B cells in HIV infection and disease

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Abstract
HIV infection leads to perturbations of all of the main cell types of the immune system, including B cells. Most B-cell perturbations are associated with indirect effects of ongoing HIV replication, although some perturbations arise regardless of the decrease in viraemia by effective antiretroviral therapy (ART). Many B-cell defects in HIV infection are associated with alterations in the B-cell subpopulations that circulate in the peripheral blood. Most B cells in healthy individuals comprise resting naive and memory B cells, whereas several minor subpopulations are over-represented in HIV-infected individuals, including immature transitional B cells, exhausted B cells, activated mature B cells and plasmablasts. In vitro and in vivo studies show that HIV can also interact directly with B cells, although the frequency of these interactions is unlikely to account for the extent of B-cell dysregulation that is observed in infected individuals. HIV virions complexed with complement and antibody can bind B cells through the complement receptor CD21, and HIV virions and proteins, including gp120 and Nef, can also interact directly or indirectly with B cells. Indirect effects of HIV viraemia on B cells include B-cell hyperactivity (as manifested by hypergammaglobulinaemia), increased B-cell polyclonal activation, increased B-cell turnover, increased expression of activation markers and of markers that are associated with activation-induced apoptosis, increased frequency of B-cell malignancies and increased differentiation of B cells to plasmablasts. HIV viraemia with CD4+ T-cell lymphopenia is associated with an increased frequency of immature transitional B cells. This increase is also associated with increased serum levels of interleukin-7. HIV viraemia leads to B-cell exhaustion, as manifested by increased expression of multiple inhibitory receptors, altered expression of homing receptors, decreased cell division and somatic hypermutation in vivo, decreased proliferative and effector properties in vitro and enrichment of HIV-specific responses in the exhausted B-cell compartment. Although most B-cell perturbations in HIV-infected individuals are attributed to viraemia and are reversible by ART, one important exception is the loss of memory B cells. All stages of HIV infection are associated with a decrease both in the frequency of resting memory B cells and of B-cell responses against T-cell-dependent and T-cell-independent antigens. Many B-cell perturbations observed in HIV infection also arise in various infectious and non-infectious disease settings that involve immune dysfunction. Several human diseases that affect B cells show dysregulation of immature transitional B cells, whereas others show dysregulation of memory B cells, with alterations that are similar to B-cell exhaustion and activation-induced terminal differentiation.