Molecular mechanisms of natural resistance to mycobacterial infections.

Natural resistance to infection with intracellular parasites is controlled by a dominant gene on mouse chromosome 1, called Bcg. Bcg affects the capacity of macrophages to destroy ingested intracellular parasites early during infection. Reactive nitrogen intermediates (RNI) have been implicated in the interferon-gamma (IFN-gamma)-induced antimicrobial action of macrophages against a wide variety of pathogens. To determine whether Bcg (Nramp) is involved in the production of RNI, these studies have taken advantage of the recent cloning of the Bcg candidate gene, designated Nramp. The expression of Bcg has been down-regulated in the B10R (Bcgr) macrophage cell line using a ribozyme hybrid to site-specifically cleave the Nramp mRNA. Following activation with IFN-gamma, the secretory activity [nitric oxide (NO) and tumor necrosis factor-alpha (TNF-alpha)] and surface marker expression (la antigen) of these Bcg(Nramp) ribozyme-transfected macrophages were markedly lower than in activated control mock-transfected macrophages (B10R-CTL). However, there was no difference in NO production of B10R-Bcg(Nramp)Rb and B10R-CTL macrophages if the treatment with IFN-gamma occurred in the presence of lipopolysaccharide (LPS). These studies support the hypothesis that the Bcg(Nramp) gene is involved in the regulation of early signaling that occurs in macrophages activated with IFN-gamma. Furthermore, it seems that IFN-gamma, but not LPS-induced activation is affected by the inhibition of Bcg(Nramp) gene expression. Definitive evidence will be provided by transfection experiments that will show whether the Bcgr allele of Bcg(Nramp) can restore NO production of the Bcgs macrophage.