January 31, 2000
Volume 52, No. 19
Tetramers an emerging tool for immunology research
By Poul Olson
Measuring the effectiveness of new vaccines or drugs to fight disease has always been a big challenge for immunologists. Until a few years ago, the only way to accomplish this was a cumbersome, time-consuming and often inaccurate test known as a limiting dilution assay.
A new technology called a "tetramer assay," developed in part by John Altman of the Vaccine Research Center, is providing immunologists with a new tool in their research. Last summer the National Institutes of Health launched a unique facility at Yerkes focused on building the synthetic structures needed to perform the test, today considered the "new gold standard" for measuring specific immune responses.
Tetramer technology applies basic principles of immunology to measure the body's response to a foreign invader. When a virus enters the body, two lines of defense are marshaled to attack the pathogen both inside and outside cells; a type of white blood cell called a T-lymphocyte is activated to target and kill infected cells. For a vaccine to work, these killer T-cells must be present at sufficient levels, and a tetramer assay tells immunologists if this has been achieved.
To construct a tetramer, a piece of protein from a virus is first bound to four specialized molecules found on the surface of all normal cells. The tetramer binds to the killer T-cells that carry a specific receptor-much like a key fitting into a lock. When the tetramer is placed in a pool of lymphocytes or blood, only those T-cells with the correct receptor will bind to it. A device called a flow cytometer sorts the cells one by one to determine those T-cells that have made the match.
"Tetramers will tell you if a vaccine protocol has worked," Altman said. "For a large-scale trial, it would take too much time to perform a limited dilution assay on every subject, and almost no aspect of these tests can be automated. In contrast, because the tetramer assay is so quick and robust, it can be applied to samples from large numbers of patients, and some of the data acquisition can be automated."
In its five months of operation the Tetramer Core Facility, under the direction of John Lippolis, has produced some 150 tetramers for scientists all over the world. A majority of these have been for vaccine trials for HIV and SIV (Simian Immunodeficiency Virus, the HIV equivalent for nonhuman primates).
Tetramers also have been made for studies involving T-cells that attack tumors, Epstein-Barr virus, herpes simplex, hepatitis B and C, melanoma and various bacteria.
In addition to their accuracy, tetramers are quick-they can tell scientists in three to four hours whether the desired immune response has occ-urred. Conventional limited dilution assays often take three to four weeks to produce a result.
Another advantage is tetramers do not require cells to be functional. Many diseases, such as SIV and HIV, render T-cells "impotent," a state dilution assays cannot recognize. With a tetramer, however, even inactivated T-cells involved in immune response can be identified.
Nearly all of the Tetramer Core Facility's business to date has come from word-of-mouth, but recently the facility began running advertisements in the journal Immunity and Science in hopes of generating more widespread interest.
Investigators interested in having a tetramer constructed must apply through NIH's Tetramer Facility Guide web site at www.niaid.nih.gov/reposit/tetramer/index.html and have their request approved by a review committee.
Except for shipping and production of the peptide strand, the cost of
customizing a tetramer is free. Upon receipt of the peptide, Lippolis and
his technicians can produce tetramers within six to eight weeks.