Abstract

Controlled localization of class II MHC molecules is essential for proper class II MHC-restricted antigen presentation and the subsequent initiation of an adaptive immune response. Ubiquitination of class II MHC molecules on cytosolic lysine (K225) of the β-chain has been shown to affect localization of the complex. We generated mice in which the endogenous β-chain locus is replaced with a GFP tagged mutant version that lacks the cytosolic lysine residue (I-A-β-K225R-EGFP). These mice have elevated levels of class II MHC as compared to I-A-β-EGFP mice, and immature bone marrow-derived dendritic cells show redistribution of class II MHC to the cell surface. Nonetheless, in these same cells efficiency of antigen presentation is unaffected in I-A-β-K225R-EGFP mice, as assayed for presentation of ovalbumin to appropriately specific T cells. The I-A-β-K225R-EGFP animals have normal CD4 T cell populations and are capable of generating antigen-specific antibody in response to model antigens and viral infection. We therefore conclude that in our experimental system modulation of trafficking by ubiquitination of residue K225 of the β-chain is not essential for the function of class II MHC products in antigen presentation or antibody production.

Highlights

  • Class II major histocompatibility complex (MHC) molecules are expressed by professional antigen presenting cells (APCs), and are necessary for the presentation of antigenic peptides to CD4 T cells and the subsequent initiation of an adaptive immune response

  • Generation of I-A-b-K225R-EGFP mice Ubiquitination of class II MHC regulates the localization of class II MHC molecules in both B cells and dendritic cells (DCs) [4,6,7]

  • In order to study the role of ubiquitination on the function of class II MHC molecules in vivo and in vitro, we generated a knock-in mouse model in which the endogenous class II MHC locus was replaced by a construct that encodes a mutant class II MHC b-chain that cannot be ubiquitinated (K225R)

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Summary

Introduction

Class II major histocompatibility complex (MHC) molecules are expressed by professional antigen presenting cells (APCs), and are necessary for the presentation of antigenic peptides to CD4 T cells and the subsequent initiation of an adaptive immune response. Following engagement of Toll like receptors, and possibly other receptors as well, this ubiquitination is turned off or reversed, which results in relocalization of class II MHC from internal compartments to the cell surface of DCs [5] This differential ubiquitination is achieved through regulation of the ubiquitin ligase responsible for the addition of ubiquitin to the class II MHC, the membraneassociated RING-CH I (MARCH-I) [7,8,9]. Expression of MARCH I is downregulated in human dendritic cells upon maturation [8], leading to a corresponding increase in the cell surface levels of class II MHC It appears that only a minor fraction of class II MHC products is modified by ubiquitination, with the caveat that biochemical methods are likely to underestimate the true extent of modification

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