Objective: The preclinical evaluation of novel immune checkpoint modulators is dependent on tumor models in mice with functional immune cells. In previous experiments, we have demonstrated, that hematopoietic stem cells (HSC) can proliferate and differentiate in vivo to form a functional humanized immune system. Further we have shown engraftment of PDX on these humanized mice. In our translational research project we were interested, whether the PD1 checkpoint inhibitors can block tumor growth in this human model system and if there is a correlation of response and PD-L1 expression.

Methods: Hematopoetic stem cells (HSC) from cord blood were transplanted intravenously into 3 week old irradiated nod scid gamma mice. After 12 weeks, blood was collected and screened by FACS for human immune cell engraftment (huCD45+). PDX were screened for PD-L1 expression on RNA level by PCR and protein level by FACS, western blot and immunhistochemistry. PD-L1 positiv and negative PDX models were subcutaneously co-transplanted into these humanized mice and followed for growth. Response to the CTLA-4 checkpoint inhibitor ipilimumab and PD-1 checkpoint inhibitor nivolumab was evaluated. Tumor and organs (bone marrow, spleen, thymus) were investigated at the end of the experiments by FACS and exemplary by immunhistochemistry for T cells and other immune cells and the expression of PD-1 and CTLA-4.

Results: We confirmed successful engraftment after 12 to 14 weeks, when up to 20% of the human immune cells in the blood were T-cells. Engraftment in different organs has been detected, with up to 15% CD45+ cells in spleen and thymus and 50% in the bone marrow. We engrafted successfully 14 PDX models on the humanized mice. 12 from 14 showed no difference in tumor growth compared to non-humanized mice. These PDX models seem to be tolerated by the human immune cells. Treatment with ipilimumab or nivolumab alone or in combination delayed the tumor growth, accompanied by an increase of “activated” T-cells in blood and tumor. We found a correlation between tumor growth inhibition by the checkpoint inhibitor treatment and tumor expression of PD-L1. Furthermore PDX tumors with a reduced growth on humanized mice (due to innate immune response) showed a stronger response to the checkpoint inhibitors.

Conclusion: HSC can be transplanted in immunodeficient mice and establish a “functional” human immune system. PDX models can be successfully engrafted on these humanized mice - generating a fully human preclinical test system for immuno-oncology. Within these models checkpoint inhibitors demonstrate tumor growth inhibition in correlation with tumor PD-L1 expression.

Citation Format: Maria Stecklum, Annika Wulf-Goldenberg, Bernadette Brzezicha, Iduna Fichtner, Jens Hoffmann. Correlation of tumor growth inhibition by check point inhibitors with PD-L1 expression in preclinical patient derived xenograft (PDX) models [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr B126.