RESUMEN
All organisms have unique immune systems that help them identify and eliminate invading microorganisms. A group of evolutionary ancient molecules, the thioester-containing proteins (TEP) superfamily, are known to play an important immune role by aiding animal hosts in the recognition, destruction, and elimination of hazardous microorganisms and their products. Our laboratory focuses on studying the role of the immune system in the mutualistic relationship between the sepiolid squid, Euprymna scolopes and its bioluminescent symbiont Vibrio fischeri. In the present study, we report the identification of a novel TEP-like transcript expressed in the light organ of squid. Characterization of the full-length coding sequence showed a molecule of 4218 nucleotides, corresponding to 1406 amino acids. Further sequence analysis revealed it contained structural characteristics of A2M molecules, including the thioester and receptor-binding domains. Analysis using the predicted amino acid sequence suggested this transcript was a homologue of CD109 molecules, thus we named it E. scolopes-CD109 (Es-CD109). In addition to the light organ, we were able to detect and amplify Es-CD109 in 12 out of 14 adult squid tissues tested. Quantification experiments showed that Es-CD109 expression levels were significantly lower in the light organ of symbiotic compared to aposymbiotic juveniles, suggesting a possible down-regulation of the host immune response in the presence of the bacterial symbiont.