U937 macrophage cells using a cytometric bead array (CBA) technique. Human TH1/TH2 kits consisting of a mixture of six cytokines were used for the assay and analysed using flow cytometry. The intracellular efficacy of the O. speciosus extract against Mycobacterium-infected macrophages was investigated using RAW 264.7 mouse macrophage cell line. Mouse macrophages were infected with M. fortuitum with a multiplicity of infection at 10 mycobacteria per cell. The result obtained from this study revealed that the acetone extract of O. speciosus increased the expression of IL-2 at 0.1 mg/mL while rifampicin supressed the expression of this pro-inflammatory cytokine. At the tested concentration the crude extract of O. speciosus, inhibited the stimulation of IL-4 and IL-5 while it markedly increased the expression of IL-10. The acetone extract of O. speciosus did not show cytotoxicity to RAW 264.7 macrophages at the highest tested concentration (1 mg/ml). On day 6 post-infection, the intracellular antimycobacterial activity of the acetone crude extract of O. speciosus at 1X to 4X MIC was superior to that of rifampicin, showing more than 90% reduction in colony forming units. In conclusion, the extract of O. speciosus had a mixed Th1/Th2 effect. The bactericidal activity observed was both dose and time-dependent.