ACTIVATION BY MITOGENS AND SUPERANTIGENS OF AXOLOTL LYMPHOCYTES - FUNCTIONAL-CHARACTERIZATION AND ONTOGENIC STUDY

Urodele amphibians have weak and slow immune responses compared to mam mals and anuran amphibians. Using new culture conditions, we tested th e ability of lymphocytes of a well-studied salamander, the Mexican axo lotl (Ambystoma mexicanum) to proliferate in vitro with diverse mitoge nic agents. We demonstrated that the axolotl has a population of B lym phocytes that proliferate specifically and with a high stimulation ind ex to the lipopolysaccharide (LPS) known as a B-cell mitogen in mammal s. This proliferative capacity is observed without significant changes throughout ontogenesis. In the presence of LPS, axolotl B lymphocytes are able to synthesize and secrete both isotypes of immunoglobulin de scribed in this species, IgM and IgY. Moreover, a distinct lymphocyte subpopulation is able to proliferate significantly in response to the mitogens usually known as T-cell specific in mammals, phytohaemaggluti nin (PHA) and concanavalin A (Con A). The activated cells are T lympho cytes, as shown by depletion experiments performed in vitro with monoc lonal antibodies, and in vivo by thymectomy. Splenic T lymphocytes of young axolotls (before 10 months) do not have this functional ability, which suggests maturation and/or migration phenomena during T-cell on togenesis in this species. Axolotl lymphocytes are able to proliferate in vitro with a significant stimulation index to staphylococcal enter otoxins A and B (SEA and SEE). These products act on mammalian lymphoc ytes as superantigens: in combination with products of the major histo compatibility complex (MHC), they bind T-cell receptors with particula r V beta elements. The fact that these superantigens are able to activ ate lymphocytes of a primitive vertebrate suggests a striking conserva tion of molecular structures implied in superantigen presentation and recognition.