The ontogeny of interleukin production and responsivity in the frog, Xenopus

The production of and responsivity to leukocyte-derived lymphokine-rich culture supernatants (SNs) was examined during the ontogeny of the frog, Xenopus. Thymocytes and splenocytes from adult frogs are capable of responding to the T-cell mitogen, phytohemagglutinin-P (PHA). Larval thymocytes are unresponsive to this lectin, whereas larval splenocytes are not. PHA-unresponsive thymocytes can be costimulated with PHA plus either a T-cell growth factor (TCGF)-rich SN or an interleukin-1 (IL-1)-rich SN (from cultures of lipopolysaccharide (LPS)-treated macrophage-enriched peritoneal cells (PCs). Stimulation of larval thymocytes with PHA does not produce a TCGF-rich SN as assayed by proliferation of lymphoblasts. Larval as well as adult splenocytes treated with PHA, however, do produce TCGF. These data are consistent with the hypothesis that the factor limiting mitogen responsiveness of larval thymocytes is the ability of cell populations in the thymus to produce rather than respond to either IL-1 or IL-2.