Persistent Mixed Chimerism in Plasma Cells Following Allogeneic Stem-Cell Transplantation in Patients with Acute Leukemia Is a Surrogate Marker for Leukemia Relapse.

Chimerism within cellular subsets following allogeneic stem cell transplantation (SCT) has been studied extensively, yet there is only limited data on chimerism kinetics within plasma cells (PC) and its prognostic significance. In this study we prospectively analyzed the relative ratios of recipient and donor-derived PC in patients (pts) with acute leukemia following SCT from a sex-mismatched donor in relation to SCT outcomes. Bone-marrow preparations were evaluated by the Duet system (Bioview LtD, Israel) for combined cytogenetic/morphologic analysis. The system first scans marrow preparartions and saves all cell coordinates. PC are detected and marked automatically (and confirmed manually) by their morphology. The stain is then removed and FISH for X and Y markers is applied to the same slide. The system can then recover the coordinates of the marked PC and differentiate recipient and donor PC by their gender. Pts were studied at 1 and 3 months after SCT and then at 3 month intervals or as was clinically indicated. The study included 50 pts, 24 male and 26 female, median age 49 years (17–69). Diagnosis was AML (n=41), ALL (n=7) or CML in blastic crisis (n=2). The donor was HLA-matched sibling (n=28), 1-Ag mismatched related (n=6) or matched unrelated (n=16). Twenty-two had myeloablative and 28 had reduced-intensity conditioning. Thirty-six pts (72%) had recipient PC detected early after SCT, constituting 0.01–1.6% of bone marrow cells. This was often associated with low level recipient chimerism (<1%) among lymphocytes. Early detection of recipient PC was not related to age, gender, disease, donor type, conditioning regimen or the occurrence of acute GVHD, and had no prognostic significance in predicting survival. The estimated median time to disappearance of recipient PC was 12 months from SCT. There was a trend for early disappearance (before 6 months) of recipient PC among pts having unrelated or mismatched donor SCT (p=0.07), but there was no association with chronic GVHD. In 16 of the 36 pts with recipient PC they persisted beyond 6 months (and up to >18 months), in 10 they disappeared by this time period, 6 died before 6 months with recipient PC and 4 have insufficient follow-up. Persistence of recipient PC beyond 6 months was not associated with mixed-chimerism in lymphoid or other lineages at this stage. Marrow tests beyond 6 months post SCT are available in 30 pts of all 50 pts in this study. The outcome of 16 pts with recipient PC detected beyond 6 months was significantly inferior to 14 pts with no recipient PC at this stage. Among the 16 pts with recipient PC, 8 relapsed (5 systemic, 3 extra-medullary). In contrast, among the 14 pts with no recipient PC, only 1 relapsed. The 2-year disease-free survival from SCT was 35% (7–62), and 91 % (74–100), respectively (p=0.02). Donor derived PC were detected during the course in 27 pts. The estimated median time to first detection of donor PC was 6 months (1–15) and the median time to full donor chimerism in PC was 13 months (3–25). Unlike the kinetics of disappearance of recipient PC, engraftment kinetics of donor PC had no relation to SCT outcomes. In conclusion, recipient PC may persist for long duration after allogeneic SCT and are relatively resistant to conditioning and to allogeneic responses. Persistence of recipient PC beyond 6 months is a surrogate marker for ineffective GVL, even in pts with GVHD, therefore associated with increased risk for leukemia relapse.