Chimerism of Blood T-Cells and Leukemia Lineage Cells Following Myeloablative Hematopoietic Cell Transplantation Is a Risk Factor for Relapse and Chronic Graft-Versus-Host Disease

Background: Mixed chimerism of blood leukemia lineage cells has been reported to be highly predictive of relapse (Mattsson J et al. Leukemia 2001), but the utility of T-cell chimerism is controversial.

Methods: Chimerism of T-cells (CD3 ⁺) and leukemic lineage cells (CD13 ⁺/CD33 ⁺ for myeloid malignancies and CD19 ⁺ for B-lymphoid malignancies) was measured in the peripheral blood for 600 hematopoietic cell transplant (HCT) recipients at 3-months post-transplant. Conditioning was myeloablative (fludarabine+busulfan+4GyTBI) and GVHD prophylaxis was with ATG+CsA+MTX.

Results: Mixed (<95% donor) chimerism was present in 6% of patients in the leukemic lineage and 16% of patients in T-cells. Compared to patients with complete chimerism (≥95%), mixed chimerism predicted a significantly greater incidence of relapse (52% vs. 27%, P=0.044), and surprisingly, also a greater incidence of cGVHD (43% vs. 23%, P=0.028). Patients with mixed leukemic lineage chimerism also had poorer cGRFS (7% vs. 39%, P<0.001) and OS (29% vs. 55%, P<0.001). Mixed T-cell chimerism predicted a significantly greater incidence of relapse (46% vs. 24%, P<0.001) and lower cGVHD incidence (9% vs. 31%), with no difference in cGRFS or OS. The sensitivity/specificity/positive predictive value (PPV)/negative predictive value (NPV) of mixed leukemic lineage and T-cell chimerism for relapse were 65%/55%/35%/81% and 66%/57%/60%/79%, respectively. Sensitivity/specificity/PPV/NPV was similarly poor for cGVHD. In patients with complete leukemic lineage chimerism at 3-months, a ≥5% drop in donor chimerism at any future timepoint had sensitivity/specificity/PPV/NPV of 82%/79%/79%/82% for subsequent relapse.

Conclusion: In the setting of myeloablative conditioning and ATG-based GVHD prophylaxis, mixed chimerism at 3-months post-transplant is a risk factor for subsequent relapse. However, the utility of these measurements in guiding medical interventions may be limited by insufficient predictive values. Nevertheless, patients with mixed chimerism may be candidates for more intensive surveillance.