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J. Exp. Med., Volume 188, Number 2, July 20, 1998 351-364

Host Resistance to Intracellular Infection: Mutation of Natural Resistance-associated Macrophage Protein 1 (Nramp1) Impairs Phagosomal Acidification

By David J. Hackam,*Dagger Ori D. Rotstein,Dagger Wei-jian Zhang,*Dagger Samantha Gruenheid,§ Philippe Gros,§ and Sergio Grinstein*

From the * Division of Cell Biology, The Hospital for Sick Children, Toronto M5G 1X8, Ontario, Canada; the Dagger  Department of Surgery, Toronto Hospital, University of Toronto, Toronto M5G 1X8, Ontario, Canada; and the § Department of Biochemistry, McGill University, Montreal, Quebec, Canada H3G 1Y6

The mechanisms underlying the survival of intracellular parasites such as mycobacteria in host macrophages remain poorly understood. In mice, mutations at the Nramp1 gene (for natural resistance-associated macrophage protein), cause susceptibility to mycobacterial infections. Nramp1 encodes an integral membrane protein that is recruited to the phagosome membrane in infected macrophages. In this study, we used microfluorescence ratio imaging of macrophages from wild-type and Nramp1 mutant mice to analyze the effect of loss of Nramp1 function on the properties of phagosomes containing inert particles or live mycobacteria. The pH of phagosomes containing live Mycobacterium bovis was significantly more acidic in Nramp1- expressing macrophages than in mutant cells (pH 5.5 ± 0.06 versus pH 6.6 ± 0.05, respectively; P <0.005). The enhanced acidification could not be accounted for by differences in proton consumption during dismutation of superoxide, phagosomal buffering power, counterion conductance, or in the rate of proton "leak", as these were found to be comparable in wild-type and Nramp1-deficient macrophages. Rather, after ingestion of live mycobacteria, Nramp1-expressing cells exhibited increased concanamycin-sensitive H+ pumping across the phagosomal membrane. This was associated with an enhanced ability of phagosomes to fuse with vacuolar-type ATPase-containing late endosomes and/or lysosomes. This effect was restricted to live M. bovis and was not seen in phagosomes containing dead M. bovis or latex beads. These data support the notion that Nramp1 affects intracellular mycobacterial replication by modulating phagosomal pH, suggesting that Nramp1 plays a central role in this process.

Key words: mycobacterium tuberculosis;  phagosome;  phagocytosis;  macrophage;  proton pump


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