Human natural killer (NK) cells produce a late-acting B-cell differentiation activity.

The supernatant of unstimulated purified NKH-1 bearing human natural killer (NK) cells was found to enhance ongoing immunoglobulin synthesis. This NK-Cell supernatant (NKSN) enhanced IgE, IgG, and IgA synthesis from corresponding B-cell lines without increasing thymidine incorporation or cell number. Separation of NKH-1+ cells into CD3- or CD3+ cells showed that this activity was produced by the CD3- population. Recombinant human interleukin (IL)-1, IL-2, IL-4, interferon (INF)-beta 1, INF-gamma, granulocyte-macrophage colony-stimulating factor (GM-CSF), tumor necrosis factor (TNF)-alpha, or partially purified low molecular weight B-cell growth factor (BCGF) failed to provide the same enhancement of Ig synthesis. While the NKSN contained small amounts of IL-6 (0.1 U/ml) and IL-6 could increase Ig synthesis in vitro, the optimal IL-6 enhancement was far less than that observed with NKSN. NKSN also enhanced ongoing Ig synthesis from in vivo activated B cells obtained from peripheral blood or bone marrow but failed to induce Ig synthesis from resting or in vitro activated B cells. These results demonstrate that human NK (CD3-, NKH-1+) cells can produce B-cell differentiation activity capable of regulating Ig production in vivo, which appears to be distinct from the activity of previously described cytokines.