Differential ability of human blood monocyte subsets to release various cytokines

We have shown that two human monocyte subsets can be isolated from the peripheral blood of healthy donors; these subsets possess different morphological, cytochemical, functional, and in vivo trafficking properties [1]. In this report, these two subsets were further characterized. One subset (intermediate monocytes, IM) has been shown to have significantly lower acid phosphatase activity and total cellular protein content as well as lower peroxidase activity when compared with another subset (regular monocytes, RM). The overall activation status of the two subsets (as determined by their alkaline phosphodiesterase activity) was identical. We also examined the capacity of these subsets to release various cytokines with or without polyriboinosinic and polyribocytidylic acid (Poly I:C) stimulation. There was no appreciable difference in their ability to release interferon (IFN), interleukin 1 (IL-1), and prostaglandin E (PGE) without stimulation, while IM produced slightly, but significantly, higher amounts of colony-stimulating factor (CSF) than RM. The amount of IFN released by IM in response to poly I:C was approximately three times higher than the amount of IFN released by RM. IL-1 was also released in higher amounts by IM than by RM in response to poly I:C. IM were also found to release more CSF than RM in response to poly I:C. In contrast, it was noted that IM secrete significantly less PGE response to poly I:C than do RM. These findings indicate that two purified human monocyte subsets, distinguishable by maturation markers, differ significantly in their ability to release various cytokines after stimulation; this difference may be relevant to potential in vivo roles of these immunoregulatory cells.