NATURAL-RESISTANCE TO INFECTION WITH INTRACELLULAR PARASITES - MOLECULAR-GENETICS IDENTIFIES NRAMP1 AS THE BCG/ITY/LSH LOCUS

Natural resistance to infection with intracellular parasites is controlled in the mouse by the expression of a locus or group of loci on chromosome 1 alternatively named Beg, Lsh, and Ity. Beg affects the capacity of mature tissue macrophages to restrict the intracellular proliferation of ingested parasites in the reticuloendothelial organs of the host during the early phase of infection. This review summarizes our molecular genetic approach to the isolation and characterization of the Beg locus. We have used a positional cloning strategy based on genetic and physical mapping, YAC cloning, and exon trapping to isolate a candidate gene for Bcg, named Nramp1, which codes for a macrophage-specific polytopic protein with 12 predicted transmembrane domains and a consensus transport motif. Sequence analysis of Nrampl cDNA clones from 27 Bcg(s) and Bcg(r) mouse strains reveals that susceptibility to infection (Bcg(s)) is associated with a single nonconservative Gly to Asp substitution at position 169 within predicted transmembrane domain 4 of the Nramp protein. Cloning experiments and homology search in available databases demonstrated that the Nramp1 gene belongs to a small gene family with several members in vertebrates and in such distantly related species as yeast and plants. Nramp proteins share a remarkable degree of similarity, with strong amino acid sequence conservation in the transmembrane domains, suggesting a common transport function for the Nramp family. Finally, we generated Nramp1(-/-) gene knockout mice, and analysis of their phenotypic characteristics established that (1)Nramp1 plays a key role in natural defense against infection with intraceullar parasites and therefore demonstrated allelism between Nrampl and Bcg/Ity/Lsh, (2) Nramp1 functions by a novel cytocidal/cytostatic mechanism distinct from those expressed by the activated macrophage, and (3) the Nramp1 Asp(169) allele of Bcg(s) inbred strains is a null allele, pointing to a critical role of this residue in Nramp1 function.