Specificity of natural serum antibodies present in phylogenetically distinct fish species

The following fish orders, at different stages of phylogenetic development, were tested for serum antibody activity against actin, myosin, tubulin, thyroglobulin, haemocyanin, single-stranded DNA and trinitrophenyl (TNP): Elasmobranches (sharks), Chondrosteans (sturgeons), and Teleosts (bony fish). Significant antibody activity was found against all these self and non-self antigens but, in particular, high titres of natural anti-TNP antibodies were noted. Anti-TNP antibodies were then isolated by affinity chromatography from pooled sera of several representative fish species, and studied by a non-competitive and a competitive enzyme immunoassay. When the isolated antibodies were tested by the non-competitive assay, Teleost (tench) anti-TNP antibodies bound almost exclusively to TNP, whereas Chondrostean and Elasmobranche antibodies bound mainly to TNP but also to one or more other antigens. When the isolated antibodies were examined by the competitive assay, they could be separated into three groups: (i) anti-TNP antibodies whose binding could be inhibited by TNP only (Teleosts); (ii) anti-TNP antibodies mainly inhibited by TNP but also significantly inhibited by two or three other antigens (Chondrosteans); and (iii) anti-TNP antibodies binding to different antigens but only slightly inhibited by them (Elasmobranches). These antibodies were found by immunocytochemical staining to bind to normal cell constituents, and especially to cytoskeletal proteins. The results strongly suggest that natural antibodies against the seven antigens examined are present in normal fish serum, and that their specificities differ with the stage of phylogenetic development.