Mechanism of nitric oxide-dependent killing of Mycobacterium bovis BCG in human alveolar macrophages.

We demonstrated that products of the L-arginine-dependent pathway of human alveolar macrophages (AM) effectively kill the Mycobacterium bovis bacillus Calmette-Guérin (BCG) in vitro. The formation of products was triggered by inoculation with BCG itself. Many reports have shown that activated rodent AM could produce an amount of nitric oxide (NO) sufficient to suppress the growth of mycobacteria. However, there have been no definitive results as to whether human AM might have the NO-dependent mechanism for the killing of mycobacteria. Therefore, we have undertaken some experiments to answer this question. Immunofluorescence assays showed an increased production of inducible nitric oxide synthase (iNOS) and peroxynitrite in BCG-inoculated AM from patients with pulmonary fibrosis. Reverse transcriptase-PCR also revealed the higher expression of iNOS-coding mRNA. Colony assays demonstrated that these human AM effectively killed BCG in their cytoplasm. However, treatment of AM with N(G)-monomethyl-L-arginine monoacetate resulted in markedly reduced killing activity. These results clearly show that BCG-induced NO and its reactive product with the oxygen radical peroxynitrite could play an important role in the killing of BCG in human AM.