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CDw78 Defines MHC Class II-Peptide Complexes That Require Ii Chain-Dependent Lysosomal Trafficking, Not Localization to a Specific Tetraspanin Membrane Microdomain¹

Neil J. Poloso^*, Lisa K. Denzin and Paul A. Roche^2,*

^* Experimental Immunology Branch, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892; and Sloan-Kettering Institute, Immunology Program, Memorial Sloan-Kettering Cancer Center, NY 10021

MHC class II molecules (MHC-II) associate with detergent-resistant membrane microdomains, termed lipid rafts, which affects the function of these molecules during Ag presentation to CD4⁺ T cells. Recently, it has been proposed that MHC-II also associates with another type of membrane microdomain, termed tetraspan microdomains. These microdomains are defined by association of molecules to a family of proteins that contain four-transmembrane regions, called tetraspanins. It has been suggested that MHC-II associated with tetraspanins are selectively identified by a mAb to a MHC-II determinant, CDw78. In this report, we have re-examined this issue of CDw78 expression and MHC-II-association with tetraspanins in human dendritic cells, a variety of human B cell lines, and MHC-II-expressing HeLa cells. We find no correlation between the expression of CDw78 and the expression of tetraspanins CD81, CD82, CD53, CD9, and CD37. Furthermore, we find that the relative amount of tetraspanins bound to CDw78-reactive MHC-II is indistinguishable from the amount bound to peptide-loaded MHC-II. We found that expression of CDw78 required coexpression of MHC-II together with its chaperone Ii chain. In addition, analysis of a panel of MHC-II-expressing B cell lines revealed that different alleles of HLA-DR express different amounts of CDw78 reactivity. We conclude that CDw78 defines a conformation of MHC-II bound to peptides that are acquired through trafficking to lysosomal Ag-processing compartments and not MHC-II-associated with tetraspanins.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ This work was supported by the Intramural Research Program of the National Institutes of Health and also National Institutes of Health Grant AI046202 (to L.K.D.).

² Address correspondence and reprint requests to Dr. Paul A. Roche, National Institutes of Health, Building 10, Room 4B36, Bethesda, MD 20892. E-mail address: paul.roche{at}nih.gov

³ Abbreviations used in this paper: MHC-II, MHC class II; pMHC-II, peptide-loaded MHC-II; DC, dendritic cell; IP, immunoprecipitate; siRNA, small interfering RNA; Ii, invariant chain; cRPMI, complete RPMI; cDMEM, complete DMEM.

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