Viral Load in Blood and Lymph Tissues of HIV-Infected Individuals

Our laboratory has previously demonstrated that lymph nodes are a major reservoir for human immunodeficiency virus (HIV) and a major site of active virus replication in infected individuals(1-3). There is at least a 10 fold greater viral burden per given number of CD4+ T lymphocytes obtained from the lymph nodes versus the peripheral blood in the same infected individual. These data have been accumulated predominantly in individuals with progressive generalized lymphadenopathy (CDC Class A1 and A2). It is unclear at present whether this pattern holds true for all categories of HIV infected individuals. We have proposed that the seeding of lymph nodes by HIV early in the course of HIV infection and the persistent production of virus in lymph nodes throughout the course of infection are major factors in the pathogenesis of HIV in virtually all infected individuals. In addition, it is likely that the selective perturbations of various T cell subsets (i.e., V-B classes of CD4+T cells) that have been observed in peripheral blood are much more dramatic in the lymph node given the greater viral burden in the lymph node compared to the peripheral blood. In order to investigate this hypothesis, it is essential that we study simultaneously lymph nodes and peripheral blood from the same individuals and that we study different individuals at various stages of disease from early in the course of infection (CDC Class A) to advanced disease (CDC Class B and C). If, as we suspect, there is active virus replication in the lymph node early in the course of infection, even at a time when there is little virus burden or active replication in the peripheral blood, this would justify anti-retroviral therapy at the earliest possible time in the course of infection. In addition, in certain patients who are about to initiate treatment with an anti-retroviral agent such as zidovudine or didanosine through their private physician, it would be important to know whether treatment actually reduces the viral burden and virus replication in lymph nodes. The effect of therapy on viral burden and replication will be compared in the lymph node versus peripheral blood mononuclear cells and both of these parameters will be compared with the level of plasma viremia.

Several years ago, we and others demonstrated that lymph nodes are a major reservoir for human immunodeficiency virus (HIV) and a major site of active virus replication in infected individuals. Subsequent studies from our group showed that virologic cross talk between B cells and CD4+ T cells occurs within the microenvironment of lymphoid tissues (LT), and that immunosuppressive CD25+CD4+ regulatory T (Treg) cells enriched in the LT. Furthermore, Treg cells isolated from the LT are particularly effective in suppressing HIV-specific cytolytic activity. More recently, follicular helper (Tfh) CD4+ T cells have been described in LT and found to play an important role in providing help to B cells during germinal center reactions. These Tfh cells are expanded in HIV-infected viremic individuals and their numbers correlate with frequencies of germinal center B cells, consistent with longstanding observations of HIV-induced GC hyperplasia in untreated infected individuals. Despite the increased frequencies of Tfh and GC B cells in LT, there is also evidence for reduced immune function due to over-expression of negative regulatory molecules. We are currently investigating several issues related to the impact of HIV infection/replication on the immune competence and homing profiles of numerous cell types within the LT. Given the paucity of Tfh and GC B cells in the peripheral blood, these studies are more appropriately conducted with tissue samples. We will also pursue immunological, migrational and virologic characteristics of various cell types including B cells and their subsets and CD4+, CD8+ and NK cells in the LT and bone marrow (BM) tissue. In this regard, the BM is both the site of B-cell development and the only known long-lived repository of plasma cells that are responsible for maintaining humoral immunity. While HIV infection has been shown to impair hematopoiesis in the BM, relatively little is known regarding the effect of HIV infection on plasma cells that home back to the BM after maturation.