Brain extracellular quinolinic acid in chronic experimental hepatic encephalopathy as assessed by in vivo microdialysis: acute effects of L-tryptophan

Increased brain quinolinic acid (QUIN) levels have been suggested to play a role in hepatic encephalopathy (HE). Previous brain tissue studies have been unable to confirm this hypothesis. Because QUIn is a potent NMDA-receptor agonist, it also is relevant to determine brain extracellular QUIN levels in HE. For this purpose, we assessed frontal neocortical extracellular QUIN levels by in vivo microdialysis in rats subjected to a portacaval shunt (PCS). We also evaluated the acute effects of altered L-tryptophan (L-TRP) availability on brain extracellular QUIN levels. The basal extracellular L-TRP levels were significantly (p < .001) higher in the PCS rats than in the sham-operated controls. However, the QUIN level (p < .05) and the QUIN to L-TRP ratio (p < .01) were significantly lower in the PCS rats. Elevated L-TRP availability increased the QUIN levels to a similar degree in both sham and PCS rats. This study, in conjunction with our previous results, does thereby not support a major involvement of QUIN in the pathogenesis of HE.

Macrophage Activation Factors in the Brains of AIDS Patients

HIV, soluble HLA class I (sHLA-I), quinolinic acid (QUIN), and the monokines IL-1&beta;, IL-6, and TNF-&#945; were measured by ELISA and PCR in brain tissue of 60 AIDS autopsies without evidence of CNS opportunistic infections. Individual cases showed good interrogational correlations for the factors measured. There was a positive correlation between concentrations of IL-1&beta; and IL-6. Brain viral burden correlated with intraparenchymal levels of sHLA-I, IL-1&beta;, and IL-6. Comparison of neuritic damage and levels of immune mediators implicates macrophage activation factors in the etiology of neurologic damage in AIDS.

Quinolinic acid in patients with systemic lupus erythematosus and neuropsychiatric manifestations

OBJECTIVE

To evaluate the relationship between quinolinic acid, a neuroactive metabolite of L-tryptophan, and neuropsychiatric manifestations of systemic lupus erythematosus (SLE).

METHODS

Forty specimens of cerebrospinal fluid (CSF) were obtained from 39 patients with SLE who were evaluated for 40 episodes of neuropsychiatric dysfunction. The diagnosis of the neuropsychiatric dysfunction was determined clinically. CSF and serum specimens were analyzed for levels of quinolinic acid without knowledge of the clinical diagnosis.

RESULTS

Neuropsychiatric dysfunction attributed to SLE (NPSLE) was confirmed in 30 patient-episodes (Group 1), whereas in the other 10 (Group 2) other etiologies were felt to explain their CNS dysfunction. The median levels of CSF quinolinic acid for Group 1 (232.5 nmol/l) were significantly higher than those for Group 2 (median 38.2 nmol/l) (p < 0.014). CSF and serum quinolinic acid levels correlated significantly (p < 0.003) but there was not correlation between CSF quinolinic acid and CSF protein concentrations or white blood cell counts.

CONCLUSION

We conclude that elevated quinolinic acid levels in the CSF and serum may be associated with NPSLE and could possibly play a role in its pathogenesis.

The kynurenine pathway and neurologic disease. Therapeutic strategies

The neurotoxic effects of QUIN have been well established. Clinical conditions have been identified where substantial elevations in CNS QUIN levels occur. There is a relationship between the severity of neurologic impairments and macrophage activation, with the magnitude of the increases in QUIN. The magnitude of QUIN increases in experimental immune activation, and macrophages in vitro, are highest in non-human primates, intermediate in gerbils and guinea pigs, and lowest in mice and rats. Macrophages in vitro are a useful screening system to evaluate potential inhibitors of the kynurenine pathway. Several models of CNS inflammation are available, including brain injury in post-ischemic gerbils and spinal cord injury in guinea pigs. 4-Chloro-3-hydroxyanthranilate is a potent inhibitor of QUIN production by macrophages and reduces QUIN accumulations in spinal cord injury. Such reductions are associated with significant neurologic improvements in the early post-injury period. The results support further investigation of QUIN as a mediator of neurologic dysfunction and damage in neurologic diseases.

