A murine model of the eosinophilia-myalgia syndrome induced by 1,1'-ethylidenebis (L-tryptophan)

The eosinophilia-myalgia syndrome (EMS) is a recently described disease that has been associated with the ingestion of L-tryptophan containing trace amounts of several impurities. The first such contaminant to be identified and linked epidemiologically to the EMS epidemic was 1,1'-ethylidenebis(L-tryptophan) (EBT), but its role in the etiology and pathogenesis of the syndrome has been controversial. We report the development of inflammation and fibrosis affecting the dermis and subcutis, including the fascia and perimyseal tissues, after the daily intraperitoneal administration of EBT to female C57BL/6 mice. Such changes are accompanied by increased numbers of mast cells, many of which appear to be degranulating. Plasma levels of quinolinic acid, a metabolic product of L-tryptophan via the kynurenine pathway, are reduced initially, and then become elevated when inflammation and fibrosis are more pronounced. The nature and location of the inflammatory cell infiltrate and fibrosis, as well as the presence of mast cells and alterations of L-tryptophan metabolism, are consistent with findings reported in patients with EMS. This murine model suggests that EBT may have been one of the mediators of EMS and should facilitate studies of the pathogenesis of EMS.

Cytopathologic and neurochemical correlates of progression to motor/cognitive impairment in SIV-infected rhesus monkeys

Neurochemical, pathologic, virologic, and histochemical correlates of simian immunodeficiency virus (SIV)-associated central nervous system (CNS) dysfunction were assessed serially or at necropsy in rhesus monkeys that exhibited motor and cognitive deficits after SIV infection. Some infected monkeys presented with signs of acquired immunodeficiency disease (AIDS) at the time of sacrifice. Seven of eight animals exhibited motor skill impairment which was associated with elevated quinolinic acid in cerebrospinal fluid (CSF). Examination of the brains revealed diffuse increases in glial fibrillary acidic protein immunoreactivity in cerebral cortex in all animals, regardless of evidence of immunodeficiency disease. Reactive astrogliosis preceded or was coincident with the onset of neuropsychological impairments. Virus rescue from CSF of six of eight infected animals showed that one of three animals with AIDS and none of three animals without AIDS at necropsy had virus rescue-positive CSF. Multinucleated giant cells were seen in the brain of only one animal with end-stage AIDS and high systemic virus burden at death. Neither systemic nor CNS virus burden was associated with the onset of CNS dysfunction. SIV-associated motor/cognitive impairment is associated with subtle, widespread changes in CNS cytology and neurochemistry, rather than with large increases in brain virus burden or widespread virus-associated brain lesions.

Kynurenine pathway metabolites in cerebrospinal fluid and serum in complex partial seizures

The kynurenine pathway metabolites, quinolinic acid (QUIN) and L-kynurenine are convulsants, whereas kynurenic acid (KYNA) is an antagonist of excitatory amino acid receptors. Imbalances in the concentrations of these metabolites have been implicated in the etiology of human seizure disorders. In the present study, L-kynurenine and QUIN concentrations in both cerebrospinal fluid (CSF) and serum were reduced in patients with intractable complex partial seizures (CPS) in both the postictal period (15-75 min after a seizure) and the interictal period (absence of seizure for > 24 h) as compared with neurologically normal control subjects. Linear regression analyses and analysis of covariance showed that the reductions in serum QUIN and L-kynurenine were correlated to blood antiepileptic medication. L-Tryptophan (L-TRP) levels also tended to be lower in both CSF and serum of the seizure patients. CSF KYNA and serum 3-hydroxykynurenine concentrations were not affected in seizure patients, whereas serum levels of KYNA were reduced. 3-Hydroxykynurenine was not detected in the CSF of either control or seizure patients. The results do not support a role for a generalized reduction in KYNA concentrations or an increased ratio of QUIN:KYNA, or increases in CSF L-kynurenine in initiation and maintenance of intractable CPS humans.

