Glutathione and Growth Inhibition of Mycobacterium Tuberculosis in Healthy and HIV-infected Subjects

Tuberculosis (TB) is a major global health problem, especially in developing countries. Approximately one-third of the world’s population is latently infected with Mycobacterium tuberculosis (LTBI). Individuals with LTBI have a 5–10% lifetime risk of developing active disease.

HIV positive subjects with LTBI are at very high risk of developing active tuberculosis. Development of active TB in HIV patients is due not only to reactivation of latent M. tuberculosis infection but also due to increased susceptibility to primary progressive M. tuberculosis infection.

Innate and adaptive immune responses are required for successful control of M. tuberculosis infection. Macrophages provide first line defense against M. tuberculosis infection. Murine macrophages can be activated to kill intracellular M. tuberculosis by treatment with LPS (a stimulus for TNF-α expression, via triggering of toll-like receptors) and IFN-γ (a product of activated lymphocytes).

The Antioxidant Glutathione (GSH)

Glutathione (GSH) is an antioxidant and plays a vital role in cellular detoxification and enhancement of immune functions. Interestingly, HIV-infected people have subnormal GSH levels in their plasma, lung epithelial lining fluid, peripheral blood mononuclear cells (PBMC), and other blood cells. It has been recently reported that the decreased GSH levels in PBMC of HIV-infected individuals is associated with a poorer prognosis.

Immunodeficiency due to HIV-1 represents the greatest recognized threat to successful containment of latent M. tuberculosis infection. The aim of this study was to examine the role of GSH in immunity against TB in samples derived from healthy and HIV-infected subjects.

In previous studies using macrophages from different sources, the authors of the present study have demonstrated that GSH plays a vital role in innate immunity against TB infection. In our recent studies we have shown that GSH has static effect on M. tuberculosis H37Rv growth in vitro. The mechanism of toxicity of GSH to mycobacteria is not yet known.

In the present study, we reexamined the extent to which GSH levels are decreased in HIV positive subjects. We also examined the relationship between GSH levels and the ability to kill intracellular M. tuberculosis, in association with other immune functions, such as cytokine production. GSH levels were modulated by treating blood samples with N-acetyl cysteine (NAC) to increase or buthionine sulphoximine (BSO) to decrease intracellular GSH pools. The results suggest that the inability of immune cells from healthy and HIV subjects to contain TB growth may be a consequence of the inability of their macrophages to maintain adequate GSH levels during in vitro infection.

N-acetyl cysteine treatment decreased the levels of IL-1, TNF-α, and IL-6, and increased the levels of IFN-γ in blood cultures derived from human immunodeficiency virus-infected subjects, promoting the host immune responses to contain M. tuberculosis infection successfully.

The researchers demonstrate growth inhibition of intracellular H37Rv in our in vitro studies using NAC-treated blood cultures from HIV patients. Furthermore, treatment of blood cultures with NAC modulated the production of cytokines in favor of the host.

The results of this study strongly indicate that the immune cell enhancing and antimycobacterial functions of GSH are important for growth control of M. tuberculosis H37Rv in blood cultures from healthy and HIV-infected subjects.

Additionally, NAC treatment down-regulated the synthesis of IL-10 and pro-inflammatory cytokines in blood cultures from HIV-infected subjects favoring immune activation.

Conclusions and Discussion

Current interventions to prevent tuberculosis in areas where TB and HIV are endemic, such as sub-Saharan Africa, have serious limitations. ART is limited by its cost and by its requirement for a sophisticated health care delivery system.

Isoniazid chemoprophylaxis has limited efficacy in regions of high TB transmission, particularly in highly susceptible individuals with advanced HIV infection. In addition, isoniazid is ineffective against INH-resistant TB strains, which may account for 10–20% of all cases in some areas.

NAC is inexpensive and non-toxic (it is considered a food supplement in the US, and is available without prescription in health food stores). The findings from this study may lead to long-term research that will be of potential importance for control of TB worldwide.

04/11/06

Reference
V Venketaraman, T Rodgers, R Linares, and others. Glutathione and growth inhibition of Mycobacterium tuberculosis in healthy and HIV infected subjects. AIDS Research and Therapy 3: 5. Epub February 2006.