An antibody against T cell receptor beta constant 1 (TRBC1) has recently become available for clinical flow cytometry and has qualitatively improved the classification of T cell populations (Berg et al., 2020 (in press); Shi et al., 2020). In the present manuscript, we present a patient for whom assessment of TRBC1 expression was instrumental in classifying a mediastinal mass.

The patient is a 71-year-old male with a history of hypertension and recent COVID-19 pneumonia, which required critical care including mechanical intubation. During this episode, an anterior mediastinal mass was seen on computed tomography. The patient was sent for bronchoscopy-guided fine-needle aspiration through the institutional rapid lung cancer screening program. The sample was submitted for cytometric and histopathological analysis, preserved in normal saline and was immediately processed (Table 1 outlines the methods and reagents). Figure 1 shows the results of the flow cytometric analysis. All cells analyzed were of T cell lineage, and several populations were readily apparent. The largest population consisted of CD3^dim, CD5^dim, and CD2/CD4/CD8-positive cells. As can be seen, these cells were evenly split between TRBC1^dim and TRBC1-negative. Two other populations include a normal CD4-positive T cell population and a normal CD8-positive T cell population. Finally, a smaller double-negative T cell population and a γδ T cell population (without TRBC1 expression) were seen. This was deemed consistent with normal thymic maturation (Figure 1). Similarly, T cell receptor (TCR) gene rearrangement analysis was polyclonal. Subsequently, histopathological analysis was notable for a proliferation of epithelial cells in a background of thymic TdT-positive lymphocytes, establishing the diagnosis of thymoma. The patient is currently awaiting surgical excision of the tumor.

The thymus is the primary site of T cell maturation. As such, distinct T cell populations (and different from those present in blood and secondary lymphoid organs, reflecting different maturation stages) are expected to be present. However, these populations may not be easily distinguished from abnormal, malignant ones. Indeed, in older patients, lymphoma is more common than thymoma and would have been one of the most likely diagnoses in a mediastinal mass.

An anti-TRBC1 antibody has recently become available. Reported experience suggests reliable distinction between clonal and polyclonal T cell populations (Berg et al., 2020 (in press); Shi et al., 2020). In the case presented here, a T cell malignancy could be easily and rapidly ruled out with inclusion of anti-TRBC1 within a larger T cell panel. It confirmed the absence of clonality within the different population subsets by showing the expected expression pattern in each maturation stage. TCR gene rearrangement and histopathological analysis confirmed the results.

Of interest, we note the consistency between intensity of CD3 and that of TRBC1, which has been suggested in tissue lymphomas but has not been consistent in leukemic T cell disorders (Berg et al., 2020 (in press); Shi et al., 2020).

To conclude, this case report builds on previous data supporting the inclusion of anti-TRBC1 in clinical flow cytometry.

一种针对 T 细胞受体 β 常数 1 (TRBC1) 的抗体最近可用于临床流式细胞术，并在质量上改进了 T 细胞群的分类（Berg 等人，  2020 年（出版中）；Shi 等人，  2020 年）。在本手稿中，我们介绍了一名患者，其对 TRBC1 表达的评估有助于对纵隔肿块进行分类。

患者是一名 71 岁男性，有高血压病史和近期 COVID-19 肺炎病史，需要包括机械插管在内的重症监护。在这一事件中，计算机断层扫描显示前纵隔肿块。通过机构快速肺癌筛查计划，患者被送去进行支气管镜引导下的细针穿刺。将样品提交细胞计数和组织病理学分析，保存在生理盐水中并立即进行处理（表 1 概述了方法和试剂）。图 1 显示了流式细胞仪分析的结果。分析的所有细胞都是 T 细胞谱系，并且有几个细胞群很明显。最大的种群由 CD3 ^dim , CD5 ^dim, 和 CD2/CD4/CD8 阳性细胞。可以看出，这些细胞在 TRBC1^暗淡和 TRBC1 阴性之间均匀分布。其他两个群体包括正常 CD4 阳性 T 细胞群体和正常 CD8 阳性 T 细胞群体。最后，观察到较小的双阴性 T 细胞群和 γδ T 细胞群（没有 TRBC1 表达）。这被认为与正常的胸腺成熟一致（图 1）。同样，T 细胞受体 (TCR) 基因重排分析是多克隆的。随后，组织病理学分析显示在胸腺 TdT 阳性淋巴细胞背景中上皮细胞的增殖，从而确定了胸腺瘤的诊断。患者目前正在等待手术切除肿瘤。

胸腺是 T 细胞成熟的主要部位。因此，预计会出现不同的 T 细胞群（并且不同于血液和次级淋巴器官中存在的那些，反映不同的成熟阶段）。然而，这些人群可能不容易与异常的恶性人群区分开来。事实上，在老年患者中，淋巴瘤比胸腺瘤更常见，并且可能是纵隔肿块最可能的诊断之一。

最近出现了一种抗 TRBC1 抗体。报告的经验表明克隆和多克隆 T 细胞群之间存在可靠的区别（Berg 等人，  2020 年（出版中）；Shi 等人，  2020 年）。在这里介绍的情况下，通过在更大的 T 细胞组中包含抗 TRBC1，可以轻松快速地排除 T 细胞恶性肿瘤。它通过显示每个成熟阶段的预期表达模式来证实不同群体子集中不存在克隆性。TCR基因重排和组织病理学分析证实了结果。

有趣的是，我们注意到 CD3 的强度和 TRBC1 的强度之间的一致性，这在组织淋巴瘤中已被提出，但在白血病 T 细胞疾病中并不一致（Berg 等人，  2020 年（出版中）；Shi 等人，  2020 年）。

总而言之，本病例报告建立在支持将抗 TRBC1 纳入临床流式细胞术的先前数据的基础上。