B cell precursors located in embryonic mesenchyme enter the bursa between about days 8 and 15 of incubation. The presence of the Bu-1 antigen on at least some of these precursors (prebursal stem cells) suggests that they are already committed to the B cell lineage. Antibody variability is generated by gene conversion in the bursa (see below), probably until the organ involutes at sexual maturity. There is no evidence for development of B cells from Ig-negative precursors after hatching in any anatomical site. Instead, B cells arise in the bursa from Ig-positive bursal stem cells, and later in the periphery from postbursal stem cells, which can restore the peripheral, but not bursal B cell compartment after transfer to a cyclophosphamide-treated host. In their early formation and capacity for self-maintenance, chicken B cells differ from the majority of mouse and human B lymphocytes, although they do share some properties, e.g. CD5 expression, with the B-l cells of these species. In spite of these differences, chicken B lymphocytes are functionally very similar to their mammalian counterparts in terms of capacity to differentiate into plasma cells and secrete antibodies at high levels. Chicken B cell subsets differing in surface phenotype and physiological criteria, such as mode of emigration from the bursa and turnover rate, have been described.

Chicken T cells develop from precursors which enter the thymus in several waves, starting at about day 6 of embryonic development, and formation of mature T cells appears to follow a course similar to that observed in mammals. Cells expressing a(3 T cell receptors (TCRs) have helper, cytotoxic or suppressor activities, and their functions, at least for helper and cytotoxic T cells, are MHC-restricted. Cells with a helper phenotype (carrying the CD4 antigen), as well as virally transformed CD8-bearing cells have been cloned. T cell-mediated suppression is most clearly shown in models where transfer of T cells from bursectomized birds into normal birds can induce B cell deficiency in the recipients; however, the mechanism of this suppression is not clear. Natural killer (NK) cell and antibody-dependent cellular cytotoxicity (ADCC) activities are readily detectable in chicken spleen and blood. Chicken NK cells have been identified as cytoplasmic CD3+, surface CD3/TCR- and CD25+ cells. A subset of these cells expresses CD8a.

Avian nonlymphoid hematopoietic cells (monocytes, granulocytes, thrombocytes, erythrocytes) are clearly defined morphologically and functionally, and bear characteristic cell surface markers detectable by monoclonal antibodies. All of these are nucleated cells, including the erythrocytes and thrombocytes, so that separation procedures designed for mammalian lymphocytes may need modification for avian cells.

In experiments involving cell or organ transfers, the types of cell marker used to follow cellular origin include strain- or species-specific monoclonal antibodies, the sex chromosome marker, and the quail-chick nuclear marker, useful because of its independence of cell cycle and lineage.

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