Crayfish hemocytes develop along the granular cell lineage.

Despite the central role of hemocytes in crustacean immunity, the process of hemocyte differentiation and maturation remains unclear. In some decapods, it has been proposed that the two main types of hemocytes, granular cells (GCs) and semigranular cells (SGCs), differentiate along separate lineages. However, our current findings challenge this model. By tracking newly produced hemocytes and transplanted cells, we demonstrate that almost all the circulating hemocytes of crayfish belong to the GC lineage. SGCs and GCs may represent hemocytes of different developmental stages rather than two types of fully differentiated cells. Hemocyte precursors produced by progenitor cells differentiate in the hematopoietic tissue (HPT) for 3 ~ 4 days. Immature hemocytes are released from HPT in the form of SGCs and take 1 ~ 3 months to mature in the circulation. GCs represent the terminal stage of development. They can survive for as long as 2 months. The changes in the expression pattern of marker genes during GC differentiation support our conclusions. Further analysis of hemocyte phagocytosis indicates the existence of functionally different subpopulations. These findings may reshape our understanding of crustacean hematopoiesis and may lead to reconsideration of the roles and relationship of circulating hemocytes.

Rapid regulation of hemocyte homeostasis in crayfish and its manipulation by viral infection.

In crustaceans, the number of circulating hemocytes changes rapidly in response to pathogen infection and injury, but the underlying mechanism remains unclear. In this study, we investigated the regulation of hemocytes homeostasis in crayfish after hemolymph withdrawal. We showed that the circulating hemocytes increased by over 2 folds within 1 h post hemolymph withdrawal and returned to normal level within 8 h. New hemocytes produced by hematopoiesis accounted for <6.5% of the total replenishment, implying a major role of sessile hemocytes in rapid hemocyte supply. Moreover, when hemocytes were transplanted, the extra cells were efficiently stored, mainly in the gill. These stored cells could be released into circulation immediately on demand. Notably, the rapid regulation of hemocyte homeostasis was abolished by white spot syndrome virus infection. These data indicate that the adjustment between the sessile and circulating pools of hemocytes may be the major route for the rapid regulation of circulating hemocytes in crayfish, and this process may be altered by pathogen infection.