Sensorineural hearing loss from quinolinic acid: a neurotoxin in middle ear effusions

Quinolinic acid (QUIN) is an endogenous metabolite that exerts a neurotoxic effect by binding to specific neuronal receptors. Studies involving a broad spectrum of infectious and inflammatory central nervous system diseases have suggested a role for QUIN in causing neuronal injury. Since there is evidence for presence of the QUIN receptor in mammalian cochleas, QUIN was measured in middle ear effusions (MEEs). Gas chromatography/mass spectrometry detected QUIN in each of 65 diluted human MEEs, with a mean of 482 +/- 75 (SEM) nmol/L and a range from 15 to 2667 nmol/L. QUIN was also detected in each of 197 chinchilla MEEs from five different models of otitis media, with a mean of 10.6 +/- 1.3 (SEM) mumol/L and a range from 0.23 to 146.0 mumol/L (corrected for dilution). To determine whether QUIN causes sensorineural hearing loss (SNHL), QUIN solutions were placed on round window membranes (RWM) for 20 to 240 minutes, in 20 chinchillas. SNHL was detected by electrocochleography in QUIN-exposed animals, but not in saline controls. We conclude that QUIN is present in MEEs and that QUIN in the middle ear has the potential to cross the RWM and cause sensorineural hearing loss, possibly by binding to specific neuronal receptors in mammalian cochleas.

Increased levels of the excitotoxin quinolinic acid in spinal cord following contusion injury

Products of inflammatory phagocytes are potential contributors to secondary pathology following spinal cord trauma. In the present study we quantified the levels of the neurotoxin and product of activated macrophages, quinolinic acid (QUIN), in the lower thoracic spinal cord of adult guinea pigs 5 days after brief compression injury. At the injured site (T13), elevations in tissue QUIN levels (> 10-fold) accompanied proportional increases in the activity of indoleamine-2,3 dioxygenase (> 2-fold) and the concentrations of L-kynurenine (> 2.5-fold). In contrast, no significant changes occurred in two uninjured regions examined compared to controls, namely cervical spinal cord (C2) and the somatosensory cortex. Further studies of QUIN as a potential contributor to spinal cord injury are warranted.

Quinolinic acid in the cerebrospinal fluid of children with symptomatic human immunodeficiency virus type 1 disease: relationships to clinical status and therapeutic response

Quinolinic acid (QUIN) is a neurotoxin implicated in the neurologic deficits associated with human immunodeficiency virus type 1 (HIV-1) infection. Forty children with symptomatic HIV-1 disease had elevated (P < .001) cerebrospinal fluid (CSF) QUIN levels (55.8 +/- 8.9 nM) compared with controls (14.9 +/- 3.0 nM). Age-adjusted CSF QUIN concentrations in HIV-1-infected children were predicted by the general index of mental abilities (GIMA, from an age-appropriate intelligence test; r = -0.45, P < .01). Zidovudine therapy reduced CSF QUIN from 64.1 +/- 16.3 to 19.7 +/- 5.2 nM (P < .01; N = 16) and increased GIMA from 76.8 +/- 5.2 to 87.2 +/- 6.3 (P < .001). Encephalopathic HIV-1-infected patients had higher CSF QUIN levels than patients without encephalopathy (79.6 +/- 16.1 vs. 32.7 +/- 6.7 nM, P < .01). CSF QUIN concentrations were also higher (P < .001) in patients who died < or = 3 years after their baseline assessment, compared with those who were still alive. These results warrant further investigation of CSF QUIN in HIV-infected children as a mediator of neurologic dysfunction and a supplemental marker of neurologic disease, particularly when combined with measures of neurocognitive functioning.

Kynurenine pathway enzymes in brain: responses to ischemic brain injury versus systemic immune activation

