CXCL13 and neoplasm: Dysfunction in tumor infiltrating CD8 T cells is also associated with marker characteristics indicative of tumor reactivity, including CD4/CD8 double positivity (24), co-expression of CD103 and CD39 (52), expression of CXCL13 (53), and upregulation of 4-1BB, which reflects recent antigen encounter (54) and was reported to aid in discriminating tumor-reactive CD8 T cells from bystander CD8 T cells in the TME (55).