Chimerism

The Chimerism investigation is carried out to monitor the engraftment of CD3 cells, myeloid cells and CD19 cells following haemopoietic stem cell transplant. Each lineage must be requested separately. The microsatellite regions which the individual primer sets are designed to amplify, consist of repetitive sequences (short tandem repeats STRs) of DNA 3-7 base pairs in length. These STRs are well distributed throughout the human genome and are highly polymorphic. There are many advantages of STR typing. It is more tolerant of the use of degraded DNA templates than other typing methods because the amplification products are less than 500bp long. STR typing is also amenable to a variety of rapid DNA purification techniques, which are compatible with PCR but do not provide enough DNA of appropriate quality for Southern blot-based analyses. Donor and Pre-transplant recipient samples must be obtained before the Post transplant sample can be analysed. The donor and recipient samples are run in conjunction with all post transplant specimens (BM, PB or fractionated cells) to illustrate the individual polymorphic alleles which distinguish the donor population from the recipient. Analysis of chimerism is valuable in assessing patients response to transplantation. Observing a complete donor profile in the post transplant samples indicates a successful transplant. Chimeric profiles, (the presence of both donor and recipient allelic peaks) depending on the donor to recipient ratio (%), may indicate a less successful engraftment or progression to relapse. Following analysis, donor and recipient profiles can be semi-quantified in the post transplant sample by obtaining the values for the areas underneath respective peaks and calculating the percentages of each. A value is obtained for each of the informative genes and the mean is calculated to give a more accurate indication of the recipient status. The 16 gene profiles for each patient are printed and handed to the consultant haematologist at least 24 hours before the weekly chimerism meeting, where the individual cases are discussed with the BMT co-ordination team.