Maturation of human dendritic cells by monocyte-conditioned medium is dependent upon trace amounts of lipopolysaccharide inducing tumour necrosis factor alpha

We investigated the ability of monocyte-conditioned medium (MCM), generated by monocytes cultured on plastic-immobilised immunoglobulin, to stimulate maturation of human monocyte-derived dendritic cells (DC). Earlier reports suggest that MCM is a strong inducer of irreversible DC maturation, whereas we find, that adding a small amount of lipopolysaccharide (LPS) to the MCM-generating cultures is required for the production of a DC-stimulatory MCM. Moreover, compared with addition of LPS directly to the DC cultures, stimulation via MCM cultures increases by several fold the DC-stimulatory potency of LPS. Maturation by this procedure is mediated mainly by tumour necrosis factor alpha secreted from monocytes during the medium-conditioning period.