Accumulation of L-kynurenine and quinolinic acid (QUIN) in the brain occurs after either ischemic brain injury or after systemic administration of pokeweed mitogen. Although conversion of L-[13C6]tryptophan to [13C6]-QUIN has not been demonstrated in brain either from normal gerbils or from gerbils given pokeweed mitogen, direct conversion in brain tissue does occur 4 days after transient cerebral ischemia. Increased activities of enzymes distal to indoleamine-2,3-dioxygenase may determine whether L-kynurenine is converted to QUIN. One day after 10 min of cerebral ischemia, the activities of kynureninase and 3-hydroxy-3,4-dioxygenase were increased in the hippocampus, but local QUIN levels and the activities of the indoleamine-2,3-dioxygenase and kynurenine-3-hydroxylase were unchanged. By days 2 and 4 after ischemia, however, the activities of all these enzymes in the hippocampus as well as QUIN levels were significantly increased. Kynurenine aminotransferase activity in the hippocampus was unchanged on days 1 and 2 after ischemia but was decreased on day 4, at a time when local kynurenic acid levels were unchanged. A putative precursor of QUIN, [13C6]anthranilic acid, was not converted to [13C6]QUIN in the hippocampus of either normal or 4-day post-ischemic gerbils. Gerbil macrophages stimulated by endotoxin in vitro converted L-[13C6]tryptophan to [13C6]QUIN. Kinetic analysis of kynurenine-3-hydroxylase activity in the cerebral cortex of postischemic gerbils showed that Vmax increased, without changes in Km. Systemic administration of pokeweed mitogen increased indoleamine-2,3-dioxygenase and kynureninase activities in the brain without significant changes in kynurenine-3-hydroxylase or 3-hydroxyanthranilate-3,4-dioxygenase activities. Increases in kynurenine-3-hydroxylase activity, in conjunction with induction of indoleamine-2,3-dioxygenase, kynureninase, and 3-hydroxyanthranilate-3,4-dioxygenase in macrophage infiltrates at the site of brain injury, may explain the ability of postischemic hippocampus to convert L-[13C6]tryptophan to [13C6]QUIN.

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 [13C6]L-tryptophan into L-KYN when stimulated by gamma-interferon, but very little [13C6]QUIN was formed unless macrophages (THP-1 cells) were first added to the cultures (to model a key component of brain inflammation). [13C6]L-Tryptophan was converted into [13C6]QUIN 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 [13C6]QUIN formation by macrophages, and when co-infused with [13C6]L-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.

Kynurenine 3-hydroxylase in brain: species activity differences and effect of gerbil cerebral ischemia

We have developed a rapid and highly sensitive assay for brain kynurenine 3-hydroxylase activity. The present study determined some characteristics and species differences of kynurenine 3-hydroxylase activity in brain and evaluated the response of this enzyme to cerebral ischemia. The kynurenine 3-hydroxylase assay is based on the conversion of L-kynurenine to 3-hydroxykynurenine in vitro and the quantification of 3-hydroxykynurenine by high-performance liquid chromatography. Kynurenine 3-hydroxylase activity was detected in human, macaque, rat, mouse, and gerbil brain. Regional gerbil brain activities ranged from 20 to 50 nmol/g/h, while kynurenine 3-hydroxylase activities in other species were one order of magnitude lower. Kynurenine 3-hydroxylase was also detected in lung, kidney, spleen, intestine, and liver of gerbils, with activities larger than in brain. Delayed increases in the activity of kynurenine 3-hydroxylase occur in several brain regions following transient ischemia in gerbils. These changes are particularly marked in regions that showed the most extensive brain damage (hippocampus and striatum). Kynurenine 3-hydroxylase may have an important role in determining the flux of kynurenine in brain.

Quinolinic acid in children with congenital hyperammonemia

Levels of the excitotoxin quinolinic acid (QUIN) were measured in the cerebrospinal fluid of infants and children with congenital hyperammonemia. Twofold to tenfold elevations of QUIN were found in 4 neonates in hyperammonemic coma (QUIN range, 250-990 nM; control mean, 110 +/- 90 nM; p < 0.005). Similar elevations of neopterin were found (range, 24-75 nM; control mean, 9.0 +/- 4.9 nM; p < 0.005). In addition, significant elevations of QUIN were found in 14 older children with congenital hyperammonemia (mean, 50 +/- 20 vs 17 +/- 6 nM; p < 0.05). Neopterin levels were not elevated in these children. The QUIN may originate from an increase in tryptophan transport across the blood-brain barrier or from induction of indolamine-2,3-dioxygenase activity. These findings support a role for QUIN in the neuropathology of congenital hyperammonemia. They also suggest the potential utility of N-methyl-D-aspartate receptor-blocking agents or inhibitors of QUIN synthesis in the treatment of hyperammonemic coma.

