Maintenance of a low viral load minimizes the development of AIDS in an HIV-infected individual.1,2 A soluble factor, HIV-Suppression Factor (HIV-SF), released by activated CD8+ T-cells has been reported to mediate suppression of HIV replication in CD4+ T-cells.3 A decline in the ability of CD8+ cells to produce this factor correlates with progression of disease.3 What is this factor that appears to limit progression of HIV infection to AIDS? It is, at least in part, a combination of chemoattractant proteins, RANTES, MIP-1 alpha, MIP-1 beta4, and IL-16.5
To identify HIV-SF, Gallo, Lusso, and colleagues4 used two types of lymphocyte-derived cell lines. A line of CD4+ cells that are readily infected with HIV in vitro served as a tool for measurement of HIV replication, and immortalized CD8+ cells, producers of the factor, as an adequate source of material for purification.
Three separate CD8+ cell lines produced HIV-SF at different levels. The best was subcloned at limiting dilution, producing several clones that produced the factor in substantially larger amounts than the parent cell line. From one of these clones, they were able to purify active fractions.
Three proteins with HIV-SF activity were purified. Partial sequence analysis of these proteins revealed that they were RANTES, MIP-1 alpha, and MIP-1 beta, members of the C-C family of chemokines.
The full inhibitory activity was attributed to a combination of the three chemokines. The culture supernatant solution of the most active clone inhibited viral replication in cultured CD4+ cells more than 95%. A neutralizing anti-RANTES antibody reduced this inhibition to about 80%, while anti-MIP-1 alpha or anti-MIP-1 beta had little effect. But a mixture of the three antibodies completely blocked the inhibition. Supernatant solutions from CD8+ cells isolated from HIV-infected patients were tested in the same way. While the amount of HIV-SF activity in these supernatant solutions varied among patients, again the combination of antibodies against RANTES, MIP-1 alpha and MIP-1 beta, but no one alone, completely blocked the suppressor activity.
Recombinant human chemokines were tested for inhibition of HIV replication in the cultured CD4+ cells. The potencies were generally in the order RANTES > MIP-1 beta > MIP-1 alpha, though MCP-1, another member of the C-C subfamily of chemokines, had no effect. These relationships were not identical for each HIV isolate.
The concentrations of chemokines in the CD8-culture supernatant solutions and the concentrations of recombinant proteins required for inhibition were within the ranges required for other physiological actions of these chemokines.
In a related study, Kurth and colleagues5 compared HIV to SIV, a closely related simian counterpart that infects many monkeys without detectable ill effects to the monkey. In analogy with the reported HIV-suppressor activity reported to be released by CD8+ lymphocytes3, Ennen et al.6 reported that SIV replication in CD4+ cells from African green monkeys was inhibited by a soluble factor released from autologous CD8+ cells. Baier et al.5 found that at least part of the suppressor activity is due to IL-16.
IL-16 (previously referred to Lymphocyte Chemoattractant Factor) is a cytokine without homology with other known cytokines.7 It is secreted from CD8+ cells8, and it induces a migratory response in cells (lymphocytes, monocytes, and eosinophils) expressing the CD4 molecule.7
Baier et al.5 cloned IL-16 from the African green monkey and found that it differed from human IL-167 by only seven non-clustered amino acid residues out of the total 130 residues. Either human IL-16 or African green Monkey IL-16 inhibited HIV-1 replication in CD8+-depleted peripheral blood mononuclear cell preparations.5
A high concentration was required for inhibition, and the human IL-16 was less potent than the monkey protein, a fact that the authors suggest is due to improper folding of the bacterial-derived recombinant proteins. It is also possible that the monkey protein is just more effective, explaining why monkeys do not succumb to SIV, and the high concentrations may be because the protein normally acts in synergy with other factors, such as the chemokines described previously.
These studies open an important window into HIV and AIDS. Like most seminal reports, they raise as many new questions as they answer old ones. What is the mechanism of suppression of HIV replication? Are chemokines general antiviral agents, or do they have a special role with HIV and related viruses? Are there other factors? Do levels of these cytokines in serum of HIV-infected individuals correlate inversely with progression to AIDS?