# In vivo labeling reveals that degranulation is increased under supraphysiological TCR stimulation, but not infection, in CD8+ T cells from old mice

## TL;DR

Old mice CD8+ T cells show increased degranulation under strong TCR stimulation but not during infection, suggesting age-related changes in immune response.

## Contribution

The study reveals age-specific differences in CD8+ T cell degranulation under different activation conditions in vivo.

## Key findings

- Old mice CD8+ T cells degranulate more under supraphysiological TCR stimulation compared to young mice.
- During infection, degranulation in old mice CD8+ T cells is reduced, matching their lower cytotoxic capacity.
- Age-related increase in CD8+ CD44+ CD62L− cells partially explains the in vitro degranulation differences.

## Abstract

CD8+ T cells exhibit distinct changes with aging, including a diminished naïve cell pool, an expansion of memory and exhausted cells, and altered effector molecule production, altogether leading to increased susceptibility to infection. They have reduced cytotoxicity in vivo, but increased granule content and faster cytotoxic kinetics to target cells in vitro. Whether CD8+ T cells from old mice degranulate when activated in vivo, within the aged environment, is unknown. This study investigates in vitro and in vivo degranulation of CD8+ T cells from young and old mice during supraphysiological aCD3 stimulation and two types of infection. Actively degranulating CD8+ CD44+ T cells were identified by positive labeling after a two-hour exposure to granule-specific fluorescent antibodies (CD107a and CD107b). Surprisingly, CD8+ T cells from old mice challenged with supraphysiological TCR-specific stimulation exhibited higher levels of degranulation as compared to their young counterparts. This effect is more prominent in vitro and can be partially explained by the age-specific increase in CD8+ CD44+ CD62L− cells. However, during microbial exposure or LCMV Armstrong infection, we show that CD8+ CD44+ and antigen-specific T cells from old mice have reduced degranulation, consistent with the diminished cytotoxic capacity. These data highlight the preserved intrinsic cytotoxic capacity of memory CD8+ T cells from old mice and suggest that the aged microenvironment and type of stimulation are contributing factors to the lower degranulation and cytotoxic capacity of these cells. This provides insight into the potential of increasing T cell activation to improve vaccine approaches in the elderly.

The online version contains supplementary material available at 10.1007/s11357-025-01723-5.