Quinolinic acid levels in a murine retrovirus-induced immunodeficiency syndrome

Mice infected with the retrovirus mixture designated LP-BM5 murine leukemia virus (MuLV) develop an immunosuppressive disease. Quinolinic acid (QUIN) is an endogenous neurotoxic N-methyl-D-aspartate agonist that may contribute to the pathogenesis of HIV-associated neurologic disease. In the present study, the levels of QUIN in brain and blood were measured in mice infected with LP-BM5 MuLV and compared with those in uninfected mice and mice infected with the nonpathogenic strain of ecotropic MuLV (helper component of LP-BM5 MuLV). Infection with LP-BM5 MuLV resulted in progressive increases in blood QUIN levels beginning 2 weeks after inoculation that peaked by 16 weeks postinfection. QUIN levels were also increased in cerebral cortex, hippocampus, and striatum. In systemic tissues, QUIN levels were increased in lung, liver, and spleen. In contrast, infection with the ecotropic viral component of the LP-BM5 MuLV mixture was not associated with any changes in brain, blood, or systemic tissue QUIN levels, even though helper virus burdens were comparable to those in mice infected with LP-BM5 MuLV. Treatment of LP-BM5 MuLV-infected mice with the antiretroviral agent zidovudine (azidothymidine) significantly reduced blood and brain QUIN levels in association with reductions in viral load in brain and spleen. These observations suggest that elevated QUIN production is not attributable to productive infection with retrovirus per se but occurs in response to an agent or agents, such as cytokines, that are produced by the host in response to virus infection.

Evidence of excitotoxicity in the brain of the ornithine carbamoyltransferase deficient sparse fur mouse

Ornithine carbamoyltransferase deficiency (OCTD) is the most common inborn error of urea synthesis. An X-linked disorder, OCTD males commonly present with hyperammonemic coma in the newborn period. There is a high rate of mortality and morbidity, with most survivors sustaining severe brain damage and resultant developmental disabilities. Although ammonia is presumed to be the principal neurotoxin, there is evidence that other neurochemical alterations may also be involved. The OCTD sparse fur (spf/Y) mouse has proven to be a useful model of this disease with similar metabolic and neurochemical alterations to those found in the human disease. In this study, the levels of the tryptophan derived excitotoxin quinolinic acid were examined in the brains of spf/Y mice. In addition, the neuropathology was examined using both light and electron microscopic approaches. Consistent with reports in children with urea cycle disorders, the levels of tryptophan and quinolinic acid were increased two-fold in various brain regions of the spf/Y mouse. Quinolinic acid, an agonist at the N-methyl-D-aspartate (NMDA) receptors, is known to produce selective cell loss in the striatum. We found a significant loss of medium spiny neurons and increased numbers of reactive oligodendroglia and microglia in the striatum of spf/Y mice. These neurochemical and neuropathological observations are consistent with an excitotoxic influence on brain injury in OCTD. It leads us to suggest that administration of NMDA receptor antagonists may ameliorate brain damage in children with inborn errors of urea synthesis.

Quinolinic acid production is related to macrophage tropic isolates of HIV-1

We sought to determine whether the neurotoxin quinolinic acid (QUIN) was produced by macrophages or lymphocytes infected with isolates of HIV-1 with varying degrees of macrophage tropism derived from patients with varying stages of AIDS dementia complex (ADC). Highly macrophage tropic isolates and minimally macrophage tropic isolates were used to inoculate macrophages and QUIN production was measured. Similarly, QUIN production from macrophages was monitored using a purified cell free highly macrophage tropic isolate and laboratory isolates SF33 and SF2. Each of these experiments was also performed with lymphocytes. We found that macrophages infected with macrophage tropic isolates of HIV-1 led to QUIN production while lymphocytes did not produce QUIN. The ability of the HIV-1 infected macrophages to produce QUIN was related to the viral inoculum and the degree of macrophage tropism of the isolate. The severity of ADC in the patient from whom a particular isolate was derived was not per se a determining factor for QUIN production. Purified cell free ADC isolates also led to QUIN production by macrophages thereby suggesting that HIV-1 infection alone is capable of inducing QUIN production.

