INTRACELLULAR ACCUMULATION OF UNCONJUGATED BILIRUBIN INHIBITS PHYTOHEMAGGLUTININ-INDUCED PROLIFERATION AND INTERLEUKIN-2 PRODUCTION OF HUMAN-LYMPHOCYTES

Decreased immune responses have been documented in hyperbilirubinemic patients. This study investigates the effects of intracellular bilirub in accumulation on lymphoproliferative response to phytohemagglutinin A (PKA). Human peripheral blood mononuclear cells (PBMNC) were preincu bated with unconjugated bilirubin dissolved in bovine albumin solution at pathological levels seen in clinical hyperbilirubinemia (0-12 mg/d l), washed, and further cultured with PHA. DNA synthesis was measured by [H-3]thymidine uptake. Interleukin-2 (IL-2) activity was determined by the CTLL proliferation assay. The amount of intracellular bilirubi n and expression of cell surface antigens were analyzed by flow cytome try. In vitro exposure of normal PBMNC to bilirubin resulted in the ac cumulation of intracellular bilirubin and a decrease in DNA synthesis after PHA stimulation in a time- and dose-dependent manner. Addition o f autologous untreated monocytes could not correct the decreased DNA s ynthesis of bilirubin-treated lymphocytes. IL-2 production by bilirubi n-treated PBMNC after PHA stimulation was significantly decreased comp ared to bilirubin-untreated PBMNC. However, addition of exogenous IL-2 to pretreated PBMNC could not correct the decreased DNA synthesis. Ex pression of Tac antigen and transferrin receptor on bilirubin-treated lymphocytes after PHA stimulation was not significantly different from bilirubin-untreated cells. These results suggest that decreased PHA-i nduced T-lymphocyte proliferation following bilirubin-pretreatment may result from impairment of proliferation at a step beyond transferrin receptor expression. These observations may help explain the increased susceptibility to infection often observed in hyperbilirubinemic pati ents.