High resolution crystal structures of DNA polymerase intermediates are needed to study the mechanism of DNA synthesis in cells. Here we report five crystal structures of DNA polymerase I that capture new conformations for the polymerase translocation and nucleotide pre-insertion steps in the DNA synthesis pathway. We suggest that these new structures, along with previously solved structures, highlight the dynamic nature of the finger subdomain in the enzyme active site.

The mechanism of DNA synthesis has been inferred from static structures, but the absence of temporal information raises longstanding questions about the order of events in one of life’s most central processes. Here we follow the reaction pathway of a replicative DNA polymerase using time-resolved X-ray crystallography to elucidate the order and transition between intermediates. In contrast to the canonical model, the structural changes observed in the time-lapsed images reveal a catalytic cycle in which translocation precedes catalysis. The translocation step appears to follow a push-pull mechanism where the O-O1 loop of the finger subdomain acts as a pawl to facilitate unidirectional movement along the template with conserved tyrosine residues 714 and 719 functioning as tandem gatekeepers of DNA synthesis. The structures capture the precise order of critical events that may be a general feature of enzymatic catalysis among replicative DNA polymerases.

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TNFRSF25, also known as Death Receptor 3 (DR3), is a cell surface receptor of the tumor necrosis factor receptor superfamily (TNFRSF) which is expressed on T cells, innate lymphoid cells, and B cells. Upon binding with TL1A ligand, this co-stimulatory molecule induces T cell activation, leading either to inflammation or cell death. Mouse PTX-35 (mPTX-35) is a TNFRSF25 functional agonist derived from a CDR that was humanized and affinity matured, then fused to a mouse IgG1-Fc backbone. Since costimulatory agonist antibodies have shown limited efficacy in clinical trials, our aims were to (i) determine the anti-tumor efficacy of mPTX-35, (ii) elucidate any mechanisms of resistance, and (iii) identify immune conditions that provide effective anti-tumor immune responses to mPTX-35 treatment. We studied the anti-tumor immune effects of mPTX-35 in both cold and hot tumor models. In a TC-1 cold tumor, we found that treatment with an anti-TNFRSF25 agonist antibody does not affect tumor growth or survival. In contrast, vaccination with an E7 antigen-specific vaccine leads to an environment that reverses primary resistance to the agonist antibody, inhibiting the tumor growth, and enhancing overall survival. Interestingly, we found that similar to other co-stimulatory agonist molecules, the immune effects of the anti-TNFRSF25 agonist are dose-dependent with lower (0.2 mg/kg) and higher (5 mg/kg) doses of mPTX-35 showing a reduced anti-tumor response than a dose of 1 mg/kg, resulting in a bell-shaped response in all tumor models. Similar immune-mediated anti-tumor effects of mPTX-35 were found in a CT26 (hot) tumor model. In conclusion, these results show that anti-TNFRSF25 agonist antibody demonstrates high anti-tumor efficacy in tumors that are infiltrated with T cells while the absence of properly antigen activated CD8 T cells (such as in cold tumors) incurs primary resistance to TNFRSF25 agonist that is reversed by proper priming of CD8 T cells. These results highlight the importance of optimal priming to limit primary resistance mechanisms and drive an efficacious immune response via the TNSFR25 axis. Citation Format: Dareen Sarhan, Lara Rabah, Vikas Tahiliani, Anh M. Trinh, Rahul R. Jasuja, Eric P. Dixon, Matthew Seavey, Vivek Verma, Seema Gupta, Samir N. Khleif. Proper priming of T cells is crucial for the anti-tumor effect of mPTX-35, an anti-TNFRSF25 agonist antibody [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2881.

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