Dual Roles for Regulatory T-cell Depletion and Costimulatory Signaling in Agonistic GITR Targeting for Tumor Immunotherapy

• Published in: Cancer research (Chicago, Ill.) Vol. 77; no. 5; pp. 1108 - 1118
• Main Authors: Mahne, Ashley E, Mauze, Smita, Joyce-Shaikh, Barbara, Xia, Jane, Bowman, Edward P, Beebe, Amy M, Cua, Daniel J, Jain, Renu
• Format: Journal Article
• Language: English
• Published: United States American Association for Cancer Research, Inc 01-03-2017
• Subjects: Animal models; Animals; Antibodies, Monoclonal - immunology; Antibodies, Monoclonal - pharmacology; Cancer; Cancer immunotherapy; CD223 antigen; CD8 antigen; Cell fate; Cell Line, Tumor; Colonic Neoplasms - immunology; Colonic Neoplasms - therapy; Depletion; Exhaustion; Fate maps; Foxp3 protein; Genotype & phenotype; Glucocorticoid-Induced TNFR-Related Protein - agonists; Glucocorticoid-Induced TNFR-Related Protein - immunology; Humans; Immunotherapy; Immunotherapy - methods; Lymphocyte Depletion - methods; Lymphocytes; Lymphocytes T; Melanoma - immunology; Melanoma - therapy; Mice; Mice, Inbred C57BL; Monoclonal antibodies; PD-1 protein; Signal Transduction; T cell receptors; T-Lymphocytes, Regulatory - immunology; Tumors
• ISSN: 0008-5472; 1538-7445
• DOI: 10.1158/0008-5472.CAN-16-0797

Agonistic monoclonal antibodies (mAb) targeting the T-cell receptor coregulatory molecule GITR exert potent therapeutic activities in preclinical tumor models. Although anti-GITR mAb are thought to act by depleting and destabilizing the intratumoral T regulatory cell (Treg) population, the precise mechanism of action is obscure. Here, we addressed this issue using a Treg fate-mapping approach, which revealed that Treg loss was primarily due to cell depletion, with minimal evidence of Treg conversion to a non-Foxp3-expressing population. Further characterization of persisting Tregs following anti-GITR mAb treatment showed that a highly activated subpopulation of CD44
ICOS
intratumoral Tregs were preferentially targeted for elimination, with the remaining Tregs exhibiting a less suppressive phenotype. With these changes in the Treg population, intratumoral CD8
T cells acquired a more functional phenotype characterized by downregulation of the exhaustion markers PD-1 and LAG-3. This reversal of CD8
T-cell exhaustion was dependent on both agonistic GITR signaling and Treg depletion, as neither mechanism by itself could fully rescue the exhaustion phenotype. Tests of anti-human GITR antibody MK-4166 in a humanized mouse model of cancer mimicked many of the effects of anti-mouse GITR mAb in syngeneic tumor models, decreasing both Treg numbers and immune suppressor phenotype while enhancing effector responsiveness. Overall, our results show how anti-GITR mAb shifts Treg populations to enable immune attack on tumors, with clinical implications for molecular markers to modify emerging treatments.
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