CD8 Cells: Suppressive Factors Discovered

Nine years ago researchers at the University of California San Francisco Medical Center reported that CD8 cells could produce a soluble substance or substances which could slow or stop the growth of HIV.(1) When the CD8 cells were taken out of the infected cell culture, HIV grew again; when they were added back, HIV growth stopped. Direct contact was not needed, as the CD8 cells could be in a separate compartment, separated from the infected cells by a filter, and they still stopped HIV growth. Laboratory studies of samples from patients showed that in those whose disease progressed, the CD8 cells lost much of their ability to inhibit HIV in this way. Unfortunately, efforts to identify the substance or substances were unsuccessful; however, other research indicated that the mechanism of action seemed to be inhibition of the LTR (long terminal repeat) of HIV.

In December 1995, four such inhibitory substances were reported. Three were found by researchers at the U.S. National Cancer Institute Laboratory of Tumor Cell Biology, then run by Robert Gallo, M.D.; these need to work together, as each substance alone had little or no effect.(2) Also in December, a separate research team in Germany reported a fourth inhibitory substance, in a letter to NATURE. All four of these turned out to be substances which were previously known. (Note: Several members of the research team at the NCI are now joining Dr. Gallo at the Institute for Human
Virology, a new research center at the University of Maryland.)

Gallo's laboratory used fairly straightforward procedures to discover three of the substances, which are proteins and members of a class called chemokines, substances involved with inflammation and which cause cells to move. First, the researchers set up a laboratory test to measure the amount of suppressive activity in a given sample. Then they selected cells which produced large amounts of this factor. To find
out what it was, cell-free material from the cultures was chemically fractionated (separated) and purified in various ways, and then tested to see which fractions kept the suppressive activity and which lost it. Finally, two fairly pure substances could be analyzed by standard methods to
determine what amino-acid sequence a protein contains. The sequences turned out to be identical to those of substances already known. Additional tests confirmed that these two were indeed correctly identified. (The third substance was tested for because it was very similar to one of the first two. It also was found to be present in the samples.)

As additional confirmation, antibodies to the three proteins were prepared. In cultures from three of four patients tested, they blocked all of the suppressive activity; in the fourth patient, they blocked 80% of it. Antibodies are quite specific in what proteins they recognize; the antibodies in action of the three proteins they were targeted against, showing that those proteins were responsible for the suppressive activity.

No one knows if these proteins themselves would be effective treatments. Some substances (such as IL 2) are produced by the body and tend to be used locally, by nearby cells; injecting a large amount systemically may not work the same way. But the identification of these three substances is certainly important for new-drug development, whether or not a final drug turns out to be a cocktail of these three, or something else.

Also, measurement of these substances in the blood might serve as a marker of AIDS progression, or of the therapeutic effects of certain drugs, or of the protective effect of preventive vaccines. The researchers noted that clinical studies of these newly identified proteins "will be critical to define their role in the natural history of HIV infection."

References

1. Walker CM, Moody DJ, Stites DP, Levy JA. CD8+ lymphocytes can control HIV infection in vitro by suppressing virus replication. SCIENCE December 19, 1986; volume 234, pages 1563-1566.

2. Cocchi F, DeVico AL, Garzino-Demo A, Arya SK, Gallo RC, and Lusso P. Identification of RANTES, MIP-1-a, and MIP-1-b as the major HIV-suppressive factors produced by CD8+ T cells. SCIENCE December 15, 1995; volume 270, pages 1811-1815.