Quinolinic acid in tumors, hemorrhage and bacterial infections of the central nervous system in children

A potential mechanism that may contribute to neurological deficits following central nervous system infection in children was investigated. Quinolinic acid (QUIN) is a neurotoxic metabolite of the kynurenine pathway that accumulates within the central nervous system following immune activation. The present study determined whether the levels of QUIN are increased in the cerebrospinal fluid of children with infections of the CNS, hydrocephalus, tumors or hemorrhage. Extremely high QUIN concentrations were found in patients with bacterial infections or the CNS, despite treatment with antimicrobial agents. CSF QUIN levels were also elevated to a lesser degree in patients with hydrocephalus or tumors. CSF L-kynurenine levels increased in parallel to the accumulations in QUIN, which is consistent with increased activity of the first enzyme of the kynurenine pathway, indoleamine-2,3-dioxygenase. The CSF levels of neopterin, a marker of immune and macrophage activation, were also increase in patients with infections. The cytokines tumor necrosis factor-alpha and interleukin-6 were also detected in some patients' samples, and were highest in patients with infection. These results suggest that QUIN is a sensitive marker of the presence of immune activation within the CNS. Further studies of QUIN as a potential contributor to neurologic dysfunction and neurodegeneration in children with CNS inflammation are warranted.

Metabolism of L-tryptophan to kynurenate and quinolinate in the central nervous system: effects of 6-chlorotryptophan and 4-chloro-3-hydroxyanthranilate

The metabolism of L-tryptophan to the neuroactive kynurenine pathway metabolites, L-kynurenine, kynurenate and quinolinate, and the effects of two inhibitors of quinolinate synthesis (6-chlorotryptophan and 4-chloro-3-hydroxyanthranilate) were investigated by mass spectrometric assays in cultured cells and in vivo. Cell lines obtained from astrocytoma, neuroblastoma, macrophage/monocytes, lung, and liver metabolized L-[13C6]-tryptophan to L-[13C6]kynurenine and [13C6]kynurenate, particularly after indoleamine-2,3-dioxygenase induction by interferon-gamma. Kynurenine aminotransferase activity was measurable in all cell types examined but was unaffected by interferon-gamma. These results suggest that many cell types can be sources of kynurenate following immune activation. In vivo synthesis of L-[13C6]kynurenine and [13C6]kynurenate from L-[13C6]tryptophan was studied in the CSF of macaques infected with poliovirus, as a model of inflammatory neurologic disease. The effects of 6-chlorotryptophan and 4-chloro-3-hydroxyanthranilate on the synthesis of kynurenate were different. 6-Chlorotryptophan attenuated formation of L-[13C6]kynurenine and [13C6]kynurenate and was converted to 4-chlorokynurenine and 7-chlorokynurenate. It may be an effective prodrug for the delivery of 7-chlorokynurenate, which is a potent antagonist of NMDA receptors. In contrast, 4-chloro-3-hydroxyanthranilate did not reduce accumulation of L-[13C6]kynurenine and [13C6]kynurenate. 6-Chlorotryptophan and 4-chloro-3-hydroxyanthranilate are useful tools to manipulate concentrations of quinolinate and kynurenate in the animal models of neurologic disease to evaluate physiological roles of these neuroactive metabolites.

