Background Recently, TIGIT+PD1 blockade was shown to confer additive survival benefits in orthotopic glioblastoma and colon cancer mouse models1 2—notably, this included experiments3 that used the 1G9 TIGIT monoclonal antibody (mAb) that has potentially agonistic effects.1 Herein we investigated the TIGIT/CD226/CD155 axis and effects of combining clinically analogous PD-1/PD-L1 mAbs with TIGIT-targeting 1G9 mAb in murine glioblastoma models.
Methods The overall survival (OS) associated with TIGIT (non-depleting IG9; 200μg every 3days for 4 doses),1 PD-1 (8H3; initial 500μg followed by 250μg every 3days for 7 doses), PD-L1 (6A2; initial 500μg followed by 250μg every 3days for 7 doses), and/or IgG mAbs was assessed in immunocompetent C57BL/6 albino mice intracranially implanted with syngeneic GL261-luc2 or CT2A-luc.4 5 The roles of T cells and NK cells were examined using depletion with CD8a, CD4, or NK1.1 mAbs. Expression of TIGIT/CD226/CD155 and PD-1/PD-L1/PD-L2 by tumor and tumor-infiltrating immune cells was evaluated using flow cytometry and RT-qPCR.
Results In vitro, GL261-luc2 and CT2A-luc tumor cells moderately expressed PD-L1, PD-1, and TIGIT; but strongly expressed TIGIT’s inhibitory ligand CD155.(Figure1A-B) Ex vivo, >83% of CD8+ and CD4+TILs in GL261-luc2 co-expressed TIGIT+/CD226+: ≥2x the proportions in CT2A-luc.(Figure 2A) CD155 and PD-L1 were highly co-expressed on tumor-infiltrating macrophages: greater in GL261-luc2 than CT2A-luc tumors.(Figure 2B)In GL261-luc2 mice, anti-TIGIT monotherapy displayed minimal OS improvement; whereas anti-PD-1 and anti-PD-L1 monotherapies demonstrated robust OS responses.(Figure 3A) Adding anti-TIGIT to PD-1/PD-L1 blockade demonstrated synergism with anti-PD-1. Anti-TIGIT plus anti-PD-1 displayed nominally improved OS in CT2A-luc compared to anti-PD-1 monotherapy (p=0.11).(Figure 3B).Given robust T-cell expression of TIGIT and PD-1, we examined how CD4+ or CD8+ depletion affected responses in GL261-luc2 mice: depletion completely abrogated anti-PD-1’s benefits.(Figure 4) Although CD4/CD8 depletion also reduced anti-TIGIT+anti-PD1’s efficacy, the resulting OS matched that of non-depleted anti-PD-1 monotherapy. Additionally, NK cell depletion had no effect on anti-TIGIT+anti-PD1’s efficacy.
Conclusions Our results recapitulate published findings regarding the synergistic benefits of combining TIGIT 1G9 mAb with anti-PD-1 using the clinically-relevant 8H3 mAb in syngeneic mouse glioblastoma, and extend those findings to anti-TIGIT+anti-PDL1 combinations. TIGIT/CD226 was highly co-expressed by immuno-responsive GL261-luc2’s tumor-infiltrating lymphocytes (TILs); wheres CD155/PD-L1 expression predominated in tumor-infiltrating myeloid cells. Depletion of CD8+ or CD4+ TILs modestly reduced anti-TIGIT+anti-PD1’s efficacy—suggesting a mechanism that is at least partially independent of T (and NK) cells. Our preliminary results suggest a complex interplay between TIGIT/CD226/CD155 and PD-1/PD-L1/PD-L2 axes in tumors and their microenvironmental constituents that warrants further investigation; plus, careful consideration of antibody clones’ functionality is necessary for designing immunotherapy combinations.
Acknowledgements We gratefully acknowledge the support of the The Jennifer Oppenheimer Cancer Research Initiative; The Ben and Catherine Ivy Foundation; Hope It’s A Beach Thing; and the Pan Mass Challenge (Erica’s Entourage and CRUS11TOUR), and the NCI (P01CA236749; K12CA090354).
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