ROC analysis confirmed that IL-6 showed the best trend in the diagnosis of AA. However, the diagnosis of AA was not greatly improved by any of the new serum markers as single on-admission tests.
We have applied polymerase-chain-reactiondirected immunoglobulin gene analysis to study the embryonic differentiation of chicken B cells. Immunoglobulin light chain DNA segments in the rearranged configuration were amplified from cells of the intraembryonic mesenchyme as early as day 7 of incubation. We showed by sequencing that the rearranged variable region genes in these early B-cell progenitors were not different from the germ-line VA1 gene (the single functional light chain variable region gene in chickens). In the bursal B lymphocytes, on the other hand, clear gene conversion events were first observed at day 15 of embryonic development. The present data indicate that rearrangement of light chain genes in the chicken occurs independently of the bursa of Fabricius and that diversification of the variable region begins only later, when the surface immunoglobulin-positive B cells are proliferating in the bursal follicles.
SummaryEmbryonic chimeras wereused to demonstrate an early separation of chicken T and B cell precursors . Genetically polymorphic cell surface antigens, Bu-1 and Ov, which are expressed on cells of the B and T lineage, respectively, are useful markers in adoptive cell transfer studies. Allelic products Bu-1a and Bu-1b can be detected with monoclonal antibodies (mAbs) L22 and 11G2, respectively, and the Ov antigen with mAb 11A9. Chimeric chickens were constructed by reconstituting irradiated 14-d Ov -H .B19 embryos with the sorted Bu-1+ or Bu-1 -fractions of spleen cells from age-matched H .B19 Ov' embryos . Chimeras were analyzed, 3-4 wk after hatching, for the presence of Ov+ cells in the bursa, thymus, spleen, and peripheral blood lymphocytes. T cell precursors giving rise to thymocytes and peripheral T cells were present only in the Bu-1 -, but not in the Bu-1+, fraction. We previously demonstrated that, in contrast, all B cell precursors in spleen from 14-d embryos are exclusively present in the Bu-1+ fraction. We also analyzed the immunoglobulin light chain gene rearrangement in these populations by polymerase chain reaction. We show here that VJ recombination occurs in the Bu-1+, but not in the Bu-1 -, fraction of spleen. These data demonstrate an early commitment to the B cell lineage, which occurs before the colonization of the bursa of Fabricius . Segregation of B cell precursors from the other hemopoietic precursors, and consequently separation of T and B cell precursors, occurs before the colonization of the primary lymphoid organs. T he avian embryo provides a unique model for the analysis of the immune system development because of the accessibility of the embryo and the anatomical separation of B and T lymphocytes in the bursa of Fabricius and thymus, respectively (1-3) . Sex-chromosomal or chick-quail nuclear markers were used in chimera studies to show that bloodborne hemopoietic stem cells migrate into the bursal and thymic rudiment at precise stages of embryonic development (4-8) . However, colonization of the thymus and bursa follows strictly different mechanisms. While the thymus receives several waves of stem cell influx separated by nonreceptive intervals (5, 6), the bursa is colonized by a single wave of precursors between day 8 and 14 in chick embryos (7) . Moreover, the colonization of the bursa of Fabricius is restricted to the embryonic life as the precursors able to seed the bursal rudiment are no longer present in the peripheral blood or in the bone marrow (BM)' of chickens after hatching (9-12) .Several lines of evidence now suggest an early segregation of the B cell lineage during chicken ontogeny. The cell surface alloantigen Bu-1 expressed on all B cells and on a subset of macrophages (13, 14) is also present on B cell precursors able to home to the bursa in cell transfer experiments (15). These Bu-1+ B cell precursors appear independently of the bursa as they exist in early bursectomized chick embryos (16). We previously demonstrated, using the Bu-1a and Bu-1b allotypic markers...
Aiolos is an Ikaros-related lymphoid regulatory protein involved in B cell development and function. To evaluate the role of Aiolos in avian B lymphopoiesis, we have cloned and characterized the first non-mammalian Aiolos ortholog in the avian. In sharp contrast to the avian Ikaros, expressed already at the multipotential stage and prior to the colonization of the lymphoid rudiments, Aiolos transcripts were not expressed in early ontogeny and were first detected in cells isolated from the embryonic bursa of Fabricius and thymus. In accordance with Ikaros, the avian Aiolos is also highly related to the mammalian homolog, thus suggesting an evolutionarily conserved function in lymphocyte development. Interestingly, in contrast to the mammalian Aiolos, at least one alternatively spliced form of avian Aiolos is detected in addition to the primary full-length transcript. Both of these alternate transcripts are expressed in bursa, thymus and peripheral lymphoid cells, and no major differences in the expression were detected during lymphocyte development. Furthermore, avian Aiolos is clearly expressed at various stages of B cell development, thus supporting recent evidence for the importance of Aiolos in B cell development and function.
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