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Original Article |
Correspondence to: Marieke van Ham, Division of Tumor Biology, The Netherlands Cancer Institute, Plesmanlaan 121, 1066 CX Amsterdam, The Netherlands. Tel:31-20-512-2023 Fax:31-20-512-2029 E-mail:vanham{at}nki.nl.
Antigen presentation by major histocompatibility complex class II molecules is essential for antibody production and T cell activation. For most class II alleles, peptide binding depends on the catalytic action of human histocompatibility leukocyte antigens (HLA)-DM. HLA-DO is selectively expressed in B cells and impedes the activity of DM, yet its physiological role remains unclear. Cell surface iodination assays and mass spectrometry of major histocompatibility complex class IIeluted peptides show that DO affects the antigenic peptide repertoire of class II. DO generates both quantitative and qualitative differences, and inhibits presentation of large-sized peptides. DO function was investigated under various pH conditions in in vitro peptide exchange assays and in antigen presentation assays using DO- and DO+ transfectant cell lines as antigen-presenting cells, in which effective acidification of the endocytic pathway was prevented with bafilomycin A1, an inhibitor of vacuolar ATPases. DO effectively inhibits antigen presentation of peptides that are loaded onto class II in endosomal compartments that are not very acidic. Thus, DO appears to be a unique, cell typespecific modulator mastering the class IImediated immune response induced by B cells. DO may serve to increase the threshold for nonspecific B cell activation, restricting class IIpeptide binding to late endosomal compartments, thereby affecting the peptide repertoire.
Key Words: antigen presentation, immune response, selection, HLA-DM, autoimmunity
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