Semi-quantitative polymerase chain reaction analysis of cytokine and cytokine receptor gene expression during thymic ontogeny

Semi-quantitative, polymerase chain reaction (PCR) is used to uncover the patterns of cytokine transcription in the mouse thymus from day 14 to day 20 of gestation, a time period which includes many of the important events in thymic ontogeny. Interleukin 4 (IL-4), IL-7 and interferon γ (IFN-γ) mRNA is abundant from fetal day (Fd) 14-16, corresponding with the period of rapid proliferation of immature thymocytes in vivo. As the level of mRNA for these cytokines diminishes, the induction and increased expression of IL-3 and IL-2 occurs. The transcription of these cytokines correlates temporally with the period of proliferation-dependent phenotypic differentiation between Fd 16 and 20. The thymic epithelium (TE)-derived cytokines including IL-1α, IL-6 and granulocyte-macrophage colony-stimulating factor (GM-CSF) begin to be transcribed between Fd 14-15 and show peak mRNA abundance from Fd 16-20, IL-5, tumour necrosis factor α (TNF-α) and LT (lymphotoxin or TNF-β) constitute a fourth group of cytokines, along with the IL-4 receptor (IL-4R), which are transcribed at an even level throughout the fetal period. The IL-2 receptor beta chain (IL-2Rβ) and IL-10 show abundant mRNA from Fd 14-20 and have a peak level of mRNA content on Fd 16. Taken together, these studies uncover complex, overlapping patterns of cytokine gene expression. The mRNA abundance and pattern of expression of each cytokine or cytokine receptor may indicate the relative contribution that it makes to different stages of fetal thymic ontogeny.