STEP PERSPECTIVE, Volume 9, No. 1 - Winter 1997; A Publication

From examining the human immune system, scientists have characterized a number of chemical " messengers. " These messengers play a fundamental role in directing a variety of necessary paths in the body's natural response to the invasion of organisms and pathogens which include HIV. In particular, one group of chemical messengers more definitively defined as cytokines, are hormone-like proteins which govern cells of the immune response. Naturally produced in the body by certain white blood cells of the immune system (such as T-helper/CD4 cells), cytokines are an essential link in the communication between cells of the immune system and of the rest of the body. A number of cytokines have been identified and characterized; however, not all of their effects are known. Yet it is apparent that an increase and/or decrease in the amount of particular cytokines is correlated with certain states of disease.

HIV disease causes a decrease in the function of immunity most apparent by an increase in the amount of HIV in the body and a subsequent decline in the number of T-helper/CD4 cells. As activated T-helper cells decrease in number, cytokines that are secreted by them also decline. Therefore, it appears that yet another factor related to this deficiency in immunity is the apparent absence and incapacity of particular cytokines to stimulate the further proliferation of T-cells. As immunologists have become more aware of the identities and effects of various cytokines, AIDS researchers have become more cognizant of the great significance that these chemical messengers play in natural immunity. In particular, two cytokines that have been identified as Interleukin-12 (IL-12) and Interleukin-2 (IL-2) are found to play central roles in one type of central immune reaction called the cell-mediated response. After being invaded by foreign virus or bacteria, the body's response is for particular white blood cells (notably the T-helper/CD4 cells) to prompt one of two distinct immune responses (cell-mediated response vs. humoral antibody response). However, it appears that in the case of HIV infection, balance between these two forms of immune responses becomes gradually upset. As HIV disease progression occurs, there is ultimately a decrease in the cell-mediated response and the humoral response becomes more dominant. Apparently, it is the cell-mediated immune response which appears to be most influential in the control of HIV infection. With the naturally enhancing effects that IL-12 and IL-2 present, there is an increasing interest in the potential for these cytokines to play a part in manipulating the immune system in order to treat HIV and AIDS. Apparently, there is a direct correlation between the levels of IL-12, IL-2 and cellular immunity as the activity of these cytokines stimulates the cell-mediated response. Since there is an evident decline in the production of these cytokines due to a decrease in the cell-mediated immune response, the notion of perhaps countering this suppression by treatment with a recombinant (genetically engineered) form of the lacking cytokine has become more veritable. By treating with IL-12 or IL-2, researchers are encouraged that the weakened human immune response to HIV may be empowered and gradually decrease or stop the pace of disease development. The immunomodulatory qualities of IL-12 and IL-2 have provided us with another hope for perhaps combating HIV infection and AIDS. Their natural abilities to stimulate the number of T-cells (CD4 and CD8) allow for the possibility to replete deficiencies found in the immune systems of HIV-positive persons. Present and future clinical trials involving the administration of recombinant forms of IL-12 or IL-2 examine this premise of maintaining, increasing or stimulating the cell-mediated immune response in HIV-infected individuals.

IL-12 was first discovered in the late 1980's, and by 1991 it was identified by scientists at Hoffman-LaRoche and Wistar Institutes. IL-2 was also extensively studied during the 1980's, but it was first distinguished in 1976 and later cloned in 1983. Originally termed the T-cell growth factor, IL-2 has been studied to date for use in the fields of both cancer and HIV treatment for several years. IL-12 is a chemical messenger which enhances the production and activity of various cells of the immune system such as T-cells, B-cells (which in turn promote antibody production) and natural killer cells (NK cells). Activated natural killer cells are known to have the ability to destroy HIV infected T-cells. IL-2 also has the capability to stimulate NK cells, but additionally enhances the degree of " secondary " cytokines found in the body.

Recombinant forms of IL-12 and IL-2 have been developed as hopeful treatments for cancer, HIV and other infectious diseases. IL-2 is currently approved by the FDA as a form of cancer treatment that physicians have already begun to use in their practice. Yet aside from past use in the management of cancer, recombinant forms of IL-12 and IL-2 are also being studied to examine their safety and efficacy in the treatment of HIV: especially when administered in conjunction with standard antiretroviral treatment. Working alongside the effects of antiretroviral therapy, the immunomodulatory enhancements of IL-12 and IL-2 may promote the efficacy of drugs such as protease inhibitors or AZT.

Interleukin -2 For over ten years, IL-2 has been studied in Phase I and II AIDS clinical trials for the management of HIV infection and AIDS. The general notion that HIV disease progresses due to a caused decline in T-cells, and a subsequent reduction in the amount of IL-2, has allowed researchers to examine the possibility that perhaps the immune system can be enhanced by active administration of IL-2. In exploring the use of IL-2 as treatment for HIV and AIDS, two forms of this interleukin have been studied. The first, Proleukin, is a recombinant human form of IL-2 that was manufactured using DNA technology. The second, IL-2 fusion toxin, is formulated to transfer toxin to cells infected with HIV.

