It has

been suggested that CD127− Treg and foxp3+ Treg possibly represent different populations [9]. In our study, a correlation between these two Treg subsets was found only in the control group. In a study of HIV infection, the positive correlation between foxp3+CD127− and CD25+CD127− CD4+ T cells found in healthy HIV-negative subjects was not present in the early chronic stage of HIV infection [23]. Together these data indicate that different Treg may contribute in various stages of chronic infections. It has been shown that depletion of CD4+CD25high and CD4+CD25+foxp3+ cells from PBMCs from patients with TB, results in increased production of IFN-γ upon TB stimulation [10, 11, 24], indicating that there is an inverse correlation between Treg and immune check details activation. In contrast, although the immunosuppressive function of Treg was not characterized in our study, we found a positive correlation between the fractions of Treg and activated CD4+ T cells. DC can initiate immune responses and stimulate induction and expansion

of Treg [14]. Absolute numbers of DC have been shown to decrease in patients with Dabrafenib nmr TB compared to healthy controls [17]. Still, although the numbers of pDC and mDC were not estimated, in our study, we did not find any differences in the fraction of DC subsets among the various groups or any correlation between DC and Treg subsets. Altogether, these data suggest that different Treg subsets may have different capability to regulate immune activation and that modulation may be induced by different signals in the various stages of TB infection. As we found gradually higher fractions of CD127− Treg throughout the various stages of TB infection correlating to immune activation, a possible theory is that higher bacterial burden and inflammation

stimulate to increased levels of Treg to balance between anti-TB T cell responses and immune-mediated pathology. In support of this, in a study of macaques, there were increased frequencies of Treg cells in blood as the animals developed disease [25]. An alternative explanation may be that Treg inhibit protective PAK6 Th1 responses facilitating mycobacterial replication and act as a causative factor in the progression to active disease [12]. We found an increase in foxp3+ Treg after preventive anti-TB treatment. Our very limited data demonstrate that this was most dominant in patients converting to QFT negative and with reduced CD8+ T cell activation after treatment, possibly indicating that expansion of this Treg subset contributes to suppression or eradication of TB. Apoptosis of TB reactive T cells may account for the depression of TB-induced T cell responses seen in active TB, but data are conflicting [3, 26]. CD95 (Fas receptor), which upon ligation with Fas ligand induces an apoptotic death signal, was expressed by a higher proportion of CD8+ T cells and a lower proportion of CD4+ T cells in patients with pulmonary TB [3].