Six novel subtypes of haemocyte defined in the Pacific oyster Crassostrea gigas

Abstract

The primitive innate immune cells (termed haemocytes) initially evolved in molluscs, which are analogous to vertebrate leukocytes, displaying significant morphological and functional heterogeneity. Elucidating the diversity morphology and functions of haemocytes is crucial to understanding the ancient immune system. In the present study, six novel haemocyte subtypes, including pro-haemocytes, larger agranulocytes, secretory haemocytes, amoeba phagocytes, macrophages and dendritic (DC)-like haemocytes were identified by their specific morphological and functional characteristics in oyster circulating haemolymph. Pro-haemocytes, characterized by their minuscule size and high proliferation, were identified as haemocyte precursors to be essential for ensuring the appropriate number and ratio of haemocytes. Amoeba phagocytes were designated for their stronger migration activity, continuous morphological changes, and capability to engulf cell debris or apoptotic cells. The phagocytic index of amoeba phagocytes, macrophages and granulocytes was assessed to evaluate their phagocytic capabilities, which were seven, two and ten, respectively, indicating that granulocytes exhibited the strongest phagocytic ability. DC-like haemocytes were identified based on their specific dendritic pseudopodia and potential role in antigen processing. The results enhanced our cognizing of the complexity of primitive immune cells in molluscs and offered insights into the evolution of the innate immune system.