Emergence of Dominant T-Cell Populations in Patients with CLL after Allogeneic Transplantation: Mediators for Gvl Effects?

Allogeneic stem cell transplantation (SCT) is the only known curative treatment for high-risk CLL. We have recently shown that minimal residual disease (MRD) monitoring can identify patients with graft versus leukemia (GvL)-induced disease response to either reduction of immunosuppression (IS) or to administration of donor lymphocyte infusions (DLI), suggesting that those patients are potentially cured by an ongoing immunologic antileukemic effect induced by donor immune cells (Leukemia 22:1377). It is uncertain, however, which cell population maintains this process; although T as well as NK-cell mediated effects are discussed. The present study addressed the question whether disease response upon immunomodulation after SCT is associated with the occurrence of dominant T cell clones.

Methods: 32 patients allografted for high-risk CLL who had MRD follow-up by clone-specific PCR or MRD-flow available were included in this investigation. We used the BIOMED T-cell receptor multiplex PCRs (TCR-PCR) to search for T cell clones which might be involved in the documented GVL effects. TCR rearrangements were sequenced and analyzed using the IMGT database.

Results: 16 of 32 patients showed MRD response after IS reduction or DLI. GVL-induced MRD clearance was associated with onset of chronic GVHD in almost all instances. Twenty-four different dominant TCR rearrangements could be identified in 15/32 patients by BIMOD TCR-PCR. Most of the T cell populations show rearranged gamma/delta TCRs suggesting that regulatory gamma/delta T cells might be involved in this process. TCR sequences employed were TRGV9 (13), TRGV2 (2) and TRGV1, TRGV4, TRGV8, TRGV10, TRGV11, TRBV5, TRBV6, TRBV12, TRBV15. In 4 patients with a potential productive TCR rearrangement (TRGV4+TRDV1, TRBV6, TRGV2, TRGV11+TRGV9) we were able to design a TCR-specific real-time PCR for quantitative follow-up of this clonal T cell population. This data was compared to flow cytometric monitoring of T-cell subpopulations and MRD kinetics post SCT. In those 4 patients we could demonstrate an inverse correlation of the kinetics of MRD and the kinetics of clonal T cell expansions. T cell clones emerging during this phase remained on a stable level throughout the whole follow-up in patients showing durable MRD negativity.

Conclusion: In CLL, MRD clearance after SCT is correlated to the emergence of dominant T cell clones, suggesting that GVL activity is based on allo- or CLL-specific T cell expansion. Further studies are needed to clarify the role of these T cell clones for GVL and GVHD development.