Humoral and formed elements of blood modulate the response of peripheral blood monocytes. I. Plasma and serum inhibit and platelets enhance monocyte adherence.

Human and rabbit peripheral blood monocytes normally adhere to plastic tissue culture plates in vitro when they are suspended in Hanks' media. Increasing amounts of autologous serum or heat-inactivated plasma in the cell suspensions prevented the adherence of both monocytes and lymphocytes. The inhibitory effect of plasma was separated into three areas of activity by chromatography on Sephacryl S-200. The profile of inhibitory activity did not coincide with the protein elution profile, suggesting that inhibition was not a nonspecific protein effect. A layer of adherent platelets overcame the inhibitory effect of plasma on monocyte adherence. Platelets selectively increased monocyte as opposed to lymphocyte adherence and this was specific for platelets in that neither neutrophils nor fibroblasts could substitute for platelets. Both plasma and platelets acted directly on monocytes.