Brain quinolinic acid in chronic experimental hepatic encephalopathy: effects of an exogenous ammonium acetate challenge

Elevated brain concentrations of the neurotoxin and NMDA receptor agonist quinolinic acid (QUIN) have been demonstrated in portacaval-shunted (PCS) rats, a chronic hepatic encephalopathy (HE) model. Increased brain QUIN levels have also been shown in acute hyperammonemic rats. In the present study, the plasma and brain (neocortical) QUIN levels in chronic PCS rats were investigated. The study also included a single exogenous ammonium acetate (NH4Ac; 5.2 mmol/kg, i.p.) challenge to precipitate a reversible hepatic coma. Compared with sham-operated controls, chronic PCS rats exhibited decreased rather than increased plasma and brain QUIN levels. The plasma-to-brain QUIN ratio was not found to be altered. The NH4Ac administration induced coma in all of the PCS rats 20-25 min after the challenge, and this coma was resolved within 60-75 min. No relevant temporal relationship between changes in brain QUIN levels and the neurological status in the PCS rats was observed. Therefore, our results do not support the contention that increased brain QUIN levels per se are involved in the pathogenesis of HE.

Increased human immunodeficiency virus (HIV) type 1 DNA content and quinolinic acid concentration in brain tissues from patients with HIV encephalopathy

Levels of human immunodeficiency virus type 1 (HIV-1) DNA and quinolinic acid were examined in areas of the central nervous system (CNS) and lymphoid organs (LN) from 5 AIDS patients with no clinically apparent CNS compromise (group I), 7 with CNS opportunistic diseases (group II), and 8 with HIV encephalopathy (group III). The brains from patients with HIV encephalopathy not only contained higher levels of HIV-1 DNA (cerebrum, P < .01; cerebellum, P < .05) as assessed by quantitative polymerase chain reaction but also showed a higher rate of viral pol region mutations suggestive of zidovudine or didanosine resistance than brains from patients in group I or II (P < .01). CNS quinolinic acid concentrations were significantly higher in group II and III patients than in group I (P = .03), even though quinolinic acid levels in LN were comparable among the 3 groups. These data suggest that CNS inflammatory changes associated with HIV encephalopathy may be triggered by a local productive HIV-1 infection within the CNS.

Zidovudine treatment prolongs survival and decreases virus load in the central nervous system of rhesus macaques infected perinatally with simian immunodeficiency virus

To assess the potential therapeutic effects of zidovudine, rhesus macaques were inoculated with simian immunodeficiency virus (SIV) strain SMM/B670 at birth and infused either continuously or intermittently with zidovudine for 6-7 months. Zidovudine did not prevent infection but did significantly increase survival time, which was associated with lower serum p26 viral core antigen levels, a lower virus burden in the cerebrospinal fluid (CSF), and lower CSF quinolinic acid levels than in untreated monkeys. Two of 5 infected, untreated monkeys developed motor impairment within 6 months following infection, whereas motor impairments did not occur in infected, zidovudine-treated monkeys until after the drug was discontinued. Zidovudine treatment was well tolerated by rhesus infants with minimal, transient side effects. These results demonstrate that zidovudine treatment significantly decreases virus load within the central nervous system (CNS) and delays the onset of CNS dysfunction and immune disease in rhesus monkeys perinatally infected with SIV.

Quinolinic acid accumulation and functional deficits following experimental spinal cord injury