Induction of pterin synthesis is not required for cytokine-stimulated tryptophan metabolism

Activation of the immune system which occurs in inflammatory disease leads to parallel increases in pterin synthesis and increased production of neuroactive L-tryptophan metabolites. Several model systems were studied to determine whether pterins, which are cofactors for hydroxylation reactions, could be required in the oxidative kynurenine pathway of L-tryptophan degradation. Treatment of mice with interferon-gamma increased L-tryptophan metabolism without any corresponding change in tissue biopterin concentrations. Cytokine-treated human fibroblasts, macrophages and glioblastoma cells all showed increases in kynurenine production, which were completely independent of pterin synthesis. When pterin synthesis de novo was blocked, either by an inhibitor of GTP cyclohydrolase or because of a genetic deficiency of one of the enzymes of the pathway of pterin biosynthesis, cytokine-stimulated increases in tryptophan metabolism were unaffected. Furthermore, increasing intracellular tetrahydrobiopterin concentrations by treating cells with sepia-pterin also had no effect on markers of tryptophan metabolism. Therefore, both normal and cytokine-stimulated L-tryptophan metabolism appears to be completely independent of pterin biosynthesis.

Effect of antiretroviral therapy on the cerebrospinal fluid of patients seropositive for the human immunodeficiency virus

Elevated levels of beta 2-microglobulin and neopterin in cerebrospinal fluid (CSF) have been associated with neurologic complications of infection with the human immunodeficiency virus (HIV). The effect of zidovudine (ZDV) on these markers was assessed by studying the effect of ZDV treatment duration on CSF levels in a cohort of 145 HIV-positive men who were receiving ZDV. CSF beta 2-microglobulin and neopterin levels were significantly lower in those who had been taking ZDV for an intermediate period of time (46-365 days) than in those who had received ZDV either long term (> 365 days) or short term (1-45 days). CSF quinolinic acid levels were independent of duration of ZDV administration. A second CSF evaluation was available after 1 year for 54 HIV-positive men (19 of whom were also in the first cohort) and 11 HIV-negative controls. Patients who had started ZDV between lumbar punctures showed a significant decrease in CSF beta 2-microglobulin, but in those who had been receiving ZDV for > 1 year beta 2-microglobulin increased (p = 0.001). The effect was not observed with neopterin (p = 0.14). (Quinolinic acid levels were not studied longitudinally.) Finally, we observed that CSF levels of beta 2-microglobulin, neopterin, and quinolinic acid correlated strongly with each other in HIV-positive individuals (r = 0.7, p < 0.0001), even though ZDV might have different effects on these markers. In conclusion, we report that initiation of ZDV therapy is associated with a transient decrease in CSF levels of beta 2-microglobulin and neopterin.(ABSTRACT TRUNCATED AT 250 WORDS)

Virus isolation and quinolinic acid in primary and chronic simian immunodeficiency virus infection

OBJECTIVE

In this 2.5-year study of simian immunodeficiency virus (SIVsm) infection in rhesus monkeys, quinolinic acid (QUIN) levels and virus isolation determinations were made in serial cerebrospinal fluid (CSF) and blood samples to evaluate the relationship between these parameters over the course of infection.

METHODS

Eight rhesus monkeys were inoculated in the saphenous vein with SIVsm. Four animals were maintained as uninoculated controls. CSF and blood samples were obtained every 1-4 weeks over the course of study. SIV isolation was determined in H9 cells for the CSF and in primary rhesus lymphocyte co-cultures for peripheral blood mononuclear cells (PBMC). QUIN was quantitated in CSF and serum by electron-capture negative chemical ionization gas chromatography mass spectrometry.

RESULTS

All SIV-inoculated animals became CSF and PBMC isolation-positive by 1-3 weeks post-inoculation. Control animals remained SIV-negative. One SIV-positive animal was humanely euthanized at 2 weeks post-inoculation. The three SIV-inoculated animals that were CSF isolation-negative after the fifth week post-inoculation maintained CSF QUIN values < 100 nM, remained CSF and PBMC isolation-negative, and clinically healthy in the chronic course of disease. In contrast, the four SIV-inoculated animals that were CSF isolation-positive 6-8 weeks post-inoculation had CSF QUIN levels as high as 153-565 nM during the second month post-inoculation and remained CSF virus isolation-negative, persistently PBMC isolation-positive, and experienced clinical symptoms of SIV in the chronic course of disease. Three of these four animals have succumbed to SIV infection.

DISCUSSION

Initial QUIN responses and viral isolation status in the first month post-inoculation were consistent among SIV-inoculated animals with CSF and serum QUIN values significantly higher than those of controls. A divergence within the SIV-inoculated group of animals became apparent within the second month of primary SIV infection and was maintained throughout the course of infection. Persistent PBMC viral isolation and marked elevations of QUIN were linked to symptomatic disease and a poor prognosis for survival. Predominantly negative PBMC viral isolation and slight, but significant, elevations of QUIN were linked to asymptomatic disease with a favorable prognosis for survival.

