Double Haploidentical Hematopoietic Stem Cell Transplantation (HSCT) Results In Successful Engraftment of Bone Marrow From Both Donors without Graft Versus Host or Graft Versus Graft Effects.
Authors: Biswas Chandra , Rittenberg Daniel , Luo Xiaoling , Sauter Christopher , Grosso Dolores , Flomenberg Neal , Alpdogan Onder 
Published: October 2019

Abstract
Abstract 1453
The use of haploidentical donors extends the potential clinical application of HSCT. However, relapse of resistant malignancy contributes to low success rates in high risk patients. Relapse may be due to the ability of leukemic cells to immunologically escape a single donor's GVL effects. We hypothesized that the use of two haploidentical donors, each targeting a different recipient haplotype, will increase anti- leukemia activity after double haploidentical SCT. We performed murine studies to establish new single haploidentical (SH) and double-haploidentical (DH) murine models that mimic the possible scenarios which might be encountered clinically rather that using more traditional Parent → F1 models.
We first established a haploidentical transplant model using two different hybrid mouse strains as donor and recipient in the experiments. Lethally irradiated B6CBAF1 (H2Kb/k) recipients were transplanted with T cell depleted (TCD) bone marrow (BM) from B6D2F1 (H2Kb/d) donors. Recipient mice harvested at days 28, 42 and 56, showed more than 90% donor cell engraftment, including donor derived lymphopoiesis and myelopoiesis, without evidence of graft versus host disease. Subsequently, lethally irradiated B6CBAF1 (H2Kb/k) recipients were transplanted with TCD-BM from two haploidentical donors (DH model) including B6SJF1 (H2Kb/s) (donor 1 - D1) and B6D2F1 (H2Kb/d) (donor 2 - D2). We observed recipients for 90 days and all mice survived without evidence of GVHD or weight loss. Analyses of blood collected retro-orbitally at day 90 revealed that recipients of DH transplants had significantly higher WBC and neutrophil counts than recipients of SH HSCT from either D1 or D2 respectively. DH recipients consistently showed successful engraftment with mixed chimerism in both bone marrow and spleen. There was no difference in thymopoiesis, B cell and myeloid cell reconstitution compared to SH transplants. In contrast, the number of splenic T cells was higher in SH recipients of D1 marrow (B6SJF1). We then explored the effects of low dose T cell infusions (1×105) on chimerism of donor cells. Low dose T cell infusion from either D1 or D2 did not affect the BM cellularity, but did increase the degree of dominance of that donor's cells in the BM and spleen.
A similar outcome was observed when this study was extended to other models such as B6C3 + C3D2F1 → B6D2F1 and B6CBAF1 + B6SJF1 → CB6F1 models where all recipients of DH transplants survived without evidence of GVHD.
Recipients of TCD DH transplants were challenged with P815 tumor cells. We used B6SJF1 (D1) + B6CBAF1 (D2) → B6D2F1 model in tumor experiments. Interestingly, recipients of TCD-DH transplants exhibited a significantly better survival than recipients of D1 SH or D2 SH transplants. However, after a low dose T cell infusion (1 ×105), recipients of D2 BM survived significantly better than recipients of D1 BM. In contrast to the TCD model, recipients of DH BM + DH T cells show similar probability of survival with recipients of D2 SH BM + D2 SH T cells.
We conclude that TCD DH HSCT results in successful engraftment of both types of BM cells. Additionally, infusion of low dose haploidentical T cells improves the anti-tumor effect without stimulating GVHD. Double haploidentical HSCT may be an ideal platform to enhance GVL effects after transplantation.

Disclosures:
No relevant conflicts of interest to declare.