CD4 and neoplasm: Here we use our well-characterized, syngeneic mouse sarcoma models to show that, whereas synthetic long peptide (SLP) neoAg vaccines containing tumour-specific MHC-I neoAgs and low inputs of tumour-specific MHC-II neoAgs efficiently promote tumour rejection, similar vaccines expressing high quantities of the same MHC-II neoAgs unexpectedly induce immunosuppressive CD4+ T cells distinct from Treg cells that inhibit tumour elimination.