Immunodeficiency

The study of mice and patients with immune deficiencies has greatly enhanced our understanding of the molecular processes involved in developmental aspects of the immune system. However, mammalian immunology is handicapped by the fact that the earliest steps in the ontogeny of the immune system occur in-utero, and are therefore difficult to study in an in vivo system. For example, mutants of GATA-3, a transcription factor of pivotal importance for T cell development, die in-utero due to an accompanying brain hemorrhage. Prominent among these early steps are thymus organogenesis and T-cell development, crucial interdependent processes that establish a functional vertebrate immune system. Current understanding of vertebrate thymus development during embryogenesis remains incomplete and would benefit from novel approaches. Others and we have initiated screens in zebrafish looking for defects in adaptive immunity. The lineage of choice is the T cell, because the organ in which they develop, the thymus, is a superficial organ that can be readily uncovered by in-situ hybridization or fluorescent microscopy. In addition, by looking for T cell specific gene expression in the zebrafish thymus, these screens pick up not only mutants of T cell, but also of thymus development. Figure 1 shows examples of defective expression of the T cell gene lck in different mutants: bloodless (bls) has a defect in the primitive wave of hematopoiesis, and therefore has no T cells on day 4, but initiates normal hematopoiesis and lymphopoiesis on day 5; cloche (clo) has a defect in vasculogenesis and hematopoiesis and therefore has neither red blood cells nor T cells; vanGogh (vgo) is a zebrafish model for DiGeorge syndrome, and has no T cells based on a thymus defect. Figure 2 shows a pure lymphoid mutant based on an inactivated Rag-1 gene. This was the first zebrafish mutant generated by the reverse genetic approach TILLING. As in mammals, the Rag mutation arrests T cells development and proliferation, which results in a reduced Rag-1 signal in the thymus.