An investigation into the role of antigen presenting cells in immunoglobulin synthesis in vitro.

Controversy surrounds the respective roles of the monocyte-macrophage series of cells and dendritic (veiled) cells (DC) in the processes involved in antigen presentation. To try to examine this question we have investigated the influence of these accessory cell populations in vitro on pokeweed mitogen (PWM) driven IgG synthesis by peripheral blood mononuclear cells (PBM). Macrophages and dendritic cells obtained by Percoll and hypertonic metrizamide density gradient centrifugation respectively were cultured with lymphocytes and their role in inducing IgG synthesis examined. Additionally phenotypic analysis of the cell populations based both on cell size and cell surface antigen characteristics using the monoclonal antibodies OKM1 and HLA-DR was performed by FACS. Whilst macrophage-depleted lymphocytes secreted negligible amounts of IgG, DC depletion of PBM had little impact on IgG synthesis. When these DC-depleted lymphocytes were further depleted of macrophages by Percoll gradient centrifugation their IgG secretion was dramatically reduced. This response could be minimally reconstituted by adding back DC but was more successfully reestablished with the add back of a population of macrophage-enriched cells. Phenotypic analysis of the cell populations involved did not allow clear discrimination of the various cell groups. On the basis of the functional studies the macrophage cell series would seem to have the central role in driving PWM reduced IgG synthesis though the possible "contamination" of these cells by DC can still not be excluded. Until such time as reliable markers are available for the clear discrimination of these 2 cell populations it is difficult to envisage resolution of the controversy on their respective roles.