LOCALIZATION OF TRK NEUROTROPHIN RECEPTOR-LIKE PROTEINS IN AVIAN PRIMARY LYMPHOID ORGANS (THYMUS AND BURSA OF FABRICIUS)

The avian thymus and bursa of Fabricius are the specific organs where the maturation and differentiation of T- and B-lymphocytes, respective ly, take place. In the mammalian lymphoid organs mRNAs of the neurotro phins and their receptors have been identified but their localization at the protein level remains still unknown. This study was undertaken to analyze the localization of the Trk family of tyrosine kinase recep tors in the avian primary lymphoid organs (thymus and bursa of Fabrici us) during the posthatching development using immunohistochemistry. Th ese proteins serve as essential constituents of the high affinity rece ptors for neurotrophins. In the thymus of all groups of age specific i mmunoreactivity (LR) was observed for all three Trks: TrkA-like IR was found labelling medullary epithelial cells and a subpopulation of cor tical epithelial cells; TrkB-like IR was found in the medullar dendrit ic cells and cortical macrophages; TrkC-like IR labelled the cortical epithelial cells and scattered medullar clusters of epithelial cells ( including Hassal's corpuscles). Quantitative analysis revealed age-dep endent decrease in the area occupied by TrkA-like IR in the cortex, an d age-dependent increase in the medulla; no changes were detected in t he area occupied by TrkB-like LR; the TrkC-like immunoreactive cells i ncrease from 7 to 30 days and then decrease. Regarding to the bursa of Fabricius, TrkA- and TrkC-like IR were exclusively found in the epith elial cells of the follicle associated and the interfollicular epithel ia, as well as TrkC-like LR in some medullary reticular epithelial cel ls of adult animals. Nevertheless, TrkB-like IR labelled extrafollicul ar unidentified cells in 7 days old animals, and the follicular secret ory dendritic cells at 30 and 60 post-hatching, The area occupied by t he medullary TrkB-like IR cells increased between 30 and 60 days. No i mmunostaining of lymphocytes was observed for any of the assessed anti gens. The blood vessels of both the thymus and the bursa of Fabricius were immunoreactive for TrkA- and TrkC-like proteins, The present resu lts provide evidence for the localization of Trks in the non-lymphoid cells (epithelial and dendritic) of the avian primary lymphoid organs, suggesting a role for neurotrophins in these cells. Moreover, the sel ective cell localization of each Trk protein, and the absence of appar ent overlapping, claims for a differential role of the specific Trk li gands. Whether or not these findings have functional relevance for T- and B-lymphocytes processing in avian primary lymphoid organs is discu ssed.