Mycobacterium bovis BCG but not Mycobacterium leprae Induces TNF-a Secretion in Human Monocytic THP-1 Cells

Vol. 96(7): 973-978, October 2001

Martha M Oliveira, Rosane Charlab*, Maria Cristina V Pessolani/⁺

Laboratório de Hanseníase, Instituto Oswaldo Cruz-Fiocruz, Av. Brasil 4365, 21045-900 Rio de Janeiro, RJ, Brasil *Centro Brasileiro de Pesquisas Físicas, CNPq, Rio de Janeiro, RJ, Brasil

In this study, we compared the level of TNF-a secretion induced in monocytic THP-1 cells after phagocytosis of Mycobacterium leprae, the causative agent of leprosy, and M. bovis BCG, an attenuated strain used as a vaccine against leprosy and tuberculosis. The presence of M. leprae and BCG was observed in more than 80% of the cells after 24 h of exposure. However, BCG but not M. leprae was able to induce TNF-a secretion in these cells. Moreover, THP-1 cells treated simultaneously with BCG and M. leprae secreted lower levels of TNF-a compared to cells incubated with BCG alone. M. leprae was able, however, to induce TNF-a secretion both in blood-derived monocytes as well as in THP-1 cells pretreated with phorbol myristate acetate. The inclusion of streptomycin in our cultures, together with the fact that the use of both gamma-irradiated M. leprae and heat-killed BCG gave similar results, indicate that the differences observed were not due to differences in viability but in intrinsic properties between M. leprae and BCG. These data suggest that the capacity of M. leprae to induce TNF-a is dependent on the stage of cell maturation and emphasize the potential of this model to explore differences in the effects triggered by vaccine strain versus pathogenic species of mycobacteria on the host cell physiology and metabolism.

Key words: Mycobacterium leprae - BCG - THP-1 - TNF-a

Mononuclear phagocytes are target cells for pathogenic mycobacteria that generally require an intracellular environment in which they survive and replicate. Although these cells can offer a quite hostile environment to the entering pathogen, they can also provide unique advantages to the infectious organism. They are long-living cells and thus provide a potential long-term habitat for the bacterial invader. In addition, macrophages play a pivotal role in host defense against infection, primarily due to the vast array of mediators that these cells can secrete, offering in this way to the invader organism an opportunity to manipulate the immune system to its own advantage (Russel 1995). Immunity to intracellular pathogens depends mainly upon the activation of IFN-g producing CD4⁺ T cells, the so called Th1 response, which increases the microbicidal capacity of macrophages (Kaufmann 1995). In this context, microorganisms that supress or avoid, early during infection, infected macrophages to produce cytokines such as TNF-a and IL-12, cricial for the emergency of a Th1-like response (Trinchieri 1993, Flesch et al. 1995), will favor the development of a susceptible phenotype.

The early molecular events that occur during the interaction between monocytic cells and mycobacteria remain poorly defined. Experiments comparing attenuated versus virulent strains and live versus dead bacteria have revealed critical differences in their effects on the host cell suggesting the importance of these events in mycobacterial pathogenesis. To gain a better understanding of the mycobacteria-host cell interaction, we have employed an in vitro model of infection using THP-1 cells, a human immature monocytic cell line. As a first approach, we compared the level of TNF-a secretion induced in these cells after phagocytosis of Mycobacterium leprae, the causative agent of leprosy, and M. bovis BCG, an attenuated strain used as a vaccine against leprosy and tuberculosis. Blood mononuclear cells of both normal individuals or leprosy patients secrete TNF-a in response to M. leprae and BCG (Santos et al. 1993, Suzuki et al. 1993, Lima et al. 2000). Besides its role in the differentiation of Th1 cells, TNF-a is involved in the formation of granulomas that play an essential role in preventing the extension and dissemination of mycobacterial infections (Kindler et al. 1989). The protective function of TNF-a has been emphasized in a report that describes the deleterious effects of deletion in TNF-a gene on mycobacterial infections (Flynn et al. 1995). Our results demonstrate that BCG but not M. leprae induces TNF-a secretion in THP-1 cells, emphasizing the potential of this model to explore differences in signaling pathways triggered by vaccine versus pathogenic species of mycobacteria.

MATERIALS AND METHODS

RESULTS

DISCUSSION

REFERENCES

Fig. 1 | Fig. 2 | Fig. 3 | Table

This work was supported by a Papes 2 grant from the Oswaldo Cruz Foundation and CNPq (Brazil).

⁺Corresponding author. Fax: +55-21-2270.9997. E-mail: cpessola@ioc.fiocruz.br

Received 14 November 2000

Accepted 3 May 2001