Characterization of a non-T, non-B human lymphocyte (L cell) with use of monoclonal antibodies. Its regulatory role in B lymphocyte function.

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Abstract
These studies investigate the role of L lymphocytes in regulating terminal B lymphocyte differentiation. L cells have abundant Fc IgG receptors and comprise 10--15% of human peripheral blood mononuclear cells (PBMC). L cells lack the conventional markers of B and T lymphocytes and in culture, do not develop into B cells, T cells, or macrophages. Additionally, use of monoclonal antibodies failed to detect on L cells, surface antigens specific for B cells, T cells, and macrophages. In these studies, purified L cell subpopulations depleted of macrophages were co-cultured with autologous PBMC in the presence of pokeweed mitogen and at the end of 8 d, development of intracytoplasmic immunoglobulin (Ig) was determined. L cells were depleted of B and T cells by rosetting techniques and, in addition, by cytotoxicity techniques using monoclonal-specific antisera to T cells. In 14 individuals, L cells when co-cultured with PBMC, enhanced Ig synthesis by 83% +/- 62 SD, and also enhanced cell proliferation. Radiated L cells lost enhancing properties. To study the role of their high density Fc IgG receptors, L cells pretreated with IgG antibody-sensitized erythrocytes were used (i.e., after lysis of rosettes). Such L cells significantly inhibited Ig synthesis (by greater than 50%) despite promoting cell proliferation. Antibody-sensitized erythrocyte-rosetted macrophages did not inhibited Ig synthesis. Thus, positive and negative influences can be mediated by the same cell, depending on the state of Fc-receptor stimulation. Such cells may play a more prominent role in "feed-back" regulation of Ig synthesis by virtue of having abundant Fc IgG receptors.