Monocyte mediated modulation of the antibody response in vitro

Human peripheral blood monocytes (PBMC) were isolated by Ficoll-Hypaque density gradient and then fractionated by differential adhesion to plastic surface. Adherent cell-depleted PBMC, non-readherent fraction and firmly adherent fraction so obtained from PBMC, PBMC themselves and a mixture of the above cells, were then sensitized in vitro with sheep erythrocytes (SRBC) so as to produce a primary antigen-dependent, antigen-specific antibody response. It appears that adherent cell-depleted PBMC produce about twice as many haemolytic areas as compared to total PBMC (from 43 to 85). If depleted PBMC are co-cultured with firmly adherent or non-readherent cells, the number of haemolytic areas goes down to 19 or up to 102, respectively. Functional, histochemical, immunochemical and morphological data suggest that the inhibiting firmly adherent fraction is composed of typical phagocytizing cells, while the enhancing cells of the non-readherent fraction are similar to the dendritic cells described in human blood and some lymphoid organs, which do not exhibit active pinocytic activity, but are the principal accessory cells needed to stimulate lymphocyte responses.