USE OF CD200-TRANSGENIC MICE AND DECOY CD200R TO DEFINE ROLE OF CD200:CD200R IN ALLOREJECTION

P1107 Aims: CD200 is a type I membrane glycoprotein, ubiquitously expressed on a variety of cells, including those important in inflammation and immunity. We and others have documented that CD200 delivers an immunosuppressive signal following engagement of a member of the CD200 receptor family, CD200R. We have constructed a CD200-transgenic mouse, where the CD200 gene is expressed under control of a doxycycline-inducible promoter. Alloimmune responses of lymphoid cells, or skin graft rejection of tissue, from these mice following exposure to doxycycline in vivo, was assessed to explore further the immunological effects of forced expression of CD200 in vivo. Methods: The mouse CD200 gene, linked to a GFP reporter under the control of a promoter with a Tetracycline Responsive Element (TRE), was introduced into fertilized embryonic cells (FVJ origin). CD200 transgenic founder mice were backcrossed to C57BL/6. The CD200 transgenic mice were mated with commercial transgenic mice constitutively expressing the rtTA, a transactivator of TRE, under control of a CMV promoter (also on a BL/6 background). Thererafter, exposure of the F1 (double transgenic) mice to doxycycline in the water supply resulted in activation of the TRE-CD200-GFP promoter by rtTA, and expression of both GFP (assessed by immunoflourescence and Western blots) and CD200 (assessed by PCR and Western blots). In the experiments to be reported, F1 mice from the N6 generation were exposed to doxycycline, and CD200 and GFP expression in different tissues was characterized. Spleen cells of these mice were used for allogeneic MLRs (in the presence/absence of doxycycline), and tail skin from individual mice used for tissue grafts to normal C3H recipients (again with/without further doxycyline in vivo). Results: Double-transgenic mice showed doxycycline-inducible expression of CD200 (and GFP) in multiple tissues, including spleen, thymus, liver, skin and heart. In MLR cultures initiated using these spleen cells as either responder (measuring lysis vs P815, following stimulation with BALB/c cells) or mitomycin-c treated stimulator (measuring lysis of EL4 target cells), forced expression of CD200 in the presence of doxycycline led to significant attenuation of immune responses compared with cultures initiated using cells from littermate control mice. In addition, skin grafts from CD200-expressing transgenic mice survived longer, and failed to induce alloimmunity in vivo (as assessed by direct lysis of EL4 targets using spleen cells from grafted mice) following grafting to naïve C3H recipients. Conclusions: Our data are consistent with the hypothesis that CD200 delivers an important immnosuppressive signal inducing hyporesponsiveness in vivo, which can be used in a model to promote prolongation of allograft survival.

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