PD-1 blocking antibodies moonlighting as killers.

Therapeutic antibodies that block PD-1-mediated inhibition of T cells have revolutionized cancer therapy. Murine cancer models are an essential tool for testing the efficacy of PD-1 blockers alone or in combination with other treatments. Depending on the isotype of the antibody and the host species, blocking antibodies can also exert cytotoxic activity towards cells expressing the target molecule. In the current issue of the European Journal of Immunology [Eur. J. Immunol. 2021. 51: 1473-1481], Polesso et al. demonstrate that depletion of PD-1+ T cells by "blocking" PD-1 antibodies can greatly impact the outcome of preclinical immunotherapy experiments. Whereas some PD-1 antibodies promoted activation and proliferation of PD-1-expressing murine T cells, the authors report that administration of a particular PD-1 antibody can result in a significant loss of antigen-specific CD8 T cells in different in vivo models. These findings once more highlight that a comprehensive characterization of antibodies is warranted to avoid misinterpretation of immunotherapy studies.