Immunoglobulin-bearing stem cells for clones of B (bursa-derived) lymphocytes.

Treatment of neonatal chickens with cyclophosphamide depletes bursal lymphocytes while maintaining the bursal epithelium intact. The bursae of normal young chickens contain "bursal stem cells" which can reconstitute the lymphoid compartment in the bursa of the cyclophosphamide-treated recipient. Using bursal stem cells from IgM allotype-heterozygous donors we show that most bursal follicles in the reconstituted host are colonized by single stem cells which are committed to the expression of one or other IgM allotype. In addition we show that the reconstituting bursal stem cells express allelically excluded surface IgM at the time of transfer. Our results suggest that B lymphocyte numbers in hatched chickens are maintained by self-renewal of committed precursors rather than by de novo production from multipotential stem cells.