CD4 and neoplasm: Yet biochemical reality complicates epitope utility: cysteinylation of PSMA459 under physiologic cystine blunted CD4+ recognition, whereas engineering tumor cells to express GILT (a lysosomal thiol reductase) enhanced class II processing and restored helper responses, pinpointing redox-dependent peptide chemistry as a gatekeeper of the class II immunopeptidome in PCa (155).