Prenatal and adult growth hormone gene expression in rat lymphoid organs

Growth hormone (GH) exerts its immune effects on mature lymphocytes through an autocrine/paracrine mechanism. We investigated the prenatal synthesis o f GH mRNA in rat lymphoid organs using the sensitive in situ RT-PCR methodo logy. We show that GH transcripts are detectable in the thymus and liver of the 18-day fetus. At this stage, all thymocytes are immature and express t he GH gene. In fetal liver, GH gene expression was localized in circulating lymphocytes and in hematopoietic cells surrounding GH mRNA-negative hepato cytes. In situ GH gene expression in fetal lymphoid organs was confirmed by in vitro RT-PCR showing that the amplified product from fetal lymphoid tis sues was similar to the product obtained from the pituitary. Moreover, GH g ene expression was detected in the thymus, spleen, and ileum Peyer's patche s of adult rat, with a localization restricted to the lymphocytes and endot helial and smooth muscle cells of blood vessels. The autocrine/paracrine ex pression of the CH gene by lymphoid and hematopoietic cells during fetal gr owth might influence the generation of regulatory cells involved in immunit y and hematopoiesis.