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Identification of a Subpopulation of Macrophages in Mammary Tumor–Bearing Mice That Are Neither M1 nor M2 and Are Less Differentiated

¹ Department of Microbiology and Immunology, University of Miami School of Medicine, ² Sylvester Cancer Center, Miami, Florida; ³ Department of Biomedical Sciences, Florida Atlantic University, Boca Raton, Florida; and ⁴ Institute of Molecular Cancer Research, University of Zurich, Zurich, Switzerland

Requests for reprints: Diana M. Lopez, University of Miami School of Medicine, P.O. Box 016960 (R-138), Miami, FL 33101. Phone: 305-243-6632; Fax: 305-243-4409; E-mail: dlopez{at}med.miami.edu.

Key Words: M1 and M2 macrophages • inflammation • tumor-induced immune suppression

Systemic and local immune deficiency is associated with cancer, and the role of M2 tumor-associated macrophages in this phenomenon is well recognized. However, the immune status of macrophages from peripheral compartments in tumor hosts is unclear. Peritoneal macrophages (PEM) are derived from circulating monocytes and recruited to the peritoneal cavity where they differentiate into macrophages. We have previously shown that PEMs from mice bearing D1-DMBA-3 mammary tumors (T-PEM) are deficient in inflammatory functions and that this impairment is associated with diminished expression of transcription factors nuclear factor B and CAAT/enhancer-binding protein. We now provide evidence that T-PEMs display neither M1 nor M2 phenotypes, yet exhibit deficiencies in the expression of several inflammatory cytokines and various proinflammatory signaling pathways. Moreover, due to nuclear factor B down-regulation, increased apoptosis was observed in T-PEMs. We report for the first time that macrophage depletion is associated with increased macrophage progenitors in bone marrow. Furthermore, T-PEMs have a lower expression of macrophage differentiation markers F4/80, CD68, CD115, and CD11b, whereas Gr-1 is up-regulated. Our results suggest that T-PEMs are less differentiated and represent a newly derived population from blood monocytes. Lastly, we show that transforming growth factor-β and prostaglandin E₂, two immunosuppressive tumor-derived factors, may be involved in this phenomenon. [Cancer Res 2009;69(11):4800–9]