Unconjugated bilirubin inhibits in vitro cytotoxic T lymphocyte activity of human lymphocytes.

Septic complications have been major problems in the management of patients with obstructive jaundice and neonatal jaundice. This study investigates effects bilirubin on human T lymphocyte responses against allogeneic mixed lymphocyte reaction. In vitro exposure of human peripheral blood mononuclear cells (PBMNC) with unconjugated bilirubin at pathological levels (6 to 12 mg/dl) did not alter the subsets of CD3, CD4, CD8, CD14, CD19 and CD56 positive populations, or expression of costimulatory surface molecules CD2, CD3, CD4 and CD8. Further incubation of bilirubin-treated PBMNC with irradiated B lymphoid Raji cells after removal of the extracellular bilirubin resulted in a dose-dependent decrease of cytotoxic T lymphocyte (CTL) activity, DNA synthesis, and expression of Tac antigen (CD25) and transferrin receptor (CD71). However, no significant change of interleukin-2 (IL-2) production was observed after this incubation between bilirubin-treated and -untreated PBMNC. These results suggest that bilirubin inhibits the induction of CTL activity, and this defect may result from the impaired responsiveness against IL-2. These observations may help explain the increased infection observed in hyperbilirubinemic patients.