Abstract 268: Engineering of transgenic mice expressing human mesothelin for investigation of mesothelin-targeted therapeutics

Abstract

Most monoclonal antibody-based therapeutics are specific for the human isoform of their targets and do not cross-react with orthologous mouse isoforms. This frequently limits pre-clinical modeling to human-derived tumors growing in immune compromised mice and precludes combination studies with immune modulating agents which rely on an active immune system for activity. Although introduction of human cDNA into mouse tumor cells will make these cells susceptible to targeting, a syngeneic mouse may reject or partially reject cells bearing the foreign transgene, impacting anti-tumor efficacy studies. Mesothelin (MSLN) is the target of many cell- and antibody-based therapies that have reached clinical trials due to its lack of expression in critical normal tissues and robust expression in many solid tumors. We have generated two distinct transgenic C57/Bl6 mouse models with compartment-limited expression of human MSLN (hMSLN) for use in pre-clinical testing of MSLN-targeted therapeutics: 1) TPO/hMSLN mice express full-length hMSLN from an insulated rat thyroid peroxidase gene promoter. Analysis by RT-PCR, immunoblot and IHC demonstrate strong thyroid-specific expression hMSLN. 2) In Nor/hMSLN mice, the hMSLN coding region was inserted as a full-length cDNA into the native mMSLN locus via a homologous recombination knock-in technique such that hMSLN is expressed in place of the mouse isoform under the control of endogenous murine MSLN transcription regulatory elements. IHC studies show expected expression of hMSLN in mouse pleura, pericardium and peritoneum. Shed hMSLN can be readily detected in the serum of both transgenic lines using ELISA, similar to human patients. Treatment of either model with MSLN-targeted immunotoxin LMB-100 at the maximum dose tolerated by non-transgenic C57/Bl6 mice resulted in no gross toxicity. Histologic analysis revealed subclinical pericarditis in Nor/hMSLN mice. No post-treatment thyroid damage was observed in TPO/hMSLN mice even upon detailed histologic examination of thyroid tissue. Syngeneic mouse pancreatic cancer cells expressing hMSLN transgene implanted orthotopically into TPO/hMSLN or Nor/hMSLN grew as well or better as those grown in non-transgenic C57/Bl6 mice. We have established two immunologically proficient transgenic mouse models expressing hMSLN that can be used for evaluating immune effect of hMSLN-targeted therapeutics as well as their on-target/off-tumor toxicities. Citation Format: Xianyu Zhang, Brendan Hagerty, Theresa Guerin, Norene O9Sullivan, Serguei Kozlov, Christine Alewine. Engineering of transgenic mice expressing human mesothelin for investigation of mesothelin-targeted therapeutics [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 268.