The heterogeneous morphologic features of canine plasmacytomas (PCTs) can make their differentiation from other round cell tumors challenging. Immunohistochemistry (IHC) for lambda (λ) and kappa (к) immunoglobulin (Ig) light chains is often equivocal because of high background staining. The chromogenic in situ hybridization (CISH) technique for light chains has shown higher sensitivity compared to IHC in human plasma cell tumors. Therefore, we aimed to validate automated CISH for light chains in canine tissues and to evaluate its diagnostic potential in canine PCTs, in conjunction with routinely used IHC markers. CISH for light chains demonstrated a clear signal in plasma cell populations of canine control tissues (lymph nodes, lymphoplasmacytic inflammation) showing a polyclonal pattern with a prevalence of λ-producing cells. CISH detected monotypic light chain expression in 33 of 53 (62%) PCTs, 31 expressing λ and 2 expressing к. CISH was more sensitive than IHC for λ light chain (58% vs. 47%, respectively) and more easily interpretable given the absence of confounding background staining. The absence of CISH staining for both λ and к in a considerable subset of tumors may be the result of lower light chain production by neoplastic cells. Multiple myeloma oncogene 1 (MUM1) was expressed by all but 2 PCTs (96%), which showed λ expression by CISH and IHC. The identification of poorly differentiated canine PCTs requires the assessment of a panel of IHC markers, with the potential support of CISH for Ig light chains.

犬浆细胞瘤（PCT）的异质形态特征使其与其他圆形细胞肿瘤的区分具有挑战性。由于高背景染色，lambda (λ) 和 kappa (к) 免疫球蛋白 (Ig) 轻链的免疫组织化学 (IHC) 通常是模棱两可的。与 IHC 相比，轻链显色原位杂交 (CISH) 技术在人浆细胞肿瘤中表现出更高的灵敏度。因此，我们的目的是验证犬组织中轻链的自动化 CISH，并结合常规使用的 IHC 标记物评估其在犬 PCT 中的诊断潜力。轻链的 CISH 在犬对照组织（淋巴结、淋巴浆细胞炎症）的浆细胞群中显示出清晰的信号，显示出多克隆模式，其中普遍存在 λ 产生细胞。 CISH 在 53 个 PCT 中的 33 个 (62%) 中检测到单型轻链表达，其中 31 个表达 λ，2 个表达 к。对于 λ 轻链，CISH 比 IHC 更敏感（分别为 58% 和 47%），并且由于不存在混杂的背景染色，因此更容易解释。在相当大的肿瘤亚群中，λ 和 к 均缺乏 CISH 染色，这可能是肿瘤细胞产生的轻链较低的结果。除 2 个 PCT 外（96%），所有 PCT 均表达多发性骨髓瘤癌基因 1 (MUM1)，CISH 和 IHC 显示 λ 表达。鉴定低分化犬 PCT 需要评估一组 IHC 标记物，并有 Ig 轻链 CISH 的潜在支持。