Distinct functions of antigen-specific CD4 T cells during murine Mycobacterium tuberculosis infection

The immune response elicited after Mycobacterium tuberculosis (Mtb) infection is critically dependent on CD4 T cells during both acute and chronic infection. How CD4 T-cell responses are maintained throughout infection is not well understood, and evidence from other infection models has suggested that, under conditions of chronic antigen stimulation, T cells can undergo replicative exhaustion. These findings led us to determine whether subpopulations of CD4 T cells existed that displayed markers of terminal differentiation or exhaustion during murine Mtb infection. Analysis of antigen-specific effector CD4 T cells revealed that programmed death-1 (PD-1) and the killer cell lectin-like receptor G1 (KLRG1) delineated subpopulations of T cells. PD-1-expressing CD4 T cells were highly proliferative, whereas KLRG1 cells exhibited a short lifespan and secreted the cytokines IFNγ and TNFα. Adoptive transfer studies demonstrated that proliferating PD-1-positive CD4 T cells differentiated into cytokine-secreting KLRG1-positive T cells, but not vice versa. Thus, proliferating PD-1-positive cells are not exhausted, but appear to be central to maintaining antigen-specific effector T cells during chronic Mtb infection. Our findings suggest that antigen-specific T-cell responses are maintained during chronic mycobacterial infection through the continual production of terminal effector cells from a proliferating precursor population.