By David N. Leff
Remember ¿The Godfather¿? That Mafia don never forgave or forgot an insult or injury. He carefully briefed his hit men to find and execute the victims of his revenge.
A similar scenario describes how the mammalian immune system is organized to terminate its pathogenic targets with extreme prejudice. Its hit men are called memory T cells. Once one of these killer cells encounters and destroys, say, an invading virus, it remembers that enemy¿s face ¿ its antigen ¿ for the lifetime of its human or animal host.
Where in the body these cytotoxic T lymphocytes come from is something of a baffler. ¿Their origin is very vague and obscure,¿ observed molecular immunologist Philip Ashton-Rickart, at the University of Chicago. He spelled out the two schools of thought vying to explain this basic question, and tested the validity of each.
¿We asked, Do memory cells directly derive from T lymphocytes carrying CD8 antigens?¿¿ he said. ¿Are they the progeny of effector [capable of cell-killing] lymphocytes? Or do they rise from a different developmental pathway, which runs parallel to the development of cytotoxic effectors?¿¿
One of the two competing models sees hordes of cell-killing T lymphocytes attacking the alien intruder, then dying off by apoptosis after they¿ve routed the foe. But a small cohort of these T cells stands aside from the engagement. It¿s pre-programmed to survive, and sworn never to forget the contours of the invader¿s antigens.
The contrary concept involves only a single population of killer T cells taking part in the counterattack, with some of the survivors developing into ever-vigilant memory cells, ready to take on the enemy if it ever shows its face again.
Together with his associates, plus cohorts of knockout/transgenic mice, Ashton-Rickart confirmed the second model¿s validity.
Besides the intellectual challenge of adjudicating between these two interpretations on a molecular level, he pursued a more practical purpose: ¿If you want to make a good vaccine which induces T-cell memory, you have to know where those memory cells come from,¿ he said. ¿So, we found that memory cytotoxic T cells were the progeny of differentiated [fully developed] cytolytic effectors.¿
Today¿s issue of Science, dated March 12, 1999, reports how Ashton-Rickart and his co-authors solved that long-standing puzzle. Its title: ¿Linear differentiation of cytotoxic effectors into memory T lymphocytes.¿
However deadly they are in action, T memory cells are slow learners. They have to ¿see¿ large quantities of antigen before graduating from naove lymphocytes to mature memory cells. Their training course exposes them to large quantities of the antigen they must memorize.
¿What that means in a practical situation,¿ Ashton-Rickart said, ¿is that if you want to make a memory cytotoxic T cell, you first have to get to the effector stage, which requires a large amount of antigen to force the differentiation all the way through. And that is difficult or impossible to achieve with traditional vaccine techniques, because the amount of antigen you¿d have to give to a person to achieve that would probably kill them.
Current Vaccines Lack Memory T Cells
¿But it works pretty well for B cells and their antibody production,¿ he added, ¿because, indeed, B cells use the second mechanism of differentiation. There you have this separate lineage to make memory cells, which require low antigen doses.¿
That finding, he suggested, ¿could explain why traditional vaccine technology ¿ which uses inactivated antigen, or just a part of the pathogen ¿ works for B cells, which can use low antigen doses, but doesn¿t work to make T-cell memory. And the final thing with our findings was that the possible applications may be obvious to treat viral infections, which are chronic and untreatable. One obvious candidate is HIV.
¿Evidence now implicates T lymphocytes as quite important in controlling HIV,¿ Ashton-Rickart said. ¿Our procedure wouldn¿t be used to immunize the entire population; that would be too expensive. But, once a patient gets infected, you can take their white blood cells ¿ their lymphocytes ¿ and make memory cells in vitro, as we do in mice. Then, [you can] put them back into the person, where they should work by killing virally infected cells.¿
Ashton-Rickart made an added point: ¿Where people have had problems with HIV treatments, they have tried putting CD8 cells ¿ not enriched for memory cells ¿ back into patients. These work for a short time, but the resistance to the virus wanes, and that is presumably because they haven¿t got memory cells. But, in our hands, the numbers are going up, so they could potentially last the lifetime of the individual. Our memory cells don¿t require any CD4 help, which is what¿s lacking in an AIDS patient. They will immediately kill any cell presenting the antigen.¿
The same strategy, he observed, ¿could be applied to the development of antitumor as well as antiviral vaccines.¿
Tests Star Double-Duty, Knockout/Transgenic Mice
The co-authors¿ in vitro procedure for vaccinating mice against a symbolic pathogen begins with the target antigen, expressed in the transgenic aspect of their dual-function mice. It¿s a human, male-specific protein called H-Y. ¿This is a transplantation antigen that only males make,¿ Ashton-Rickart explained. ¿Even if you have two individuals who are matched for immune type, the female recipient will still reject the male donor ¿ or vice versa ¿ because the male H-Y antigen is encoded by a gene on the males-only Y chromosome.¿
The knockout feature of the mice, called RAG-1 (for recombination activation gene-1) lacks a protein required for rearrangement of T-cell receptor and immunoglobulin genes. ¿If you can¿t rearrange these genes,¿ Ashton-Rickart said, ¿then you don¿t get any T or B lymphocytes developing. These mice, which lack this gene, are devoid of T and B cells. So, they¿re a kind of immunological empty test tube ¿ the same as people used to use SCID or nude mice for.¿
During the next year, he intends to ¿see if this preclinical work has any clinical utility, by collaboration with physicians treating HIV, using my technique to restore immunological memory to AIDS patients. We seem to have stumbled into the world of clinical relevance.¿ n