Detection of microchimerism after allogeneic blood transfusion using nested polymerase chain reaction amplification with sequence-specific primers (PCR-SSP): a cautionary tale.

In bone marrow transplantation, the detection of chimerism is an important adjunct to the repertoire of tests available for determining acceptance of the graft. In solid organ transplantation, there is currently intense interest in the role that chimerism plays in both short- and long-term host reactivity to the graft. Allogeneic blood transfusion has been associated with a subtle immunosuppressive effect in renal transplantation and chimerism is implicated as a possible mechanism for this effect. To assess the survival of allogeneic cells after blood transfusion or transplantation, we have developed a technique based on molecular typing for HLA class II alleles, which enables the detection of donor-derived cells in patients receiving blood transfusions. While developing this technology, we investigated why we and others observe false amplification. Sequencing of false products has shown that they arise from amplification of both pseudogenes and non-pseudogenes present in the DNA under test. Elucidation of this phenomenon allows the amplification of these false products to be predicted in any given combination and hence avoided by the judicious selection of primers. Validation has been achieved by following donor alleles after transfusion of blood containing defined numbers of leukocytes expressing selected mismatched antigens.