Differential expression of cytokines in human blood monocyte subpopulations

Cytokine expression was analyzed in CD14++ regular monocytes and in the novel subset of CD14+/CD16+ small monocytes. Biologic activity for tumor necrosis factor (TNF), interleukin-1 (IL-1), and IL-6 in the supernatant of elutriator-enriched, cell sorter-purified small monocytes was about 10-fold lower compared with regular monocytes when stimulated with lipopolysaccharide (LPS) for 12 hours. In CD14++ regular monocytes levels were 1,157 U x 10(-3)/mL, 158 U/mL, and 1,337 U/mL for TNF, IL-1, and IL-6, respectively. By contrast, CD14+/CD16+ small monocytes exhibited 137 U x 10(-3)/mL, 14 U/mL, and 60 U/mL for TNF, IL-1, and IL-6, respectively. Additional treatment with interferon-gamma enhanced production of TNF in both subsets, but CD14+/CD16+ small monocytes still exhibited lower levels. Stimulation of the monocyte subsets by platelet-activating factor gave the same pattern of results. Hybridization with 32P-labeled oligonucleotides specific for the respective cytokine messenger RNAs (mRNAs) showed a 10-fold lower prevalence of transcripts for TNF, IL-1, and IL-6, as well. By contrast, the constitutive expression of Glyceraldehyde-3-phosphate-dehydrogenase mRNA was 1.7-fold higher in the CD14+/CD16+ small monocytes. These data indicate that the novel subset of small monocytes is selectively suppressed in the expression of the cytokines TNF, IL-1, and IL-6, suggesting that these cells may comprise a deactivated type of cell. The expression of class II transcripts in the small monocytes is, however, similar to the regular monocytes, and the cell surface expression of class II protein about threefold increased. Thus, the novel subset of small monocytes appears to be a functionally distinct type of cell.