Researchers at the National Institute of Allergy and Infectious Diseases (NIAID) have shown that throughout the symptomless stage of HIV infection, when little HIV is detectable in a person's bloodstream, the virus is sequestered in the lymph nodes and related organs, replicating and accumulating in large quantities.
The new findings demonstrate that significant viral activity occurs even during the prolonged period of apparent disease quiescence when patients feel well and damage to the immune system is not yet severe," says Anthony S. Fauci, M.D., NIAID director and chief of NIAID's Laboratory of Immunoregulation. "This information may prompt a rethinking of the optimal time to initiate treatment of HIV-infected patients." Dr. Fauci and his colleagues report their findings in the March 25 "Nature."
The study results help explain a central enigma in AIDS research: why patients with HIV infection progressively lose crucial immune cells, even when the amount of virus and level of viral replication in the bloodstream is low.
We must rethink the concept of clinical latency," says Dr. Fauci. "Our findings clearly demonstrate that a true state of microbiological latency does not exist even during the early stages of HIV disease."
It has been known for years that HIV can be found in lymphoid tissue. However, Dr. Fauci and his group compared blood and lymphoid tissue samples taken simultaneously from the same patient. For each of 12 patients in various stages of HIV infection, the investigators analyzed the samples to determine the relative numbers of HIV-infected cells and levels of HIV replication in the blood and the lymphoid tissue. The investigators used the polymerase chain reaction (PCR), electron microscopy and a probe for viral activity called in situ hybridization in their study.
In patients in the so-called clinically latent stage of HIV infection, we found low viral burden and replication in the bloodstream, but higher levels of virus and replication in the lymphoid tissues," says lead author Giuseppe Pantaleo, M.D., visiting scientist in the Laboratory of Immunoregulation. "Our studies suggest that the lymphoid organs, and not the bloodstream, are the critical sites of the immune destruction seen in HIV disease."
The uniqueness of this study is that it simultaneously compares HIV replication in the bloodstream and lymphoid tissue from the same individuals, and demonstrates a striking dichotomy between blood and lymphoid tissue in viral burden and replication," says Dr. Fauci. "In addition, this study provides insight into the potential mechanisms of this dichotomy. This advance in our understanding of HIV pathogenesis provides a sound rationale for studies of early treatment strategies, especially with combinations of non-toxic drugs."
Currently, antiretroviral therapy is initiated when a patient's count of the crucial immune system cells called CD4+ T cells falls below 500 per cubic millimeter (mm3) of blood, or when signs or symptoms of HIV infection are evident.
Lymphoid tissues include the lymph nodes, spleen, tonsils, adenoids and other organs. Immune activity is concentrated in regions called germinal centers within these tissues, where thread-like tentacles of follicular dendritic cells (FDCs) form networks that trap invaders and present them to immune cells that congregate there.
CD4+ T cells are progressively depleted during HIV infection. A healthy, uninfected person usually has 800 to 1200 cells/mm3. When a person's CD4+ T cell count falls below 200/mm3, he or she becomes particularly vulnerable to opportunistic infections and cancers.
The Course of HIV Infection
Once it enters the body, HIV replicates rapidly, and disseminates. During this stage, called the acute or primary stage of infection, large numbers of viral particles spread throughout the body, seeding themselves in various organs, particularly the lymphoid tissues. Three to six weeks after exposure to the virus, up to 50 to 70 percent of HIV-infected persons suffer flu-like symptoms related to acute infection, such as fever, malaise, headaches and swollen lymph nodes.
A week to a month later, the patient's immune system fights back and the patient generally goes into a symptomless stage of infection lasting an average of 10 years.
Large amounts of virus become trapped in the FDC networks in the germinal centers, even early in infection. Surrounding the germinal centers are paracortical areas rich in CD4+ T cells. These cells become infected in increasingly large numbers, and viral particles accumulate. Many of the CD4+ T cells are activated by chemical signals called cytokines, allowing them to be more easily infected, the researchers speculate.
For years, even when little virus is present in the blood, significant amounts of HIV accumulate in the germinal centers, both in infected cells and as free virus. "At least 10 times more virus per any given number of CD4+ T cells may be present in the lymph nodes than is present in the bloodstream," says Dr. Fauci. "Paradoxically, the "filtering system" in the lymphoid organs, so effective at trapping pathogens and initiating an immune response, may be an important reason why HIV is so effective at destroying the immune system. A steady stream of CD4+ T cells become vulnerable to HIV infection as they traffic to the lymphoid tissue in response to infections."
Using techniques such as electron microscopy, the NIAID team has shown that the germinal centers eventually are overwhelmed by HIV. The FDC networks break down in late-stage disease and virus trapping is impaired, allowing spillover of large quantities of virus into the bloodstream.
"During this burned-out stage the follicular dendritic cells can no longer effectively filter and trap the virus," Dr. Fauci explains. "The breakdown of lymph node structure seen late in HIV disease may preclude a successful immune response against any pathogen."
Drs. Pantaleo and Fauci's co-authors include Cecilia Graziosi, Ph.D. and James F. Demarest, B.S., also of the Laboratory of Immunoregulation; Luca Butini, M.D. and Maria Montroni, M.D., of the University of Ancona, Italy; Cecil H. Fox, Ph.D., of Yale University School of Medicine; Jan M. Orenstein, M.D., of George Washington University; and Donald P. Kotler, M.D., of St. Luke's-Roosevelt Hospital Center, New York.
NIAID, a component of the National Institutes of Health, supports research on allergy, immunology and infectious diseases. NIH is an agency of the U.S. Public Health Service, Department of Health and Human Services.
Reference: Pantaleo, G., C. Graziosi, J.F. Demarest, L. Butini, M. Montroni, C.H. Fox, J.M. Orenstein, D.P. Kotler, and A.S. Fauci. HIV INFECTION IS ACTIVE AND PROGRESSIVE IN LYMPHOID TISSUE DURING THE CLINICALLY LATENT STAGE OF THE DISEASE. Nature 362: 355-358 (1993).
Further Reading: Drs. Pantaleo, Graziosi, and Fauci review the HIV disease process in THE IMMUNOPATHOGENESIS OF HUMAN IMMUNODEFICIENCY VIRUS INFECTION. New England Journal of Medicine 328: 327-335 (1993).