A mechanism for increased quinolinic acid formation following acute systemic immune stimulation

Mechanisms for increased levels of quinolinic acid (QUIN) following systemic immune stimulation were investigated. In gerbils, systemic administration of pokeweed mitogen (PWM) increased plasma and cerebrospinal fluid QUIN levels, while plasma kynurenic acid levels were decreased and cerebrospinal fluid kynurenic acid levels were unchanged. PWM also increased the QUIN concentrations of brain and systemic tissues. In slices of spleen, lung, liver, duodenum, and kidney, PWM caused marked increases in [13C6]QUIN formation from L-[13C6]tryptophan (but not from [13C6]anthranilic acid). PWM also increased QUIN excretion in the urine and enhanced the formation and excretion of [13C6]QUIN following an intraperitoneal injection of L-[13C6]tryptophan. Indoleamine-2,3-dioxygenase activity was increased in the brain, kidney, lung, spleen, and duodenum while hepatic L-tryptophan-2,3-dioxygenase activity was reduced, data consistent with in vitro L-kynurenine formation from L-tryptophan. Kynurenine-3-hydroxylase activity was increased in the duodenum, lung, and spleen, but not in the brain, kidney, or liver. Kynureninase activity was increased in the brain, lung, and duodenum, but not in the spleen, kidney, or liver. 3-Hydroxyanthranilate-3,4-dioxygenase activity was unchanged in the brain, lung, and liver. No change in kynurenine aminotransferase activity was observed in the brain or lung, while liver kynurenine aminotransferase activity was reduced. We conclude that increased activities of kynurenine pathway enzymes in various tissues following systemic immune stimulation, in conjunction with macrophage infiltration of the affected tissue, provide a mechanism to account for increased concentrations of QUIN.

Quantitation of human immunodeficiency virus, immune activation factors, and quinolinic acid in AIDS brains

HIV encephalitis is unusual in that neurologic damage occurs in the absence of significant infection of neuronal or glial cells. Because the predominant infected cell in the brain is the macrophage, it has been proposed that release of viral or immune activation factors from macrophages may mediate neurologic damage. Numerous studies have examined the concentration of immune activation factors in the cerebrospinal fluid (CSF), however, there has been no correlation between these CSF measurements and severity of HIV encephalitis (Wiley, C.A., C.L. Achim, R.D. Schrier, M.P. Heyes, J.A. McCutchen, and I. Grant. 1992. AIDS (Phila.). 6:1299-1307. Because CSF measurements may not represent tissue concentrations of these factors, we examined the concentrations of HIV p24, quinolinic acid (QUIN), IL-1, IL-3, IL-6, TNF-alpha, and GMCSF within the brains of 10 AIDS autopsies. Homogenization and extraction of cortical gray, cortical white and deep gray matter showed a good correlation between the amount of HIV gp41 immunostaining and extracted HIV gag protein p24. The concentrations of cytokines were low in the tissue extracts and showed no correlation with severity of HIV encephalitis. Brain extracts from mild cases of HIV encephalitis showed elevated levels of TNF-alpha in deep gray matter, while in more severe cases, elevated TNF-alpha levels were also found within cortical white and cortical gray matter. Brain tissue and CSF QUIN concentrations were substantially increased compared to control values. QUIN concentrations were not correlated with the severity of HIV encephalitis. We conclude that increased tissue levels of TNF-alpha and QUIN may have a role in the etiology of HIV-related neurologic dysfunction.

4-Chloro-3-hydroxyanthranilate, 6-chlorotryptophan and norharmane attenuate quinolinic acid formation by interferon-gamma-stimulated monocytes (THP-1 cells)

