Demonstration of a circulating suppressor factor of thymocyte proliferation during endotoxin fever in humans

Sub-pyrogenic levels of human leukocytic pyrogen (LP) have been shown to enhance phytohemagglutinin-induced murine thymocyte proliferation. It was concluded that LP is similar to lymphocyte-activating factor (LAF). Since endotoxin stimulates the production of LP and LAF, we attempted to employ in vitro thymocyte proliferation to detect circulating LP/LAF in 12 normal human subjects during experimental endotoxin fever. Sera obtained before and during fever were first mixed with an immunoadsorbent that binds human LP/LAF, and then the dissociated material was added to thymocyte cultures. Material derived from sera obtained during the maximum fever (3 to 4 hr after endotoxin) was markedly suppressive for thymocyte proliferation in vitro. The appearance of this suppressive effect correlated with the profound lymphopenia observed in the subjects. This suppressor factor(s) was nondialyzable and was destroyed at 70 degrees C, and its suppressive effects inhibited the lymphocyte-activating property of LP/LAF. In addition, the suppression of PHA responses did not appear to be modulated by prostaglandin synthesis. The results demonstrate that a factor circulates during endotoxin fever in humans that suppresses in vitro thymocyte proliferation.