MACROPHAGE HETEROGENEITY AND DIFFERENTIATION - DEFINED SERUM-FREE CULTURE CONDITIONS INDUCE DIFFERENT TYPES OF MACROPHAGES INVITRO

Macrophages (MAC) are important effector cells of the immune system. They arise from circulating blood monocytes (MO), which undergo further maturation upon leaving the vasculature and migrating into the various tissues and body cavities. A similar differentiation process can be followed in vitro when monocytes are cultured in the presence of serum. In this study, different factors and serum proteins, either alone or in combination, were tested for their ability to promote the survival and/or maturation of blood MO in the absence of serum. Elutriation-purified MO cultured for 8 days on hydrophobic teflon foils in the presence of 5% human serum differentiated into large, well-spread MAC, whereas in the absence of serum, MO rapidly died. The serum-induced maturation of MAC was accompanied by a strong expression of CD16, CD14 and MAX antigens. Secretion of TNF-alpha and neopterin increased about 10-fold as compared with freshly isolated MO. The replacement of serum by either M-CSF (100 ng/ml) or immunoglobulin (0.5-5 mg/ml) had a marked effect on MO survival (about 50% of serum-cultured MO), but cells were smaller, less spread out and had low expression of CD16, CD14 and MAX antigens. Their functional competence in terms of TNF-alpha and neopterin release was reduced to 10-20% as compared with MAC cultured in the presence of serum. Both albumin (0.5-5 mg/ml) and 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) (10(-8) M) as substitutes for serum were less effective in terms of MO survival (20-30% of serum-cultured MO), but were comparable to serum with respect to morphology and phenotype; however, they induced MAC with lower secretory activity. A combination of 1,25(OH)2D3 or albumin (0.5 mg/ml) with immunoglobulin (0.5 mg/ml) resulted in MAC showing serum-cultured characteristics in terms of phenotype, but lower secretory capacity and survival rate. However, the combination of the three factors together could substitute for serum in all parameters tested. The same result was obtained by cultivation of MO with high concentrations of albumin (5 mg/ml) and immunoglobulin (5 mg/ml). Other factors tested had no effect on the MO into MAC differentiation process (transferrin, vitamin A, fibronectin, vitamin D2). In summary, we describe defined serum-free culture conditions for the generation of distinct types of MAC, which differ in terms of phenotype, morphology and function. Our system may be useful to study regulatory signals operative during the ontogeny of human MAC. It may be especially helpful to distinguish between competence/survival and maturation-inducing activities.