Induction of NOS in human monocytes infected with different Legionella species

The contribution of nitric oxide (NO) radicals to the suppression of intrac ellular replication of Legionella has been well established in rodents but remained questionable in humans. Considering the fact that human monocytes do not exhibit a high-output NO production, we used sensitive methods such as detection of inducible NO synthase (iNOS) mRNA by reverse transcript ion -PCR and demonstration of iNOS protein expression by means of flow cytometr y and Western blot to compare the levels of iNOS induced by Legionella spec ies which, in accordance to their human prevalence, show different multipli cation rates within human monocytic cells. The expression of iNOS in Mono M ac 6 (MM6) cells showed an only moderate inverse correlation to the intrace llular replication rate of a given Legionella species in the protein expres sion assays. However, stimulation of host cells with 1,25-dihydroxyvitamin D-3 to enhance NO production and inhibition of NO production by treatment o f host cells with ATG-methyl-L-arginine were not able to modify the intrace llular multiplication of legionellae within MM6 cells. Therefore, NO produc tion does not seem to play a crucial role for the restriction of intracellu lar replication of Legionella bacteria within human monocytic cells. Rodent models in investigations which are supposed to clarify the involvement of NO radicals in defense mechanisms against Legionella infections in humans a re of doubtful significance. (C) 2001 Federation of European Microbiologica l Societies. Published by Elsevier Science B.V. All rights reserved.