Progesterone Receptor in the Chick Bursa of Fabricius: Characterization and Immunohistochemical Localization

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Abstract
A high affinity progesterone-binding site was studied in the chick bursa of Fabricius. The Km for progesterone was 1.4 nM, and the concentration of progesterone-binding sites increased with estradiol treatment. In estradiol-treated bursas, the receptor concentration was about 240 fmol/mg protein. The binding site was specific for progestins, with the following order of affinities: ORG 2058 [16-.alpha.-ethyl-21-hydroxy-19-nor-4-pregnene-3,20-dione] > progesterone > promegestone. Androgens, dexamethasone and estradiol were weak competitors for progesterone binding in the bursa cytosols from estradiol-treated chicks. IgG fraction of antiserum (IgG-RB) raised in rabbit against the B-subunit of chick oviduct progesterone receptor (PR) was used for an immunohistochemical study. The PR was found only in the interfollicular cells, which were most probably nonlymphoid cells. Staining was localized exclusively in the elongated nuclei of these cells. No staining was seen in the bursal epithelium or inside the lymphoid follicles. Apparently, the interfollicular cells of the bursa contain specific PRs which are under estrogen regulation as in the oviduct. These cells might be under direct progesterone regulation.