In the war between Homo sapiens and Mycobacterium tuberculosis, the battle has turned in TB's favor, thanks to two logistical defeats for mankind:
* One chemical-warfare weapon after another is falling to the enemy, as the bacterium develops resistance to one antibiotic after another.
* Host defenses over large parts of the earth are crumbling, as the immunodeficiencies of HIV and malnutrition spread through Third-World populations and First-World inner cities.
In fact, with the earth's population pushing 6 billion people, around 2 billion of its men, women and children are going through life playing host to infection by M. tuberculosis microbes.
"In many of these people," said immunologist Carl Nathan of Cornell University Medical College in New York, "the primary infection -- which is where the disease appears shortly after the organism enters the body -- is not clinically apparent. They don't get sick from that, and the organism stays quiescent for the lifetime of the individual."
But not always. "In maybe five percent of these people," Nathan continued, "at some time later in life, the microbe escapes the undefined mechanism of control that has been exerted on it for so many years, and starts to proliferate. Its numbers increase, and it begins to cause tissue damage and clinically active disease."
Early in the 1990s, Nathan and his colleagues identified and cloned a gene that expresses nitric oxide synthase (NOS). This enzyme produces the gaseous free radical, nitric oxide (NO). (See BioWorld Today, Nov. 5, 1996, p.1)
Nathan is senior author of a paper in today's Proceedings of the National Academy of Sciences (PNAS), dated May 13, 1997. Its title: "Identification of nitric oxide synthase as a protective locus against tuberculosis."
This enzyme, INOS (I for 'induced') is induced by various lymphocytes and cytokines in the immune system, Nathan told BioWorld Today, as well as by lipopolysaccharide -- the bacterial endotoxin that causes septic shock. "Induced nitric oxide synthase [INOS] can be widely expressed in different cell types in mammalian hosts," Nathan pointed out, "provided one finds the appropriate stimuli to induce it. We've concentrated on its expression in macrophages," he went on, "because those are the cells that actually eat the M. tuberculosis, and are responsible for controlling it.
"So INOS is a host-defense weapon against TB," Nathan observed, adding, "We have found the enzyme in human macrophages, from patients with pulmonary TB. And we can infer that those macrophages, in which the organism can lie dormant for decades, express INOS. Also we have cloned NO-resistance genes from human isolates of the bacterium."
From these findings, he continued, "it's our speculation, not tested yet, that these could be new antibiotic targets. That is, if one could inhibit the pathogen's defense against our own innate antimicrobial mechanism, this might make those mechanisms more effective."
Therapeutic Drug Vista Over Horizon
The scenario he pictures would "find compounds that inhibit NO resistance genes in the bacteria, and test them in mice. If they made the mice better able to rid themselves of TB through the high-output NO pathway, then those candidate drugs might be of interest for experimental trials in patients."
From his studies in the PNAS paper, Nathan infers "that in mice, both stages of the disease -- primary and reactivated -- require INOS for their control. That is, without the enzyme the primary disease is going to be rapidly lethal instead of initially contained.
Also," he continued, "if you don't have that enzyme, the prolonged phase of clinical dormancy is going to be interrupted at the moment that the INOS ceases to be active. Then, recrudescence of the disease will occur."
Thence, another speculation: "That in people one reason for reactivation could be that the host, for whatever reason, ceases to maintain active expression of INOS in the lungs, or other TB lesions."
This concept comes back full-circle to the reason, or reasons. "It could be that age or malnutrition prevents the ongoing signals of INOS activation," Nathan observed. "Or else, steroid therapy could supervene and prevent the expression of INOS in response to those signals."
Beware Long-Term Steroid Medication
Steroid medication is a salient risk factor for TB infection in the First World of sophisticated drug therapies. "Clinically," Nathan pointed out, "if you have people who for some reason must take steroids for an extended period of time, they are very vulnerable to reactivating the TB they've been carrying around unbeknowst to themselves for some years. The well-known ability of steroids to block expression of INOS could account for their making people susceptible to both primary and reactivation infection."
The PNAS paper demonstrated this nefarious role of steroids in knockout mice that Nathan's group constructed, which lacked the INOS gene. These INOS-minus-minus homozygous knockouts "proved highly susceptible to TB, resembling wild-type littermates immunosuppressed by high-dose glucocorticoids, and allowed Micobacterium tuberculosis to replicate as fast or faster in the lungs than reported for other gene-deficient hosts."
Total absence of the nitric-oxide releasing enzyme "led to rapid bacterial growth, necrotic granulomatous pneumonitis, and death." But animals with only one NOS gene, rather than none, "still retained sufficient INOS activity to extend their survival as long as mice possessing the full chromosomal complement."
Molecular microbiologist C. Kendall Stover, who works on TB pathogenesis, is senior director of biological research at PathoGenesis Corp., in Seattle. (See BioWorld Today, June 16, 1996, p. 1) He is acquainted with Nathan's work, and commented to BioWorld Today:
"Nitric oxide production in macrophages has been thought for some time to be involved in killing of intracellular pathogens such as TB. That's been shown in cells, but in mice it had never really been shown directly for TB. While this PNAS report is an important finding in mice, it's highly controversial still as to whether a NOS or NO killing mechanism is operational for pathogens in people. Mice are much more innately resistant to TB than humans are."
Stover concluded: "I think it's a significant finding, and bears further study, but extending those findings to infections in humans has still got to be done." *