P1.12-05 Microenvironment Characterization of Small Cell Lung Cancer Xenografts Implanted in Hematopoietic Humanized Mice

Abstract

With the high mutational burden seen in small cell lung cancer (SCLC) there is the theoretical potential to improve SCLC outcome through immunotherapy. Recent immunotherapeutic clinical trials in SCLC have demonstrated promising results. However, to better understand the immune response and the potential role of immunotherapy in SCLC, immunocompetent models are needed. To this end, we have applied a hematopoietic humanized mouse model, Hu-CB-BRGS to investigate the mechanisms underlying SCLC immunotherapy and develop novel strategies to improve therapeutic efficacy. BALB/c-Rag2nulllll2rynullSIRPαNOD (BRGS) pups were humanized through transplantation of cord blood (CB)-derived CD34+ cells. SCLC flank tumors were initiated in BRGS mice using two characterized SCLC cell lines (CDX) and 1 patient derived xenograft (PDX). Upon verification of human T-cell chimerism in the Hu-CB-BRGS mice, SCLC tumors grown in BRGS mice were engrafted into the flanks of Hu-CB-BRGS mice by trocar transfer. Tumor growth was quantified by twice weekly measurement and harvested on reaching 1200 mm3. At harvest, tumor tissue as well as host immune organs (lymph node, spleen) were collected for immunological assessment. Humanized immune system and tumor were evaluated by flow cytometry and immunohistochemistry (IHC). Flank tumors from two CDX tumors (H82 and H187) and one PDX tumor (LX-95) were successfully developed in the Hu-CB-BRGS mice with take rates averaging 82%. SCLC tumor growth rate in Hu-CB-BRGS mice was comparable to that seen in BRGS mice. Although human T cells were well represented in the lymph nodes and spleens of the Hu-CB-BRGS mice, we detected very few tumor infiltrating immune cells in the engrafted SCLC tumors by IHC and flow cytometry as defined by CD45 and CD3, which is consistent with the observations in SCLC patient tumor tissue. Tumor cells, identified by EpCAM expression, expressed low levels of MHC class I, II and PD-L1. PD-1 was expressed by human T cells found in the lymph nodes and spleen of Hu-CB-BRGS mice, while the SCLC xenografts expressed varying levels. We demonstrate that both SCLC PDX and CDX tumors can be grown in the context of a humanized immune system within mouse recipients. SCLC Hu-CB-BRGS mice demonstrate persistence of human immune cells, including T cells and B cells in the immune organs. The xenograft tumor microenvironment included variable human immune infiltrates. The SCLC Hu-CB-BRGS mouse model may be a valuable preclinical platform for testing human specific immune-oncology therapeutics for SCLC patients.