Potential role of nitric oxide in the normal human immune response to malaria and in the immunopathogenesis of cerebral malaria.

Abstract

The pathogenesis of cerebral malaria, a clinical syndrome with a high mortality rate despite the best available chemotherapy, is complex and imperfectly understood. Both parasite sequestration and cytokines have been shown to be important. It has been hypothesized, but not proven, that high levels of tumor necrosis factor alpha induce nitric oxide synthase, and increased nitric oxide contributes to the pathogenesis of cerebral malaria by inhibiting neurotransrnission.

Objectives: To assess the role of nitric oxide in the immune response to P. falciparum and its role in the immunopathogenesis of cerebral malaria, and to look for clinical and laboratory prognostic indicators in children with cerebral malaria. Setting: Children hospitalized into wards in general .paediatric and 'p,aediatric surgery at Muhimbili Medical Centre, Dar es Salaam, Tanzania. Patients and study design: A prospective cross sectional case control study was done. Markers of nitric oxide production was compared in 86 children with cerebral malaria with those in 45 children with non-cerebral malaria and 41 healthy controls. Urine was collected from all these participants on isopropanol. Urinary nitrates was measured by reducing -~~~~.~~~- -~ - ~--~

111 nitrates to nitrites with bacterial nitrate reductase coupled with Greiss reaction. Urinary creatinine was measured using creatinine sigma diagnostic kit, and then the spot urine nitrate: creatinine ratio was computed. Cerebrospinal fluid was obtained from all children with cerebral malaria and analysed for nitrates. Simple bed side clinical and laboratory prognostic indicators was assessed in 88 children with cerebral malaria. History of witnessed convulsions within 3 hrs of admission was obtained and presence of decerebration and corneal reflex was looked. Level of conciosness was assessed by Blantyre coma score. Total white blood cell count was obtained using a coulter counter, and differential count was obtained manually. The parasite count was calculated on a thick film stained with Fields A and B stain. Methaemoglobin was calculated by using the method of Evelyn and Malloy. Cerebrospinal fluid opening pressure was measured using a spinal fluid manometer. Results: Nitric oxide production correlated inversely with the severity of the disease, the levels being lowest in fatal cerebral malaria, and highest in healthy controls. There was no evidence of increased nitric oxide production in the cerebrospinal fluid. The three groups were different from each other (ANOVA; P <0.0001). By-multiple comparison test, patients with cerebral malaria and those with non-cerebral malaria differed from healthy controls (P <0.05). Furthermore, patients with cerebral malaria differed from those with non-cerebral malaria (P <0.05). The clinical and laboratory features associated with poor prognosis were:- witnessed convulsions, presence of decerebration, absent corneal reflex, low level of ••

IV coma and increased total white blood cell count. Conclusions: It appears that nitric oxide production is not increased in cerebral malaria. Rather, it is suppressed in both cerebral and non-cerebral malaria. The production is below the baseline levels of healthy controls, suggesting that both the constitutive and inducible isoforms are suppressed. These results suggest that nitric oxide is protective rather than harmful in malaria. Five clinical and laboratory features were associated with poor prognosis as mentioned above. However, differential total white blood cell count, level of parasiternia, methaemoglobin, and raised cerebrospinal fluid opening pressure did not correlate with poor outcome. Thus early recognition of the factors for poor prognosis and good medical and nursing care may reduce mortality.