EXPRESSION THE GROWTH-HORMONE GENE IN IMMUNE TISSUES

It is well established that growth hormone (CH)-like proteins and mRNA, are present in immune tissues, but it is not known whether this reflects ectopic transcription of the CH gene or the expression of a closely related gene. This possibility was, therefore, investigated. Immunoreactive (IR) GH-like proteins were readily measured by radioimmunoassay and immunoblotting in the spleen, bursa of Fabricius and thymus of immature White Leghorn chickens, in which IR-CH was similar in size and antigenicity to the major CH moieties present in the pituitary gland. RT-PCR of mRNA from these immune tissues, with oligonucleotide primers spanning the coding region of pituitary GH cDNA, also generated cDNA fragments identical in size (689 bp) to pituitary CH cDNA. BamHI and Rsal cleavage sites were located in these cDNA sequences in the same position as those in pituitary CH cDNA. These amplified cDNA sequences also contained sequences that hybridized, by Southern blotting, with a chicken pituitary CH cDNA probe, thus suggesting a high degree of homology between pituitary and immune CH transcripts. The nucleotide sequence of the PCR products generated from these immune tissues, determined by a modified cycle dideoxy chain termination method, were also identical to pituitary CH cDNA. This homology extended over 593 bp of the spleen cDNA (spanning nucleotides 70-663 of the pituitary CH cDNA and its coding region for amino acids 5-201), 613 bp of the bursa cDNA fragment (spanning nucleotides 63-676 of the pituitary GH cDNA and its coding region for amino acids 3-207) and 607 bp of the thymic cDNA fragment (spanning nucleotides 61-665 of pituitary GH cDNA and its coding region for amino acids 4-203). These results clearly establish that the GH mRNA is present in immune tissues, in which GH-IR proteins are present. The local production of GH within the immune system of the domestic fowl, therefore, suggests it has paracrine or autocrine roles in modulating immune function.