In this study we perform the characterization of FALC cells of Cyprinus carpio and its relation with peritoneal haemangiosarcoma. Fat-associated lymphoid cluster in Cyprinus carpio

FALC cells are natural helper cells producing Th2-type cytokines, which express c-kit, Sca-1, IL7R and CD45 in mouse and human. These cells are involved in allergic response and contribute in the inflammatory reactions of the adipose tissue; however, a lack of information prevails about the presence of these cells in other species. The study aim was to find and characterize FALC cells in the common carp (Cyprinus carpio) using immunohistochemistry and molecular biology techniques as well as to explore their relationships with their microenvironment. Histological description of the FALC was performed using H&E and polyclonal antibodies were used against cell-surface markers such as c-kit, Sca-1 and CD45. Furthermore, gene expression of c-kit, Sca-1 and IL7R was assessed. C. carpio FALC cells express at pre and post transcriptional level the same surface markers reported in FALC of mouse. By exposure to the soluble fraction of helminthes FALC cells produce abundant Th2 cytokines (IL-5, IL-6 and IL-13) but do not synthesize IL-1α. Additionally FALC cells probably participate in vascular remodelling of the intestine vessels inducing tumours because a malignant haemangiosarcoma in the peritoneal cavity was found. In this tumour, abundant FALC with their characteristic cell-surface markers were detected. The findings of this study suggest the involvement of some proto-oncogenes such as c-kit and Sca-1, and the deregulation of Src kinases modulated by CD45 present in Cyprinus carpio FALC with ontogeny of peritoneal haemangiosarcoma in this fish species.