The immune system's army of CD4+ T cells not only declines in overall size during the course of HIV disease, but also becomes progressively less diverse as specific CD4+ T cells programmed to fight different invaders are lost, according to researchers at the National Institute of Allergy and Infectious Diseases (NIAID). These depleted cell types may not be immediately restored by therapies such as antiretroviral drugs or interleukin-2 (IL-2) that can increase an HIV-infected person's overall CD4+ T cell count. Rather, such therapy, at least in the short-term, appears to boost only the cells that were present when therapy began. The findings are reported in the May 1997 issue of Nature Medicine.
Our findings argue for treatment early in disease, before elements of the immune system are significantly depleted," says senior author H. Clifford Lane, M.D., NIAID's clinical director. "Our data also suggest that drugs to prevent opportunistic infections may remain important even for patients with CD4+ T cell counts that are rapidly increasing in response to therapy, because these individuals may be missing part of their CD4+ T cell repertoires."
Adds co-lead author Mark Connors, M.D., of the NIAID Laboratory of Immunoregulation, "The loss of CD4+ T cells is a qualitative phenomenon as well as a quantitative one. In other words, a CD4+ T cell count of 200 per cubic millimeter (mm3) of blood during the natural history of HIV infection may be very different from a CD4+ T cell count of 200/mm3 in the context of therapy. Depletions in the CD4+ T cell repertoires of HIV-infected people and hence the reduced ability of their immune systems to recognize certain antigens are probably key to the development of immunodeficiency in these people."
The current findings shed light on an observation reported by Dr. Lane and his colleagues in the mid-1980s: HIV-infected people often lose their ability to respond to "remote recall antigens": substances to which one was exposed in the past, such as the antigens in a tetanus vaccine. The new data suggest that this decreased responsiveness is due to a loss of specific CD4+ T cell types, which scientists refer to as "clones."
A loss of CD4+ T cell clones, and the resulting "holes" in a person's CD4+ T cell repertoire, rather than an active immunosuppressive phenomenon, may explain why an HIV-infected person becomes unresponsive to remote recall antigens," says co-lead author Joseph A. Kovacs, M.D.
Using molecular techniques, the NIAID researchers have demonstrated that CD4+ T cell diversity begins to decline when an HIV-infected person's CD4+ T cell count falls to approximately 150 to 200 cells/mm3 of blood. This decline accelerates when a person's count falls to 50 cells/mm3. A healthy person without HIV infection typically has a CD4+ T cell count in the range of 600 to 1500 cells/mm3.
Even when therapies boosted a person's CD4+ T cell counts to 200/mm3 or higher, CD4+ T cell diversity was not restored, the researchers observed.
However, Dr. Lane suggests that even patients with advanced disease may be able to mount adequate immune responses if antiretroviral therapy reduces HIV replication to very low levels. With potent suppression of HIV, other clones may be able to proliferate to sufficient levels to perform the job normally done by the missing clones.
"Think of a person's CD4+ T cell clones as tiles in a scrabble game," explains Dr. Lane. "As disease progresses, not only does an HIV-infected person have fewer tiles, but also fewer different tiles. If a person loses the letter "z," they will be unable to spell out the word "zebra."
However, if they have enough other tiles, they still may be able to describe a zebra by spelling a"horse-like animal with black and white stripes."
CD4+ T cells have on their surfaces a molecule called a T-cell receptor (TCR) which recognizes and binds to foreign invaders that have been ingested and processed by other specialized immune cells -- antigen-presenting cells (APC) -- and displayed on the surfaces of APCs.
Each CD4+ T cell's TCR has an alpha and beta chain. Each beta chain contains a variable region that is derived from one of at least 22 V-beta subfamilies. These V-beta subfamilies are often used by immunologists to classify T cells.
In their studies, Drs. Lane, Connors, Kovacs and their colleagues used a specialized polymerase chain reaction (PCR) technique to examine differences in size patterns in the V-beta subfamilies of CD4+ T cells taken from both HIV- infected and HIV-uninfected people.
Among these subjects were five sets of twins in which one twin was HIV-seropositive, the other HIV-seronegative. The five healthy, HIV-seronegative twins had TCR V-beta families that were virtually all normal. Their infected twins had disruptions in as many as 11 different V-beta subfamilies.
The researchers applied the same PCR technique to samples taken from HIV-infected people at different stages of HIV infection, and found that disruptions in the CD4+ T cell repertoire became more pronounced as disease progressed. In a group of eight control patients without HIV infection, fewer than 5 percent of all V-beta families had disruptions. In a group of five HIV-infected people with CD4+ T cell counts above 200 cells/mm3, approximately 15 percent of all V-beta families had disruptions. However, among six patients with CD4+ T cell counts lower than 200/mm3, nearly 40 percent of V-beta families had disruptions. In most cases these were associated with depletions within these V-beta families.
Therapy with antiretroviral drugs or interleukin-2 plus antiretroviral therapy led to only minor changes in previously disrupted V-beta repertoires, and in the relative percentages of naive and memory CD4+ T cell subsets, the researchers found.
"Most likely, the increase in a patient's CD4+ T cell count after initiating therapy represents expansion of the existing repertoire in a patient's bloodstream and lymph nodes rather than the generation of "new" CD4+ T cells by the thymus," says Dr. Kovacs. "This view is fortified by our observation that many of the patients in the study had minimal thymic tissue."
"In the future, modifications of the techniques used in this study may prove useful clinically for better defining the predictive value of a CD4+ T cell count," he adds. "Knowing a person's specific T cell repertoire might allow one to better predict the susceptibility of a person to opportunistic infections, and may one day help guide treatment decisions."
In addition to Drs. Lane, Connors and Kovacs, co-authors of the paper include Seth Krevat, Juan C. Gea-Banacloche, M.D., Michael C. Sneller, M.D., Mark Flanigan, Julia A. Metcalf, R.N., Robert E. Walker, M.D., Judith Falloon, M.D., Michael Baseler, Ph.D., Randy Stevens, Irwin Feuerstein, M.D., and Henry Masur, M.D. The techniques used for precisely analyzing CD4+ T cell repertoires were developed at the Pasteur Institute in France by Drs. Christophe Pannetier and Philippe Kourlisky.
NIAID, a component of the National Institutes of Health, conducts and supports research aimed at preventing, diagnosing and treating illnesses such as AIDS and other sexually transmitted diseases, tuberculosis, asthma and allergies. NIH is an agency of the U.S. Department of Health and Human Services.References: