CD4 and Autoimmunity: The lethal CD4-driven autoimmunity seen in CTLA4-deficient mice (10) shows that at best CTLA4 blockade must be incomplete in human cancer immunotherapy, and although the autoimmunity associated with PD1 deficiency is relatively mild (11), the human melanoma studies show that the autoimmunity is augmented by blocking both CTLA4 and PD1 pathways.