However, in the attempt to find a simple solution of treating HIV through the known effects of IL-2, further investigative steps have not withheld future questions and difficulties. Looking at past AIDS clinical trials examining IL-2 treatment, there is the apparent absence of any clear-cut evidence of IL-2's benefit for treating HIV infection - partially due to developing evidence that IL-2 may also simultaneously induce more HIV. While it has been known that IL-2 stimulates the development of more T-cells, it also increases the number of those that are already infected with HIV; therefore there is a heightened concentration of HIV as cells continue to divide. In addition, IL-2 enhances the expression of secondary cytokines, one of them being Tumor Necrosis Factor Alpha (TNF-alpha) which has also been known to magnify the concentration of HIV. Therefore, with higher doses of IL-2, growth of the system of T-cells (including infected T-cells) and the number of TNF-alpha is stimulated, and subsequently an increase in the levels of HIV occurs. In light of this impeding outcome of high dose IL-2 therapy, IL-2 trials have required concomitant treatment with an antiretroviral, usually AZT. In addition, because the higher quantity of administered IL-2 apparently promotes increased HIV replication, this has caused researchers to utilize lesser amounts of IL-2 to achieve more humble goals. These more moderate objectives desired with lower IL-2 dosages include an enhancement of T-cell capacity, stimulated NK cell activity, and heightened sensitivity of IL-2 without increasing T-cell numbers. However, in light of the difficult questions which arise in the experimental study of IL-2 administration towards reestablishing a normal immune system, the occurrence of toxicities is also another limitation that presents itself in vivo.

In comparison to IL-12, IL-2 is naturally released by T-cells at a later stage in the progression of the immune response. In attempts to reconstitute towards a normal immune system, researchers have discovered that dose-limiting toxicities can occur. In order to achieve relatively more heightened levels of CD4/Helper T-cells in the body, investigators observed that the higher dosage of required IL-2 also led to a range of side-effects which include flu-like indications, fever, swollen lymph nodes, and nausea to pneumonia. In addition, researchers have also realized that in order for treatment to be effectual in producing substantially increased CD4 T-cell counts, IL-2 must be administered in several doses over a longer period of time, due to the short half-life of IL-2 itself. One small trial which incorporated high doses of IL-2 administration through "intermittent continuous" infusion led to promising results for those with relatively higher CD4 cell counts at study entry. Six out of ten patients with CD4 > 200 at baseline maintained CD4 cell increases of greater than 50% when given five day infusions of IL-2 every eight weeks. Two out of 12 patients, with CD4 < 200 at study entry showed some T-cell increase from this particular scheme of drug administration. And with individuals with an entry CD4 cell count of less than 100, no enhanced immunological activity occurred. (Kovacs and Lane) Another trial, conducted by the National Insitute of Health, also brought about greater clarity to the nature of treating with IL-2 in combination with ARV therapy. In particular, it prompted the development of a scheme of dose and dose-reductions of administered IL-2 in order to have a positive balance between the apparent treatment advantages and the toxicities that may occur in the human immune system. In addition, a number of trials across the country have examined different dosages and dose schedules due to the foreseen natural fluctuation of IL-2 production in the body. These questions are being asked since the results of clinical trials involving IL-2 vary upon the frequency of administration. Overall, however, present studies of IL-2 treatment with HIV-positive patients now utilize reduced measurements of the drug compared to those dosages previously administered in the treatment of particular cancers such as kidney cancer.

Another study of intermittent infusions of IL-2 with AZT and/or ddI which was presented at the 1993 International AIDS Conference in Berlin gave encouraging conclusions. From this trial, the researchers concluded that by giving a series of multiple infusions of IL-2 in combination with either AZT and/or ddI, the cooperative efforts of both elements of treatment notably heightened the number of CD4 cells and the expression of IL-2 receptors. Multiple 5-day infusions of 12-18 million IU/day of IL-2 in combination with AZT and/or ddI proved safe for HIV-positive individuals. In addition, an apparent difference in administering treatment either intravenously or by subcutaneous injection was noted. Intravenous administration allows for greatest intake of IL-2; however, this in turn leads to not only a more significant level in the number of Helper T-cells, but also a subsequent risk for increased toxicity. On the other hand, subcutaneous injection of IL-2 allows for a relatively decreased risk of hostile side effects, but with less efficacy in absorption of the drug and stimulation of T-cell levels.

