Sean F. Monaghan, Rajan K. Thakkar, Mai L. Tran, Xin Huang, William G. Cioffi, Alfred Ayala, Daithi S. Heffernan

Programmed death 1 (PD-1) is an inhibitor protein receptor for the immune system and has been shown to be upregulated in animal models of critical illness as well as after trauma and in burn victims in humans. It is believed that PD-1 may play a role in the immune dysfunction seen in surgical critical illness. However, although prior studies have associated changes in PD-1 expression with altered immune cell function, it is not known if a correlation with clinical status exists. We therefore aimed to describe a potential role for PD-1 in the immune dysfunction seen in critically ill trauma and surgical patients. This is an observational cohort study. Acute Physiology and Chronic Health Evaluation II (APACHE II) scores were calculated on critically ill and injured trauma and surgical intensive care unit patients from a tertiary care/level I trauma center. Blood was drawn within 24 h of establishment of diagnosis and admission to the intensive care unit to measure circulating cytokine levels, as well as PD-1 expression on circulating cells. Main outcome measures included PD-1 expression on leukocytes and the relationship to physiological dysfunction (APACHE II) as well as the correlation of PD-1 expression and interleukin 10 levels among patients with severe physiological dysfunction. Samples were collected from 90 critically ill surgical patients. Among patients with severe physiological dysfunction (APACHE II >20), there were increased numbers of granulocytes (median, 144 vs. 90 cells/μL; P = 0.037) and monocytes (median, 12 vs. 6 cells/μL; P = 0.022) with PD-1 expression. In addition, among patients with an APACHE II score of greater than 20, there was a larger percentage of CD3 cells (44% vs. 29%; P = 0.015) expressing PD-1. When only patients with an APACHE II score greater than 20 were assessed, PD-1 expression on monocytes correlated positively with interleukin levels in the serum (r = 0.525, P = 0.05). Variability in the expression of PD-1 on leukocytes in critical surgical illness correlates with physiological dysfunction and suggests that PD-1 may be a valuable tool in the assessment of immune dysfunction following trauma or severe surgical insult.