It is known that e.g. epithelial cell, fibroblast, etc. can express MHC 2 molecules. Are they “loaded” with peptides and if so, how can such cells uptake foreign proteins?
Yes, they might. If processed peptides are present (generated by extracellular processing) and can bind to MHC II on the non-APC T cell activation might potentially occur. While there is evidence for the presentation of peptides by MHC II expressed on APC, evidence for presentation by non-APC cells are rare, but not impossible. Please take a look at our papers in Nature Communications and in Trends in Immunology
Article Divergent Paths for the Selection of Immunodominant Epitopes...
Article Determinants of Immunodominance for CD4 T cells