Nitric oxide production and nitric oxide synthase activity in malaria-exposed Papua New Guinean children and adults show longitudinal stability and no association with parasitemia

Abstract

Individuals in areas of intense malaria transmission exhibit resistance (or tolerance) to levels of parasitemia in their blood that would normally be associated with febrile illness in malaria-naive subjects. The resulting level of parasitemia associated with illness (the pyrogenic threshold) is highest in childhood and lowest in adulthood. Clinical parallels between malarial and bacterial endotoxin tolerance have led to the supposition that both share common physiological processes, with nitric oxide (NO) proposed as a candidate mediator. The hypotheses that NO mediates tolerance and blood stage parasite killing in vivo were tested by determining its relationship to age and parasitemia cross-sectionally and longitudinally in a population of 195 children and adults from Papua New Guinea encountering intense malaria exposure. Despite pharmacological clearance of asymptomatic parasitemia, NO production and mononuclear cell NO synthase (NOS) activity were remarkably stable within individuals over time, were not influenced by parasitemia, and varied little with age. These results contrast with previous smaller cross-sectional studies. Baseline NO production and NOS activity did not protect against recurrent parasitemia, consistent with previous data suggesting that NO does not have antiparasitic effects against blood stage infection in vivo. The NO indices studied were markedly higher in specimens from study subjects than in samples from Australian controls, and NOS activity was significantly associated with plasma immunoglobulin E levels, consistent with induction of NO by chronic exposure to other infections and/or host genetic factors. These results suggest that NO is unlikely to mediate killing of blood stage parasites in this setting and is unlikely to be the primary mediator in the acquisition or maintenance of malarial tolerance.

FIG. 1.

A. Blood smear results in 424 subjects from Papua New Guinea who were screened for enrollment into the study. Results represent the reading of a single daily smear. (B) Parasite density of P. falciparum in 105 enrolled subjects at baseline. Diamonds signify the higher of two measurements taken 24 h apart.

FIG. 2.

Plasma NOx levels in Darwin controls (DC) and PNG subjects not excluded at T₀. The median plasma NOx level in Darwin controls (11 μmol/liter; IQR, 7 to 24) was significantly lower than for each age group of PNG subjects (P < 0.001 for each pairwise comparison; Mann-Whitney U test). In the PNG group, age only explained 2.5% of the overall variation in plasma NOx levels (P = 0.033), with the predicted NOx level falling 3.5% for every 5 years of increasing age. The horizontal lines in the middle of the boxes represents the median, and the boxes extend from the 25th to the 75th percentile. Whiskers extend no further than the most distant data point or 1.5 times the IQR. Outlying values are represented by dots.