CTLA4 and neoplasm: If CTLA-4 inhibits T cell priming while PD-1/PD-L1 inhibits T cell effector function, one possible explanation may be that a narrowing of the immune response with PD-L1 blockade may reflect non-productive antigen recognition of already exhausted T cells, while repertoire narrowing following CTLA-4 blockade may represent generation of novel, productive T cell responses to a limited number of tumor antigens.