Role of veiled cells in lymphocyte activation.

Peripheral blood lymphocytes from normal rabbits or from rabbits hyperimmunized with human immunoglobulin were cultured in 20-microliters hanging droplets. The cultures were stimulated with concanavalin A or, in the case of cells from sensitized animals, with human immunoglobulin. The addition to the cultures of small numbers of autologous or allogeneic veiled cells separated from afferent lymph increased the responses to low doses of the stimulants, particularly when the cells were cultured at low cell densities or for short periods of time. At high cell densities or at longer periods of culture, stimulation occurred in the absence of added veiled cells and was associated with clumping of the lymphoid cells and development of cells which morphologically resembled veiled cells found in afferent lymph. The enhanced responses upon addition of small numbers of veiled cells were also correlated with the formation of lymphoid aggregates which were frequently held together by the elongated processes of a single veiled cell. The observations support the view that the veiled cells are lymph-borne precursors of dendritic cells in the lymph nodes. This may provide an in vitro model for the cellular relationships which occur in paracortical cords of lymph nodes following antigenic stimulation.