Ly1+ PRO-B lymphocyte clones. Phenotype, growth requirements and differentiation in vitro and in vivo.

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Abstract
Several clones obtained from the bone marrow of a BALB/c mouse were found to contain the heavy and light chain Ig genes in the germline configuration, to express Ly1 and to carry the B cell lineage markers B‐220, Lyb8 and BP‐1; these clones are Pgp‐1+, LFA‐1+, J11d+, Mac‐1+ and Thy1‐, Lyt2‐, L3T4‐, GM1.2‐ and Ia‐. Three clones analyzed in detail (Lyd9, LyH7 and Lyb9) have receptors for interleukin (IL) 2 and IL3 as assessed with the 7D4 and CC11 monoclonal antibodies respectively. They grow in rIL3 but not in rIL2 or rIL1; both rIL4 and rIL5 also promote their proliferation, albeit to a much lesser extent than rIL3. None of the interleukins tested alone or in various combinations promoted the clones to differentiate in vitro along the B cell pathway. Treatment with 5‐Azacytidine (5‐Aza) induced cell surface Ia expression but not rearrangement or expression of Ig genes. However, 5‐Aza‐treated Lyd9, LyH7 and Lyb9 cells co‐cultured with X‐ray irradiated accessory cells and LPS gave rise to Ly1+, IgM+ B lymphocytes (range 14‐51%) including mu + kappa + (78‐93%), and mu + lambda + (9‐25%) B lymphocytes. In vivo, the Lyd9, LyH7 and Lyb9 clones gave rise to IgM+ B lymphocytes (8.5‐17%) including mu + kappa +, and mu + lambda +, but not to Lyt2+ or L3T4+ T lymphocytes after 4‐6 weeks of transfer into Scid mice. Our results indicate that Ly1+ IgM+ cells comprise a subpopulation of B lymphocytes that is derived from IL3‐responsive Ly1+ PRO‐B lymphocytes.