Abstract
Immune and neuroendocrine functions display significant overlap in highly divergent and evolutionarily distant models such as molluscs, crustaceans, insects and mammals. Fundamental players in this crosstalk are professional phagocytes: macrophages in vertebrates and immunocytes in invertebrates. Although they have different developmental origins, macrophages and immunocytes possess comparable functions and differentiate under the control of evolutionarily conserved transcription factors. Macrophages and immunocytes share their pools of receptors, signalling molecules and pathways with neural cells and the neuro-endocrine system. In crustaceans, adult transdifferentiation of circulating haemocytes into neural cells has been documented recently. In light of developmental, molecular and functional evidence, we propose that the immune-neuroendocrine role of circulating phagocytes pre-dates the split of protostomian and deuterostomian superphyla and has been conserved during the evolution of the main groups of metazoans.
Highlights
The immune system of complex organisms is based on the equilibrated and continuous crosstalk between cellular and molecular components (Lemaitre and Hoffmann, 2007; Murphy, 2011)
Metchnikoff established, for the first time, that phagocytes represent the first line of defense against infections and that they can be detected in all organisms, from invertebrates to vertebrates
The term immunocyte has been coined for mammalian cells (Berman, 1963; Smith and Blalock, 1988; Carr and Blalock, 1988), but could be extended to invertebrates as well due to the functional similarities between phagocytes of metazoans (Ottaviani and Franceschi, 1997; Malagoli et al, 2006; Ottaviani et al, 2007, 2008; Ottaviani, 2011)
Summary
The immune system of complex organisms is based on the equilibrated and continuous crosstalk between cellular and molecular components (Lemaitre and Hoffmann, 2007; Murphy, 2011). The family of GATA transcription factors, their co-factors (U-shaped/FOG), the AML1 domain family transcription factors (Lozenge/Runx1) (Hoffmann et al, 1999; Fossett and Schulz, 2001), as well as Notch signaling (Duvic et al, 2002), are essential for immune cell determination and differentiation into specific cytotypes This brief overview on the hematopoiesis in key invertebrate models shows that the term immunocyte (or the broader term hemocyte) may include cells sharing only little similarity in terms of development and maturation. The homologies among invertebrate hemocytes and between invertebrate and vertebrate immunocytes are far from being established This notwithstanding, the interactions between immune and neural functions have been observed in several invertebrate models, including molluscs, insects and crustaceans
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