Quinolinic acid (QUIN) is an excitotoxic tryptophan metabolite that is produced by activated macrophages. Accumulations of QUIN are implicated in the aetiology of a broad spectrum of human neurological diseases, particularly inflammatory conditions. To determine whether QUIN is an endogenous neurotoxin requires agents that reduce QUIN synthesis, and animal models where QUIN levels increase in association with neurological disease. Compression injury of the spinal cord of guinea pigs results in secondary neurological deficits, related to inflammation and macrophage activation. We evaluated whether 4-chloro-3-hydroxyanthranilate (4Cl-3HAA), an inhibitor of 3-hydroxyanthranilate-3,4-dioxygenase, reduces QUIN accumulations in this model and influences the progression of neurological deficits. Intraperitoneal injections of 4Cl-3HAA (100 mg/kg every 12 h) attenuated QUIN accumulations in spinal cord following injury, and reduced the severity of delayed functional deficits. Intraperitoneal injections of the macrophage toxin, silica, also reduced QUIN levels and attenuated neurological deficits. A direct subdural infusion of Cl-3HAA into the injured spinal cord (50 microM, 1 microliter/h) promptly exacerbated functional impairments, which suggests that the infusate had direct toxic effects. These studies demonstrate that guinea pigs with spinal cord injury constitute a useful model to study the mechanisms that increase central nervous system (CNS) QUIN levels in conditions of CNS inflammation, and to evaluate the neurochemical and neurological effects of agents designed to reduce the accumulations of QUIN and other potential pathogenic mediators within the CNS. The results are consistent with a contributory role for QUIN in the pathogenesis of secondary functional impairments following spinal cord injury, although the possibility that 4Cl-3HAA had additional effects independent of QUIN cannot be excluded. Further studies are required to determine whether the beneficial effects of 4Cl-3HAA are sustained. While it is unknown whether secondary inflammatory processes contribute significantly to neurological deficits in human spinal cord injury, strategies that reduce the accumulation of QUIN are worthy of consideration and evaluation as a therapeutic target.

The relationship between plasma and brain quinolinic acid levels and the severity of hepatic encephalopathy in animal models of fulminant hepatic failure

Quinolinic acid is an excitatory, neurotoxic tryptophan metabolite proposed to play a role in the pathogenesis of hepatic encephalopathy. This involvement was investigated in rat and rabbit models of fulminant hepatic failure at different stages of hepatic encephalopathy. Although plasma and brain tryptophan levels were significantly increased in all stages of hepatic encephalopathy, quinolinic acid levels increased three-to sevenfold only in the plasma, CSF, and brain regions of animals in stage IV hepatic encephalopathy. Plasma-CSF and plasma-brain quinolinic acid levels in rats and rabbits with fulminant hepatic failure were strongly correlated, with CSF and brain concentrations approximately 10% those of plasma levels. Moreover, there was no significant regional difference in brain quinolinic acid concentrations in either model. Extrahepatic indoleamine-2,3-dioxygenase activity was not altered in rats in stage IV hepatic encephalopathy, but hepatic L-tryptophan-2,3-dioxygenase activity was increased. These results suggest that quinolinic acid synthesized in the liver enters the plasma and then accumulates in the CNS after crossing a permeabilized blood-brain barrier in the end stages of liver failure. Furthermore, the observation of low brain concentrations of quinolinic acid only in stage IV encephalopathy suggests that the contribution of quinolinic acid to the pathogenesis of hepatic encephalopathy in these animal models is minor.

Early intrathecal events in rhesus macaques (Macaca mulatta) infected with pathogenic or nonpathogenic molecular clones of simian immunodeficiency virus

BACKGROUND

Encephalitis is a common and devastating sequela of HIV infection in humans and of simian immunodeficiency virus (SIV) infection in rhesus macaques. We used the SIV-infected rhesus macaque model to study early intrathecal events in the pathogenesis of lentiviral encephalitis.

EXPERIMENTAL DESIGN

To examine early events and to compare the neuroinvasiveness and neurovirulence of pathogenic (SIVmac239) and nonpathogenic (SIVmac1A11) molecular clones of SIV and the role of host immunity in the early postinfection period, we inoculated groups of rhesus macaques with each of these clones and compared them with a third group of animals inoculated with pathogenic uncloned SIV (SIVmac). We collected paired cerebrospinal fluid and sera before and at intervals after inoculation and determined albumin and IgG concentrations, SIV-specific humoral immune response, and concentrations of quinolinic acid. Two animals from each group were killed and necropsied at 2, 8, 13, and 23 weeks after inoculation. Routine histopathology and semi-quantitative in situ hybridization were performed on tissue from multiple levels of the central nervous system (CNS).

