By John Easton
Medical Center Public Affairs
April 15, 1999
Vol. 18 No. 14
Researchers from the University reported in the March 12 issue of the journal Science that the cells that are crucial players for any vaccine against HIV-infected or cancerous cells are distressingly slow learners.
The problem, the researchers demonstrate, is that it takes several generations of intense instruction to make a lasting impression on a T cell. Creating large numbers of “memory” T cells that can recognize a troublemaker they have seen before and attack when they see it again requires prolonged, continuous exposure to high levels of the intruder.
“This finding suggests that the typical approach to vaccines for treatment of cancer or AIDS is not often likely to produce the desired result,” said author Philip Ashton-Rickardt, Assistant Professor in Pathology at the University. “But it also shows us how we can get around the problem.”
Ashton-Rickardt’s team––including immunology graduate student Joseph Opferman and postdoctoral fellow Bertram Ober, all from the Gwen Knapp Center for Lupus and Immunology Research at the University––set out to answer a central question in immunology: Where do the T cells responsible for “remembering” a previous infection and fighting it off a second time come from?
When stimulated by an invader, T lymphocytes multiply and attack the infecting foreigner. Once they get the upper hand, most of these T cells are no longer needed and die off. A small percentage, however, survives and stands guard in case this particular invader comes back.
Immunologists have proposed two models for this process. One requires two parallel tracks for cell-killing T lymphocytes. Most of the T cells attack the foreigner then die off soon after victory. But a smaller group of T cells is preprogrammed to survive and to remember the invader. B lymphocytes, the immune cells that secrete antibodies, follow this pattern.
The competing model involves just one initial population of cell-killing T cells. Most of these cells die off after defeating the invader, but some cells survive and develop into memory cells that remain eternally vigilant for any subsequent attack.
The research team confirmed the second model. They found that it required intense, prolonged stimulation-high levels of antigen for at least five cell divisions-to create a significant number of memory T cells. Without strong stimulation for three to four days, few memory cells emerged.