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Published online before print April 5, 2007

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(Journal of Leukocyte Biology. 2007;82:152-160.)
© 2007 by Society for Leukocyte Biology

Circulating CD14^–CD36⁺ peripheral blood mononuclear cells constitutively produce interleukin-10

Immunology Program, Division of Basic Medical Sciences, Faculty of Medicine, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada

¹ Correspondence: H1809-Immunology, Faculty of Medicine, Memorial University of Newfoundland, 300 Prince Philip Drive, St. John’s, Newfoundland, Canada, A1B 3V6. E-mail: mgrant{at}mun.ca

The impact of immune regulatory imbalance covers surprising physiological breadth. Although dominance of anti-inflammatory cytokines such as IL-10 is associated with reduced immune responsiveness and susceptibility to persistent infection, conditions such as cardiovascular disease and diabetes are linked to chronic inflammation and lower IL-10 levels. An appropriate threshold for immune activation is critical for optimal protection from infection and conversely, from short- and long-term side-effects of immune effector mechanisms. To assess the possibility that IL-10 plays a role in setting this threshold and that healthy maintenance of immune silence may involve low-level immune suppression, we sought out and characterized human peripheral blood cells constitutively producing the immunosuppressive cytokine IL-10. We determined the surface phenotype of circulating PBMC constitutively producing IL-10 by surface and intracellular flow cytometry and visualized their ultrastructure by electron microscopy. The frequency of IL-10-producing and -secreting cells was estimated by ELISPOT and flow cytometry. Up to 1% of PBMC constitutively produce IL-10. These CD14^–CD36⁺CD61⁺ nonadherent cells expressed general markers of hematopoietic and progenitor cells (CD45 and CD7) but no stem cell, T cell, B cell, NK cell, monocytes or dendritic cell markers. Inflammation-associated TLRs were also absent. The IL-10-producing cells had prominent nuclei, multiple mitochondria, and abundant rough endoplasmic reticulum. Healthy individuals have PBMC constitutively producing IL-10. Although the lineage of these cells remains unclear, their properties and frequency suggest a potential role in homeostatic or innate immune suppression.

Key Words: immune regulation • hematopoietic cells • cytokines