Thymus-replacing activity from the metamorphic spleen of Xenopus laevis

Anuran metamorphosis offers an interesting vertebrate immunological paradigm, for adult cells that arise within the immunocompetent larval body are MHC Class I disparate from those of the larva. The animals, in order to avoid immune self-destruction during this transition period, are made unresponsive to these modified-self cells by an impairment of T-cell functions. However, it remains to be discovered how an animal with compromised T-cell functions can protect itself from those environmental pathogens protection from which is thymus dependent. During metamorphosis, larval and adult immunocytes capable of reacting to each other coexist within the animal. Their interaction might stimulate the secretion of a cytokine capable of circumventing T-cell functions by acting directly on B cells. Here, we report that such an activity is released in vitro by metamorphic and not by adult splenocyte suspensions. This activity will amplify in vitro anti-hapten responses by immunized, but not carrier-primed, adult splenocytes. The activity is unaffected by dexamethasone and, since it will amplify anti-hapten responses in T-cell-depleted immunized adult splenocyte suspensions, the antibody-producing (B) cell population may be affected directly. Two radiolabeled protein peaks of 65 kD and 40 kD were obtained by SDS-PAGE analysis from secreting, metamorphosing, but not from adult, splenocytes.