GITR Ligand on Leukemic Myeloid Dendritic Cells Suppresses Induction of Leukemia-Associated Antigen-Specific CTLs from Naïve CD8⁺ T Cells.

Glucocorticoid-induced TNFR-related protein (GITR), a member of the TNF receptor superfamily, is expressed at low levels on resting T cells and is up-regulated following activation. Triggering of GITR with its ligand (GITRL) has been described to abrogate the function of CD4⁺CD25⁺ regulatory T cells and stimulate effector T cells in mice, but little is known about roles of GITR-GITRL interaction in humans. Recent evidence suggests that the interaction between GITR on NK cell and GITRL, which is constitutively expressed on tumor cells, negatively regulates NK cell-mediated anti-tumor activity in cancer patients. Leukemic myeloid dendritic cells (mDCs) can induce cyclin-dependent kinase (CDK2)- specific cytotoxic T lymphocytes (CTLs) from naïve T cells and CDK2-specific CTLs are detectable in MRD⁺ patients with leukemia after allo-SCT (Blood. 110 (11): a3228. 2007). The GITR-GITRL interaction may affect this process and a modification of the interaction may improve the efficiency of inducing CDK2-specific CTLs. Therefore, the expression of GITRL on leukemic cells and leukemic mDCs was examined to determine whether an engagement of GITR controls the priming of leukemia-associated-antigen (LAA)-specific CD8⁺ T cells. When cryopreserved BMMCs obtained at diagnosis from 5 patients with AML were assessed using flow cytometry, GITRL expression was detectable on leukemic cells in 3 patients. Leukemic mDCs were enriched from PBMCs of HLA-A24⁺ patients with anti-CD1c mAb-conjugated magnetic beads and were assessed for their ability to stimulate HLA-A24⁺ naïve CD8⁺ T cells to acquire cytotoxicity specific to CDK2-peptides (CDK2 _158–166, CDK2 _178–186). A three days culture of immature leukemic mDCs in the presence of TNFα up-regulated the expression of GITRL along with CD83 and CD40 expression. Naïve CD8⁺ T cells isolated from healthy individuals and cord blood were cultured with the GITRL-expressing leukemic mDCs in the presence or absence of anti- GITR monoclonal antibodies (mAbs) for 14 days and stained for CDK2 _158–166/A24, CDK2 _178–186/A24 pentamers. Blocking of GITR with the mAbs augmented induction of CDK2 _158–166- and CDK2 _178–186- specific CD8⁺ T cells from 0.37% to 1.17% and from 0.45% to 1.64%, respectively (Fig). Anti-GITR mAbs did not enhance induction of CDK2-specific T cells by peptide-pulsed monocyte-derived DCs which do not express GITRL. These data suggest that the expression of GITRL on circulating leukemic mDCs may suppress induction of CTLs specific to LAAs and induce cancer immunoediting in patients with leukemia. Administration of anti-GITR mAb after allo-SCT may enhance graft versus leukemia effect by CDK2-specific CTLs without vaccination of CDK2-peptides.

Fig

View largeDownload slide

Blocking of GITR on T cells augments induction of CDK2-specific CTLs

Fig

View largeDownload slide

Blocking of GITR on T cells augments induction of CDK2-specific CTLs

Close modal