The processing of antigen and anti-Ig by antigen-specific B cells

B lymphocytes are very efficient antigen-presenting cells when they bear surface receptors for the antigen in question. However, there is very little known about the intracellular events occurring between the time B cells bind native antigen and present it in processed form to T cells. To analyze internalization and degradation of antigen versus anti-Ig or anti-MHC antibodies by B cells, we have used highly purified resting antigen-specific B lymphocytes that bind the hapten, trinitrophenyl. These cells were treated with [¹²⁵I]-labeled trinitrophenylated antigens or with [¹²⁵I]-labeled antibodies reactive with either cell surface immunoglobulin or major histocompatibility complex (MHC) (class I or class II) molecules. The fate of these proteins was followed by measuring the amount of acid soluble and insoluble radioactivity associated with the cells or released into the incubation medium. The cell-associated and released radioactivity was further analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Our results demonstrate that the kinetics of degradation of specific antigen into acid soluble fragments which are released into the culture medium closely parallel those with which B cells acquire the ability to specifically conjugate to carrier-specific T cells. In contrast, degradation of anti-Ig is more complete than the degradation of antigen but requires a longer period of time to reach completion. Furthermore, the initial release of soluble fragments of anti-Ig as compared to antigen into the culture medium is marginally slower. Finally, there is significant intracellular accumulation of degradation intermediates when anti-Ig is processed, but this is not the case with antigen. These findings suggest that the intracellular routing of anti-Ig vs antigen differ and that anti-Ig may be routed predominantly to lysosomes where it is degraded slowly, but completely, whereas antigen is processed more rapidly, but less completely, in endosomes.