Accumulation of quinolinic acid and L-kynurenine occurs in the brain and/or blood following immune activation, and may derive from L-tryptophan following induction of indoleamine 2,3-dioxygenase and other kynurenine-pathway enzymes. In the present study a survey of various cell lines derived from either brain or systemic tissues showed that, while all cells examined responded to interferon-gamma by increased conversion of L-[13C6]tryptophan into L-kynurenine (human: B-lymphocytes, neuroblastoma, glioblastoma, lung, liver, kidney; rat brain: microglia, astrocytes and oligodendrocytes), only macrophage-derived cells (peripheral-blood mononuclear cells; THP-1, U-937) and certain liver cells (SKHep1) synthesized [13C6]quinolinic acid. Tumour necrosis factor-alpha enhanced the effects of interferon-gamma in THP-1 cells. Norharmane, 6-chloro-DL-tryptophan and 4-chloro-3-hydroxyanthranilate attenuated quinolinic acid formation by THP-1 cells with IC50 values of 51 microM, 58 microM and 0.11 microM respectively. Norharmane and 6-chloro-DL-tryptophan attenuated L-kynurenine formation with IC50 values of 43 microM and 51 microM respectively, whereas 4-chloro-3-hydroxyanthranilate had no effect on L-kynurenine accumulation. The reductions in L-kynurenine and quinolinic acid formation are consistent with the reports that norharmane is an inhibitor of indoleamine 2,3-dioxygenase, 6-chloro-DL-tryptophan is metabolized through the kynurenine pathway, and 4-chloro-3-hydroxyanthranilate is an inhibitor of 3-hydroxyanthranilate 3,4-dioxygenase. These results suggest that many tissues may contribute to the production of L-kynurenine following indoleamine 2,3-dioxygenase induction and immune activation. Quinolinic acid may be directly synthesized from L-tryptophan in both macrophages and certain types of liver cells, although uptake of quinolinic acid precursors from blood may contribute to quinolinic acid synthesis in cells that cannot convert L-kynurenine into quinolinic acid.

Mechanism of delayed increases in kynurenine pathway metabolism in damaged brain regions following transient cerebral ischemia

Delayed increases in the levels of an endogenous N-methyl-D-aspartate receptor agonist, quinolinic acid (QUIN), have been demonstrated following transient ischemia in the gerbil and were postulated to be secondary to induction of indoleamine-2,3-dioxygenase (IDO) and other enzymes of the L-tryptophan-kynurenine pathway. In the present study, proportional increases in IDO activity and QUIN concentrations were found 4 days after 10 min of cerebral ischemia, with both responses in hippocampus > striatum > cerebral cortex > thalamus. These increases paralleled the severity of local brain injury and inflammation. IDO activity and QUIN concentrations were unchanged in the cerebellum of postischemic gerbils, which is consistent with the preservation of blood flow and resultant absence of pathology in this region. Blood QUIN and L-kynurenine concentrations were not affected by ischemia. Brain tissue QUIN levels at 4 days postischemia exceeded blood concentrations, minimizing a role for breakdown of the blood-brain barrier. Marked increases in the activity of kynureninase, kynurenine 3-hydroxylase, and 3-hydroxyanthranilate-3,4-dioxygenase were also detected in hippocampus but not in cerebellum on day 4 of recirculation. In vivo synthesis of [13C6]QUIN was demonstrated, using mass spectrometry, in hippocampus but not in cerebellum of 4-day postischemic animals 1 h after intracisternal administration of L-[13C6]tryptophan. However, accumulation of QUIN was demonstrated in both cerebellum and hippocampus of control gerbils following an intracisternal injection of 3-hydroxyanthranilic acid, which verifies the availability of precursor to both regions when administered intracisternally. Notably, although IDO activity and QUIN concentrations were unchanged in the cerebellum of ischemic gerbils, both IDO activity and QUIN content were increased in cerebellum to approximately the same degree as in hippocampus, striatum, cerebral cortex, and thalamus 24 h after immune stimulation by systemic pokeweed mitogen administration, demonstrating that the cerebellum can increase IDO activity and QUIN content in response to immune activation. No changes in kynurenic acid concentrations in either hippocampus, cerebellum, or cerebrospinal fluid were observed in the postischemic gerbils compared with controls, in accordance with the unaffected activity of kynurenine aminotransferase activity. Collectively, these results support roles for IDO, kynureninase, kynurenine 3-hydroxylase, and 3-hydroxyanthranilate-3,4-dioxygenase in accelerating the conversion of L-tryptophan and other substrates to QUIN in damaged brain regions following transient cerebral ischemia. Immunocytochemical results demonstrated the presence of macrophage infiltrates in hippocampus and other brain regions that parallel the extent of these biochemical changes.(ABSTRACT TRUNCATED AT 400 WORDS)

Relationship of cerebrospinal fluid immune activation associated factors to HIV encephalitis

OBJECTIVES AND DESIGN

Because macrophages are the predominant immune cell and the predominant infected cell in the brains of patients with HIV encephalitis, we studied macrophage and immune activation-associated factors in the cerebrospinal fluid (CSF) from 39 autopsied AIDS cases for whom complete neuropathologic evaluation of the brain was available.

