In an ex vivo experiment, populations of splenocytes harvested from allogeneic transplant-challenged mice treated with rapamycin (an immunosuppressant routinely used to facilitate allogeneic transplants) were rendered significantly less active as responder cells in a primary allo-mixed lymphocyte culture assay (measured on the vertical axis as the percentage of target mastocytoma cells lysed by cytotoxic T lymphocyte induction) when the mice in question received concurrent treatment every third day with PEG-CCS13 (red bar), as well as with the natural agonist fragment, CD200Fc (a large, antibody-like protein fragment), but not with vehicle alone (PBS), rapamycin alone (Rapa Only) nor with a PEGylated, control aptamer comprising a random sequence (PEG-cApt). These results indicate that mice subjected to allogeneic skin grafts that normally trigger potent immunorejection instead experience significant dampening of the immune response when treated with CD200R1 agonists such as PEG-CCS13.