IL-15/NKG2D signaling-mediated activation of CD8+ T cell bypass pathways in sepsis-induced immune injury.

BACKGROUND: Sepsis is a systemic inflammatory response syndrome (SIRS) caused by infection, accompanied by immune dysregulation, leading to a high mortality rate. Memory CD8+ T cells, as core effector cells for long-term immune surveillance, function through the classical activation pathway, mediated by the T cell receptor (TCR)'s recognition of antigens. This study aimed to explore the function and mechanism of the interleukin (IL)-15/NKG2D signaling in mediating bypass initiation of memory CD8+ T cells in sepsis-induced immune injury. METHODS: A mouse sepsis model was set up using cecum ligation and puncture (CLP), and the progression of sepsis was observed through IL-15 intervention. Serum inflammatory factors and organ damage markers [aspartate aminotransferase (AST), alanine aminotransferase (ALT), lactate dehydrogenase (LDH), blood urea nitrogen (BUN), creatinine (CREA)] were detected by ELISA and biochemical analysis. HE staining was employed to assess pathological alterations in the spleen and multiple organ tissues. Immunohistochemical analysis was performed to detect CD8 expression in the liver, lung, kidney, and spleen. Flow cytometry was used to analyze the expression of NKG2D, initiation markers (CD38/HLA-DR), and effector molecules [Granzyme B, interferon-gamma (IFN-γ), tumor necrosis factor-alpha (TNF-α)] in splenic CD44high memory CD8+ T cells. In in vitro experiments, isolated CD3+CD8+CD44high T cells were stimulated with IL-15 and IL-15+ anti-NKG2D antibody, and their activation status and cytotoxicity to NKG2D-sensitive YAC-1 target cells were detected. RESULTS: Exogenous IL-15 significantly aggravated multi-organ damage (elevated AST, ALT, LDH, BUN, and CREA) and systemic inflammatory response in septic mice. Histopathological examination revealed splenic lymphocyte necrosis and worsening liver and kidney damage. Exogenous administration of IL-15 significantly enhances the recruitment and infiltration of CD8+ T cells in major visceral organs, including the liver, spleen, kidneys, and lungs. Mechanistic studies found that IL-15 significantly promoted NKG2D expression on splenic CD44high memory CD8+ T cells and increased the levels of initiation markers and effector molecules (Granzyme B, IFN-γ, and TNF-α) in NKG2D+ T cells. In vitro experiments further confirmed that IL-15-induced T cell activation (increased CD38+/HLA-DR+ expression) and enhanced killing function against YAC-1 cells could be markedly inhibited by anti-NKG2D antibody. CONCLUSIONS: IL-15 activates a TCR-independent bypass activation pathway by inducing NKG2D expression on memory CD8+ T cells, thereby enhancing cytotoxicity and exacerbating sepsis-induced multi-organ immune damage. This study suggests that the IL-15/NKG2D signaling may be a promising molecular target for sepsis-associated immunopathological damage.