Characterization of natural killer cells and their precursors in the murine bone marrow.

We have fractionated murine bone marrow cells according to their density on bovine serum albumin (BSA) gradient and studied (a) the NK activity against YAC-1 targets, (b) the proportion of asialo GM1+ lymphocytes, (c) and the presence of large granular lymphocytes (LGL) in the different fractions (A, B, C, D). The NK activity was found mainly in the C fraction, but the proportion of asialo GM1+ cells was the same in every fraction. No LGLs were found in the bone marrow. Cells from the various fractions were also transplanted into irradiated recipients. Seven days later the highest NK activity was found in the spleens of mice injected with cells from the A + B fractions indicating that the immediate precursors for NK cells reside in the low density fractions of the BSA gradient. Mice transplanted with C or D fractions needed longer time to develop normal NK levels. The treatment of bone marrow cells before transplantation with anti-asialo GM1+ complement did not inhibit the development of NK activity, so it can be concluded that the precursor for NK is asialo GM1-.