Relevance of Chimerism Testing

Chimerism is a vital part of post-transplant testing after Hematopoietic Stem Cell Transplantation (HSCT). Chimerism monitoring can be used to obtain accurate information on a patient’s’ engraftment status post stem cell transplant by quantitatively determining the proportion of donor and recipient derived cells in the patient post-transplant. This requires the use of DNA markers that are informative enough to distinguish donor derived DNA from recipient derived DNA. Most H&I laboratories use Short Tandem Repeats (STR’s) for chimerism monitoring. STR’s are short sequences of DNA, distributed throughout the genome, which are repeated in tandem a variable number of times. The number of repeats of different STR markers varies between individuals, from 4 to 50 repeats for some STRs, giving a highly polymorphic system that can be used to uniquely identify donor derived DNA from patient derived DNA. With the exception of monozygotic twins, careful selection of a number of STR markers will enable most individuals to be uniquely identified and this technique has been in use in forensic science for just such a purpose. Examples of STR markers include TH01, a tetra nucleotide repeat of AATG located on chromosome 11 and FES/FPS a tetra nucleotide repeat of AAAT located on chromosome 15. Chimerism is important clinically as it can highlight when engraftment has taken place and also when it may have started to fail. Treatment decisions can be based on the results of chimerism to try to rescue a failing graft. In the case of rising patient chimerism after stem cell transplant the clinicians may decide to lower the immunosuppression of the patient to try and promote engraftment. Other investigations which could be undertaken would be a post-transplant antibody screening of the patient in case the patient has produced HLA antibodies against any mismatches as may be the case with a cord blood or mismatched adult stem cell transplant. If this is the case the patient could undergo plasma exchange to reduce the antibody titre.

Principles of Chimerism Testing by Short Tandem Repeat (STR)

Chimerism monitoring can be used to obtain accurate information on a patient’s engraftment status post stem cell transplant by quantitatively determining the proportion of donor and recipient derived cells in the patient post-transplant. H&I laboratories typically report chimerism results as % donor chimerism. A 100% donor chimera implies complete engraftment. A 0% donor chimerism implies no donor engraftment with all other percentages reported as mixed chimerism showing the proportion of donor engraftment. A longitudinal study of chimerism is of more value than a single static result and the H&I laboratory would typically test at agreed intervals and report a history of the chimeric status of the patient since transplant rather than a single test report. Relative changes in the magnitude of donor chimerism provides key information which helps clinicians to intervene and to monitor the patients’ response to such intervention. Intervention options include changes in immunosuppression regimes and donor lymphocyte infusion (DLI). Many H&I laboratories offer lineage specific chimerism monitoring, separating T and B cells from myeloid cells. This approach proves useful in some cases of mixed chimerism, where one cell line may dominate and mask clinically significant changes in other cell subsets.

The use of chimerism monitoring has also proved useful in the case of double cord transplants where it is possible to see a mixed chimera consisting of patient and one or both cords early in the post-transplant period before one cord eventually expands to 100% present in the patient.