High levels of inducible nitric oxide synthase mRNA are associated with increased monocyte counts in blood and have a beneficial role in Plasmodium falciparum malaria.

To date, there have been conflicting reports concerning the clinical significance of nitric oxide (NO) in Plasmodium falciparum malaria. Some authors have proposed that NO contributes to the development of severe and complicated malaria, while others have argued that NO has a protective role. To investigate these apparently contradictory reports, reverse transcription-coupled PCR was used to study inducible NO synthase (iNOS) in whole-blood RNA samples from patients with severe and complicated malaria or uncomplicated malaria and from healthy donors. This work produced three principal findings. First, samples of patients with severe and complicated malaria were variably positive, with weak to moderate intensity. Markedly higher iNOS RNA levels were observed in samples of patients with uncomplicated malaria than in patients with severe and complicated malaria. Samples of healthy donors were uniformly negative. Second, since we initially demonstrated iNOS expression in whole-blood RNA samples, we extended our investigations to individual blood cells such as monocytes, lymphocytes, neutrophils, and platelets to identify the cellular source of iNOS. We found that iNOS was expressed predominantly in monocytes. Third, retrospective statistical analysis of monocyte counts clearly demonstrated that patients with uncomplicated malaria had higher monocyte counts at the time of presentation than patients with severe and complicated malaria. Taken together, our findings give room to the interpretation that NO may have a beneficial rather than a deleterious role in falciparum malaria.