The Mantoux Test: a model for tracking T cell differentiation in the skin during secondary immune challenge

In this study, we hypothesized that T cells differentiate in the skin as a consequence of localised proliferation at the site of secondary immune challenge in humans. One consequence of cellular proliferation is telomere shortening, which can be counteracted by telomerase. It is recognised that telomerase is up-regulated in activated T cells that are resident in lymphoid tissues or the blood. We have used the Mantoux Test (MT), a secondary immune response, to determine the degree of cellular differentiation in the skin and its consequences during an episode of cutaneous inflammation. We have established a skin suction blister technique to isolate lymphocytes from MTs up to 19 days after induction for flow cytometric analysis, heteroduplex analysis, cell culture and measurement of telomerase activity (TRAP assay). Skin biopsies were also collected for immunohistochemical staining. Marked antigen-specific CD4+ T cell expansion with preserved clonality was observed in the skin during the MT. In contrast to recent murine studies, we found that this expansion was mediated in part by the extensive proliferation of CD4+ T cells in the skin. This was associated with substantial telomeric shortening in the antigen-specific CD4+ T cells in the skin, indicating accelerated cellular differentiation. Despite the development of a highly differentiated population of CD4+ T cells during the MT, there appeared to be no increase in the proportion of anergic / suppressive CD4+CD25+ T cells, which might mediate the down-regulation of inflammation during resolution. The erosion of telomeres appeared to be mediated by the reversible inhibition of telomerase by type 1 interferons in vivo. This reversible inhibition was distinct from the irreversible down-regulation of telomerase that was observed when MT skin T cells were repeatedly activated in vitro culminating in cell senescence. The inhibition of telomerase activity in T cells during secondary immune responses in the skin represents a possible control checkpoint that may limit uncontrolled T cell expansion in non-lymphoid tissues in vivo