Activation by mitogens and superantigens of axolotl lymphocytes: functional characterization and ontogenic study

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Abstract
Urodele amphibians have weak and slow immune responses compared to mammals and anuran amphibians. Using new culture conditions, we tested the ability of lymphocytes of a well‐studied salamander, the Mexican axolotl (Ambystoma mexicanum) to proliferate in vitro with diverse mitogenic agents. We demonstrated that the axolotl has a population of B lymphocytes that proliferate specifically and with a high stimulation index to the lipopolysaccharide (LPS) known as a B‐cell mitogen in mammals. 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 described 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, phytohaemagglutinin (PHA) and concanavalin A (Con A). The activated cells are T lymphocytes, as shown by depletion experiments performed in vitro with monoclonal 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 ontogenesis in this species. Axolotl lymphocytes are able to proliferate in vitro with a significant stimulation index to staphylococcal enterotoxins A and B (SEA and SEB). These products act on mammalian lymphocytes as superantigens: in combination with products of the major histocompatibility complex (MHC), they bind T‐cell receptors with particular Vβ elements. The fact that these superantigens are able to activate lymphocytes of a primitive vertebrate suggests a striking conservation of molecular structures implied in superantigen presentation and recognition.