CD8A and melanoma: For example, it was difficult to establish a reliable experimental method to induce cancer-recognizing CD8+ T cells in vivo, and we could not establish well-matched mouse cell lines for our experiments because PD-L1 was expressed at a low level or not at all in the cell lines available to us, i.e., breast cancer 4T1 cells [30], melanoma B16-BL6 cells [31], and colorectal Colon-26 cells (our unpublished data).