Polyreactive antigen-binding B cells in the peripheral circulation are IgD(+) and B7(-)

Polyreactive antibodies are naturally occurring antibodies, primarily of the IgM isotype, that are capable of reacting with a wide variety of different self and nonself antigens. Previously we reported that a B cell capable of making polyreactive antibody has Ig receptors on its surface that can bind different antigens. The present investigation was initiated to characterize these polyreactive antigen-binding B cells further. A panel of fluorescein isothiocyanate-labeled antigens (insulin, IgG Fe fragment or beta-galactosidase) served as probes to select polyreactive antigen-binding B cells by cell sorting. Our experiment revealed that these polyreactive antigen-binding B cells were mainly of the IgD isotype. They expressed high levels of CD40 and major histocompatibility complex class II molecules, but little or no B7-1, B7-2, or Fas. In contrast to the binding of antigens to monoreactive receptors (usually high affinity), the binding of antigens to polyreactive receptors (usually moderate or low affinity) did not up-regulate the expression of B7-1 or B7-2. Antigens that bound to polyreactive receptors, however, were internalized and degraded, although not as efficiently as antigens that bound to monoreactive receptors. Despite the ability of these B7(-) cells to process antigens, they were not able to activate T cells in a mixed leukocyte reaction. It is concluded that polyreactive antigen-binding B cells have properties that are consistent with the ability to induce immunological tolerance.