title: Antigen-presenting function of B lymphocytes.
abstract: Here we review our current results studying B cells as APC and the mechanisms by which processed antigen is transported to and held on the cell surface for recognition by the specific T cell along with the MHC class II molecules. These studies were carried out using the globular protein cytochrome c as antigen for which the T-cell antigenic determinant was localized to a C-terminal 10-amino acid peptide fragment. For certain analyses, native cytochrome c or antigenic peptide fragments were covalently coupled to antibodies directed toward B-cell surface structures, allowing the targeting of antigen to the APC surface. Our findings indicate that all B cells function as APC and that the APC function is not differentially regulated in defined B-cell subpopulations. Using cytochrome c-antibody conjugates, it was shown that the surface Ig plays two significant roles in augmenting the B-cell APC function following antigen binding: signalling for enhanced APC function and concentrating antigen for subsequent internalization and processing. Both IgM and IgD appear to function identically in facilitating antigen processing in both immune and nonimmune B-cell populations. Furthermore, the surface Ig does not appear to be specially differentiated to function in concentrating antigen, as antigen artificially bound to other B-cell surface structures including MHC class I and class II molecules is also effectively presented. Lastly, evidence is presented that a previously described B-cell activating factor activity is strongly associated with the membranes of activated but not unactivated helper T cells, providing a mechanism by which the T-cell helper function can be focused on the specific antigen-presenting B cell. Concerning the mechanism by which processed antigen is presented at the B-cell surface, evidence is presented suggesting a role of peptide-binding chaperone proteins which may function to transport peptide to the APC surface and facilitate its association with the appropriate Ia. One candidate protein, PBP72/74, is described which binds peptides but not native antigens, is a member of the hsp70 family and appears to play a role in antigen presentation by the ability of antisera raised against it to block APC functions. Peptide-antibody conjugates were used to explore the spacial restrictions on MHC-restricted peptide presentation and it was shown that peptides covalently coupled to antibodies specific for Ig, class I or class II molecules are effective antigens in vitro even in the absence of processing.(ABSTRACT TRUNCATED AT 400 WORDS)
authors.affiliation: Department of Biochemistry, Molecular Biology and Cell Biology, Northwestern University, Evanston, IL 60208.
authors.forename: S K
authors.initials: SK
authors.lastname: Pierce
authors.forename: J F
authors.initials: JF
authors.lastname: Morris
authors.forename: M J
authors.initials: MJ
authors.lastname: Grusby
authors.forename: P
authors.initials: P
authors.lastname: Kaumaya
authors.forename: A
authors.initials: A
authors.lastname: van Buskirk
authors.forename: M
authors.initials: M
authors.lastname: Srinivasan
authors.forename: B
authors.initials: B
authors.lastname: Crump
authors.forename: L A
authors.initials: LA
authors.lastname: Smolenski
mesh_terms: D000818:Animals; D000936:Antigen-Antibody Complex / Q000378:metabolism; D000938:Antigen-Presenting Cells / Q000378:metabolism*; D001402:B-Lymphocytes / Q000378:metabolism*; D000949:Histocompatibility Antigens Class II; D008213:Lymphocyte Activation; D011489:Protein Denaturation; D011817:Rabbits; D011947:Receptors, Antigen, B-Cell / Q000276:immunology*; D011948:Receptors, Antigen, T-Cell / Q000276:immunology; D013601:T-Lymphocytes / Q000378:metabolism
chemical_list: D000936:Antigen-Antibody Complex; D000949:Histocompatibility Antigens Class II; D011947:Receptors, Antigen, B-Cell; D011948:Receptors, Antigen, T-Cell