Peripheral blood lymphocytes of critically ill patients show signs of late stage apoptosis

Among the immunological events, apoptosis plays an important role in sepsis. Inhibition of apoptosis in mouse models of sepsis improved survival [1]. Phosphatidyl serin externalisation has been detected in peripheral blood leukocytes of patients suffering from sepsis as an early stage of apoptosis [2, 3]. It has not been shown if they undergo complete apoptosis. Therefore, the aim of the current study was to examine if circulating lymphocytes of septic patients display DNA degradation as a sign of late apoptosis.

Isolated mononuclear cells of 11 critically ill patients (three with severe sepsis) were compared with eight healthy controls. Phosphatydil serin externalisation was evaluated by annexin V binding. Necrotic cells were excluded by propidium iodine stain. DNA fragmentation was detected by TUNEL staining using flow cytometry.

In critically ill patients annexin binding was increased to 12.1 ± 7.5% compared with a basal population of 3.8 ± 0.9% in healthy controls (P < 0.05). The TUNEL-positive population increased from 0.5 ± 0.1% to 2.3 ± 1.5% in critically ill patients (P < 0.05).

The study demonstrates that mononuclear cells of critically ill patients show signs of early apoptosis and to a lesser degree also signs of DNA fragmentation. This may be explained by rapid clearing of phosphatydil serin expressing cells from the circulation by phagocytosis.