Increased apoptosis, curtailed expansion and incomplete differentiation of CD8+ T cells combine to decrease clearance of L. monocytogenes in old mice

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Abstract
Aging is accompanied by altered immunity, resulting in a variable state of poorly understood immunodeficiency. While both the numbers and the functionality of naïve T cells are decreased by aging, the impact of these changes upon immune defense against bacterial pathogens in vivo remains understudied. Using a model of Listeria monocytogenes (Lm), where the primary CD8+ T‐cell response is critically important for immune defense, we show that C57BL/6 (B6) mice exhibit an age‐dependent reduction in survival, with delayed bacterial clearance in old animals. Kinetic analysis of antigen‐specific CD8+ T‐cell expansion showed that CD8+ effectors begin dividing at the same time in old and adult mice, but that the proliferative burst remained incomplete during discrete windows of time and was coupled with increased effector apoptosis in old mice. Further, antilisterial CD8+ T cells in old mice showed altered expression of key phenotypic and effector molecules and diminished polyfunctionality, measured by the ability to simultaneously produce multiple effector molecules. These results suggest that defects in functional maturation of CD8+ cells in aged mice, compounded by (or perhaps coupled to) their reduced expansion in response to infection, yield effector CD8+ T‐cell populations insufficient in size and capability to effectively clear newly encountered intracellular pathogens.