Quantify the age-related decline of CD8⁺ T cell response to a novel antigen

Introduction/Rationale: Aging impairs T cell–mediated immunity. Mechanistically, this decline is thought to result from two major factors: (1) cumulative antigenic stimulation over time driving T cell senescence, and (2) loss of appropriate antigen-specific TCRs due to insufficient usage. Although age-related contraction of the TCR repertoire has been reported, it remains unclear which specific TCRs are lost and how this loss affects responses to novel antigens. To address this question, we examined how aging alters the size, clonality, and functional quality of antigen-specific CD8⁺ T cell responses.

Methods: Young (2–3 months) and aged (20–24 months) C57BL/6J mice were infected with Listeria monocytogenes expressing chicken ovalbumin (OVA). OVA₂₅₇–₂₆₄ (SIINFEKL)-specific CD8⁺ T cells were quantified using tetramer staining and profiled by single-cell TCR sequencing. TCR diversity was evaluated using metrics of clonal richness and evenness, while TCR functional quality was assessed based on antigen-binding affinity and downstream signaling capacity.

Results: Aged mice exhibited significantly delayed clearance of Listeria compared to young mice. OVA-specific CD8⁺ T cells were markedly reduced in aged mice before infection, at peak response, and during the memory phase. Single-cell TCR sequencing revealed a substantial reduction in OVA-specific TCR repertoire diversity in aged mice, with a 57% decrease in effector cells and an 87% decrease in memory cells. Moreover, clonally expanded OVA-specific CD8⁺ T cells from aged mice showed impaired tetramer binding and diminished signaling following antigen engagement.

Conclusion: Our findings demonstrate that aging profoundly reduces CD8⁺ T cell responses to a novel antigen due to loss of both TCR quantity and functional quality. This deterioration of antigen-specific immunity may contribute to increased infection susceptibility and diminished vaccine efficacy in older individuals.