IDENTIFICATION OF CLASS-I GENES IN CARTILAGINOUS FISH, THE MOST ANCIENT GROUP OF VERTEBRATES DISPLAYING AN ADAPTIVE IMMUNE-RESPONSE

Sharks are members of the most primitive class of vertebrates (Chondri chthyes) shown to have an adaptive immune system. Surprisingly, howeve r, class I genes have not been identified unambiguously in this taxon, and absence of class I loci or a failure to express class I genes mig ht explain some of the relatively ''weak'' adaptive immune responses d ocumented in cartilaginous fish. We report here the isolation of three unique cDNA clones from two different species of sharks that encode b ona fide class I proteins. These clones exhibit different sequence and expression profiles indicating that they are likely to represent both classical and nonclassical class I lineages. In addition, our prelimi nary analysis suggests that there may be transfer of gene segments amo ng shark class I genes over evolutionary time. The cloning of shark cl ass I genes completes the identification of molecules that define the adaptive immune system (including Ig, TCR, and MHC class II proteins) in this taxon. Thus, simple models invoking a total absence of certain molecular hallmarks of the immune system to account for poor immune r esponsiveness in cartilaginous fish should be abandoned.