Abstract
BackgroundThe success of agents that reverse T-cell inhibitory signals, such as anti-PD-1/PD-L1 therapies, has reinvigorated cancer immunotherapy research. However, since only a minority of patients respond to single-agent therapies, methods to test the potential anti-tumor activity of rational combination therapies are still needed. Conventional murine xenograft models have been hampered by their immune-compromised status; thus, we developed a hematopoietic humanized mouse model, hu-CB-BRGS, and used it to study anti-tumor human immune responses to triple-negative breast cancer (TNBC) cell line and patient-derived colorectal cancer (CRC) xenografts (PDX).MethodsBALB/c-Rag2nullIl2rγnullSIRPαNOD (BRGS) pups were humanized through transplantation of cord blood (CB)-derived CD34+ cells. Mice were evaluated for human chimerism in the blood and assigned into experimental untreated or nivolumab groups based on chimerism. TNBC cell lines or tumor tissue from established CRC PDX models were implanted into both flanks of humanized mice and treatments ensued once tumors reached a volume of ~150mm3. Tumors were measured twice weekly. At end of study, immune organs and tumors were collected for immunological assessment.ResultsHumanized PDX models were successfully established with a high frequency of tumor engraftment. Humanized mice treated with anti-PD-1 exhibited increased anti-tumor human T-cell responses coupled with decreased Treg and myeloid populations that correlated with tumor growth inhibition. Combination therapies with anti-PD-1 treatment in TNBC-bearing mice reduced tumor growth in multi-drug cohorts. Finally, as observed in human colorectal patients, anti-PD-1 therapy had a strong response to a microsatellite-high CRC PDX that correlated with a higher number of human CD8+ IFNγ+ T cells in the tumor.ConclusionHu-CB-BRGS mice represent an in vivo model to study immune checkpoint blockade to human tumors. The human immune system in the mice is inherently suppressed, similar to a tumor microenvironment, and thus allows growth of human tumors. However, the suppression can be released by anti-PD-1 therapies and inhibit tumor growth of some tumors. The model offers ample access to lymph and tumor cells for in-depth immunological analysis. The tumor growth inhibition correlates with increased CD8 IFNγ+ tumor infiltrating T cells. These hu-CB-BRGS mice provide a relevant preclinical animal model to facilitate prioritization of hypothesis-driven combination immunotherapies.
Highlights
The success of agents that reverse T-cell inhibitory signals, such as anti-Programmed Cell Death Protein-1 (PD-1)/Programmed Cell Death Protein-1 ligand (PD-L1) therapies, has reinvigorated cancer immunotherapy research
Using hu-cord blood (CB)-BRGS mice, we demonstrated that a triple negative breast cancer (TNBC) cell line is more effectively treated with anti-PD-1 immunotherapy and an epigenetic modifier than either treatment alone
Using patient-derived colorectal cancer xenografts (CRC patient-derived colorectal cancer (CRC) xenografts (PDX)), we demonstrated that the immune system responds differentially to microsatellite instable-high (MSI-H) and microsatellite stable (MSS) tumors, consistent with data reported from clinical trials [9]
Summary
The success of agents that reverse T-cell inhibitory signals, such as anti-PD-1/PD-L1 therapies, has reinvigorated cancer immunotherapy research. Ex vivo organotypic microfluidic, spheroid culture models have been utilized to screen and identify small molecules that can be used in combination strategies to enhance efficacy of exisiting immunotherapies [11,12,13,14] These approaches are hampered by a lack of dynamic interactions between the tumor, tumor microenvironment (TME), and immune system and an inability to investigate in vivo conditions that may influence the tumor-immune system interaction [11, 15]. Hematopoietic humanized mice are generated by intravenous injection of human CD34+ stem cells derived from umbilical cord blood (CB) into immunodeficient mice lacking T, B and NK cells [19,20,21,22] These mice have been shown to accept and grow tumor cells from patient derived xenografts (PDXs) [23]
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