T Cell Development

In mouse, T cell precursors are derived from the AGM region at E8.5. The corresponding anatomic region in zebrafish may be the dorsal aorta circa 36 hours post fertilization (hpf). By ultramicroscopy T cell precursors enter the thymic anlage at about 68 hpf, and gene expression studies detect T cell specific genes such as lck and Rag-2 first at 72hpf in the thymus (Fig. 1), making it the first zebrafish organ to become lymphoid. By analogy with mammals, zebrafish T cells express genes such as Ikaros, GATA-3, Rag-1, lck and TCR alpha during early development. Figure 2 shows whole mount in situ hybridization (WISH) analysis of day (d) 3.5 wild-type zebrafish larvae with the indicated probes (top row). A grey arrowhead indicates expression of Ikaros and GATA-3 in brain, black arrows indicate thymic expression with all probes, and a grey vertical arrow indicates Rag-1 expression in the olfactory pit. Expression of the indicated probes is shown in the middle row in d7 wild-type larvae. The bottom row shows expression of TCR alpha and Pax-5 in d8 and d4 wild-type larvae, respectively. The grey arrowhead indicates expression of Pax-5 at the midbrain/hindbrain boundary. Given the transparency and positioning of the larvae, contralateral thymic staining at d3.5 with the Rag-1 and at d8 with the TCR alpha probes is seen.

In adult zebrafish, T cell progenitors are derived from the bone marrow-equivalent hematopoietic kidney, and mature T cells are found in spleen, kidney, thymus, gut, and in circulation. Some of the mutants generated in our previous screen lead to defective T cell development, and positional cloning of these mutants is underway.