RESULTS

SIVmac and SIVmac-239 invaded both the meninges and the CNS parenchyma simultaneously within 2 weeks of inoculation, whereas nonpathogenic SIVmac-1A11 was not neuroinvasive. Gross disruption of the blood-brain barrier was not detected at any time. However, elevated IgG indices and high levels of cerebrospinal fluid quinolinic acid denoted intrathecal immune activation soon after viral neuroinvasion. Virus load in the CNS declined as the immune response peaked but subsequently increased with waning immunity. One macaque that never developed an SIV-specific immune response died with severe SIV encephalitis.

CONCLUSIONS

Our findings support the following hypotheses of early events in SIV neuropathogenesis: (a) Pathogenic virus invades the CNS within days of i.v. inoculation and elicits an intrathecal immune response, including intrathecal synthesis of IgG and macrophage activation; (b) the immune response initially is associated with a decreased virus load in the CNS; (c) as immunodeficiency develops, virus load in the CNS increases once again; and (d) both virus and host factors are important in determining the course of events.

Cerebrospinal fluid levels of kynurenine pathway metabolites in patients with eating disorders: relation to clinical and biochemical variable

In brain, most L-tryptophan is metabolized to indoleamines, whereas in systemic tissues L-tryptophan is catabolized to kynurenine pathway metabolites. Among these latter compounds are: quinolinic acid, an N-methyl-D-aspartate receptor agonist; kynurenic acid, an antagonist of excitatory amino acid receptors that also reduces quinolinic acid-mediated neurotoxicity; and L-kynurenine, a possible convulsant. Because the metabolism of L-tryptophan through the kynurenine pathway is dependent upon adequate nutrition, we sought to determine whether the impaired nutrition characteristic of eating-disordered patients might be associated with specific disturbances in this metabolic pathway. Cerebrospinal fluid levels of L-tryptophan, quinolinic acid, kynurenic acid, L-kynurenine, and 5-hydroxyindoleacetic acid were measured in medication-free female patients meeting DSM-III-R criteria for either anorexia nervosa (n = 10) or normal-weight bulimia nervosa (n = 22), studied at varying stages of nutritional recovery. Eight healthy, normal-weight females served as a comparison group. Cerebrospinal fluid levels of kynurenic acid were significantly reduced in underweight anorectics, compared to normal females, but returned to normal values with restoration of normal body weight. Although cerebrospinal fluid quinolinic acid levels were not different from controls, the ratio of quinolinic acid to kynurenic acid was significantly increased during the underweight phase of anorexia nervosa. Furthermore, in the eating-disordered patients, kynurenic acid levels in cerebrospinal fluid correlated positively with percent-of-population average body weight.(ABSTRACT TRUNCATED AT 250 WORDS)

The relationship between plasma and brain quinolinic acid levels and the severity of hepatic encephalopathy

BACKGROUND/AIMS

Quinolinic acid is an endogenous neuroexcitant derived from tryptophan. Brain quinolinic acid concentrations are reportedly elevated in chronic liver failure. The aim of this study was to determine if brain quinolinic acid levels correlate with the severity of hepatic encephalopathy.

METHODS

Postmortem samples of selected brain regions and plasma samples taken at several stages of encephalopathy were obtained from patients with acute and chronic liver failure. Quinolinic acid levels were measured by mass spectroscopy using [18O]quinolinic acid.

RESULTS

Plasma quinolinic acid levels were significantly increased by stage I encephalopathy in patients with acute liver failure and by stages II and III in patients with chronic liver failure. Brain quinolinic acid levels were elevated only in patients with acute liver failure and were uniformly distributed at concentrations below those observed in plasma.

CONCLUSIONS

The uniform distribution of quinolinic acid at subplasma concentrations in the brains of patients with acute liver failure suggests that it is synthesized peripherally and enters the brain across a permeabilized blood-brain barrier. Whereas the elevation of brain quinolinic acid levels in patients who died of acute but not chronic liver failure suggests that the involvement of quinolinic acid in the pathogenesis of hepatic encephalopathy is minimal, it could predispose these patients to seizures.

