Whilst determination of specific CD4 TEM cell longevity was beyond the scope of this study; they were absent at four months following last detection of viable bacilli, indicating a lifespan of

such as the SLO; according with reports that responses to mycobacteria are initiated in the LN [43] and [44]. Despite their CX 5461 short-lived nature, CD4 TEM cells appear to make a significant contribution to inhibitors protective immunity, as the reduction in bacterial burden was reduced by up to 60% in their absence. CD4 TEM have been reported as important mediators of protection in M. tuberculosis [45] Verteporfin solubility dmso malaria [46] and Leishmania [38], among other infections. We acknowledge, however, that a direct protective, rather than associative role of these cells remains to be shown; but at present, the lack of technologies

to allow the sorting of live T cells based on cytokine production, preclude the TEM adoptive transfer experiments required to definitively demonstrate such a role. It is intriguing to speculate whether at least a proportion of the protection afforded by BCG during childhood is due to persisting bacilli and associated TEM. There is evidence that BCG may persist for many years in humans [37], [47], [48],

[49], [50], [51] and [52] and together with the observed waning of immune responses to BCG through childhood [36]; this may represent gradual clearance of bacilli and associated T cells. Long-term memory, however, is considered dependent on the generation of TCM responses. At present, few reports directly identify an antigen-specific CD4 TCM cells induced in mice by BCG alone [19] and [22]; some describe TCM-like cells after clonal expansion induced by prime-boost vaccination, challenge or reinfection [14], [21] and [53]. In humans, TCM may only appear after contraction of the BCG-specific TEM response [20]. This situation is confounded by our incomplete understanding of TCM cell phenotypes. Conflicting evidence is often published, and there is clearly below substantial plasticity between memory T cell phenotypes (reviewed in [42] and [54]). Unequivocal identification of these cells is also complicated by the weak expression of characterisitic cells markers (e.g. CCR7) and their often mutual expression by the naïve T cell population. ICS by flow cytometry is often used, but has a distinct effector bias relying immediate cytokine production, and so is unlikely optimal for TCM detection [55] and [56]. To circumvent this, we performed class II-peptide tetramer staining, but were unable to detect any CD4+CD62Lhi antigen-spepcific TCM cells.