CD4 and tuberculosis: In this study we have demonstrated that: (i) active TB induces a large increase in the highly differentiated CD27low subset of Mtb-specific CD4 T cells in the lungs and in peripheral blood; (ii) an increase in the percentages of these cells in the blood during TB is associated with lung tissue destruction and disease severity; (iii) evaluation CD27lowMtb-specific CD4 T cells provides a valuable means to assess TB activity, the degree of Mtb-induced lung destruction and lung tissue repair following TB treatment.