Phylogenetically conserved peritoneal fibrosis response to an immunologic adjuvant in ray-finned fishes

Maintenance of body homeostasis and protection from infection are fundamental to survival. While recognition of self and non-self appeared early in the evolution of metazoans, immunity remains one of the fastest evolving traits to keep up with dynamic challenges from parasites. The immune system thus intertwines ancient innate immune pathways with recently evolved adaptive pattern-recognition units. Here, we focus on peritoneal fibrosis, an effective, yet costly, defense to eliminate infection by a specialist tapeworm parasite, Schistocephalus solidus (Cestoda), observed in only some populations of threespine stickleback (Gasterosteus aculeatus, Perciformes). We asked whether stickleback fibrosis is a derived species-specific trait or an ancestral immune response that was widely distributed across ray-finned fish (Actinopterygii). First, we reviewed literature published on fibrosis in fish in general and found that peritoneal fibrosis specifically is very rarely reported in ray-finned fish. Then, we experimentally tested for peritoneal fibrosis with parasite-specific and non-specific immune challenges in deliberately selected species across fish tree of life. Peritoneal injection with a common non-specific vaccination adjuvant (Alum) showed that most of the tested species were capable to develop fibrosis. On the other hand, the species were largely indifferent to the tapeworm antigen homogenate. One specific fish clade – Characidae - did not respond to any of the treatments. We therefore show that despite being rarely reported in the literature, peritoneal fibrosis is a common and deeply conserved fish response to a non-specific immune challenge. We outline directions for further research on mechanisms and evolution of peritoneal fibrosis in fish, and also discuss new perspective on peritoneal fibrotic pathology in human patients. IMPACT STATEMENT Immunity is a crucial and rapidly evolving system due to the coevolutionary arms race between pathogens and their hosts. Yet, many key features of the vertebrate immune system are ancient. This apparent contradiction raises a key question: are immune functions widely similar among animals, or rapidly evolving to each populations’ needs? To address this question, we used experimental immune challenges to evaluate an immune response (peritoneal fibrosis) in phylogenetically diverse set of fish species. Peritoneal fibrosis can be a major form of pathology in humans as well. In some populations of threespine stickleback, peritoneal fibrosis is induced by cestode infection (or injection of cestode proteins or alum adjuvant), and serves to limit cestode growth. First, we performed a comprehensive literature search and show that peritoneal fibrosis has not been widely documented previously in other fish species. We then experimentally tested the ability of 17 species, drawn from across the fish tree of life, to initiate the peritoneal fibrosis response to artificial immune challenges. Our results show that the peritoneal fibrosis response towards a general immune challenge (Alum adjuvant injection) is an ancestral trait in fish, but has been lost entirely in some clades. In contrast, only few species initiated fibrosis when exposed to protein from a tapeworm specialized to infect stickleback. Our comparative experiment thus brings new insights into the evolution of peritoneal fibrosis in fish, showing that it is a common response to a non-specific cue, which can be employed in responding to species-specific parasites.