A Novel Scale Biopsy Method to Collect Zebrafish Malignant and Non-Malignant Lymphocytes

Zebrafish (Danio rerio) are a powerful model for studying lymphopoiesis and lymphoid cancers, since zebrafish and humans share similar adaptive immune systems, including B and T cells. Several D. rerio T-cell acute lymphoblastic leukemia (T-ALL) models are described, including fish that express lymphoblast-specific human MYC (hMYC). We reported that rag2:hMYC fish also develop B-cell acute lymphoblastic leukemia (B-ALL), making them useful to study both ALL types. A limitation of zebrafish ALL models is that no non-lethal methods exist for serial sampling of ALL. To address this, we pioneered a novel 'scale biopsy' method to collect ALL cells from live zebrafish. However, knowledge about which lymphocytes reside in normal zebrafish scales is scarce. Thus, to define normal epidermal lymphocyte identities, we performed scale biopsies on lymphocyte-labeled transgenic lines lacking rag2:hMYC. We used conventional and confocal microscopy to analyze scales, revealing abundant epidermal lymphocytes. We also analyzed scales from fish with T- and B-ALL, which demonstrated markedly increased lymphocytes in confluent sheets of ALL cells in many scales. We next used fluorescence-activated cell sorting (FACS) to purify non-malignant and ALL lymphocytes from scales for ex vivo studies. Flow cytometric and expression analyses of ALL cells from scales or other tissues of the same fish showed that scale ALL cell gene expression resembled that of ALL cells elsewhere. We also quantified the number of normal T, B, T-ALL, and B-ALL cells that could be purified per scale. Overall, scale biopsy provides a non-lethal, minimally invasive technique to study zebrafish epidermal lymphocytes, enabling ex vivo studies, including analyses of malignant samples.