AbstractNumerous alterations in CD8+T cells contribute to impaired immune responses in elderly individuals. However, the discrimination between cell‐intrinsic dysfunctions and microenvironmental changes is challenging. TCR transgenic OT‐I mice are utilized to investigate CD8+T‐cell immunity, but their immunodeficient phenotype hampers their use especially in aging. Here, we demonstrate that using a heterozygous OT‐I model minimizes the current limitations and provides a valuable tool to assess antigen‐specific T‐cell responses even at old age. We analyzed phenotypic and functional characteristics of CD8+T cells from OT‐I+/+and OT‐I+/−mice to prove the applicability of the heterozygous system. Our data reveal that OVA‐activated CD8+T cells from adult OT‐I+/−mice proliferate, differentiate, and exert cytolytic activity equally to their homozygous counterparts. Moreover, common age‐related alterations in CD8+T cells, including naive T‐cell deterioration and decreased proliferative capacity, also occur in elderly OT‐I+/−mice, indicating the wide range of applications for in vivo and in vitro aging studies. We used the OT‐I+/−model to investigate cell‐intrinsic alterations affecting the cytotoxic behavior of aged CD8+T cells after antigen‐specific in vitro activation. Time‐resolved analysis of antigen‐directed target cell lysis confirmed previous observations that the cytotoxic capacity of CD8+T cells increases with age. Surprisingly, detailed single cell analysis revealed that transcriptional upregulation of perforin in aged CD8+T cells shifts the mode of target cell death from granzyme‐mediated apoptosis to rapid induction of necrosis. This unexpected capability might be beneficial or detrimental for the aging host and requires detailed evaluation.