Seattle Treatment Education Project (STEP) Perspective, Vol. 5, No. 2 - July 1993 * 127 Broadway E. Ste 200 Seattle, WA 98102
Joel Gibson M.S

In a plenary session, Flossie Wong-Staal delivered a talk on molecular targets, i.e. genes or products of genes which would serve as good targets for the development of antiretroviral therapy. Both tat and rev are regulatory genes found in both HIV-1 and HIV-2. Like most regulatory genes, these genes are necessary for viral replication, making them good targets for antiretroviral agents. Both tat and rev act upon specific proteins. Mutant variations of these proteins have been isolated from some individuals which attack tat and rev, creating "nonsense" end products. This translates into decreased viral replication. An investigator from UCSD has learned the tumor necrosis factor (TNF) which is found in increased amounts in people with aids, can decrease the effectiveness of the tat inhibitor which is currently under study. (See discussion of ACTG 213 under "Other Antivirals".) She stresses that rev is the better of the two targets in which to develop drugs against. "The state of the art for 'gene therapy' with all its limitations is still an expensive and perhaps impractical endeavor," says Wong-Staal. A current proposal involves the use of "retroviral vectors." These vectors would contain a "therapeutic gene" having activity against one of the necessary genes involved in HIV replication. CD4 cells purified from PBMCs or CD34 cells taken from the bone marrow would be taken from an individual and placed into culture. Then these PBMCs would be treated in such a way over a period of three to four weeks, to encourage the cells to allow these retroviral vectors into them. The PBMCs would then be transfused back into the same individual. She goes on to say that "the ultimate goal is to be able to have a directly injectable vector with therapeutic gene, which would hone into the same target cells and tissues as HIV."

Jay Levy from UCSF gave a talk entitled "HIV Pathogenesis and Long Term Survival (LTS)." He began his talk with stating that the mean length of time from seroconversion to an aids-defining event is 10 years. He went on to describe two conditions which he feels fit the definition of Long Term Survival (LTS):

* Living for more than eight years with HIV infection and showing no clinical signs of disease; i.e. asymptomatic

* Living for more than eight years with HIV and not only showing no clinical disease signs, but also having normal CD4 counts

The earliest immune response to initial infection with HIV may play a critical role in the future development of clinical disease, or lack thereof. Also, which cell type becomes infected first, can be a determinant of the rate of HIV replication and in turn, disease progression. In work done by Embretson, Haase, and colleagues, although up to 30% of CD4 cells are infected in the lymph nodes of asymptomatic individuals, only about one in 400 of these cells is actually producing virus. During the asymptomatic period, a relatively non-cytopathic viral strain can normally be found. It is thought that over time, perhaps following different "pressures" by the person's immune system response, mutation of the virus occurs, becoming a high replicating, pathogenic strain. It is of upmost importance to delineate which factors lead to such a change. The type of cytokines which are produced during a particular stage of HIV infection has been found to be of great significance. For example, the production of tumor necrosis factor-alpha (TNF-a) and loss of interleukin-1 (IL-1) production from macrophages lead to increased CD4 cell death. Conversely, IL-2, IL-12, and gamma interferon (IFN-gamma) have beneficial effects on a specific subset of CD4 cells (TH-1), one of which is enhancement of CD8 activity. CD8 cells, in addition to their ability to kill virally-infected cells, have been found to inhibit HIV production by stopping a key step (transcription) in the viral replication cycle. This response does not correlate with the number of CD8 cells, but instead is tied to a subset of CD8 cells with expression of a particular genetic makeup (phenotype).

Levy goes on to specify four characteristics which all LTS have (albeit to different degrees):

* By quantitative assays of viral load by polymerase chain reaction (PCR) and measuremnet of infectious virus in PMBCs and the lymph nodes, it has been found that these individuals have a relatively low level of infectious viral load.

* The viral strains found in LTS show properties which are different than found in those people with advanced disease. For example, viral stains from LTS do not replicate to very concentrated amounts and do not grow in a wide variety of cell types. They are classified as "NSI" viral strains in that they do not cause cell fusion in tissue culture. Cell fusion has been positively linked to increased cell death and more rapid disease progression. Basically, the viral strains from LTS are considered not as virulent.

* A particular type of antibody called "enhancing antibodies" which have deleterious effects in any viral infection, have not been found in the blood of LTS. This contrasts to finding these enhancing antibodies in those with advancing disease.

* With regard to cytokine production, the subset of CD4 cells (TH-1) which produce "beneficial" cytokines for the maintenance of asymptomatic infection, can be found readily in LTS. The other subset of CD4 cells (TH- 2), having a negative effect on keeping the virus at low levels of replication, can not be found.

Levy feels that maintenance of the TH-1 subset of CD4 cells and strong anti-HIV activity by CD8 cells could lead to long term survival in all HIV infected persons.

