CD107a and CD107b

Dear all,
We are trying to use the CD107a and CD107b as markers for cytolytic activity of activated
functional CD8 T cells as part of an assay for monitoring the immune competence of
transplant patients to CMV. We use MHC class I tetramers to gate out the CMV specific CD8
T cells and look for IFN-gamma and CD107a/b expression after activating it with the CMV
peptide. 
The problem we are running in to is that we have the % of CD8 T cells that are expressing
CD107a/b is more in the unstimulated sample rather than the activated sample.
Any suggestions to negate or correct this problem would be highly appreciated. 
Warm regards,
Vijay
******************************************
Vijay Nandakumar, 
R&D Associate,
Cellular and Molecular Immunology Laboratory
Mayo Clinic
200, 1st street SW,
Rochester, MN, 55905
*****************************************