Transforming growth factor-β decreases survival of Mycobacterium bovis-activated T cells

Background. A comprehensive understanding of the immune response induced by Mycobacterium bovis Bacille Calmette-Guérin in activation of protective T cells against tuberculosis is important to develop effective therapies to combat this disease. In this study, our experiments were designed to determine effects of transforming growth factor (TGF)-β on M. bovis-induced T-cell activation and survival. Methods. Fluorescence-activated cell sorter (FACS) analysis was used for detection of apo-ptotic cells by three different methods: 1) scattered light change during early phase of apoptosis; 2) detection of hypodiploid DNA, or 3) terminal deoxynucleotidyl transferase dUTP nick end-labeling (TUNEL) technique. Quantification of positively stained population was based on samples stained with isotype control antibodies analyzed on a FACScan. Results. TGF-β added at initiation of culture did not alter percentage of viable cells. By contrast, TGF-β added 72 h post-activation decreased percentage of viable cells. This effect was statistically significant (p <0.05). Furthermore, addition of anti-TGF-β MoAb together with TGF-β abolished the ability of this cytokine to decrease survival in post-activated human T cells. Role of TGF-β on post-activated human T cells was further confirmed by staining apoptotic nuclei with propidium iodide, which detects late events of apoptosis, and by DNA fragmentation determined using TUNEL assay. Interestingly, TGF-β did not promote Fas-mediated killing. Finally, TGF-β increased apoptosis of CD4⁺ T cells after mycobacterial stimulation. Conclusions. This study indicated an important role for TGF-β in suppression of protective immune response against M. bovis by promoting elimination of post-activated T cells. Furthermore, results showed that TGF-β had no direct effect on M. bovis-induced up-regulation of Fas (CD95).