Trafficking of major histocompatibility complex class II molecules in human B-lymphoblasts deficient in the AP-3 adaptor complex

Abstract

The major histocompatibility complex class II subunits (MHC-II) α and β assemble with the invariant chain (Ii) in the endoplasmic reticulum and are transported to endosomal–lysosomal organelles known as MHC class II compartments (MIICs). Although it has been shown that two dileucine-based signals in the cytosolic tail of Ii, as well as a dileucine-based signal in the tail of the β chain mediate sorting to MIICs, the molecular mechanisms by which αβIi complexes are sorted have yet to be resolved fully. The AP-3 adaptor complex stands out as a particularly good candidate for mediating this targeting because: (i) it has a proven role in the trafficking of membrane proteins to lysosome-related organelles; and (ii) it has the ability to interact with dileucine-based signals in vitro. To investigate the potential role of AP-3 in transport of MHC-II to MIICs, we have examined MHC-II trafficking in human B-lymphoblast lines from patients with Hermansky–Pudlak syndrome type 2 (HPS-2), which are deficient in the AP-3 complex. Pulse-chase analyses revealed no significant alteration in the kinetics of synthesis and degradation of either MHC-II subunits or Ii. Moreover, we observed neither impairment of the formation of compact SDS-resistant αβ dimers, nor delay in the appearance of a conformational epitope indicative of a mature, Ii-free αβ dimer. Finally, we demonstrated that in HPS-2 patients’ cells, there was no delay in the expression of the αβ dimers on the cell surface. Thus, AP-3 does not seem to be essential for normal trafficking of MHC-II. These findings have important implications for HPS-2 patients, because they suggest that the recurrent bacterial infections suffered by these patients are not likely due to impaired antigen processing and presentation by MHC-II.