A mechanism of quinolinic acid formation by brain in inflammatory neurological disease: Attenuation of synthesis from L-tryptophan by 6-chlorotryptophan and 4-chloro-3-hydroxyanthranilate

Quinolinic acid (QUIN), kynurenic acid (KYNA) and L-kynurenine (L-KYN) are neuroactive kynurenine pathway metabolites that accumulate in inflammatory neurological diseases. These increases were attributed to the induction of indoleamine-2,3-dioxygenase (IDO), the enzyme that converts L-tryptophan into L-KYN. Direct conversion of L-tryptophan into QUIN by brain tissue occurs in conditions of CNS inflammation, but not by normal brain tissue. To investigate whether increased activity of enzymes distal to IDO may determine L-KYN conversion to QUIN, rhesus macaques were inoculated with poliovirus directly into the spinal cord, as a model of focal inflammatory neurological disease (FASEB J. 6, 2977-2989, 1992). Induction of spinal cord IDO (35.9-fold) accompanied smaller, but proportional increases in kynurenine-3-hydroxylase (2.4-fold) and kynureninase (2.3-fold) activities, which were correlated to CSF and tissue QUIN levels, as well as to measures of inflammatory lesions. 3-Hydroxyanthranilate-3,4-dioxygenase activity was unchanged. Cerebrospinal fluid KYNA levels increased in proportion to both IDO activity and L-KYN accumulation, though kynurenine aminotransferase activity was unaffected. Cerebrospinal fluid neopterin, a marker of macrophage and immune activation, accumulated in proportion to the responsive enzymes and metabolites. The cell types involved in producing QUIN were investigated in vitro. Human foetal brain cultures consisting of astrocytes and neurons converted large quantities of 13C6L-tryptophan into L-KYN when stimulated by gamma-interferon, but very little 13C6QUIN was formed unless macrophages (THP-1 cells) were first added to the cultures (to model a key component of brain inflammation). 13C6L-Tryptophan was converted into 13C6QUIN by either gamma-interferon stimulated macrophages, or following intracisternal administration into poliovirus-infected macaques. Inhibitors of the kynurenine pathway, 6-chlorotryptophan and 4-chloro-3-hydroxyanthranilic acid, attenuated 13C6QUIN formation by macrophages, and when co-infused with 13C6L-tryptophan into poliovirus-infected macaques. These results suggest roles for increased activities of IDO, kynurenine-3-hydroxylase and kynureninase in accelerating the synthesis of QUIN, L-KYN and KYNA in conditions of brain inflammation. Macrophage infiltrates, and perhaps microglia, are important sources of QUIN, whereas constitutive brain cells and macrophages are sources of L-KYN. Drugs that inhibit kynurenine pathway enzymes attenuate QUIN formation in the CNS, and provide tools to examine the consequences of reduced QUIN accumulation.