The expression of the thyroid hormone (TH) receptor genes a (TRa) and p (TRp) in Xenopus laevis begins after the embryo hatches. The TRa mRNA increases throughout the premetamorphosis stage of tadpole development, is maximal by prometamorphosis, and falls after climax of metamorphosis to a lower level in frogs. The TRp mRNA is barely detectable during premetamorphosis. In synchrony with the onset of endogenous TH synthesis by the thyroid gland (prometamorphosis), the level of TRp mRNA rises in parallel with endogenous TH, reaching a peak at the climax of metamorphosis (stage 61) and drops to -10% of its peak level after metamorphosis. As suggested by this correlation, exogenous TH up-regulates TRp mRNA as much as 20-fold during premetamorphosis, whereas TH up-regulates TRa mRNA by ~ 2-fold during the same period. Up-regulation of TRp mRNA is the earliest response to exogenous TH by competent tadpoles yet detected.

Complementary DNA encoding the IgG1 induction factor, the first lymphokine directed to B lymphocytes, from a murine T-cell line has been cloned using a new strategy. The putative primary amino-acid sequence was deduced from the nucleotide sequence determined. The lymphokine synthesized by the direction of this cloned cDNA has many other functions, such as production of B-cell growth factor-1 and induction of Ia on B cells.

Proliferation and maturation of antigen-stimulated B cells are regulated by several soluble factors derived from macrophages and T cells. These soluble factors are functionally divided into two groups: B-cell growth factor (BCGF), thought to be involved in B-cell proliferation; and B-cell differentiation factor (BCDF), responsible for maturation of activated B cells into immunoglobulin-secreting cells. This classification needs to be re-examined in the light of the recent cloning of complementary DNA encoding IgG1 induction factor (interleukin-4, IL-4) from the 2.19 mouse T-cell line. Recombinant IL-4 has BCGF and BCDF activities and affects B cells, T cells and mast cells (refs 7, 8; our unpublished data). Another well-characterized B-cell factor is T-cell replacing factor (TRF), which, when secreted by the murine T-cell hybridoma B151K12, is defined by two activities: induction of IgM secretion by BCL1 leukaemic B-cell line; and induction of secondary anti-dinitrophenol (DNP) immunoglobulin G (IgG) synthesis in vitro by DNP-prime B cells. Although TRF from B151K12 was classified as BCDF, purified TRF has BCGF-II activity. To elucidate the molecular properties of TRF we isolated cDNA encoding TRF from the 2.19 T-cell line and report here the structure and multiple activities of this lymphokine.