Immune checkpoint receptor-ligand interactions are essential for down-regulating immune responses and maintaining self-tolerance. Functional antagonist antibodies to PD-1 and CTLA-4 enhance existing immune responses and are approved therapies in multiple oncology indications. We hypothesize that many human autoimmune diseases occur due to dysregulated checkpoint signaling, leading to uncontrolled T cell responses. Agonist antibodies to checkpoint receptors that mimic the function of natural ligands have the potential to suppress human autoimmune/inflammatory disease and reinstate tolerance. Two functional anti-checkpoint receptor antibodies that down-regulate human immune responses were discovered and optimized. Antibody #1 is a humanized hybridoma-derived antibody that was optimized for affinity and functional activity using mutations identified from deep sequencing of the splenic repertoire. To avoid potential antagonist activity Antibody #1 was non-blocking for checkpoint ligand binding. Antibody #1 was highly efficacious in a human PBMC-NOD-scid IL2rγnull (NSG) graft versus host disease model, and efficacy in the model was not dependent on Fc effector function. Antibody #2 is a humanized anti-checkpoint receptor antibody derived from single antigen-specific B cell sorting and PCR. It was increased in affinity by incorporating mutations identified from sequences of 3 related single B cell clone antibodies. Optimized Antibody #2 demonstrated improved functional activity in inhibiting human T cell activation in vitro and potently inhibited tetanus toxoid recall responses in human whole blood. These results indicate that functional anti-checkpoint antibodies that down-modulate immune responses and lack antagonist activity can be discovered and optimized and may have the potential to restore immune balance in autoimmune and inflammatory diseases.