By David N. Leff

Science Editor

Editor's note: Science Scan is a roundup of recently published, biotechnology-relevant research.

Like a schoolyard bully who picks only on small, weak kids during recess, the human cytomegalovirus (CMV) singles out and infects only people with weakened immune defenses. In particular, it can cause life-threatening disease in organ-transplant patients and individuals with AIDS.

In late-stage AIDS patients, CMV inflicts retinitis, which leads to blindness in 20 percent of them. But 90 percent of the human population harbors latent cytomegalovirus, most of whom are unaware of it. Yet human CMV infects a wide variety of cells and tissues in people with compromised immune systems. Among its targets of opportunity are the endothelial cells that line blood vessels. Thus, the viral pathogen promotes atherosclerosis and rapidly progressing coronary artery disease in cardiac transplant patients and in the development of coronary restenosis after angioplasty .

What's more, increased endothelial cell and vascular smooth muscle proliferation may result in thickening of the arteries, unless offset by increased apoptosis - programmed cell death. Researchers would like to get to the root of the ways and means by which CMV beats up on endothelial cells, but they're frustrated by the fact that laboratory strains of the human virus don't replicate in such cell cultures.

That's the bad news. The good news is that human and murine CMV share a similar pathobiology, and have co-linear genomes. These were recently cloned in E. coli as infectious bacterial artificial chromosomes (BACs), where they can be readily mutated.

A paper in the current issue of Science, dated Jan. 12, 2001, focuses on the genetic basis of this viral predilection for endothelia. The article's title: "A ribonucleotide reductase homolog of cytomegalovirus and endothelial cell tropism." Its co-authors are at Princeton University in New Jersey and the Ludwig-Maximillan University in Munich, Germany.

They generated and recovered 199 viable mutant mouse viruses, and discovered a CMV gene called M45 that controls the ability of the virus to grow on endothelial cells. What's more, cells infected with these M45 mutant proteins die off rapidly by self-suicide. From this, the co-authors construe that normal, nonmutant M45 protects the virus from apoptosis by thwarting the destruction of infected cells. When they infected endothelia at a high multiplicity of infection, most cells infected with mutant virus were dead 30 hours later. Normal, nonmutant virus showed cytopathic effects but were still alive.

The paper suggests that "the M45 gene encodes or activates an inhibitor of apoptosis that is indispensable for virus growth and spread in endothelial cells." However, it adds, "it is conceivable that the observed phenotype is not strictly confined to endothelial cells and macrophages, but generally applies to cells that are more prone to undergo apoptosis."

Rapid-Response 'Innate' Immunity Yields Its Molecular Secrets, A Putative Boon To Vaccinologists - Maybe

The immune defenses we rely on to ward off or kill off the pathogenic viruses, bacteria and fungi - not to mention allergenic proteins - that besiege the human race comprise an arsenal of two basic weapons - antibodies and T cells. Besides marching off against the enemy's attacks, these two arms of the immune system - A and B - beef up our preparedness by means of vaccines against invading microorganisms.

But it takes time to mobilize and deploy these defenses. Which is why an advance force, the innate immune system, goes first and fastest into the fray. These innate, rapid-response elements don't exist just to protect Homo sapiens; they show up in all life forms - from bacteria, insects and birds to mammals. They, too, are assailed by pathogens, for which they need innate countermeasures.

The immune system recruits receptor proteins that recognize a microbial pathogen by the molecular patterns displayed on its surface. Many of these receptors are involved in innate responses to microbial lipids and carbohydrates. Now, Japanese scientists have identified the mammalian protein that triggers an innate immune reaction to bacterial DNA. They report this finding in Nature, dated Dec. 7, 2000.

Their paper bears the title: "A Toll-like receptor recognizes bacterial DNA." It shows how the mammalian immune system distinguishes bacterial DNA - itself a potent inducer of innate immunity on its own behalf - from that of the host organism.

The co-authors, mainly at Osaka University's Research Institute for Microbial Diseases, describe the mammalian protein - 'Toll-like receptor 9' (TLR9) - that recognizes a specific pattern in bacterial DNA. They point out that "DNA from bacteria depend on the presence of an unmethylated pair of nucleotides - CpG (cytosine and guanine) - to stimulate mammalian immune cells. In contrast, these have few CpG dinucleotides and are mostly methylated, so they lack DNA stimulation.

"Accumulating evidence," the paper points out, "has revealed the therapeutic potential of CpG DNA as adjuvants for vaccine strategies for cancer, allergy and infectious diseases." Their studies in knockout mice devoid of TLR9 reveal that "vertebrate immune systems appear to have evolved a specific Toll-like receptor that distinguishes bacterial DNA from self-DNA."

But before planning to turn over this finding to the vaccinologists, consider a caveat from the editorial accompanying the Osaka article, titled: "A Toll for DNA vaccines." Its author, immunologist Robert Modlin at the University of California at Los Angeles, cautions: "The activation of Toll-like receptors can also be detrimental to the host. It can contribute to tissue injury in the form of apoptosis and the life-threatening symptoms of septic shock."