Immunology Letters, 11 (1985) 29-37 Elsevier Imlet 652 MONOCYTE MEDIATED MODULATION OF THE ANTIBODY RESPONSE IN VITRO Maria L. VILLA, Giovanna RAPPOCCIOLO, Paolo PIAZZA and Enrico CLERICI Cattedra di Immunologia e Istituto Nazionale per 1o Studio e la Cura dei lbmori, Via Venezian, 1, 20133 Milano, Italy (Received 27 November 1984) (Modified version received 10 May 1985) (Accepted 6 June 1985) 1. Summary 2. Introduction Human peripheral blood monocytes (PBMC) were isolated by Ficoll-Hypaque density gra- dient and then fractionated by differential adhe- sion to plastic surface. Adherent ceil-depleted PBMC, non-readherent fraction and firmly ad- herent 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 pro- duce about twice as many haemolytic areas as compared to total PBMC (from 43 to 85). If depleted PBMC are co-cultured with firmly ad- herent or non-readherent ceils, the number of haemolytic areas goes down to 19 or up to 102, respectively. Functional, histochemical, immunochemical and morphological data suggest that the inhibit- ing firmly adherent fraction is composed of typi- cal 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 ac- tive pinocytic activity, but are the principal ac- cessory cells needed to stimulate lymphocyte responses. Key words: monocyte - dendritic cell - primary in vitro re- sponse Macrophages and monocytes play a pivotal part in the initiation and regulation of the im- mune responses. They are capable of processing and presenting antigen and also control, both positively and negatively, lymphocyte growth and differentiation. Experiments performed by several authors using human phagocytic mononuclear cells isolated from peripheral blood, have provid- ed reproducible results. Indeed, mononuclear ceils are, in low and intermediate concentrations, an absolute requirement for the initiation and the enhancement, respectively, of the immune re- sponse in vitro, while when they are added in ex- cess to cultures, a suppression of the antibody production occurs. The dose-dependent, enhancing or inhibiting effect of human mononuclear cells, has been ob- served in the proliferative responses of T and B lymphocytes to pokeweed mitogen (PWM), concanavalin A (Con A), and phytohaemaggluti- nin (PHA), in the non-specific immunoglobulin production by B lymphocytes treated with PWM, in the B lymphocyte colonies induced in vitro by staphylococcal protein A (SPA) and in the specif- ic primary antibody production against trinitrophenylated polyacrylamide beads (TNP- PAA) and sheep red blood cells (SRBC) [1-4]. Evidence is accumulating that the above an- tithetic influences, rather than being simple dose- dependent effects, are caused by functionally different subpopulations of blood mononuclear 0165-2478 / 85 / $ 3.30 © 1985 Elsevier Science Publishers B.V. (Biomedical Division) 29