RESULTS

CSF HIV p24 antigen was present in less than one-third of cases (11 out of 39). Less than half of the autopsies with moderate to severe parenchymal infection by HIV had high levels of CSF p24, although all autopsies with elevated levels of HIV p24 had moderate to severe HIV encephalitis. Elevated levels of cytokines, beta 2-microglobulin, neopterin, and quinolinic acid were observed.

CONCLUSIONS

Although many of the CSF findings showed a strong correlation with each other, none showed a strong correlation with the severity of HIV infection of the brain itself. The absence of a close association between CSF abnormalities and HIV encephalitis could reflect the abundance of complicating opportunistic infections in these terminally ill patients or the inadequacy of CSF as a marker of basal ganglia involvement in HIV encephalitis. These findings complicate interpretation of clinical studies of CSF in patients with AIDS where neuropathologic evaluation is unavailable.

Effects of immune activation on quinolinic acid and neuroactive kynurenines in the mouse

Accumulation of quinolinic acid and neuroactive kynurenines derived from tryptophan are of potential significance in human neuropathologic diseases because of their neurotoxic and convulsant properties. Clinical studies have established that sustained elevations of quinolinic acid, L-kynurenine and kynurenic acid within the cerebrospinal fluid occur in patients with a broad spectrum of inflammatory diseases and correlate with markers of immune activation and interferon-gamma activity. The present study describes an animal model that replicates these clinical observations and investigates the role of interferon-gamma as a mediator between immune activation and increased kynurenine pathway metabolism. Marked elevations in quinolinic acid, L-kynurenine and 3-hydroxykynurenine as well as an increased ratio of quinolinic acid: kynurenic acid in brain occurred 24 h after systemic pokeweed mitogen administration to C57BL6 mice. In plasma, L-tryptophan and kynurenic acid levels were reduced by pokeweed mitogen, while the concentrations of L-kynurenine, 3-hydroxykynurenine and quinolinic acid were increased. Interferon-gamma, pokeweed mitogen and lipopolysaccharide induced indoleamine-2,3-dioxygenase, the first enzyme of the kynurenine pathway, and increased both L-kynurenine and quinolinic acid concentrations of brain and systemic tissues, particularly in the lung, gastrointestinal tract and spleen. In contrast, hepatic tryptophan-2,3-dioxygenase activity was either reduced or unaffected. Increases in kynurenine pathway metabolism were sustained in mice given daily injections of interferon-gamma for seven days and subsequent responses to interferon-gamma were further enhanced. In contrast, daily administration of lipopolysaccharide was associated with subsequent attenuated responsiveness (tolerance) to lipopolysaccharide, pokeweed mitogen and interferon-gamma. Systemic administration of a monoclonal antibody to mouse interferon-gamma either attenuated or abolished the responses of kynurenine pathway metabolism to pokeweed mitogen and interferon-gamma. We conclude that acute and chronic increases in quinolinic acid and neuroactive kynurenines follow immune stimulation in mice, and result from indoleamine-2,3-dioxygenase induction. The results demonstrate that interferon-gamma is an important mediator between immune stimulation and indoleamine-2,3-dioxygenase induction. These increases in kynurenine pathway metabolism closely parallel the responses documented in patients with a broad spectrum of inflammatory diseases. Mice treated with immune stimuli are a useful model to investigate the relationships between immune activation and kynurenine pathway metabolism.

Quinolinic acid in culture media used for in vitro neurotoxicology studies

Serum from several species is frequently added to the incubation media of cells in vitro. The excitotoxin and N-methyl-D-aspartate receptor agonist quinolinic acid was found to be present in serum in concentrations ranging from 59 to 4895 nM. Some neuronal systems are reported to be particularly sensitive to the neurotoxic effects of quinolinic acid. Therefore, quinolinic acid in serum should be considered as a potentially confounding variable in neurotoxicology studies in vitro.

