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Susceptibility to infectious diseases is under genetic control in humans. Animal models provide an ideal tool to study the genetic component of susceptibility and to identify candidate genes that can then be tested for association or linkage studies in human populations from endemic areas of disease. The Nramp1 gene was isolated by positional cloning the host resistance locus Bcg/Ity/Lsh, and mutations at this locus impair the resistance of mice to infections with intracellular parasites, such as Salmonella, Leishmania, and Mycobacterium. Allelic variants at the human Nramp1 homologue have recently been found to be associated with susceptibility to tuberculosis and leprosy in humans. The Nramp1 protein is an integral membrane protein expressed exclusively in the lysosomal compartment of monocytes and macrophages. After phagocytosis, Nramp1 is targeted to the membrane of the microbe-containing phagosome, where it may modify the intraphagosomal milieu to affect microbial replication. Although the biochemical mechanism of action of Nramp1 at that site remains unknown, Nramp homologues have been identified in many other animal species and actually define a protein family conserved from bacteria to humans. Some of these homologues have been shown to be divalent cation transporters. Recently, a second member of the mammalian Nramp family, Nramp2, was discovered and shown to be mutated in animal models of iron deficiency. The Nramp2 protein was subsequently shown to be the major transferrin-independent iron uptake system of the intestine. Together, these results suggest that Nramp1 may control intracellular microbial replication by actively removing iron or other divalent cations from the phagosomal space.

The identification and characterization of genetic factors influencing natural susceptibility to infectious diseases in humans and in model organisms, such as the laboratory mouse, can provide new insight into the basic mechanisms of host defense against infections. In the mouse, resistance or susceptibility to infection with intracellular pathogens such as Salmonella, Mycobacterium and Leishmnania is controlled by the Natural resistance associated macrophage protein (Nramp1) gene on chromosome 1, which influences the rate of intracellular replication of these parasites in macrophages. Nramp1 codes for an integral membrane protein, which is expressed exclusively in macrophage/monocytes and polymorphonuclear leukocytes. The protein is localized to the endosomal/lysosomal compartment of the macrophage and is rapidly recruited to the membrane of the particle-containing phagosome upon phagocytosis. Nramp defines a novel family of functionally related membrane proteins including Nramp2, which was recently shown to be the major transferrin-independent uptake system of the intestine in mammals. This observation supports the hypothesis that the phagocyte-specific Nramp1 protein may regulate the intraphagosomal replication of antigenically unrelated bacteria by controlling divalent cation concentrations at that site. Recent genetic studies have found that allelic variants at the human NRAMP1 locus are associated with susceptibility to leprosy (Mycobacterium leprae) and tuberculosis (Mycobacterium tuberculosis) and possibly with the onset of rheumatoid arthritis.

In mice, natural resistance or susceptibility to infection with intracellular parasites is determined by a locus or group of loci on chromosome 1, designated Bcg, Lsh, and Ity, which controls early microbial replication in reticuloendothelial organs. We have identified by positional cloning a candidate gene for Bcg, Nramp1, which codes for a novel macrophage-specific membrane transport protein. We have created a mouse mutant bearing a null allele at Nramp1, and we have analyzed the effect of such a mutation on natural resistance to infection. Targeted disruption of Nramp1 has pleiotropic effects on natural resistance to infection with intracellular parasites, as it eliminated resistance to Mycobacterium bovis, Leishmania donovani, and lethal Salmonella typhimurium infection, establishing that Nramp1, Bcg, Lsh, and Ity are the same locus. Comparing the profiles of parasite replication in control and Nramp1-/- mice indicated that the Nramp1Asp169 allele of BcgS inbred strains is a null allele, pointing to a critical role of this residue in the mechanism of action of the protein. Despite their inability to control parasite growth in the early nonimmune phase of the infection, Nramp1-/- mutants can overcome the infection in the late immune phase, suggesting that Nramp1 plays a key role only in the early part of the macrophage-parasite interaction and may function by a cytocidal or cytostatic mechanism distinct from those expressed by activated macrophages.

Natural resistance to infection with unrelated intracellular parasites such as Mycobacteria, Salmonella, and Leishmania is controlled in the mouse by a single gene on chromosome 1, designated Bcg, Ity, or Lsh. A candidate gene for Bcg, designated natural resistance-associated macrophage protein (Nramp), has been isolated and shown to encode a novel macrophage-specific membrane protein, which is altered in susceptible animals. We have cloned and characterized cDNA clones corresponding to the human NRAMP gene. Nucleotide and predicted amino acid sequence analyses indicate that the human NRAMP polypeptide encodes a 550-amino acid residue membrane protein with 10-12 putative transmembrane domains, two N-linked glycosylation sites, and an evolutionary conserved consensus transport motif. Identification of genomic clones corresponding to human NRAMP indicates that the gene maps to chromosome 2q35 within a group of syntenic loci conserved with proximal mouse 1. The gene is composed of at least 15 exons, with several exons encoding discrete predicted structural domains of the protein. These studies have also identified an alternatively spliced exon encoded by an Alu element present within intron 4. Although this novel exon was found expressed in vivo, it would introduce a termination codon in the downstream exon V, resulting in a severely truncated protein. Northern blot analyses indicate that NRAMP mRNA expression is tightly controlled in a tissue-specific fashion, with the highest sites of expression being peripheral blood leukocytes, lungs, and spleen. Additional RNA expression studies in cultured cells identified the macrophage as a site of expression of human NRAMP and indicated that increased expression was correlated with an advanced state of differentiation of this lineage.