To investigate lineage-specific chimerism of plasma cells after allogeneic transplantation by real-time PCR based on bi-allelic single-nucleotide polymorphism (SNP) or, in case of female-to-male transplantation, on the detection of the DFFRY-gene, 48 samples from bone marrow samples and peripheral blood from 34 non-myeloma patients were analyzed at different times after transplantation. Plasma cells and T-cells were obtained with CD138- or CD3-microbeads and MiniMACS columns. The median chimerism for T-cells at day +100 was > 99.9 % and remained stable on day +180 and one year after transplantation. In contrast, the median donor-plasma-cell chimerism at day +100 was 95.5 %, at day +180 98,6 %, at day +360 99.8 %, and two or more years after transplantation > 99.9 %. There was a trend for more patients with full donor-plasma-cell chimerism at day 180 who received standard conditioning in comparison to reduced-intensity conditioning (56 % vs. 28 %, p=0.3) and who experienced acute graft-versus-host disease (43 % vs. 18 %, p=0.2).

To evaluate whether the quantitative measurement of donor-plasma-cell chimerism after allogeneic stem cell transplantation for patients with multiple myeloma might be a useful tool to determine minimal residual disease, we investigated 48 samples from 21 myeloma patients at different times after allogeneic stem cell transplantation, and we compared it with immunofixation and, in some cases, with molecular methods by using patient-specific primers. All patients with molecular remission measured by patient-specific primers had a plasma-cell chimerism of more than 99.5 %. Furthermore, in all but one patient plasma-cell chimerism of more than 99.3 % was associated with a negative immunofixation. In six patients with positive immunofixation after allogeneic stem cell transplantation treatment with bortezomib or donor-lymphocyte infusion was started and monitored by plasma-cell chimerism and immunofixation. All patients experienced an increase of donor-plasma-cell chimerism with negative immunofixation if chimerism exceeded > 99.3 %. Dilution experiments with the TIB-196 cell line showed sensitivity of 104 using real-time PCR and SNPs, 105 in case of Y-PCR and 105 for nested PCR with patient-specific primers. We conclude that plasma-cell chimerism after allogeneic stem cell transplantation is delayed in comparison to T-cell chimerism, suggesting a reduced graft versus hematopoiesis effect on plasma cells. Quantitative measurement of plasma cells after allogeneic stem cell transplantation with highly sensitive real-time PCR allows monitoring of residual host-tumor cells in patients with multiple myeloma and allows guiding adoptive immunotherapy strategies to enhance remission status and to prevent relapse.

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