Effects of adherence, activation and distinct serum proteins on the in vitro human monocyte maturation process

Elutriator-purified human monocytes were cultured in a serum-free (SF) medium, and various serum proteins and functional activating agents were assessed for their effects on the in vitro maturation of human monocytes to macrophages. Following 3 days of suspension culture in Teflon labware, 60% of the monocytes were easily recovered. When varying concentrations of human AB serum (HuAB) were employed, human monocyte maturation progressed rapidly; the kinetics of this maturation process during cell suspension culture were very similar to the pattern observed following adherence culture. In contrast, when SF medium was employed, a marked retardation of the monocyte maturation process was observed; this could not be attributed to any changes in cell recovery and/or viability. Thus, cells could be maintained in their monocytoid form for 3 days when cultured in SF medium. When HuAB was added after 3 days of culture, human monocyte maturation into macrophages proceeded at a normal rate. We attempted to characterize certain of the serum protein(s) found in HuAB which promoted the monocyte maturation process. Human immunoglobulin G (IgG) was found to be the most potent serum protein in increasing 5'-N activity and decreasing peroxidase activity of suspension cultured monocytes. Immunoglobulin M (IgM) and albumin (Alb) were shown not to have significant monocyte maturation activity. Heat-treated human gamma globulin and IgG purified by high-performance liquid chromatography (HPLC) were shown to have patterns identical with that of untreated HGG and IgG with regard to promoting monocyte maturation; F(ab')2 was not an active maturation promoter, indicating the need for an intact Fc portion of the IgG molecule. Fibrinogen and fibronectin also had maturation promoting activity. Finally, addition of the potent monocyte functional activators, muramyl dipeptide (MDP), polyriboinosinic:polyribocytidilic acid (Poly I:C), and lipopolysaccharide (LPS) had no effect on the monocyte maturation process. Thus, neither cell adherence or activation appear to be critical for the monocyte to macrophage maturation process. Instead, we hypothesize that in addition to proper nutritional support, a group of serum proteins (unified mainly by their ability to interact with monocyte membrane receptors) appear to be the principal promoters of this process.