Murine plasmacytoid dendritic cells initiate the immunosuppressive pathway of tryptophan catabolism in response to CD200 receptor engagement.

In this study, using a soluble CD200-Ig fusion protein, we provide evidence that murine dendritic cells (DCs) possess a functional CD200R, whose engagement results in the reinforcement or appearance of immunosuppressive properties in these cells. In particular, the plasmacytoid subset (CD11c+B220+120G8+) of splenic DCs (pDCs) is induced by CD200-Ig to express the enzyme IDO, which initiates the tolerogenic pathway of tryptophan catabolism. As a result, pDCs are capable of suppressing Ag-specific responses in vivo when transferred into recipient hosts after treatment with CD200-Ig. IDO induction in pDCs through CD200R engagement requires type I IFNR signaling. Although the release of IFN-alpha may contribute to the full expression of CD200-Ig activity, autocrine IFN-alpha is unlikely to mediate alone the effects of CD200R engagement. These data prospect novel functions for both pDCs and the CD200-CD200R pair in the mouse. At the same time, these data underscore the possible unifying role of the IDO mechanism in immune tolerance. Copyright 2004 The American Association of Immunologists, Inc.