Maternal transfer of antibodies to eggs in Xenopus laevis

The immune system of the African clawed frog, Xenopus laevis, includes nearly the full repertoire of lymphoid organs and immune cell types found in mammals. In contrast to the mammalian immune system, the development of the amphibian immune system occurs in the open environment. Oviparity necessitates a rapid ontogeny of the immune system. X. laevis larvae become immunocompetent about 2 weeks after fertilization of the egg. During this 2-week window, larvae cannot mount an adaptive immune response to potential pathogens and presumably must depend on innate responses. In the present study, the possibility of maternal transfer of antibodies to eggs was examined. Adult female X. laevis were injected three times at weekly intervals with the hapten–carrier complex, trinitrophenylated bovine serum albumin (TNP-BSA). The sera of immunized frogs demonstrated antibody activity to BSA, TNP-BSA, and, importantly, trinitrophenylated ovalbumin (TNP-OVA) when examined by enzyme-linked immunosorbent assay (ELISA). Reactivity to TNP-OVA confirmed that antibodies were produced against TNP. The adult female frogs were induced to lay eggs by injection of human chorionic gonadotropin. Next, membrane-free extracts of the eggs were treated with protease inhibitors in order to prevent proteolysis of proteins found in the eggs. On analysis by ELISA, it was found that TNP-specific antibodies were present in the egg extracts. This demonstrated the transfer of antigen-specific antibodies from adult females to eggs in X. laevis.