Studying the biology of cytotoxic T lymphocytes in vivo with a fluorescent granzyme B-mTFP knock-in mouse.

Understanding T cell function in vivo is of key importance for basic and translational immunology alike. To study T cells in vivo, we developed a new knock-in mouse line, which expresses a fusion protein of granzyme B, a key component of cytotoxic granules involved in T cell-mediated target cell-killing, and monomeric teal fluorescent protein from the endogenous Gzmb locus. Homozygous knock-ins, which are viable and fertile, have cytotoxic T lymphocytes with endogeneously fluorescent cytotoxic granules but wild-type-like killing capacity. Expression of the fluorescent fusion protein allows quantitative analyses of cytotoxic granule maturation, transport and fusion in vitro with super-resolution imaging techniques, and two-photon microscopy in living knock-ins enables the visualization of tissue rejection through individual target cell-killing events in vivo. Thus, the new mouse line is an ideal tool to study cytotoxic T lymphocyte biology and to optimize personalized immunotherapy in cancer treatment.