A monoclonal antibody, TcR2, has been shown to recognize an avian homologue of the mammalian alpha/beta T cell receptor (TcR). The TcR2-reactive molecule was found to be a T3-associated heterodimer with relative molecular mass of 90-kDa consisting of disulfide-linked 50-kDa and 40-kDa polypeptides. The sizes of the deglycosylated TcR2 polypeptides differed from those of TcR1, an avian homologue of the mammalian gamma/delta T cell receptor. Immunofluorescence analysis revealed that TcR1 and TcR2 are expressed on separate populations of T cells during their development first in the thymus and then in the periphery. Ontogenetic studies revealed that the TcR1+ thymocytes are generated first and the generation of TcR2+ cells begins approximately 3 days later. While most TcR2+ cells in the thymus expressed both CT4 and CT8, TcR2+ cells in blood and the spleen were either CT4+ or CT8+. The TcR1+ cells in blood and thymus were CT4-CT8-, but the majority of TcR1+ cells in the spleen surprisingly expressed the CT8 marker. The data suggest that TcR1 and TcR2 cells are generated in the thymus as separate T cell sublineages.

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The lymphoid immune system is comprised of two major cell types, B cells and T cells, originally identified in avian species. Although both lineages arise from hematopoietic stem cells, avian B cells require a period of development in the bursa of Fabricius while T cells undergo development in the thymus. Each cell type expresses a lineage-specific antigen receptor encoded by genes created by the rearrangement of individual members of variable (V), diversity (D), and joining (J) gene segment families during embryonic development. In this report, we demonstrate that productive rearrangement of the TCR beta gene occurs exclusively in the thymus during normal development. TCR beta rearrangements involving gene segments from the V beta 1 gene family can be detected beginning on day 12 of development, while rearrangements involving the other family of V beta gene segments, V beta 2, were first detected on day 14 of embryogenesis. In contrast, productive rearrangements of Ig light (IgL) and heavy (IgH) chain genes were not restricted to the bursa of Fabricius. Instead, VH-DJH heavy chain rearrangements and VL-JL light chain rearrangements were detected primarily in the embryonic spleen, beginning as early as embryonic day 10, even in birds bursectomized at 60 h of development. Within the spleen, Ig rearrangement was confined to the subset of cells that express the chB6 surface protein. Unlike bursal lymphocytes, which express the recombinase activating gene (RAG)-2 but not RAG-1, splenic B cell precursors also express RAG-1. The data indicate that, while B cell precursors initiate recombination prior to migration of the bursa of Fabricius, T cell precursors undergo V(D)J recombination following migration to the thymus. Thus, distinct developmental mechanisms appear to regulate the process of receptor rearrangement during avian B and T cell development.