Abstract 1131: CD8 immuno-PET using a fully human mAb can detect increases in tumor-infiltrating T cells by a T cell activating immunotherapy (2019)

Abstract Detection and quantification of immune cell subsets and changes in the tumor immune microenvironment would be a powerful tool to correlated to response in cancer immunotherapy. Currently, biopsies used to monitor the tumor microenvironment suffer from sampling error, restricted sampling over time and to inaccessible tissues/organs, and do not reflect discordance across tumor lesions of the same patient. Immuno-PET detection of CD8-expressing T cells is a promising translational imaging approach to assess CD8+ tumor-infiltrating lymphocytes (TILs) pre- and post-therapy. We have developed a fully human anti-CD8 antibody that was radiolabeled with the positron emitting radionuclide Zirconium-89 (89Zr) using the bifunctional chelator p-SCN-Bn-Desferoxamine (DFO). CD8-genetically humanized immunocompetent mice were used to validate dose dependent targeting of 89Zr-CD8 to lymphoid tissues. Next, two models were developed to monitor the response of the T cell activating bispecific antibody REGN1979 (CD3xCD20). In the first model designed to lack a CD8 antigen sink, Raji lymphoma cells were co-implanted with human peripheral blood mononuclear cells (hPBMCs) in immune deficient NSG mice and treated with REGN1979. Here, 89Zr-CD8 detected a two-fold increase in CD8+ TILs as quantified by flow cytometry compared to control antibody treated mice. To determine if 89Zr-CD8 could detect changes in CD8+ TILs in the presence of increased circulating and splenic T cells, a different model was developed. Raji lymphoma cells were implanted subcutaneously and hPBMCs were implanted intraperitoneally in immune deficient SRG-15 mice (hSIRP KI, Rag2 KO, IL2Rg KO, hIL-15 KI). Due to the humanization of IL-15, T cells from the implanted hPBMCs rapidly expanded and produced a CD8 antigen sink. In the SRG-15 model, total tumor uptake of 89Zr-CD8 was lower and spleen uptake higher than in the NSG model due to the presence of the antigen sink. These SGR-15 mice treated with REGN1979 also demonstrated an increase in CD8+ TILs by flow cytometry that was specifically detected with 89Zr-CD8. This work demonstrates that 89Zr-CD8 can specifically detect CD8+ T cells in lymphoid tissues of CD8 humanized mice and therapy-induced alterations of CD8+ TILs. Importantly, the detection of CD8+ TILs occurs in both the presence and absence of an antigen sink. This work supports the clinical translation of 89Zr-CD8 immuno-PET to investigate its utility for predicting and monitoring response in patients undergoing immunotherapy. Citation Format: Richard Tavare, Makenzie Danton, Jason T. Giurleo, Marcus P. Kelly, Sosina Makonnen, Carlos Hickey, Tomas C. Arnold, Dangshe Ma, William C. Olson, Gavin Thurston, Jessica R. Kirshner. CD8 immuno-PET using a fully human mAb can detect increases in tumor-infiltrating T cells by a T cell activating immunotherapy [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 1131.