A T cell specific alternative is “enzyme labelling,” including granzyme B which is expressed by activated CD8 T cells and is involved in immune mediated tumour cell killing.139 In a mouse model, the PET imaging agent GZB was tested for quantifying granzyme B release at CT26 and MC38 tumour sites before and after anti-PD-1 and anti-CTLA-4 therapy.140 GZB PET signal positively correlated with CD8 T cell subpopulations and Th1 inflammatory cytokines and negatively correlated with immunosuppressive cell types at the tumour site and anti-inflammatory cytokines at draining lymph nodes for MC38. Here, CTLA4 is linked to neoplasm.