Ubiquitin-Dependent Control of Class II MHC Localization Is Dispensable for Antigen Presentation and Antibody Production

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.

MHC Class II Antigen Presentation by Dendritic Cells Regulated through Endosomal Sorting

“…Knock-in mice for a ubiquitination mutant of MHCII showed dysfunction of DCs similar to that of MARCH1 knockout mice ). Others concluded that ubiquitin-dependent control of MHCII localization is dispensable for antigen presentation and antibody production (McGehee et al 2011). In the latter study, mice in which endogenous MHCII was replaced by GFP-tagged wildtype MHCII were compared with knock-in mice for a K225 ubiquitination mutant of GFP-tagged MHCII.…”

“…MHCII peptide loading and ubiquitinaton are not necessarily mechanistically linked, and the sequence of events could merely be dictated by differences in kinetics. A second indication for MHCII peptide loading at the endosomal-delimiting membrane is that loading of wild-type MHCII and mutant K225 MHCII were equally efficient (Shin et al 2006;van Niel et al 2006), and that antigen presentation was unaffected in ubiquitination-deficient MHCII knock-in mice (McGehee et al 2011). Third, interference with MHCII sorting at MVBs with a dominant-negative mutant of VPS4 also did not affect peptide loading (T ten Broeke, R Wubbolts, W Stoorvogel, et al, unpubl.).…”

How to target MHC class II into the MIIC compartment

“…In immature dendritic cells MHC II beta chain engineered to not accept ubiquitin (by mutating single lysine in the cytoplasmic tail for arginine) remained at the plasma membrane, while a ubiquitin fused directly to the MHC II beta chain showed decreased surface expression (Ma et al, 2012). Again, McGehee et al (2011) reported a mouse with engineered MHC II molecules unable to accept uniquitin and could not observed any effect on MHC II distribution and other processes dependent of MHC II such as antibody production. Various experiments reported effects of ubiquitin modifications on endosomal behavior of various proteins, including MHC II.…”

Beyond the classical: Influenza virus and the elucidation of alternative MHC class II-restricted antigen processing pathways

“…What directs trafficking of MHC II through the early endosome? Recent evidence suggests that a lysine residue of the MHC class II b-chain (K 225 of mouse I-A b ) and its monoubiquitination by the MARCH-I E3 ubiquitin ligase regulate the trafficking of MHC class II in both mice and humans [59][60][61][62][63][64] but the issue is clearly complicated. MHC class II ubiquitination by MARCH-I results in a low level of surface MHC class II and this tightly regulated process is blocked by DC maturation [65].…”

“…Despite this, a firm link between MHC ubiquitination and T CD4? responses has yet to be established [61,62,65]. Exploration of these issues in well-defined antigen systems may provide traction.…”