Anti-immunoglobulin augments the B-cell antigen-presentation function independently of internalization of receptor-antigen complex.

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All mouse splenic B cells, including small resting B cells, process and present the native globular protein antigens, pigeon and tobacco hornworm moth cytochromes c, to a cytochrome c-specific T-cell hybrid in a major histocompatibility complex-restricted fashion, in the micromolar to nanomolar antigen-concentration range. As is the case for macrophages, treatment with paraformaldehyde or the lysosomotropic agents chloroquine and ammonium chloride blocked processing of the native pigeon protein but did not affect the presentation of a carboxyl-terminal peptide fragment of pigeon cytochrome c (residues 81-104) which contained the T-cell antigenic determinant. However, in contrast to macrophages, whose antigen-processing and -presentation functions are insensitive to radiation, radiation blocked the processing of the native protein but not the presentation of the peptide fragment. The processing and presentation function of the B cells was augmented by F(ab')2 of rabbit anti-mouse Ig antibodies, in that 1/10th to 1/30th as many cells and 1/10th as much antigen were required to maximally activate the T-cell hybrid. This augmentation did not appear to be due to either crosslinking of the Ig receptors or to B-cell activation, as monovalent Fab fragments were nearly as effective as the bivalent reagent, and the concentrations of F(ab')2 anti-Ig used did not induce measurable proliferative responses. Furthermore, enhancement can occur in the absence of cytochrome c binding and internalization, since B cells that were fixed with paraformaldehyde after treatment with F(ab')2 anti-Ig were more effective in presenting the carboxyl-terminal peptide than were untreated fixed cells. The same phenomenon followed the binding of an irrelevant antigen (carboxydinitrophenylated bovine serum albumin) by antigen-binding B cells, resulting in enhanced processing and/or presentation of native pigeon cytochrome c. Thus, nonspecific enhancement of antigen processing and presentation can be obtained by either antigen or anti-Ig binding to the B-cell antigen receptor, both treatments presumably delivering the same signal without requiring internalization of the specifically bound antigen for subsequent processing.

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