CD19 and neoplasm: A variant example of selective agonism is the creation of CD3 T cell engagers (TCEs) that incorporate antibody binding regions of 4-1BB with CD3, and checkpoint blockade (PD-L1), and a tumor target such as CD19 or EGFR (Figure 2), with the aim of only engaging 4-1BB on a T cell in the TME (53, 107).