Papel do fator de necrose tumoral α(TNF-α) e seus receptores na modulação da apoptose de macrófagos durante a infecção com Mycobacterium bovis

Abstract

Tuberculosis (TB) is a disease of worldwide importance. The tubercle bacilli enter the organism mainly via respiratory tract and initial interaction in the lungs is with the alveolar macrophages, which serve as host cells. Apoptosis of the infected macrophages constitutes a host defense alternative capable of removing the environment supporting bacterial growth. However, virulent strains of mycobacteria appear to be capable of modulate this process. Some studies have highlighted the importance of tumor necrosis factor-a (TNF-α) in the modulation of apoptosis of infected macrophages. TNF-a exerts its biological activities via two distinct cell surface receptors, TNFR1 and TNFR2, whose extracellular domain can be released by proteolysis forming soluble TNF receptors (sTNFR-1 and sTNFR-2). The signaling through TNFR1 initiates the majority of the biological functions of TNF-a, leading to either survival or cell death, whereas TNFR2 induces survival. The aim of this study was to evaluate the influence of TNF-TNFR signaling in the modulation of apoptosis of alveolar macrophages during early stage of infection by virulent and attenuated strains of Mycobacterium bovis, as well as the possible interference of mycobacteria in this signaling. C57BL/6 mice were intratracheally infected with attenuated (BCG Moreau) or virulent (ATCC19274) strains of M. bovis. After 3 and 7 days of infection, the following parameters were assessed: (1) expression of TNF receptors (TNFR1 and TNFR2) on the surface of alveolar macrophages, (2) expression mRNA for TNF receptors, (3) levels of soluble receptors sTNFR1 e sTNFR2 in BAL and levels of TNF-a in lung (4) frequency of apoptosis alveolar macrophage and (5) number of bacilli in alveolar macrophages. Furthermore, the effect of abrogation of TNF-TNFR1 signaling in the modulation of macrophage apoptosis was assessed in TNFR1 deficient mice infected with M. bovis BCG. A significant increase of apoptosis and high expression of TNFR1 were observed in alveolar macrophages at 3 and 7 days after infection with attenuated M. bovis but only on day 7 in infection with the virulent M. bovis. Low surface expression of TNFR1 and increased levels of sTNFR1 on day 3 after infection by the virulent strain were associated with reduced rates of apoptotic macrophages. In addition, a significant reduction in apoptosis of alveolar macrophages was observed in TNFR1-/- mice at day 3 after BCG infection. These results suggest a potential role of TNFR1 in macrophage apoptosis during infection by mycobacteria. In this context, the shedding of TNFR1 appears to contribute to the modulation of macrophage apoptosis.