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

The major histocompatibility complex class II subunits (MHC-II) alpha and b eta assemble with the invariant chain (Ii) in the endoplasmic reticulum and are transported to endosomal-lysosomal organelles known as MHC class II co mpartments (MIICs). Although it has been shown that two dileucine-based sig nals in the cytosolic tail of Ii, as well as a dileucine-based signal in th e tail of the beta chain mediate sorting to MIICs, the molecular mechanisms by which alpha beta Ii complexes are sorted have yet to be resolved fully. The AP-3 adaptor complex stands oat as a particularly good candidate for m ediating this targeting because: (i) it has a proven role in the traffickin g of membrane proteins to lysosome-related organelles; and (ii) it has the ability to interact with dileucine-based signals in vitro. To investigate t he 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 Hermans ky-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 ob served neither impairment of the formation of compact SDS-resistant alpha b eta dimers, nor delay in the appearance of a conformational epitope indicat ive of a mature, Ii-free alpha beta dimer. Finally, we demonstrated that in HPS-2 patients' cells, there was no delay in the expression of the alpha b eta dimers on the cell surface. Thus, AP-3 does not seem to be essential fo r normal trafficking of MHC-II. These findings have important implications for HPS-2 patients, because they suggest that the recurrent bacterial infec tions suffered by these patients are not likely due to impaired antigen pro cessing and presentation by MHC-II. (C) 2000 Published by Elsevier Science B.V. All rights reserved.