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gC1q Receptor Ligation Selectively Down-Regulates Human IL-12 Production through Activation of the Phosphoinositide 3-Kinase Pathway¹

Stephen N. Waggoner^*,, Michael W. Cruise^*,, Rachel Kassel^*, and Young S. Hahn^2,*,,

^* Beirne Carter Center for Immunology Research, Department of Microbiology and Department of Pathology, University of Virginia, Charlottesville, VA 22908

gC1qR, a complement receptor for C1q, plays a pivotal role in the regulation of inflammatory and antiviral T cell responses. Several pathogens, including hepatitis C virus, exploit gC1qR-dependent regulatory pathways to manipulate host immunity. However, the molecular mechanism(s) of gC1qR signaling involved in regulating inflammatory responses remains unknown. We report the selective inhibition of TLR4-induced IL-12 production after cross-linking of gC1qR on the surface of macrophages and dendritic cells. Suppression of IL-12 did not result from increased IL-10 or TGF-, but was dependent on PI3K activation. Activation of PI3K and subsequent phosphorylation of Akt define an intracellular pathway mediating gC1qR signaling and cross-talk with TLR4 signaling. This is the first report to identify signaling pathways used by gC1qR-mediated immune suppression, and it establishes a means of complement-mediated immune suppression to inhibit Th1 immunity crucial for clearing pathogenic infection.

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