David N. Leff
Editor's note: Science Scan is a roundup of recently published, biotechnology-relevant research.
At 2 months of age, a newborn baby gets its first shots - a vaccination cocktail against diphtheria, whooping cough, tetanus and polio. At 15 months, a measles-mumps-rubella vaccine beefs up its immunity against the main remaining infections of childhood.
What those needles are pumping into the infant's body are mixtures of the whole or partial, inactivated or attenuated, bacteria or viruses that impart their respective diseases. But grown-ups, too, can get immunized against the hazards of influenza, hepatitis B, rabies - and now anthrax, for the U.S. military. Not to mention ongoing efforts to perfect vaccines against malaria and AIDS.
What all these menacing microorganisms have in common is some sort of epitope - an antigenic bull's eye facing the immune system's antibodies. One way that defense system rattles its antibody's chains is its dendritic cells. (A dendrite's principal gig is to bring such foreign antigens to the attention of the immune system's humoral B-cell arm, which proliferates its antigen-killing cytotoxic T cells to order.)
All B cells fly flags called CD40 on their membrane surface. These send signals to one subset of T cells, called CD4+, or T helper cells, which display their opposite-number signal, the C40 ligand. It takes those two to tango - that is, inspire those B cells to churn out in quantity the specific antibody programmed to attack and destroy that captive antigen.
But there was a downside. The consequences of having CD4+ T cells as the intermediates in controlling immune responses result in inadequate host reactions against infectious agents when there is an insufficient number of these cells, as in immune deficiency disorders, such as after infection with HIV.
All of which leads to a paper in the just-released October issue of Nature Medicine, titled: "Dendritic cells genetically modified to express CD40 ligand and pulsed with antigen can initiate antigen-specific humoral immunity independent of CD4+ T cells." Its senior author is pioneer gene therapist Ronald Crystal, who heads pulmonary and critical care at the Weill Medical College of Cornell University in New York.
This development, Crystal told BioWorld Today, embodies a double-barreled vaccine strategy.
"The new concept," he elaborated, "is to genetically modify dendritic cells so that we are essentially morphing them into a combination with a CD4+ helper cell. Antigens," he explained, "are engulfed and eaten by dendritic cells, presented to CD4 cells."
So he and his co-authors sought to determine if it is feasible to genetically modify dendritic cells to express CD40 ligand, and thus incorporate the functions of CD4+ cells. "This," the paper suggested, "should enable the dendritic cells to interact with microbe-specific antigens and directly activate B cells, thus generating microbe-specific protective antibodies without CD4+ T-cell help."
"The concept we asked ourselves was," Crystal went on, "if we took the helper T-cell's CD4 gene and CD40 ligand, and put them into a dendritic cell, could we convert that duet to function in the dual roles of a dendritic cell and a helper T cell? And if so, could we develop a vaccine strategy that was completely independent of CD4 cells?"
Crystal's second concept was to take the pathogenic bacterium Pseudomonas aeruginosa as his vaccine target. He pointed out that P. aeruginosa is the lifelong scourge of cystic fibrosis victims and of burn injury patients.
"The question, of course, for any vaccine," he pointed out, "is: What are the epitopes that you're going to choose for the target?" He and his co-authors decided to feed the whole P. aeruginosa bacterium, after heat-killing it, to the morphed dendritic-cum-CD4 cell. They constructed an adenovirus gene transfer vector expressing murine CD40 ligand cDNA.
"So we were theoretically eliminating the need for CD4 cells," he observed, "and allowing the immune system to choose the antigens, wherever they may be."
The gene transfer construct, pulsed with P. aeruginosa, produced significant amounts of serum antibodies against the bacterium, on the order of 300 percent to 700 percent for Immunoglobulin M, 300 percent to 500 percent for IgG - evidence of the vaccine's B-cell activation. Control animals showed negligible increases.
Vaccinated mice achieved 90 percent survival when challenged with a lethal dose of Pseudomonas, compared with 10 percent or less in controls.
"Our in vivo demonstration of this work," Crystal recounted, "was to inject the genetically modified dendritic cell plus its Pseudomonas component, into the tail veins of naive mice. Then we challenge the mice, through the respiratory route, with a lethal infection of the wild-type bacterium. The results," he added, "were very striking. Not only were the animals protected, but the experiment showed that we don't need helper T cells at all. It appears from the mouse studies that the protection is based entirely on humoral immunity - that is, B cells making an antibody or group of antibodies."
Crystal concluded: "So we're seriously considering moving this novel vaccine into humans. Probably the initial field trials would be in normal volunteers, to see whether or not we can generate the same kind of immunity."
Counterintuitive Infections Assail Recovering HIV Patients With Immune System Counterattack
"Immune reconstitution syndrome" is a paradoxical cause of death, now seen increasingly in AIDS patients on the HAART regimen. It's caused by a backlash of their restored immune systems. A survey article in the Annals of Internal Medicine dated Sept. 19, 2000, reports the unexpected surge of new opportunistic infections. It's titled: "Inflammatory reactions in persons infected with HIV-1 after initiation of highly active antiretroviral therapy."
Its authors, at the Thomas Jefferson University in Philadelphia, propose that the syndrome, which in some cases has been fatal, results from a newly bounced-back immune system's inflammatory reaction to bacteria or viruses already present, but lying low, in the treated patient. They canvassed the strange outbreak at many centers and came up with a laundry list of pathogens that rarely infect healthy persons. Among them: Mycobacterium avium, Mycobacterium tuberculosis, cytomegalovirus, hepatitis C and B, Cryptococcus and Histoplasma species, Herpes zoster.