T he scope of B cell abnormalities in HIV-infected patients, first described almost 20 years ago (1), has come to define a pervasive paradigm in HIV infection, namely that of immunodeficiency in the setting of virus-driven immune activation. Hallmarks of B cell abnormalities in HIV disease include hypergammaglobulinemia (2-4), increased expression of activation markers (5), increased levels of autoantibodies (6, 7), increased risk of developing B cell lymphomas (8), and decreased responsiveness to in vivo vaccination and ex vivo stimulation (9, 10). The range of defects in B cell responses includes both T cell-dependent and -independent pathways (1, 11, 12), suggesting that along with defects in CD4 ϩ T cell help, HIV infection causes intrinsic B cell defects. Recent studies have shown that the majority of these abnormalities are attributable to ongoing viral replication and are reversible by the suppression of plasma viremia with effective antiretroviral therapy (2, 6, 13-15). In light of similar abnormalities described in the CD4 ϩ and CD8 ϩ T cell compartments of HIV-infected individuals (16-18), there is clear evidence that immune activation brought on by unchecked HIV-1 replication has widespread deleterious effects on the immune system.Insight into the apparent paradox that in vivo B cell hyperactivity translates into poor response to immunogens and ex vivo stimuli has remained elusive for almost two decades. However, with the availability of antiretroviral therapies that can effectively sustain the suppression of virus replication, it has become apparent that B cell hyperactivity and poor responsiveness to stimulation are directly linked to ongoing virus replication by mechanisms that may include cytokine deregulation, loss of memory B cells, and induction of terminal differentiation (15,19,20). However, most studies on B cell function have been based primarily, if not exclusively, on responsiveness of B cells to exogenous surrogates of antigenic stimulation, such as soluble anti-Ig antibodies and trimeric CD40 ligand (CD154), triggers of the B cell receptor (BCR) and CD40, respectively.The initiation of a humoral immune response to T celldependent antigens requires cognate interactions between antigen-specific B cells and activated antigen-specific CD4 ϩ T cells involving both cell-contact interactions and soluble factors. One of the most important contact-mediated interactions between B cells and CD4 ϩ T cells involves CD40 expressed constitutively on B cells and its membrane-bound ligand CD154 expressed on activated CD4 ϩ T cells (21). The expression of CD154 on activated CD4 ϩ T cells is induced after T cell receptor triggering by antigen-presenting cells, likely dendritic cells, and requires key costimulatory interactions between CD80͞CD86 on mature antigen-presenting cells and CD28 on CD4 ϩ T cells (22). These interactions also induce CD4 ϩ T cells to secrete cytokines, which, together with expression of CD154, provide CD4 ϩ T cells with full helper function. T helper (Th) cells can be divided into ...