Lanfranca Mirna Perusina, Lazarus Jenny, Shao Xia, Nathan Hari, Di Magliano Marina Pasca, Zou Weiping, Piert Morand, Frankel Timothy L
Department of Surgery, University of Michigan, Ann Arbor, Michigan.
Department of Radiology, University of Michigan, Ann Arbor, Michigan.
J Surg Res. 2018 Dec;232:570-577. doi: 10.1016/j.jss.2018.07.015. Epub 2018 Aug 7.
The tumor microenvironment of pancreatic ductal adenocarcinoma (PDAC) contains abundant immunosuppressive tumor-associated macrophages. High level of infiltration is associated with poor outcome and is thought to represent a major roadblock to lymphocyte-based immunotherapy. Efforts to block macrophage infiltration have been met with some success, but noninvasive means to track tumor-associated macrophagess in PDAC are lacking. Translocator protein (TSPO) is a mitochondrial membrane receptor which is upregulated in activated macrophages. We sought to identify if a radiotracer-labeled cognate ligand could track macrophages in PDAC.
A murine PDAC cell line was established from a transgenic mouse with pancreas-specific mutations in KRAS and p53. After confirming lack of endogenous TSPO expression, tumors were established in syngeneic mice. A radiolabeled TSPO-specific ligand ([11C] peripheral benzodiazepine receptor [PBR]28) was delivered intravenously, and tumor uptake was assessed by autoradiography, ex vivo, or micro-positron emission tomography imaging.
Resected tumors contained abundant macrophages as determined by immunohistochemistry and flow cytometry. Immunoblotting revealed murine macrophages expressed TSPO with increasing concentration on activation and polarization. Autoradiography of resected tumors confirmed [11C]PBR28 uptake, and whole mount sections demonstrated the ability to localize tumors. To confirm the findings were macrophage specific, experiments were repeated in CD11b-deficient mice, and the radiotracer uptake was diminished. Micro-positron emission tomography imaging validated radiotracer uptake and tumor localization in a clinically applicable manner.
As new immunotherapeutics reshape the PDAC microenvironment, tools are needed to better measure and track immune cell subsets. We have demonstrated the potential to measure changes in macrophage infiltration in PDAC using [11C]PBR28.
胰腺导管腺癌（PDAC）的肿瘤微环境包含大量免疫抑制性肿瘤相关巨噬细胞。高水平的浸润与不良预后相关，被认为是基于淋巴细胞的免疫治疗的主要障碍。阻断巨噬细胞浸润的努力已取得一些成功，但缺乏在PDAC中追踪肿瘤相关巨噬细胞的非侵入性方法。转位蛋白（TSPO）是一种线粒体膜受体，在活化的巨噬细胞中上调。我们试图确定放射性示踪剂标记的同源配体是否能够追踪PDAC中的巨噬细胞。
从具有KRAS和p53胰腺特异性突变的转基因小鼠建立小鼠PDAC细胞系。在确认缺乏内源性TSPO表达后，在同基因小鼠中建立肿瘤。静脉注射放射性标记的TSPO特异性配体（[11C]外周苯二氮䓬受体[PBR]28），并通过放射自显影、离体或微正电子发射断层扫描成像评估肿瘤摄取情况。
通过免疫组织化学和流式细胞术确定，切除的肿瘤含有大量巨噬细胞。免疫印迹显示，小鼠巨噬细胞表达TSPO，其浓度随活化和极化而增加。切除肿瘤的放射自显影证实了[11C]PBR28的摄取，整装切片显示了定位肿瘤的能力。为了确认这些发现具有巨噬细胞特异性，在CD11b缺陷小鼠中重复实验，放射性示踪剂摄取减少。微正电子发射断层扫描成像以临床适用的方式验证了放射性示踪剂的摄取和肿瘤定位。
随着新的免疫疗法重塑PDAC微环境，需要工具来更好地测量和追踪免疫细胞亚群。我们已经证明了使用[11C]PBR28测量PDAC中巨噬细胞浸润变化的潜力。
