Lymphopenia-induced homeostatic proliferation of CD8⁺ T cells is a mechanism for effective allogeneic skin graft rejection following burn injury

Burn patients are immunocompromised yet paradoxically are able to effectively reject allogeneic skin grafts. Failure to close a massive burn wound leads to sepsis and multiple system organ failure. Immune suppression early (3 days) after burn injury is associated with glucocorticoid-mediated T cell apoptosis and anti-inflammatory cytokine responses. Using a mouse model of burn injury, we show CD8⁺ T cell hyperresponsiveness late (14 days) after burn injury. This is associated with a CD8⁺ T cell pro- and anti-inflammatory cytokine secretion profile, peripheral lymphopenia, and accumulation of a rapidly cycling, hyperresponsive memory-like CD8 ⁺CD44⁺ IL-7R^- T cells which do not require costimulation for effective Ag response. Adoptive transfer of allospecific CD8⁺ T cells purified 14 days postburn results in enhanced allogeneic skin graft rejection in unburned recipient mice. Chemical blockade of glucocorticoid-induced lymphocyte apoptosis early after burn injury abolishes both the late homeostatic accumulation of CD8⁺ memory-like T cells and the associated enhanced proinflammatory CD8⁺ T cell response, but not the late enhanced CD8⁺ anti-inflammatory response. These data suggest a mechanism for the dynamic CD8⁺ T cell response following injury involving an interaction between activation, apoptosis, and cellular regeneration with broad clinical implications for allogeneic skin grafting and sepsis.