MHC class II epitopes require protein denaturation and removal of intra- and inter-chain disulphide bonds prior to proteolysis. The lysosomal thiol reductase gamma-IFN-inducible lysosomal thiol reductase (GILT) has been shown to facilitate MHC class II-restricted antigen (Ag) processing by breaking disulphide bonds. GILT is constitutively expressed in APCs. The reduction of disulphide bonds by mature GILT is optimal at acidic pH (Hastings et al. 2006, Arunachalam et al. 2000).