Immunomodulatory and intracellular antimycobacterial activity of Oxyanthus speciosus investigated using human (U937) and mouse (RAW 264.7) macrophage cell lines

Tuberculosis (TB), caused by Mycobacterium tuberculosis, remains a significant cause of death due to challenges associated with the present chemotherapy. Co-infection with HIV also greatly increases the risk of latent TB infection (LTBI) progressing to active disease as HIV suppresses the immune system, allowing infected individuals to become more susceptible to TB infection [1]. Medicinal plants are used in many parts of southern Africa to treat TB-related symptoms including chest pain and coughing [2]. The immunomodulatory efficacy of the acetone extract of Oxyanthus speciosus (Rubiaceae) against LPS-stimulated U937 macrophages was determined using a cytometric bead array (CBA) flow cytometry technique (human Th1/Th2 kit comprising a mixture of six cytokines). The intracellular efficacy of the extract against Mycobacterium-infected RAW 264.7 mouse macrophages was also investigated. Cells were infected with Mycobacterium fortuitum and subsequently treated with plant extract. The acetone extract of Oxyanthus speciosus increased the expression of IL-2 at 0.1 mg/mL while rifampicin suppressed the expression of this pro-inflammatory cytokine. The crude extract of O. speciosus inhibited the stimulation of IL-4 and IL-5 while markedly increasing the expression of IL-10. The acetone extract of O. speciosus was not cytotoxic to RAW 264.7 macrophages at the highest concentration (1 mg/mL) tested. On day 6 post-infection, the intracellular antimycobacterial activity of the crude acetone extract of O. speciosus at 1X to 4X MIC was superior to that of rifampicin, with more than 90% reduction in colony forming units. In conclusion, the extract of O. speciosus enhanced the production of IL-10 despite the fact that it also had a good stimulatory effect on IL-2, thus revealing a mixed Th1/Th2 effect. The intracellular bactericidal activity observed was both dose- and time-dependent.

Acknowledgements: The Medical Research Council, National Research Foundation, University of Pretoria (Institutional Research Theme for Animal and Zoonotic Diseases) provided funding. The South African National Biodiversity Institute (SANBI) is thanked for allowing collection of plant material at the Lowveld National Botanical Gardens.

Keywords: Oxyanthus speciosus, antimycobacterial, immunomodulatory, intracellular activity.

References:

[1] McShane H. Co-infection with HIV and TB: double trouble. Int J STD AIDS 2005; 16: 95 – 100

[2] McGaw LJ, Lall N, Meyer JJM, Eloff JN. The potential of South African plants against Mycobacterium infections. J Ethnopharmacol 2008; 119: 482 – 500