6-Chloro-D,L-tryptophan, 4-chloro-3-hydroxyanthranilate and dexamethasone attenuate quinolinic acid accumulation in brain and blood following systemic immune activation

Accumulations of the neurotoxin quinolinic acid (QUIN) occur in the brain and blood following immune activation and are attributed to increased metabolism of L-tryptophan through the kynurenine pathway. Systemic administration of 4-chloro-3-hydroxyanthranilate (an inhibitor of 3-hydroxyanthranilate-3,4-dioxygenase), 6-chloro-D,L-tryptophan (a substrate of the kynurenine pathway) and dexamethasone (an anti-inflammatory agent) attenuated the accumulation of QUIN in the brain and blood following systemic pokeweed mitogen administration to mice. 6-Chloro-D,L-tryptophan and dexamethasone also attenuated the increases in brain and lung indoleamine-2,3-dioxygenase activity and elevations in plasma L-kynurenine levels. We conclude that QUIN formation can be modified by drugs which act at different levels of the cascade of events that link immune stimulation to increased kynurenine pathway metabolism.

Cerebrospinal fluid nitrite/nitrate levels in neurologic diseases

Nitric oxide has been proposed to mediate cytotoxic effects in inflammatory diseases. To investigate the possibility that overproduction of nitric oxide might play a role in the neuropathology of inflammatory and noninflammatory neurological diseases, we compared levels of the markers of nitric oxide, nitrite plus nitrate, in the CSF of controls with those in patients with various neurologic diseases, including Huntington's and Alzheimer's disease, amyotrophic lateral sclerosis, and HIV infection. We found that there were no significant increases in the CSF levels of these nitric oxide metabolites, even in patients infected with HIV or in monkeys infected with poliovirus, both of which have significantly elevated levels of the neurotoxin quinolinic acid and the marker of macrophage activation, neopterin. However, CSF quinolinic acid, neopterin, and nitrite/nitrate levels were significantly increased in a small group of patients with bacterial and viral meningitis.

Viral load and its relationship to quinolinic acid, TNF alpha, and IL-6 levels in the CNS of retroviral infected mice

Mouse models of infection of the central nervous system (CNS) have been used to study retroviral-induced neurologic disease. Ecotropic-neurotropic murine leukemia virus (MuLV) infection of susceptible neonatal mice causes a neurologic disease characterized by progressive hindlimb paralysis. The lesions consist of chronic noninflammatory spongiform change predominantly involving brainstem and spinal cord. Two molecularly cloned strains of MuLV, ts-1, a temperature-sensitive mutant of Moloney MuLV, and pNE-8, derived from a feral mouse isolate Cas-Br-E, were used in this study. Infected mice were sacrificed at regular intervals postinoculation throughout the time-course of disease. The neuropathology was evaluated using standard histological and immunohistopathological techniques. Tissue concentrations of viral proteins and potentially cytotoxic factors were compared with the histopathology in select regions of the CNS. Areas of extensive vacuolation with neuronal and oligodendroglial infection were observed in spinal cord, brainstem, and cerebellum. High titers of infectious virus were observed within CNS lesions, whereas low titers were observed in morphologically uninvolved areas. Western blot analysis revealed abundant production of viral envelope proteins, which correlated well with infectious virus titers. Serum quinolinic acid (QUIN) concentrations in both groups of noninfected and infected mice were similar. However, CNS tissue concentrations of QUIN, TNF alpha, and IL-6 in ts-1 infected mice were significantly higher than in pNE-8 infected or noninfected mice. The difference in concentration of these factors may be the result of greater activation of macrophages/microglia in ts-1 infected mice. During murine retroviral encephalitis, CNS damage may be mediated by direct infection of CNS cells and may be enhanced by indirect effects of neurotoxic factors possibly secreted by infected/activated macrophages.