TNFRSF9 and cancer: Finally, a 41BBL triggered T cell 41BB signal has been shown to promote CD8+ tissue-resident memory T (TRM) cell responses (16), and co-stimulation with a 41BB-agonist like anti-41BB Ab promotes CTL expansion, cytotoxicity and survival in addition to converting CTL exhaustion, thus potentiating cancer immunotherapy (17–22).