Human Immunodeficiency Virus Type 1 Bound to B Cells: Relationship to Virus Replicating in CD4+ T Cells and Circulating in Plasma

The selection and expansion of antigen-specific B cells in follicles of lymphoid tissues constitute hallmarks of the humoral immune response. Resident follicular dendritic cells (FDC), through their capacity to trap and present antigens, play a critical role in creating an intricate network for antigen-mediated interactions with B and T cells (16, 28). During human immunodeficiency virus (HIV) type 1 (HIV-1) infection, a large number of virions can be detected in the follicular areas of lymphoid tissues in the form of diffuse in situ hybridization signals that most likely represent virus-containing immune complexes (IC) trapped on FDC (1, 10). Furthermore, the FDC networks in germinal centers have been shown to account for the bulk of the HIV-1 RNA signal during ongoing viral replication (22) and are thought to represent an important source of residual “trapped” virus after plasma viremia is suppressed following the initiation of antiretroviral therapy (12). It was recently demonstrated that lymphoid tissue-derived B cells and peripheral blood-derived B cells of HIV-1-infected viremic patients carry replication-competent virus on their surface (18). Similar to what has been proposed for FDC trapping of HIV-1 virions (14, 15), the mechanism of interaction between B cells and virus was found to involve the binding of HIV-containing IC to CD21 on the surface of the cells (18). It was also found that virus bound to B cells could efficiently infect activated CD4+ T cells through cell-cell contact (18). Recent in vitro studies have also demonstrated that HIV-1 in the form of IC bound to B cells is more stable and more efficiently passed to CD4+ T cells than its free-virion counterpart (13). Furthermore, considering that most interactions between lymphoid tissue-derived B cells and CD4+ T cells have been shown to occur at the border between follicular and T-cell-rich areas (9), B cells may constitute an important bridge between virus trapped on FDC and virus replicating in CD4+ T cells. In the present study, we considered the relationship between HIV-1 bound to the surface of B cells and HIV-1 that constitutes plasma viremia by investigating the genotypic relationship between HIV-1 variants bound to the surface of B cells and virus replicating in CD4+ T cells and HIV-1 virions present in plasma. To this end, heteroduplex mobility assays (HMA) and heteroduplex tracking assays (HTA) were performed with cell- and plasma-associated HIV-1 derived from peripheral blood and cell-associated HIV-1 derived from lymph node biopsy samples. HMA is a rapid and sensitive approach for identifying genetic diversity, as HTA is for establishing genetic relationships among HIV-1 variants between and within individuals over time (6), as a function of the state of disease progression (5, 19) and immune stimulation (21), and upon interruption of effective antiretroviral therapy (2). Differences in virus fitness (25), cytopathogenicity (20), and viral populations isolated from various compartments (23, 31) have also been measured by using HTA. Here we used heteroduplex and sequencing analyses to demonstrate a close association between B-cell- and CD4+-T-cell-derived HIV-1 and to show that virus in CD4+ T cells in the blood or in lymph nodes is not the likely source of much of the virus in plasma.