Development of a Peripheral Blood Mononuclear Cells (PBMC) ImmunoGraft Platform to Evaluate the Pharmacodynamics of Immuno-Oncology (IO) Therapeutics

Humanized immune system (HIS) mouse models enable in vivo studies in the context of the human immune cells with a human tumor and are critical for the development of next-generation immune-oncology (IO) agents. Humanization of immunodeficient mice through the adoptive transfer of normal adult PBMC leads to rapid engraftment of human T cells to study immune-modulatory agents in the context of human tumor xenografts but is limited by the development of xenogeneic graft-versus-host disease (xGVHD). In this study, we evaluated the engraftment of PBMC in β2microglobulin null superimmunodeficient mice NSG-B2M mice, that lack MHC Class I on host tissues. A cell line-derived xenograft model (CDX) co-engrafted with PBMC (PBMC-ImmunoGraft) was characterized for humanization, tumor-infiltrating leukocytes (TIL) phenotype, and tumor response to checkpoint inhibitors.

PBMC from healthy donors (N=7) were implanted and engraftment in peripheral blood was assessed by flow cytometry. T cell memory phenotypes were assessed over time in a small cohort, and costimulatory and inhibitory T cell subsets were evaluated at the terminal time point in blood and secondary lymphoid organs. Next, NSG-B2M mice were co-implanted with MDA-MB-231 breast cancer cell line s.c, humanized with PBMC, and tumor growth kinetics were monitored. Efficacy studies evaluating checkpoint inhibitors are currently ongoing.

Successful PBMC engraftment without xGVHD was observed in NSG-B2M mice up to 8 weeks, in contrast with MHC Class I expressing immunodeficient mice that developed xGVHD within 4-5 weeks. Dose and donor-dependent chimerism was observed. T cells were detectable in the periphery starting at 2 weeks with stable levels (up to 40% of live cells to 8 weeks) with increasing T effector memory cells over the course of the study. All tumors evaluated had high levels of TIL as measured by flow cytometry and
immunohistochemistry. Costimulatory and inhibitory molecules evaluated on CD4 and CD8 included 4-1BB, TIM-3, LAG-3, OX-40, as well as PD-1, which was expressed on both peripheral blood cells and in TIL. Tumor growth kinetics was unaltered by PBMC humanization through a 5-week study window.

PBMC-humanized NSG-B2M mice may represent a model for evaluating IO therapeutics with a long study window due to the lack of xGVHD. While PBMC engraftment kinetics are donor dependent, similar phenotypes are observed and T cell subsets expressing several relevant therapeutic targets, including PD-1are present. This model may permit a rapid in vivo method to study checkpoint blockade and other T-cell directed IO therapeutics.