Yet despite these questions that have arisen from years of study on the logistics of IL-2 treatment for HIV disease, the potential theoretical benefits that IL-2 presents at this stage allows it to endure as a principal form of possible treatment in reinstating the immune system. Specifically, overall conclusions have indicated that treatment of HIV with IL-2 may be most beneficial in HIV-infected individuals whose CD4 cell counts are between 100 and 300 cells/mm3 and decreasing. However, for those with a CD4 count below 100, the evident benefits of therapy with IL-2 may not be an overall advantage against the possible toxicities that may occur. Drawing from the results of a recent Phase II study on IL-2 by Italian researchers in 1992, individuals afflicted with HIV disease and lymphoma and treated with IL-2 and AZT demonstrated improvement in their cell-mediated immunity. More noticeably, this "empowerment" of the immune system appeared to occur proportionally greater in those with higher CD4 cell counts. The combination treatment of IL-2 with AZT did not allow for further progression of HIV disease in these patients.

Interleukin-12 Like IL-2, IL-12 is also a stimulating factor in the activation of certain cells of the immune system. In particular, as IL-12 incites the proliferation of T-cells, antibody-producing B-cells, and natural killer cells, this may result in yet another plausible source of treatment by which HIV may also be controlled. In fact, it appears that in comparison to IL-2, IL-12 may be less accountable for toxicities as it normally stimulates specific immune cells early in the development of the immune response. In light of this relatively lower degree of toxicity, IL-12 has become a proposed drug of great potential for enhancing immunocompromised systems. In addition, where there have been noted side effects from IL-2, adverse reactions in humans caused by IL-12 are unknown. However, in experimental lab situations, IL-12 has been shown to increase IL-2 production. This, in turn, could lead to the development of flu-like symptoms. Yet IL-12 apparently also has the capability to stay active in the body for lengthier periods of time, which makes it an even more praiseworthy option in human clinical trials. In addition, recent experiments at the National Cancer Institute on IL-12 have shown that this drug reinstated cell-mediated immunity in certain damaged cells of HIV-positive individuals, and also did not present any increase in the level of HIV as T-cell populations increased again. IL-12's ability to counteract immune dysregulation in individuals with HIV has great implications in the search for control of the disease.

In June, 1994, small trials of IL-12 involving asymptomatic HIV-positive persons were instigated. These trials were attempting to examine the safety of IL-12 at different dosages and its effectiveness when administered with AZT. Theoretically, trials involving IL-12 hold great promise as there is the possibility that concomitant treatment with IL-12 and ARVs may ultimately amend the immunodeficiencies and problems brought about by HIV. The safety issue of recombinant IL-12 administration in humans is being studied under Phase I trials of HIV-positive individuals with T-cells between 100 and 500 cells/mm3. IL-12^�s potential for therapeutic effects will be examined in future studies. In clinical trials, the recombinant human form of IL-12 (rhIL-12) is administered by injection. In 1994, the biotechnology firm, Genetics Institute, Inc., in accordance with Hoffman-LaRoche, developed rhIL-12 as possible therapy for HIV and cancer, along with other infectious diseases.

Based upon in vitro demonstrations of IL-12's effects on the defense capacity of immune cells, it appeared that immunodeficiency could be countered by the addition of IL-12. In the lab, cultures of white blood cells from HIV-positive asymptomatic persons gained their capacity to fight HIV when treated with IL-12. It is hopeful that comparable effects may be achieved in humans. In addition to IL-12's stimulating effects upon such immune cells as T-cells, B-cells, and NK cells, IL-12 also has an apparent ability to activate cellular production of a natural substance in the body called interferon-gamma. Interferon-gamma is a type of biological response modifier (BRM) which is able to expand the defensive killing capability of immune cells. In other words, interferon-gamma is another mediating target upon which IL-12 places its enhancing effects in order to heighten the cell-mediated immune response. This is yet another pathway of attempting to reverse immune dysregulation and achieve a therapeutic advantage.

By manipulating the impaired immune response with the treatment tools of IL-12 and/or IL-2 in conjunction with an antiretroviral, researchers are hoping to regain the natural balance between cell-mediated and humoral immune responses in order to halt HIV disease progression and reestablish a healthy immune system. HIV/AIDS researchers are investigating the potential value of accompanying antiretroviral treatment which disassembles further HIV replication, with IL-2 and/or IL-12 which reconstructs the dismembered immune system. Gaining further knowledge about the potential effects and optimum employment of these cytokines as immunomodulatory treatment for HIV is a continuing journey of questioning trials and greater understanding. There are still uncertainties which surround the logistics of IL-2 and IL-12 use in terms of dosage and frequency; however, great potential awaits for this form of treatment in the struggle against HIV disease. As advancement in the field of immunomodulatory therapy with IL-2 and IL-12 progresses, researchers are looking more strongly at the goal of manipulating the immune response and enhancing T-cell cytotoxicity against HIV.

Lisa Boonprakong is a Research Study Assistant at the University of Washington, AIDS Clinical Trials Unit.

These articles were provided by the Seattle Treatment Education Project - Copyright (c) 1997 - Seattle Treatment Education Project. Noncommercial reproduction encouraged. Distributed by AEGIS - http://www.aegis.com