By David N. Leff

Editor's note: Science Scan is a round-up of recently published biotechnology-related research:

Can a carrot a day keep the eye doctor away?

This brightly hued root vegetable gets its name, and its color, from carotene, a ruby-red pigment that is the chemical precursor of vitamin A. And that fat-soluble vitamin, which also tints butter, egg yolk and many other foods, is a key component of rhodopsin. Also known as "visual purple," rhodopsin is the light-sensitive pigment of the eye.

It translates light falling on the retina's rhodopsin-tipped photoreceptor rods into nerve impulses reporting images to the brain. A protein in the retinal pigment epithelium (RPE) processes vitamin A in the visual system, by converting it to a form called 11-cis-retinal, which is essential to vision. (See BioWorld Today, Feb. 6, 1995, p. 1.)

The title of a paper in the December 1998 issue of Nature Genetics reports that the Rpe65 gene "is necessary for production of 11-cis-vitamin A in the retinal visual cycle." Its first author is T. Michael Redmond, at the National Eye Institute's Laboratory of Retinal Cell and Molecular Biology, in Bethesda, Md.

He and his co-authors tackled an unanswered question: What contribution does the RPE65 protein make to mammalian vision? In their approach, they took testimony from blind mice in which they had disrupted the murine Rpe65 gene, which is 95 percent identical to the human sequence.

Their discovery of 11-cis-vitamin A's role in vision, extrapolated from mice to people, showed how defects in this gene lead to blinding retinal dystrophies. "This research," said the National Eye Institute's director, Carl Kupfer, "provides hope for some people with severe retinal dystrophy, such as Leber's congenital amaurosis, who are born blind or lose their vision in early childhood. These results may allow scientists some day to develop treatments that will restore vision to people who have been blind for most of their lives." (See BioWorld Today, March 25, 1998, p. 1.)

Redmond and his co-authors found that in their Rpe65-minus mice, vitamin A piled up unused in the retinal pigment epithelium cells. "When you take away the RPE65 protein from these cells in animals," he said, "the conversion process of vitamin A stops dead in its tracks. The mice tell us that the RPE65 protein is a major player in that conversion."

He explained the carrot connection. "When you eat a carrot, Vitamin A — which is converted from the carrot's beta-carotene — goes to the retinal pigment epithelium cells, which convert the vitamin into 11-cis-retinal. This then combines with opsin to make rhodopsin. But the defective RPE cells do not convert the vitamin A into 11-cis-retinal, so the vitamin just accumulates in those cells.

"If we can reverse the blindness in mice," Redmond concluded, "we can think about reversing the blindness in humans. Perhaps if we replace the defective Rpe65 gene with a normal gene, we can use the framework already in the eye and restore visual sensitivity. It won't be easy, but we have a starting point."

Mouse Trials Of Anti-TB Monoclonal Antibodies Refute Dogma That Only Cellular Immunity Works

Immunologists waging war against Mycobacterium tuberculosis (TB) may be fighting with one arm tied behind them.

The human immune system packs a one-two punch — cellular and humoral. The cellular arm fields battalions of T lymphocytes to kill intruding infectious agents, plus monocyte/macrophages to engulf and digest them. The humoral arm generates antibodies programmed to target specific antigens flagging particular pathogens.

"Cell-mediated immunity," said immunologist and microbiologist Barry Bloom, at Albert Einstein College of Medicine in Bronx, N.Y., "has been shown to be essential for protection from tuberculosis infection." But, he added, "Unfortunately, a possible role for antibodies against tuberculosis infection has largely been ignored." Ostensibly, this interior niche in which the disease hides out makes it inaccessible to antibodies.

Bloom is senior author of a paper in the current Proceedings of the National Academy of Sciences (PNAS), dated Dec. 22, 1998, titled: "A mAb recognizing a surface antigen of Mycobacterium tuberculosis enhances host survival."

The hosts in question are mice challenged with rapidly lethal TB organisms. The monoclonal antibody zeros in on unique antigens decorating the mycobacterium's surface.

Bloom decided to test the dogma that excuses humoral immunity from anti-TB duty after checking antibodies out in another species, Cryptococcus neoformans. This is an infective, yeast-like fungus that shares with M. TB the intracellular lifestyle of hunkering down inside an infected cell, rather than attacking it from outside.

The mice he treated with monoclonals aimed at the fungus's enveloping polysaccharide antigen experienced "markedly enhanced survival."

"These studies," Bloom's paper pointed out, "reveal that antibodies can participate in the developing immune response to intracellular pathogens. Since human tuberculosis is thought to result from infection with as few as 10-200 tubercle bacilli, we sought to explore the possibility that antibodies specific to the surface of M. tb might prevent infection, or modify the course of experimental tuberculosis."

His designated humoral hitter for immunizing the mice was a monoclonal antibody, 9d8, trained expressly to complex with the microbe's arabinomannan antigen. The precedent-setting anti-C. neoformans monoclonal was assigned to serve as an irrelevant control.

For starters, the co-authors incubated approximately 200,000 virulent TB bacilli, coated with 9b8 mAb ascites, or with control antibodies, which they administered to the mice intratracheally. Later, they upped the ante to one million bacilli and higher titers of 9b8. Other mice got the mixture intraperitoneally at intervals before and after the infectious bacterial challenge.

In four independent experiments, 35 percent to 100 percent of the mice that received the 9b8 mAb immunization were still alive 220 days later, whereas the controls were all dead.

However, the lungs and spleens of both cohorts were equally infected with M. tuberculosis. From this unexpected finding, Bloom concluded that "the 9b8 mAb appeared to confer some protection against mortality but apparently did not prevent infection."

Given that the upsurge of TB in big-city populations coincides with the AIDS epidemic, the Einstein investigators repeated their antibody experiments in immunocompromised mice. Even some of those knockout animals enjoyed enhanced survival.

Despite a slew of unanswered questions raised by these preclinical experiments, Bloom's PNAS paper concluded: "The results offer encouragement that it may be possible to harness specific humoral responses that are not lacking in individuals with HIV/AIDS for developing improved preventive vaccines or therapeutics against tuberculosis." *