When Mycobacterium tuberculosis infects a new TB victim,instead of zooming right in on its favorite target tissue, usuallypulmonary, the cagey pathogen holes up invisibly andasymptomatically in macrophages. Safe in-side these garbage-collecting immune-system cells, the TB agent bides its time,often for years or a whole human lifetime, until some cue,perhaps a sudden drop in its host's immune defenses, promptsit to exit and attack its victim's lungs directly.
How M. tuberculosis breaks and enters the macrophage is aconundrum. How it escapes the fate of the engulfed wasteparticles normally scavenged and elimi-nated by thosephagocytic cells is another riddle. A third enigma: Does itactually replicate inside the cell?
Preliminary solutions to all three mysteries surfaced in lastFriday's Science in a report titled "Cloning of an M. tuberculosisDNA Fragment Associated With Entry and Survival InsideCells." Molecular biologist Lee Riley of New York's CornellMedical College is the paper's principal author.
He and his co-authors set out to blow the TB pathogen's coverby proxy. Instead of M. tuberculosis itself as the invadingpathogen, they used a stand-in microbe, a non-pathogenicstrain of Escherichia coli, which normally has no such invasiveproperties. Instead of macrophages, Riley set up cultures ofimmortal and non-phagocytic human HeLa cells, which M.tuberculosis also penetrates in vitro. Had Riley employedmacrophages, they would have quickly engulfed the E. coli,explained Stuart Gunzberg, a post-doctoral fellow in Riley's lab.
The Cornell team transformed the imposter E. coli withplasmids con-taining a 1,535-base DNA fragment they isolatedfrom an M. tuberculosis ge-nomic library. This invasive-genesequence, somewhere along the first 850 bases of the fragment,encoded a protein that engineered the pathogen's ability topenetrate the HeLa-cell (and presumably macrophage) walls.The remaining 685 nucleotides apparently encode themicrobe's staying power in the cell, once it gets inside. Andelectron microscopy revealed larger numbers of E. coli cellswithin the macrophages than went in, documenting theirintracellular proliferation.
The Science article concluded: "Drugs designed to target an M.tuberculosis product mediating macrophage survival, orvaccines against a product encoding mammalian cell entry,could contribute substantially to the worldwide TB controlstrategies."
For openers in these directions, Gunzburg told BioWorld, "onemight get an antibody to the invasiveness protein, then see ifthis blocks the invasion. Then this protein could be used toimmunize, and therefore prevent the TB penetration."
He also suggests that linking certain drugs to the invasivenessprotein may enhance their delivery across cell membranes."We're currently looking in some depth at purifying theprotein, doing all the appropriate animal studies, thenhopefully going to human studies," he said.
Nor has it escaped Riley's notice that such a cell-trespassingDNA sequence also has potential for gene therapy.
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.