Dendritic cells induce T lymphocytes to release B cell-stimulating factors by an interleukin 2-dependent mechanism.

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Abstract
Dendritic cells (DC) are essential accessory cells for T-dependent antibody responses in culture. A 3-stage mechanism to explain the capacity of DC to stimulate primary antibody responses to heterologous erythrocytes is outlined. First, DC induced [murine] T cells to produce and to become responsive to interleukin 2 (IL-2). This stage corresponded to the syngeneic mixed leukocyte reaction and involved the clustering of DC and T cells into discrete aggregates. Isolated clusters, representing 5-10% of the culture, were critical for IL-2 release and the production of IL-2-responsive T blasts. In the 2nd stage, IL-2 directly triggered the responsive T cells to release B cell helper factors. This role for IL-2 was documented with a rabbit anti-IL-2 reagent, purified IL-2, and T cells that were rendered IL-2 responsive by an initial co-culture with DC. T cell growth was not required for IL-2-mediated helper factor release, since irradiated and untreated responders produced similar levels of factor and did so within 3 h of the addition of IL-2. In the final stage, helper factors stimulated the development of antibody-secreting cells from purified B lymphocytes. The helper factors were not H-2 restricted, but for both sheep and horse erythrocytes, the response to factors was antigen-dependent and specific. The IL-2 that was present in the DC/T cell-conditioned medium did not act on B cells, since helper activity was neither neutralized nor absorbed by the anti-IL-2 reagent. Thus, the ability of the DC to induce IL-2 release and responsiveness underlies its capacity to trigger both T and B lymphocyte reactions.