We, and other groups, have recently demonstrated that B7-H1 is essentially involved in the induction and maintenance of T-cell anergy 25. There is abundant evidence that different viruses abuse B7-H1 to RG7422 turn-off effector T-cell responses 26–28. The findings of this study imply that B7-H1-mediated inhibition of T-cell responses is, at least partly, due to its capacity to contribute to the induction of IL-35 production. Yet, B7-H1 alone was not sufficient to induce IL-35, but required co-signaling via sialoadhesin. Sialoadhesin, a member of sialic acid binding lectin family of I-type lectins, preferentially
binds to sialylated carbohydrate structures (e.g. NeuAcα2,3-Gal) 29 and CD43 Small molecule library cell line has been recently described as ligand for sialoadhesin on T cells
30. Sialoadhesin is a frequently used marker for macrophages because it is typically not expressed on monocytes, lymphocytes, and DC. Yet, type-I IFN have lately been reported to up-regulate sialoadhesin on monocytes 30–33, but also on DC (our unpublished data). Thus, sensing of viral infections by DC leads to the up-regulation of the inhibitory receptor pair B7-H1 and sialoadhesin, which is critical for the induction of IL-35+ Treg. We have discovered this novel pathway of immune-regulation by analyzing the impact of HRV on DC. HRV are specialized pathogens and only infect humans with all the well-known symptoms of a cold. HRV infection is probably the most frequent human infectious disease, which indicates that the host/HRV relationship is highly evolved. HRV utilizes a variety of tricks to blunt our immune-system and induction of IL-35+ Treg may represent a further prominent immune-evasion mechanism 13. Since induction of B7-H1 and sialoadhesin expression on DC seem to be induced by many other viruses as well, it is intriguing to suggest that the induction of IL-35+ Treg is a general theme in viral infections. Cells were maintained in RPMI 1640 (Gibco, Paisley, Scotland), supplemented with 2 mM L-glutamine, 100 U/mL penicillin, 100 μg/mL streptomycin, and 10% FBS (Sigma-Aldrich, St. Louis, MO, USA). Recombinant human GM-CSF
and IL-4 were kindly provided by Novartis Research Institute (Vienna, Austria). The HRV-blocking reagent WIN 52035-2 14 was a kind gift from Arachidonate 15-lipoxygenase the Sterling-Winthrop Research Institute (Rensselaer, NY, USA) and was used at a final concentration of 5 μg/mL. IL-10,TGF-β and IL-12 were purchased from R&D Systems (Minneapolis, MN, USA). IFN-α (isoform 2c) was purchased from Boehringer Ingelheim (Vienna, Austria). Monensin, PMA, and Ionomycin were obtained from Sigma-Aldrich. Human IL35:Fc was obtained from Alexis Biochemicals (San Diego, CA, USA). The following murine mAb were generated in our laboratory: negative control Ab VIAP (against calf intestine alkaline phosphatase), 5-272 (B7-H1), 7-239 (CD169, sialoadhesin), VIT6b (CD1a).