Trafficking of MHC Class II in Dendritic Cells is Dependent on but Not Regulated by Degradation of Its Associated Invariant Chain

In dendritic cells (DC), newly synthesized MHCII is directed to endosomes by its associated invariant chain (Ii). Here, Ii is degraded after which MHCII is loaded with peptides. In immature DC, ubiquitination of peptide-loaded MHCII drives its sorting to lysosomes for degradation. Ubiquitination of MHCII is strongly reduced in response to inflammatory stimuli, resulting in increased expression of MHCII at the plasma membrane. Whether surface exposure of MHCII is also regulated during DC maturation by changing the rate of Ii degradation remained unresolved by conflicting results in the literature. We here pinpoint experimental problems that have contributed to these controversies and demonstrate that immature and mature DC degrade Ii equally efficient at proper culture conditions. Only when DC were cultured in glutamine containing media, endosome acidification and Ii degradation were restricted in immature DC and enhanced in response to lipopolysaccharide (LPS). These effects are caused by ammonia, a glutamine decomposition product. This artificial behavior could be prevented by culturing DC in media containing a stable dipeptide as glutamine source. We conclude that Ii degradation is a prerequisite for but not a rate limiting step in MHCII processing.