TRK-LIKE PROTEINS DURING THE POSTHATCHING GROWTH OF THE AVIAN BURSA OF FABRICIUS

Neurotrophins are growth factors acting on responsive cells through me mbrane receptors identified as Trk tyrosine kinase proteins (A, B and C). Trks are present in the mammalian lymphoid organs, and indirect ev idence suggests that they are also present in the avian bursa of Fabri cius. This study was designed to analyze (a) the occurrence and locali zation of Trk proteins in the bursa of Fabricius; and (b) whether the post-hatching growth of the organ (from hatching to 75 days) involves cells expressing Trk proteins. We used pigeon bursae of Fabricius, and rabbit polyclonal antibodies against specific epitopes of TrkA, TrkB and TrkC. Cytokeratins and vimentin were studied in parallel with labe l non-lymphoid cells of the bursal follicles. Immunoreactivity (IR) fo r all assessed antigens was found in specific non-lymphoid cells. From hatching to 15 days, TrkB-like IR was found outside the follicles in cells localized beneath the follicle associated (FAE) and interfollicu lar (IFE) epithelium. Between 30-75 days TrkB-like IR labelled the med ullary secretory dendritic cells (SDC). The density of SDC displaying IR increased up to 60 days. TrkA-like and TrkC-like IR was primarily o bserved in FAE and IFE, but also in the medullary reticular epithelial cells (REC) up to 15 days. The present results provide evidence of th e occurrence, localization and post-hatching changes in Trk proteins i n avian bursa of Fabricius. Trks were localized on non-lymphoid cells which participate in providing the adequate microenvironment for B lym phocyte maturation. Furthermore, the cell segregation in the expressio n of Trks suggests specific roles for their ligands in controlling the function of medullary SDC and REC, hence bursal lymphoid follicle phy siology. (C) Elsevier Science B.V.