Evolution of innate immunity: specificity, regulation and priming of the immune response in the lophotrochozoan, Biomphalaria glabrata. – BIOMGENIM

Innate immunity is the first line of defence against pathogens in vertebrates and in invertebrates. Understanding the functional basis and the characteristics of immune processes is a major challenge in agronomical, ecological and evolutionary biology as well as in Public health. The classical view of invertebrate immune systems as non-specific, poorly regulated and lacking a 'memory', is challenged by recent discoveries revealing the existence of (i) diversification processes producing hundreds or thousands of variants of recognition proteins or immune receptors, (ii) signalling pathways specifically activated in response to particular immune challenges, and (iii) phenomenon of immune priming leading to the failure of secondary infections by the same pathogen. The limits between innate and adaptive immunity have now become blurred and the fields of comparative and evolutionary immunology are subject to controversy and questions. Our current knowledge of invertebrate immunity is mainly based on studies of the antimicrobial response of ecdysozoa (insects and crustaceans, or Caenorabditis). In this project we propose for the first time a coordinated investigation of both anti-parasitic and antimicrobial immunity in an organism belonging to the Lophotrochozoa. We anticipate that this study will reveal unexpected and original immune processes and constitute an undeniable advance in our comprehension of the evolution of the innate immune systems. The general scope of the project is to characterize the level of specificity, regulation and priming of the immune response and to analyze the underlying molecular processes in the lophotrochozoan species Biomphalaria glabrata (gastropod snail). As a first intermediate host of the human blood fluke Schistosoma mansoni, B. glabrata has been particularly studied for its anti-parasitic immune response. Our previous studies (i) yielded most of the immune-relevant candidate genes reported so far, including a potential orthologue of the vertebrate proinflammatory cytokine MIF (macrophage migration inhibitory factor), and (ii) provided evidence that the immune response of B. glabrata may be specific and 'primed ' in a sustainable manner. The specific aims of the present project are: 1- To determine the degree of specificity of the B. glabrata immune defence and to analyse the molecular mechanisms underlying the different responses. Preliminary analysis of a restricted number of candidate genes showed a differential expression after a challenge by a Gram negative bacteria and a trematode parasite, suggesting that at least two types of immune responses exist in this mollusc. Our goal is now to identify transcripts that are specifically regulated after challenge with Gram positive and Gram negative bacteria, fungi and metazoan parasites (S. mansoni), using microarray-based transcriptomic analysis. 2- To investigate immune regulation through the functional analysis of BgMIF, a potential cytokine and key regulator of the immune response. Analyses of the biological activities of a recombinant B. glabrata MIF protein (BgMIF) have been initiated and showed that it presents the three activities that are characteristic of all vertebrate MIF proteins (a tautomerase activity, a positive effect on cell proliferation and inhibition of p53-dependant apoptosis). Functional characterization of BgMIF will be achieved using expression studies, complementary analyses of biological activities, and RNA interference. 3- To assess the sustainability and specificity of immune priming and to determine the molecular processes responsible for the failure of secondary infections. Our previous studies showed that snails primarily infected with S. mansoni could not be re-infected by the same parasite. If specific and sustainable, such a phenomenon of immune-priming may have epidemiological and evolutionary implications that are similar to those of the immune memory of vertebrates. Sustainability and specificity will be analysed through series of experimental primary infections/injections and secondary infections. Underlying molecular processes will be investigated using a microarray approach aimed at identifying genes whose expression is regulated during a primary or successful infection and a secondary or unsuccessful infection. Since specificity, regulation and sustained priming are the three major features of immune systems directly affecting host-pathogen interactions, the outcome of our project is expected to generate a radical change in the perspectives for host-pathogen ecological and evolutionary studies, and to have implications for control programs centred on schistosomiasis transmission.