Beyond cellular immunity: On the biological significance of insect hemocytes

Authors: Stanley, David; HAAS, ERIC - Creighton University; KIM, YONGGYUN - Andong National University

Submitted to: Cells
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/10/2023
Publication Date: 2/12/2023
Citation: Stanley, D.W., Haas, E.J., Kim, Y. 2023. Beyond cellular immunity: On the biological significance of insect hemocytes. Cells. 12(4). Article 599. https://doi.org/10.3390/cells12040599.
DOI: https://doi.org/10.3390/cells12040599

Interpretive Summary: Insect cellular immune defenses are biologically costly because many cells are lost in the immune responses and must be replaced. We posed the hypothesis that such loss and replacement of immune cells would be a better investment if the immune cells contributed to aspects of insect biology that are unrelated to immunity. We think of this as immune cell actions beyond immunity. Here, we present information on immune cell actions beyond immunity. These include transporting iron, constructing body structures, surviving severe oxygen shortages, clearing out dead cells during nervous system development and developing specialized niches in which immune cells are created. We propose the broad biological roles of immune cells extends considerable beyond immunity.

Technical Abstract: Insect immunity is assorted into humoral and cellular immune reactions. Humoral reactions involve regulated production of anti-microbial peptides (AMPs) which directly kill microbial invaders at the membrane and intracellular levels. In cellular immune reactions, circulating and sessile hemocytes are mobilized to sites of infections, where they directly engage invading microbes. Hemocytes reduce numbers of microbes by phagocytosis and formation of melanized nodules. Nodulation reactions involve formation of microaggregates of a few hemocytes with attached microbial cells. The microaggregates grow into larger nodules that are finally melanized via active phenol oxidase and attached to internal body walls or organs. These reactions involve investments with millions of hemocytes, depending on the host sizes, that are later replaced by hematopoiesis at a high biological cost. Here, we considered the high biological costs of maintaining and replacing hemocytes would be a better investment if hemocytes carried out meaningful biological actions unrelated to cellular immunity. This idea allows us to treat a set of 10 hemocyte actions that are not directly involved in immunity, some of which, so far, are known only in Drosophila melanogaster. These include 1/ their actions in molting and development, 2/ in surviving severe hypoxia, 3/ producing PPO and its actions beyond immunity, 4/ producing vitellogenin in a leafhopper, 5/ recognition and responses to cancer in Drosophila, 6/ non-immune actions in Drosophila, 7/ clearing apoptotic cells during development of the central nervous system, 8/ developing hematopoietic niches in Drosophila, 9/ synthesis and transport of a lipoprotein, and, 10/ hemocyte roles in iron transport. We propose the biological significance of hemocytes extends considerably beyond immunity.