BCG. Rebirth of a star performer

Allelic exchange in Mycobacterium tuberculosis with long linear recombination substrates

Genetic studies of Mycobacterium tuberculosis have been greatly hampered by the inability to introduce specific chromosomal mutations. Whereas the ability to perform allelic exchanges has provided a useful method of gene disruption in other organisms, in the clinically important species of mycobacteria, such as M. tuberculosis and Mycobacterium bovis, similar approaches have thus far been unsuccessful. In this communication, we report the development of a shuttle mutagenesis strategy that involves the use of long linear recombination substrates to reproducibly obtain recombinants by allelic exchange in M. tuberculosis. Long linear recombination substrates, approximately 40 to 50 kb in length, were generated by constructing libraries in the excisable cosmid vector pYUB328. The cosmid vector could be readily excised from the recombinant cosmids by digestion with PacI, a restriction endonuclease for which there exist few, if any, sites in mycobacterial genomes. A cosmid containing the mycobacterial leuD gene was isolated, and a selectable marker conferring resistance to kanamycin was inserted into the leuD gene in the recombinant cosmid by interplasmid recombination in Escherichia coli. A long linear recombination substrate containing the insertionally mutated leuD gene was generated by PacI digestion. Electroporation of this recombination substrate containing the insertionally mutated leuD allele resulted in the generation of leucine auxotrophic mutants by homologous recombination in 6% of the kanamycin-resistant transformants for both the Erdman and H37Rv strains of M. tuberculosis. The ability to perform allelic exchanges provides an important approach for investigating the biology of this pathogen as well as developing new live-cell M. tuberculosis-based vaccines.

Recombinant Mycobacterium bovis BCG secreting functional interleukin-2 enhances gamma interferon production by splenocytes

Mycobacterium bovis BCG was genetically engineered to express and secrete mouse interleukin-2 (IL-2) and rat IL-2. Genes encoding IL-2 were inserted into an Escherichia coli-BCG shuttle plasmid under the control of the BCG HSP60 promoter. To facilitate study of proteins produced in this system, the IL-2 gene product was expressed (i) alone, (ii) with the mycobacterial alpha-antigen secretion signal sequence at the amino terminus, (iii) with an influenza virus hemagglutinin epitope tag at the amino terminus, and (iv) with both the secretion signal sequence and the epitope tag. When expressed with the alpha-antigen signal sequence, biologically active IL-2 was secreted into the extracellular medium. Western blot (immunoblot) analysis of the intracellular IL-2 and extracellular IL-2 revealed that the secretion signal was appropriately cleaved from the recombinant lymphokine upon secretion. To assess the ability of recombinant BCG to stimulate cytokine production in a splenocyte population, mouse splenocytes were cultured together with wild-type or IL-2-producing BCG. IL-2-secreting BCG clones stimulated substantial increases in gamma interferon production, which could be reproduced by the addition of exogenous IL-2 to BCG. Levels of IL-6, IL-10, tumor necrosis factor alpha, and granulocyte-macrophage colony-stimulating factor were not significantly changed, while IL-4 and IL-5 remained undetectable (less than 50 pg/ml). The enhanced production of gamma interferon in response to IL-2-secreting BCG was strain independent. Recombinant BCG expressing mammalian cytokines provides a novel means to deliver cytokines and may augment the immunostimulatory properties of BCG in immunization and cancer therapy.

Natural resistance to infection with intracellular parasites: isolation of a candidate for Bcg

Natural resistance to infection with intracellular parasites is controlled by a dominant gene on mouse chromosome 1, called Bcg, Lsh, or Ity. Bcg affects the capacity of macrophages to destroy ingested intracellular parasites early during infection. We have assembled a 400 kb bacteriophage and cosmid contig within the genomic interval containing Bcg. A search for transcription units by exon amplification identified six novel genes in this contig. RNA expression studies showed that one of them, designated Nramp, was expressed exclusively in macrophage populations from reticuloendothelial organs and in the macrophage line J774A. Nramp encodes an integral membrane protein that has structural homology with known prokaryotic and eukaryotic transport systems, suggesting a macrophage-specific membrane transport function. Susceptibility to infection (Bcgs) in 13 Bcgr and Bcgs strains tested is associated with a nonconservative Gly-105 to Asp-105 substitution within predicted transmembrane domain 2 of Nramp.