Macrophages et Développement de l’Immunité - CNRS URA 2578

We focus on the successive emergence and functional traits of the first innate immune cell types in vertebrate development, using the assets of a small tropical freshwater fish, the zebrafish. This led us to investigate also the transition from primitive to definitive hematopoiesis, including the origin and migrations of hematopoietic stem cells, using in vivo cell tracing tools unavailable in mammals (Murayama et al., 2006). Finally we use the zebrafish model to study innate responses to infections in the live animal at high resolution.

Macrophages, granulocytes, and developmental hematopoiesis (K. Kissa, D. Le Guyader, E. Murayama, M. Tauzin)
In all vertebrate embryos, the first leukocytes to arise are the ’primitive macrophages’, which we showed to originate from a distinct anterior mesoderm territory adjacent to the cardiac field. These cells differentiate in the yolk sac, then quickly invade the still unvascularized embryonic tissues, notably the brain and retina where they become primitive microglia. Recently we explored the basis for their naturally invasive behaviour and found that their colonization of the brain and retina required their expression of a specific matrix metalloprotease, presumably to help them penetrate the basal lamina of these organs.


Neutrophilic granulocytes appear a day later in development, throughout the embryo, surprisingly barely in the blood, but mostly wandering in the mesenchyme and epidermis. Using an in vivo photoactivatable cell tracer, we found that these cells arise from two origins: i) the progenitors of the primitive macrophages, which we thus discovered to be bipotent; ii) a previously undescribed caudal hematopoietic tissue (CHT), which we found to be a transitional site for the homing, expansion and differentiation of definitive hematopoietic stem cells, before their migration to the final hematopoietic organs (the thymus and kidney in fish) (Murayama et al., 2006). Within the european Integrated Project "Zebrafish Models for Human Development and Disease", we performed two large-scale screens for mutants affected in macrophage or granulocyte differentiation, or their capacity to colonize tissues. We found many, and are now analyzing further the phenotypes of the most promising ones.

Innate host responses to infections (J.-P. Levraud, E. Colucci-Guyon)
Mycobacterium marinum, the agent of fish tuberculosis, is very close to its human counterpart M. tuberculosis. The complex glycolipids of the cell wall of mycobacteria are thought to be a major factor of their pathogenicity. We inject these components purified from M. marinum in zebrafish larvae, either as such or coupled to fluorescent beads, and assess their effects on phagocytosis, cell recruitment, granuloma formation, or induction of host genes (collaboration with L. Kremer, CNRS Montpellier, and Y. Guérardel, CNRS Lille).
We also set up an experimental system of viral infection of zebrafish embryos, which makes it possible to exploit antisense (morpholino) technology to assess the anti-viral effects of any virus-induced host gene (collaboration with G. Lutfalla, CNRS Montpellier and P. Boudinot, INRA Jouy-en-Josas). This already allowed us to identify functionally the two chains of the fish interferon (IFN) receptor among all class II helical cytokine receptor genes, which led us to conclude that the ancestral innate IFN system is neither IFN-± nor -ß, but IFN lambda (Levraud et al., J. Immunol., in press).