Characterization of age-associated T cells: cytotoxic CD4+ T cells

Aging profoundly reshapes the T cell compartment. Thymic involution, myeloid skewing of haematopoiesis, and lifelong antigenic exposure collectively reduce the pool of naïve T cells, while promoting the expansion of memory-like populations with altered effector programs. Among these, CD4⁺ cytotoxic T lymphocytes (CD4⁺ CTLs) have emerged as a relevant subset that can exert either protective or pathogenic functions depending on the immune context. However, the signals that govern their differentiation and sustain their accumulation remain poorly defined. This work aimed to elucidate the mechanisms that drive CD4⁺ CTL differentiation and shape their effector functions. Adoptive transfer of CD4⁺ T cells derived from young donors into aged recipients revealed that age-associated extrinsic cues promote their conversion into CD4⁺ CTLs, a process sustained by persistent antigenic stimulation. To investigate the underlying mechanisms, we developed an in vitro model showing that repeated T cell receptor (TCR) engagement together with IL-2 is sufficient to induce CD4⁺ CTLs expressing cytotoxic molecules and exhibiting antigen-specific killing activity. We further show that activation of the cyclic GMP–AMP synthase (cGAS)–stimulator of interferon genes (STING) pathway contributes to the upregulation of CCL5, a key effector chemokine of CD4⁺ CTLs, thereby linking mitochondrial dysfunction and cytosolic DNA sensing to inflammatory features of these cells. Taken together, these findings refine our understanding of the regulatory principles that induce and sustain CD4⁺ CTL differentiation in aging tissues, offering potential strategies to modulate this population and, ultimately, mitigate immune aging and improve healthspan.