Susan Buchbinder, from the San Francisco City Clinic Cohort study, spoke on genetic testing done in their study as a method to characterize what they called healthy, long term positives (HLP). They define HLP as someone with documented HIV infection for greater than 10 years and having CD4 counts greater than 500. Using 593 men from the original cohort of 6704, the investigators described the clinical status of these men 10-15 years after seroconversion: 70% either had a disease-defining AIDS diagnosis or CD4 counts less than 200; 11% had CD4 counts between 200 and 500; and 8% (42) had CD4 counts greater than 500. (The remaining 11% of the men either died prior to an AIDS diagnosis or have not had a recent CD4 count done through the study.) Obtaining the blood of these 42 HLP, the possible role of specific genetic markers found on their cells was studied. Human Leukocyte Antigens (HLA) are found on the surface of most cells in the body. The specific HLA type of any given individual is comprised of a mixture from both parents and has similarities to any siblings. This is the reason organ transplants are often attempted within families. There are two classes of HLA, class I and class II. Of the four types of class I which have been found with significant increased frequency in HLP, after making statistical adjustments, only the C2 appears to be significantly associated with HLP. With the class II data, again four types have been associated with HLP. However, one in particular, the DRB4, seems to be the strongest candidate for being possibly responsible for delayed HIV disease progression. Their conclusions are that the risk of developing AIDS increases with duration of HIV infection; 13.8 years after seroconversion, 68% have developed aids. Conversely, roughly a third of the men have not developed aids. At least 8% of those without an AIDS diagnosis are healthy long-term positives. And, specific HLA class II, show strong association with HLP, suggesting a role in delayed progression for one or more of these genes.

In a study from Amsterdam looking at long term asymptomatic (LTA) HIV infection, various factors were explored to help explain why some individuals had disease progression, some were asymptomatic with CD4 cells between 200 and 500 (LTA-pCD4), and others were asymptomatic with greater than 500 CD4 cells (LTA-nCD4). One hundred and forty-two gay men were studied who had seroconverted prior to 1986, 13 who were LTA-nCD4 and 48 who were LTA-pCD4. The other 81 men were considered progressors due to symptomatic HIV infection. Follow up was for seven years. The biggest difference in the three groups came when analyzing the rate of decline of CD4 cells. It was not the actual number of CD4 cells which was the determinant, but how fast they were decreasing. A multivariant analysis to show whether other factors which were found to be different in the three groups were related to this rate of CD4 decline, or were independant predictors of disease progression was conducted. Three factors emerged which did show independant prediction: T- cell reactivity, especially the CD3 antibody response; presence of anti-p24 antibodies; and seronegativity for hepatitis B.

The last bit of information on pathogenesis and long term survival comes from a variety of sources including Jay Levy, the Multicenter AIDS Cohort Study (MACS), the San Francisco Men's Health Study (SFMHS), the Edinburgh cohort study of hemophiliacs, and a study of female sex workers in Nairobi, Kenya, as well as others. There are certain individuals who have most assuredly come in contact with HIV in ways which usually result in infection, and yet have no evidence of HIV infection by all currently available detection methods, including polymerase chain reaction (PCR) and other highly sensitive assays. For example, almost 10% of 260 female sex workers in Kenya have remained seronegative for three years despite ongoing unprotected sexual exposure to HIV infected men. Investigators from the MACS study have described gay men who remain uninfected despite repeated high-risk exposure to HIV. Further, scientists in Edinburgh have found hemophiliacs and blood-transfusion recipients who have remained uninfected even though they have received contaminated blood products. Individuals in this category which have been extensively studied, have found that many of them have CD8 activity against HIV in vitro, which would only be present if the person had at some previous time been exposed to HIV. Possible explanations include that there is an early strong cell-mediated anti-HIV response prior to antibody production which may have prevented the infection. It is also possible that these individuals were exposed to non- infectious viruses or viral antigens, and infection was never established. Further testing of these individuals may yield important answers to the many questions still remaining in the area of pathogenesis.

References:

1. PS-03(1). 1993. IX International Conference on aids. Berlin, Germany. June 1993.

2. PS-05(2). 1993. IX International Conference on aids. Berlin, Germany. June 1993.

3. WS-B03(2). 1993. IX International Conference on aids. Berlin, Germany. June 1993.

4. WS-C03(3). 1993. IX International Conference on aids. Berlin, Germany. June 1993.
9307
STEP5206

Copyright © 1993 - Seattle Treatment Education Project (STEP) - All rights reserved. Noncommercial reproduction is encouraged. STEP is published four times a year by the Seattle Treatment Education Project, 127 Broadway East, 3rd Floor, Seattle, WA 98102.    Email: step100@aol.com  STEP web page