Abstract
The Pacific oyster, Crassostrea (Magallana) gigas, is a bivalve mollusk of significant ecological and economic importance, and it has recently emerged as a model species for studying the innate immunity of bivalves. In recent years, oyster farming has been confronted with episodes of massive mortality, exacerbated by climate change and human activities. These mortalities, though multifactorial, share a common factor: the ability of pathogens (viruses and bacteria) to evade the oysters' immune defenses, often leading to fatal septicemia. While immune cells in vertebrates, particularly humans, are well characterized, the diversity and functional specialization of hemocytes in C. gigas remain a black box and are hotly debated within the scientific community. This knowledge gap hampers our understanding of host-pathogen interactions, thus limiting the development of strategies to reduce oyster mortality in aquaculture. In this context, the main objective of my thesis project was to characterize the circulating hemocyte types in C. gigas using cytological, functional, and single-cell transcriptomic approaches (scRNA-seq). These methods first allowed us to identify seven distinct constitutive hemocyte types in naïve animals. These hemocyte populations were characterized based on their morphological properties, gene expression profiles, and specific biological functions. Furthermore, we established a hemocyte ontology, suggesting potential differentiation pathways for the cell lineages. Using this hemocyte atlas, we then assessed the differential impact of Vibrio aestuarianus infection on hemocyte populations, both from a cytological and transcriptomic perspective, revealing alterations dependent on the circulating bacterial load. This work provides a significant contribution to the understanding of immunity in C. gigas, by offering a precise definition of hemocyte types. Our results propose a reference hemocyte atlas and emphasize the importance of studying hemocyte homeostasis in mollusks to better understand and anticipate oyster mortality crises during epizootic episodes.