CYCS and neoplasm: To establish an MHCII-restricted in vivo model to test the ability of tumour-specific B cells to specifically activate tumour-specific CD4+ T cells, we utilized Hen Egg Lysozyme (HEL) specific SWHEL B cells3 and Moth Cytochrome C residues 87–103 (MCC) T cell receptor (TCR) transgenic 5C.C7 CD4+ T cells in combination with a B16.F10 tumour line that expresses recombinant HELMCC antigen containing the relevant B and T cell epitopes.4,5 The ability of tumour-specific B cells to activate tumour-specific CD4+T cells independently of other APCs was determined.