Maturational state of lymphoid cells in long-term bone marrow cultures.

B lymphocytes are not produced in long-term bone marrow cultures (LTBMC) and controversy exists regarding T cell differentiation in the cultures. By analyzing the kinetics of B cell reconstitution in lethally irradiated mice by cells from fresh bone marrow or from LTBMC, we have determined that B cell differentiation is blocked at an early stage in the cultures. Colony-forming B cells (CFU-B) are generated more rapidly and to higher levels in recipients of fresh bone marrow cells than in animals grafted with cells from LTBMC. The data further indicate that the adherent fraction cells from LTBMC are an enriched source of B cell precursors as compared to non-adherent cells and that B cell reconstitutive potential and levels of spleen colony-forming stem cells (CFU-S) do not correlate with one another. Under appropriate culture conditions, T lymphocytes are detectable in LTBMC. These Thy-1-positive cells proliferate in the cultures and can generate cytotoxic responses. These effectors originate from Thy-1-positive cells present in the initial bone marrow inoculum used to establish the cultures and have not differentiated from mature Thy-1-negative progenitors in vitro.