Oligoclonal T Cells Associated with Gvhd Following Allogeneic Stem Cell Transplantation Conditioned with Thymoglobulin and Reduced Intensity Total Body Irradiation.
Authors: Manjili Masoud , Roberts Catherine H , Kmieciak Maciej , Gowda Madhu S , Ferreira-Gonzalez Andrea , Archer Kellie , Song Shiyu , Chung Harold M , McCarty John M. , Toor Amir 
Published: October 2019

Abstract
Abstract 4662
Patients undergoing unrelated donor stem cell transplantation following reduced intensity regimens are prone to acute graft vs host disease (GVHD). In vivo T cell depletion with rabbit anti-thymocyte globulin (r-ATG, Thymoglobulin, Genzyme inc. Cambridge MA) is consistently associated with reduced risk of acute GVHD however poor T cell reconstitution seen with current schedules results in a high incidence of opportunistic infections and relapse. We report data on immune reconstitution in patients participating in an ongoing clinical trial testing a novel conditioning regimen for allogeneic GCSF-mobilized blood stem cell transplantation. Patients were randomized to receive conditioning with either 7.5 or 5.1 mg/kg of r-ATG in divided doses between days -9 and -7, followed by 450 cGy total body irrradiation (TBI) in 3 fractions on day -1 and 0. GVHD prophylaxis was with tacrolimus (day -3 to 120) and mycophenolate mofetil (day 0-30). So far 10 heavily pre-treated (median number of prior therapies 4, prior autologous SCT n=5) patients have been transplanted; 6 from unrelated donors (1 bone marrow), 3 from matched related donors and 1 from an HLA-A mismatched sibling. Diagnosis includes MM (4), NHL (3), and CLL/PLL (3). Median patient age is 57 years. No patients have developed acute GVHD in the first 90 days. All patients achieved prompt engraftment of neutrophils and have demonstrated sustained complete myeloid donor chimerism (median <1% recipient DNA) at 3-6 months post transplant. NK cell recovery is prompt (mean±SD absolute CD56+ cell count 177±85/μL at day 30) and sustained (184±116 at day 90). T cell subset recovery is modest (absolute CD3+ cell count 861±934/μL at day 90) with predominantly cytotoxic T cells (CD3+/4+ cell count 143±116 and CD3+/8+ cell count 708±837). T cell chimerism at day 90 is mixed with either donor ('10% recipient DNA, n=5) or recipient dominance (>10% recipient DNA, n=3). Patients demonstrating dominant donor T cell chimerism at day 90 went on to develop either delayed onset acute GVHD (n=2/8 evaluable) or chronic GVHD (n=2/8) after withdrawal of immunosuppression. Patients demonstrating mixed T cell chimerism with recipient dominance did not develop chronic GVHD; one of these patients has relapsed, following an HLA-A mismatched SCT from his brother, and though he had predominantly recipient derived T cells, his granulocytes were completely donor derived indicating graft tolerance. T cell receptor beta locus was examined by RT-PCR for oligoclonality in all the donor-recipient pairs at baseline, day 90 and at onset of GVHD. Patients with GVHD demonstrated high level of expression of TCR V beta 23 and 24 (n=1/4), 11 (n= 1/4), 18 (n= 1/4), or 11 and 18 (n= 1/4) exclusively, in addition to TCR V beta 14, 16, 17, 22. The latter loci were also expressed in patients who had no GVHD with mixed T cell chimerism; this group of patients also expressed TCR V beta 4 (n=2/2), 13 and 19 (n=1/2) exclusively. All but one of the patients expressed the majority of TCR V beta loci at day 90 (with the exceptions noted above) indicating early polyclonal T cell recovery following transplantation. Asymptomatic CMV and EBV reactivation requiring therapy developed in one patient each. No patients have developed invasive fungal infections. In conclusion conditioning with Thymoglobulin and reduced intensity TBI results in stable myeloid engraftment in patients receiving unrelated and alternative donor transplants. In this small group of patients, GVHD appears to be associated with emergence of oligoclonal T cell populations which in the future may be selectively depleted ex vivo to allow engraftment without risk of chronic GVHD.

Disclosures:
McCarty: Celgene: Honoraria; Genzyme: Honoraria. Toor:Genzyme: Research Funding.