The Role of PD-1/PD-L1 and IL-7 in Lymphocyte Dynamics and Sepsis Progression: A Biomarker Study in Critically Ill Patients

Sepsis pathophysiology involves a dysregulated immune response to infection, excessive inflammation, and immune paralysis. This study explores the relationships between cell death biomarkers (serum-soluble levels of programmed cell death protein 1 (PD-1), programmed death ligand 1 (PD-L1), and interleukin-7 (IL-7)) and the percentages of various lymphocyte subsets in relation to the severity and progression of sepsis. This prospective, observational study included 87 critically ill patients. We monitored parameters on days 1 (sepsis was diagnosed according to the Sepsis-3 Consensus) and 5. We established an IL-7 cutoff value of 1.94 pg/mL by comparing levels between a healthy control group and patients with sepsis (<i>p</i> < 0.0001). Lymphopenia was observed in all patients, with negative correlations between helper T lymphocytes and cytotoxic and B lymphocytes, and positive correlations involving cytotoxic lymphocytes across all groups. We found correlations between PD-1/PD-L1 and lymphocyte subsets. IL-7 showed a statistical correlation with PD-1 in non-survivors. Assessing lymphocyte levels shows potential as a biomarker for evaluating the progression of sepsis. Monitoring IL-7 levels could help assess survival, as low levels are associated with higher mortality risk. Monitoring IL-7 levels could help assess survival, as low levels are associated with higher mortality risk. Elevated PD-1/PD-L1 expression impairs costimulatory signalling, reducing T cell responses and lymphopenia, which increases the risk of nosocomial infections.