MHC class II expression is regulated in dendritic cells independently of invariant chain degradation

We have investigated the mechanisms that control MHC class II (MHC II) expr ession in immature and activated dendritic cells (DC) grown from spleen and bone marrow precursors. Degradation of the MHC II chaperone invariant chai n (li), acquisition of peptide cargo by MHC II, and delivery of MHC II-pept ide complexes to the cell surface proceeded similarly in both immature and activated DC. However, immature DC reendocytosed and then degraded the MHC II-peptide complexes much faster than the activated DC. MHC II expression i n DC is therefore not controlled by the activity of the protease(s) that de grade Ii, but by the rate of endocytosis of peptide-loaded MHC II. Late aft er activation, DC downregulated MHC II synthesis both in vitro and in vivo.