HUMAN MACROPHAGES CONVERT L-TRYPTOPHAN INTO THE NEUROTOXIN QUINOLINIC ACID

Substantial increases in the concentrations of the excitotoxin and N-methyl-D-aspartate-receptor agonist quinolinic acid (QUIN) occur in human patients and non-human primates with inflammatory diseases. Such increases were postulated to be secondary to induction of indoleamine 2.3-dioxygenase in inflammatory cells, particularly macrophages, by interferon-gamma. To test this hypothesis, human peripheral-blood macrophages were incubated with L-[C-13(6)]tryptophan in the absence or presence of interferon-gamma. [C-13(6)]QUIN was quantified by gas chromatography and electron-capture negative-chemical-ionization mass spectrometry. [C-13(6)]QUIN was detected in the incubation medium of both unstimulated and stimulated cultures. Exposure to interferon-gamma substantially increased the accumulation of [C-13(6)]QUIN in a dose- and time-dependent manner. The QUIN concentrations achieved exceeded those reported in both cerebrospinal fluid and blood of patients and of non-human primates with inflammatory diseases. Macrophages stimulated with interferon-gamma may be an important source of accelerated L-tryptophan conversion into QUIN in inflammatory diseases.