PT - JOURNAL ARTICLE AU - Lejeune, Pascale AU - Cruciani, Véronique AU - Berg-Larsen, Axel AU - Schlicker, Andreas AU - Mobergslien, Anne AU - Bartnitzky, Lisa AU - Berndt, Sandra AU - Zitzmann-Kolbe, Sabine AU - Kamfenkel, Claudia AU - Stargard, Stefan AU - Hammer, Stefanie AU - Jørgensen, Jennifer S AU - Jackerott, Malene AU - Nielsen, Carsten H AU - Schatz, Christoph A AU - Hennekes, Hartwig AU - Karlsson, Jenny AU - Cuthbertson, Alan S AU - Mumberg, Dominik AU - Hagemann, Urs B TI - Immunostimulatory effects of targeted thorium-227 conjugates as single agent and in combination with anti-PD-L1 therapy AID - 10.1136/jitc-2021-002387 DP - 2021 Oct 01 TA - Journal for ImmunoTherapy of Cancer PG - e002387 VI - 9 IP - 10 4099 - http://jitc.bmj.com/content/9/10/e002387.short 4100 - http://jitc.bmj.com/content/9/10/e002387.full SO - J Immunother Cancer2021 Oct 01; 9 AB - Background Targeted thorium-227 conjugates (TTCs) are an emerging class of targeted alpha therapies (TATs). Their unique mode of action (MoA) is the induction of difficult-to-repair clustered DNA double-strand breaks. However, thus far, their effects on the immune system are largely unknown. Here, we investigated the immunostimulatory effects of the mesothelin-targeted thorium-227 conjugate (MSLN-TTC) in vitro and in vivo in monotherapy and in combination with an inhibitor of the immune checkpoint programmed death receptor ligand 1 (PD-L1) in immunocompetent mice.Methods The murine cell line MC38 was transfected with the human gene encoding for MSLN (hMSLN) to enable binding of the non-cross-reactive MSLN-TTC. The immunostimulatory effects of MSLN-TTC were studied in vitro on human cancer cell lines and MC38-hMSLN cells. The efficacy and MoA of MSLN-TTC were studied in vivo as monotherapy or in combination with anti-PD-L1 in MC38-hMSLN tumor-bearing immunocompetent C57BL/6 mice. Experiments were supported by RNA sequencing, flow cytometry, immunohistochemistry, mesoscale, and TaqMan PCR analyses to study the underlying immunostimulatory effects. In vivo depletion of CD8+ T cells and studies with Rag2/Il2Rg double knockout C57BL/6 mice were conducted to investigate the importance of immune cells to the efficacy of MSLN-TTC.Results MSLN-TTC treatment induced upregulation of DNA sensing pathway transcripts (IL-6, CCL20, CXCL10, and stimulator of interferon genes (STING)-related genes) in vitro as determined by RNASeq analysis. The results, including phospho-STING activation, were confirmed on the protein level. Danger-associated molecular pattern molecules were upregulated in parallel, leading to dendritic cell (DC) activation in vitro. MSLN-TTC showed strong antitumor activity (T:C 0.38, p<0.05) as a single agent in human MSLN-expressing MC38 tumor-bearing immunocompetent mice. Combining MSLN-TTC with anti-PD-L1 further enhanced the efficacy (T:C 0.08, p<0.001) as evidenced by the increased number of tumor-free surviving animals. MSLN-TTC monotherapy caused migration of CD103+ cDC1 DCs and infiltration of CD8+ T cells into tumors, which was enhanced on combination with anti-PD-L1. Intriguingly, CD8+ T-cell depletion decreased antitumor efficacy.Conclusions These in vitro and in vivo data on MSLN-TTC demonstrate that the MoA of TTCs involves activation of the immune system. The findings are of relevance for other targeted radiotherapies and may guide clinical combination strategies.Data are available upon reasonable request.