BackgroundGDF-15, a member of the TGF-beta superfamily, is a critical factor of feto-maternal tolerance, but also of local immune suppression at sites of cellular stress. Various solid tumor types secrete high levels of this immunosuppressive cytokine. In cancer patients, elevated GDF-15 serum levels correlate with poor prognosis and reduced overall survival.1 Mechanistically GDF-15 interferes with LFA-1/ICAM-1 dependent immune cell extravasation and thus limits immune infiltration into GDF-15 expressing tumors. In line with immune infiltration as a prerequisite of checkpoint-inhibitor responses GDF-15 serum levels negatively correlate with the response to anti-PD-(L)1 CPI therapy.2 3 Neutralization by clinical stage GDF-15 neutralizing antibody visugromab (CTL-002; GDFather-1/2 trial NCT04725474) restores responsiveness to anti-P(L)1 by increasing extravasation of immune cells into the tumor microenvironment.2 To be active, bispecific T-cell engagers are dependent on infiltration of T-cells into the tumor tissue. Once bound to both T-cell and tumor cell, bispecifics retain T-cells at cancer cell sites. In this study we tested the synergy of visugromab with tebentafusp, a CD3-cell redirecting bispecific fusion protein for gp100+ tumors, to enhance effector T-cell retention in the tumor.MethodsSpecific binding and cytotoxic activity of tebentafusp was qualified by flow-cytometry and cytotoxicity assays with HLA-A2+ gp100+ human SK-MEL-5-melanoma cells, respectively. In vivo T-cell retention was tested in PBMC-humanized NOG mice, treated with tebentafusp4 and isotype or a combination of tebentafusp and visugromab. After five days body weight, serum GDF-15, tumor size/weight and mouse and human immune cell infiltration were measured as a final read-out.ResultsTebentafusp bound gp100 on GDF-15 secreting HLA-A2+ SK-MEL-5 melanoma cells and mediated tumor specific killing by T cells in vitro. In vivo, treatment with tebentafusp in combination with isotype resulted in a 4.3-fold increase in T-cell numbers in s.c. SK-MEL-5 tumors in PBMC-humanized NOG mice, while in combination with visugromab, a 15.3-fold increase was observed.ConclusionsGDF-15 is an inhibitor of immune infiltration, and its neutralization enhances effector cell presence in tumors. Bispecific T-cell engagers are dependent on proper effector cell infiltration into tumors to induce significant tumor cell killing. In a PD study combining tebentafusp with anti-GDF-15 antibody visugromab significantly increased the number of intratumoral T cells in mice. Increasing the number of effector cells in the tumor microenvironment is expected to have a direct positive impact on the anti-tumor activity of different bispecific T-cell engaging treatments.ReferencesWischhusen J, et al. Growth/Differentiation Factor-15 (GDF-15): From Biomarker to Novel Targetable Immune Checkpoint. Front Immunol. 2020 May 19;11:951. PubMed PMID: 32508832Haake M, et al. Tumor-derived GDF-15 blocks LFA-1 dependent T cell recruitment and suppresses responses to anti-PD-1 treatment. Nat Commun. 2023, in press Hong G, et al. Plasma GDF15 levels associated with circulating immune cells predict the efficacy of PD-1/PD-L1 inhibitor treatment and prognosis in patients with advanced non-small cell lung cancer. J Cancer Res Clin Oncol. 2023 Jan;149(1):159–171. PubMed PMID: 36472770Chen LN, Carvajal RD. Tebentafusp for the treatment of HLA-A*02:01-positive adult patients with unresectable or metastatic uveal melanoma. Expert Rev Anticancer Ther. 2022 Oct;22(10):1017–1027. PubMed PMID: 36102132Ethics ApprovalThis study was approved by EPO Berlin’s Ethics Board; approval number E0023–23.