Quinolinate in brain and cerebrospinal fluid in rat models of congenital hyperammonemia

Children with inborn errors of urea synthesis who survive neonatal hyperammonemic coma commonly exhibit cognitive deficits and neurologic abnormalities. Yet, there is evidence that ammonia is not the only neurotoxin. Hyperammonemia appears to induce a number of neurochemical alterations. In rodent models of hyperammonemia, uptake of L-tryptophan into brain is increased. It has been reported that in an experimental rat model of hepatic encephalopathy, in the ammonium acetate-injected rat, and in patients with hepatic failure and inborn errors of ammonia metabolism, quinolinate, a tryptophan metabolite, is increased. Elevations in quinolinate are of particular concern, as quinolinate could excessively activate the N-methyl-D-aspartate subclass of excitatory amino acid receptors, thereby causing selective neuronal necrosis. We sought to identify an animal model that would replicate the increases in quinolinate that have been associated with hyperammonemia in humans. Levels of quinolinate were measured in hyperammonemic urease-infused rats and ammonium acetate-injected rats. In the urease-infused rat, brain tryptophan was doubled, and serotonin and its metabolite 5-hydroxyindoleacetic acid were significantly increased. Yet, despite the increase in tryptophan and evidence for increased metabolism of tryptophan to serotonin, there were no observed increases of quinolinate in brain, cerebrospinal fluid, or plasma. In the ammonium acetate-injected rat, significant increases of 5-hydroxyindoleacetic acid in cerebral cortex were also observed, but quinolinate did not change in cerebrospinal fluid or cerebral cortex. In summary, we were unable to demonstrate an increase of quinolinate in brain or cerebrospinal fluid in these rat models of hyperammonemia.

Quinolinic acid and kynurenine pathway metabolism in inflammatory and non-inflammatory neurological disease

Neurological dysfunction, seizures and brain atrophy occur in a broad spectrum of acute and chronic neurological diseases. In certain instances, over-stimulation of N-methyl-D-aspartate receptors has been implicated. Quinolinic acid (QUIN) is an endogenous N-methyl-D-aspartate receptor agonist synthesized from L-tryptophan via the kynurenine pathway and thereby has the potential of mediating N-methyl-D-aspartate neuronal damage and dysfunction. Conversely, the related metabolite, kynurenic acid, is an antagonist of N-methyl-D-aspartate receptors and could modulate the neurotoxic effects of QUIN as well as disrupt excitatory amino acid neurotransmission. In the present study, markedly increased concentrations of QUIN were found in both lumbar cerebrospinal fluid (CSF) and post-mortem brain tissue of patients with inflammatory diseases (bacterial, viral, fungal and parasitic infections, meningitis, autoimmune diseases and septicaemia) independent of breakdown of the blood-brain barrier. The concentrations of kynurenic acid were also increased, but generally to a lesser degree than the increases in QUIN. In contrast, no increases in CSF QUIN were found in chronic neurodegenerative disorders, depression or myoclonic seizure disorders, while CSF kynurenic acid concentrations were significantly lower in Huntington's disease and Alzheimer's disease. In inflammatory disease patients, proportional increases in CSF L-kynurenine and reduced L-tryptophan accompanied the increases in CSF QUIN and kynurenic acid. These responses are consistent with induction of indoleamine-2,3-dioxygenase, the first enzyme of the kynurenine pathway which converts L-tryptophan to kynurenic acid and QUIN. Indeed, increases in both indoleamine-2,3-dioxygenase activity and QUIN concentrations were observed in the cerebral cortex of macaques infected with retrovirus, particularly those with local inflammatory lesions. Correlations between CSF QUIN, kynurenic acid and L-kynurenine with markers of immune stimulation (neopterin, white blood cell counts and IgG levels) indicate a relationship between accelerated kynurenine pathway metabolism and the degree of intracerebral immune stimulation. We conclude that inflammatory diseases are associated with accumulation of QUIN, kynurenic acid and L-kynurenine within the central nervous system, but that the available data do not support a role for QUIN in the aetiology of Huntington's disease or Alzheimer's disease. In conjunction with our previous reports that CSF QUIN concentrations are correlated to objective measures of neuropsychological deficits in HIV-1-infected patients, we hypothesize that QUIN and kynurenic acid are mediators of neuronal dysfunction and nerve cell death in inflammatory diseases. Therefore, strategies to attenuate the neurological effects of kynurenine pathway metabolites or attenuate the rate of their synthesis offer new approaches to therapy.