Down-regulation of the immune response in aging individuals puts this population at a potential risk against infectious agents. In-depth studies conducted in humans and mouse models have demonstrated that with increasing age, the T cell immune response against pathogens is compromised and response to vaccinations is subdued. In the present study, using a mouse model, we demonstrate that older animals exhibit greater susceptibility to Encephalitozoon cuniculi infection, and their ability to evoke an Ag-specific T cell response at the gut mucosal site is reduced. The dampening of T cell immunity was due to the defective priming by the dendritic cells (DC) isolated from the mucosal tissues of aging animals. When primed with DC from younger mice, T cells from older animals were able to exhibit an optimal Ag-specific response. The functional defect in DC from older mice can be attributed to a large extent to reduced IL-15 message in these cells, which can be reversed by addition of exogenous IL-15 to the cultures. IL-15 treatment led to optimal expression of costimulatory molecules (CD80 and CD86) on the surface of older DC and restored their ability to prime a T cell response against the pathogen. To our knowledge, this is the first report which demonstrates the inability of the DC population from aging animals to prime a robust T cell response against an infectious agent. Moreover, the observation that IL-15 treatment can reverse this defect has far-reaching implications in developing strategies to increase vaccination protocols for aging populations.