CD38 assay may be better predictor of progression

Marker shows how well immune system works

Clinicians who would like a more complete picture of how well their patients’ immune systems are holding up under antiretroviral therapies now can order a test that measures CD38 molecules.

"We need markers more than viral load and CD4 counts to help us figure out how patients are doing," says Joseph C. Gathe Jr., MD, FACP, a Houston internal medicine specialist who treats more than 2,000 HIV patients.

A CD38 test shows whether a patient’s immune system is active. If it’s active, that means viral replication is taking place. "This looks to me as a clinician like one of the most important markers," Gathe says.

He points to research conducted by a Los Angeles cellular immunologist as evidence that the CD38 marker would be a good tool in helping physicians decide when to start antiretroviral drugs.

That research suggests that a CD38 assay is a better tool in predicting risk of HIV progression to AIDS than the tests measuring CD4 cell count and viral load, says Janis V. Giorgi, PhD, professor of medicine in the division of hematology-oncology and director of the University of California-Los Angeles School of Medicine.

Clinicians who use CD38 tests in combination with CD4 cell counts and viral load tests will get a very clear picture of how close a patient is to the brink of AIDS-defining illnesses, Giorgi says.

"If you look at CD4 cells, you know how much immune deficiency a person has, but if you look at CD38, you’ll see how fast they’re using up their reserves, so between the two of them you have the whole immune picture," Giorgi says.

Giorgi compares HIV infection to a perpetual war in which the patient’s immune cells are soldiers that at first are young and healthy, but begin to age, tire more easily, and eventually die as the war progresses over the years.

"The virus can’t be gotten rid of, and it can’t be eradicated, so all that can be done is to help the immune system fight it," she explains. "So the more intensely the fight has to be carried out at the beginning of infection, the faster the soldiers get killed off."

The CD38 marker might be compared to the soldiers’ armor. When the soldiers are at war, they wear their uniforms. When they’re at peace, they wear civilian clothing. So if a test shows a high number of CD38 molecules, that means the soldiers are at war. The higher the number of CD38 molecules, the harder they’re fighting and the less likely that they’ll be able to keep up that pace for long.

"The fact is that the CD38 test is even more important than viral load because even though the viral load is the cause of the CD38 count being high, the CD38 is a better reflection of the immune system’s response to the virus," Giorgi says.

Therefore, if a patient on antiretroviral therapy has a high viral load and a high CD38 marker, then a clinician will need to be concerned that the patient’s immune system is activated and may be wearing itself out — even if the person’s CD4 cell count remains high.

"If a person with a high viral load has more CD38 activation, then it’s a worse situation than if a person has the same viral load but not as much CD38 activation," Giorgi explains.

A healthy person who is not infected with HIV probably has about 1,000 CD38 molecules/CD8 cell.

In a study published in the Journal of Acquired Immune Deficiency Syndromes and Human Retrovir ology, Giorgi and fellow investigators suggested that CD38 measurements be categorized in this way:

• low = fewer than 2,500 molecules/CD8 cell;

• medium = 2,500-3,999 molecules/CD8 cell;

• high = 4,000-7,000 molecules/CD8 cell;

• very high = more than 7,000 molecules/CD8 cell.1

Giorgi says research has determined that a relative risk of developing AIDS according to the CD38 marker is as follows:

• low CD38 count = 4% of people developed AIDS in three years;

• medium CD38 count = 18% of people developed AIDS in three years;

• high CD38 count = 40% of people developed AIDS in three years;

• very high CD38 count = 73% of people developed AIDS in three years.

CD38 research also has generated these findings:

• CD38, a flow cytometric marker of T-cell activation of CD8 T cells, can predict HIV disease progression independently of plasma viral burden and CD4 T-cell number. So measuring CD38 antigen expression on CD8 T cells of HIV patients may help clinicians assess the impact of therapeutic interventions.2

• T-cell immune activation is important in determining how long a patient will survive with advanced HIV-1 infection. Shorter survival rates are associated with elevated cell surface expression of CD38 activation antigen on CD4 and CD8 T cells. However, the advanced patient’s survival duration was not associated with plasma virus burden and viral chemokine co-receptor usage.3

CD38 assay now available for clinical use

The CD38 test actually has been available for a few years, but so far it has mostly been used by researchers. "The assay is done on the same kind of instrument that the CD4 cell count is done on, so any lab doing CD4 counts using a flow cytometer can do the CD38 assay," she says.

"If physicians want the test, they can demand it because the technology is here now," Giorgi adds.

Most laboratories use the FACScan or the newer FACSCalibur flow cytometers, and they can easily adapt the instruments to measure CD38 molecules, says Todd Christian, product manager of Becton Dickinson Immunocytometry Systems in San Jose, CA. Becton Dickinson manufactures the flow cytometry equipment that was used in the CD38 research. The company also has the CD38 antibody that’s needed for the assay.

Giorgi has published on the Internet the procedure for calibrating the instrument. It’s available free to any laboratory or clinician visiting the Web site at http://cyto.mednet.ucla.edu. Once on the Web page, click on "protocols," and then click on the link to "CD38 Fluorescence Intensity Quantitation."

References

1. Liu Z, Cumberland WG, Hultin LE, et al. Elevated CD38 antigen expression on CD8+ T cells is a stronger marker for the risk of chronic HIV disease progression to AIDS and death in the Multicenter AIDS Cohort Study than CD4+ cell count, soluble immune activation markers, or combinations of HLA-DR and CD38 expression. J Acquir Immune Defic Syndr Hum Retrovirol 1997; 16:83-92.

2. Liu Z, Cumberland WG, Hultin LE, et al. CD8+ T-lymphocyte activation in HIV-1 disease reflects an aspect of path ogenesis distinct from viral burden and immunodeficiency. J Acquir Immune Defic Syndr Hum Retrovirol 1998; 18:332-340.

3. Giorgi JV, Hultin LE, McKeating JA, et al. Shorter survival in advanced human immunodeficiency virus type 1 infection is more closely associated with T lymphocyte activation than with plasma virus burden or virus chemokine co-receptor usage. J Infect Dis 1999; 179:859-870.