*University of Maryland, School of Medicine, Dept. of Radiation Oncology, Baltimore, MD; †Integrated Research Facility, Frederick, MD; ‡Naval Medical Research Center, Silver Spring, MD.
Health Phys. 2014 Jan;106(1):84-96. doi: 10.1097/HP.0b013e3182a2a9b2.
Delayed immune reconstitution remains a major cause of morbidity associated with myelosuppression induced by cytotoxic therapy or myeloablative conditioning for stem cell transplant, as well as potentially lethal doses of total- or partial-body irradiation. Restoration of a functional immune cell repertoire requires hematopoietic stem cell reconstitution for all immune cells and effective thymopoiesis for T cell recovery. There are no medical countermeasures available to mitigate damage consequent to high-dose, potentially lethal irradiation, and there are no well characterized large animal models of prolonged immunosuppression to assess efficacy of potential countermeasures. Herein, the authors describe a model of T and B cell reconstitution following lethal doses of partial-body irradiation with 5% bone marrow sparing that includes full exposure of the thymus. Rhesus macaques (n = 31 male, 5.5-11.3 kg body weight) were exposed to midline tissue doses of 9.0-12.0 Gy using 6 MV LINAC-derived photons at a dose rate of 0.80 Gy min, sparing approximately 5% of bone marrow (tibiae, ankles, and feet). All animals received medical management and were monitored for myeloid and lymphoid suppression and recovery through 180 d post-exposure. Myeloid recovery was assessed by neutrophil and platelet-related hematological parameters. Reconstitution of B and T cell subsets was assessed by flow cytometric immunophenotyping, and recent thymic emigrants were identified by RT-PCR of T cell receptor excision circles. Mortality was recorded through 180 d post-exposure. Acute myelo-suppression was characterized by severe neutropenia and thrombocytopenia, followed by recovery 30-60 d post-exposure. Total T (CD3+) and B (CD20+) cells were reduced significantly following exposure and exhibited differential recovery patterns post-exposure. Both CD4+ and CD8+ subsets of naïve T cells and total CD4+ T cell counts remained significantly lower than baseline through 180 d post-exposure. The failure of recent thymic emigrants and naïve T cell subsets to recover to normal baseline values reflects the severe radiation effects on the recovery of marrow-derived stem and early thymic progenitor cells, their mobilization and seeding of receptive thymic niches, and slow endogenous thymic regeneration.
延迟的免疫重建仍然是与细胞毒性治疗或造血干细胞移植的骨髓抑制相关的发病率的主要原因，以及全身或半身照射的潜在致命剂量。所有免疫细胞的造血干细胞重建和 T 细胞恢复的有效胸腺发生都需要功能性免疫细胞库的恢复。没有医疗对策可以减轻高剂量、潜在致命照射造成的损害，也没有经过充分特征描述的长时间免疫抑制的大型动物模型来评估潜在对策的效果。在此，作者描述了一种在保留 5%骨髓的情况下，通过半身照射致死剂量后 T 和 B 细胞重建的模型，该模型包括胸腺的完全暴露。恒河猴（n=31 只雄性，5.5-11.3 公斤体重）使用 6 MV LINAC 衍生的光子在 0.80 Gy min 的剂量率下，接受中线组织剂量为 9.0-12.0 Gy 的照射，大约保留 5%的骨髓（胫骨、脚踝和脚）。所有动物均接受医疗管理，并通过暴露后 180 天监测骨髓和淋巴抑制及恢复情况。骨髓恢复通过中性粒细胞和血小板相关的血液学参数评估。B 和 T 细胞亚群的重建通过流式细胞仪免疫表型分析评估，通过 T 细胞受体切除环的 RT-PCR 鉴定近期胸腺移居者。通过暴露后 180 天记录死亡率。急性骨髓抑制表现为严重的中性粒细胞减少和血小板减少，随后在暴露后 30-60 天恢复。暴露后，总 T（CD3+）和 B（CD20+）细胞明显减少，并表现出不同的恢复模式。暴露后 180 天，幼稚 T 细胞的 CD4+和 CD8+亚群以及总 CD4+T 细胞计数仍明显低于基线。近期胸腺移居者和幼稚 T 细胞亚群未能恢复到正常基线值，反映了骨髓来源的干细胞和早期胸腺祖细胞的严重辐射效应、它们的动员和受纳胸腺龛的播种以及内源性胸腺再生的缓慢。
