Kirubel Belay

"Characterizing the impact of post-transplant cyclophosphamide on alloreactive T-cell clones in HCT patients"

Graft versus Host Disease (GVHD) is one of the largest barriers to successful hematopoietic cell transplants (HCT). GVHD manifests when immunocompetent donor lymphocytes recognize recipient cells as foreign and react against them. Thus, minimizing the donor T-cell response to recipient antigens is imperative to successful transplants. High dose post-transplant cyclophosphamide (PTCy) has shown to be successful in reducing GVHD in allogeneic HCT and showed the ability to induce tolerance even in the setting of mismatched HLA. However, it is not yet clear if PTCy is discriminatorily deleting alloreactive T-cells or subduing the entire immune repertoire. Using mixed lymphocyte reactions and high-throughput sequencing of the TCRB chain CDR3 region, we tracked donor-derived alloreactive T-cell clones post-transplant in blood and tissue samples from HCT recipients. We compared the productive frequencies of both alloreactive and non-alloreactive clones in patients who received PTCy, and those who received other immunosuppressants. We found a sharp decline in alloreactive clones immediately following PTCy. Simultaneously, we observed an expansion of non-alloreactive clones approximately 3 to 4 weeks post-transplant. Patients who received other immunosuppressants instead saw a trend where non-alloreactive clones followed the same trajectory as alloreactive clones. We also observed similarities in trends of alloreactivity between patients who developed GVHD and received PTCy, and patients who did not develop GVHD. However, the patients who developed GVHD and did not receive PTCy did not have distinct similarities with either group. This suggests that high dose cyclophosphamide administered 3 days post-transplant may be selectively targeting alloreactive T-cells, possibly leading to tolerant immune repertoire reconstitution.