Percent Patient NK and T Cells at Day 28 Predict Graft Outcome after Haploidentical Hematopoietic Cell Transplant for Sickle Cell Disease

The only definitive cure for sickle cell disease (SCD) is hematopoietic cell transplant (HCT). Donor options are often limited due to the requirement of having a human leukocyte antigen (HLA)-matched donor. A haploidentical HCT from a family member has the potential to increase the donor pool for HCTs in SCD patients; however, conditioning regimens remain toxic. Reduced-intensity conditioning has the potential to successfully deplete the host immune system, reducing the rate of graft failure while also avoiding toxic side effects of traditional regimens. Natural killer (NK) cells are among the first cell lineages to repopulate following HCT, so sufficient depletion is necessary to ensure that the patient's NK cells will not target the donor cells post HCT. The primary objective of this study was to determine if a high percentage of patient NK cells at day 28 post-transplant could predict successful engraftment versus graft failure in SCD patients that have received a haploidentical HCT.

Whole blood was drawn from SCD patients and their donors from two clinical trials (ClinicalTrials.gov Identifier: NCT02165007, NCT03077542) at baseline (before HCT) and at days 28, 60, 100, 180, and 365 following HCT. Peripheral blood mononuclear cells (PBMCs) were isolated via gradient-dependent lymphocyte separation and frozen for future batch analysis. Once samples from all time points were received, PBMCs were thawed and stained with fluorescent antibodies against CD3, CD4, CD8, CD45, CD56, iNKT, NKG2D, and HLA and analyzed by flow cytometry. HLA antibody optimization was achieved by screening for HLA mismatches between a patient and their haploidentical donor. Mann-Wilcoxon-Whitney rank tests were used to identify significant differences in various cell populations (CD3-CD56+ NK cells, CD3+ T cells) between patients with successful engraftment and those with graft failure. Areas under the receiver operating characteristic curve (AUCs) of the cell populations were used to determine which immune cell lineage held the highest potential to predict HCT graft failure at day 28. Statistical significance was established at p<0.05.

A total of 15 patients were analyzed in this study: 11 patients had successful engraftment and 4 had graft failure. There were no statistically significant differences in the pre-HCT % NK or % T cell values in those who engrafted versus those who failed to engraft. At day 28 post-HCT, significant differences were noted between the % NK cells of all lymphocytes (p=0.0151) and % T cells of all lymphocytes (p=0.0028), as well as %-patient NK cells (p=0.0029) and %-patient T cells (p=0.0055) between the engrafted population and the graft failure population [Figure 1, A,B]. NK cells of all lymphocytes had an AUC of 0.9038 (95% Confidence Interval, CI, 0.573-1.00). T cells of all lymphocytes had an AUC of 0.9385 (95% CI 0.827-1.00). The AUC for %-patient T cells was 0.9273 (95% CI 0.779-1.00). Lastly, the AUC for %-patient NK cells was 0.9773 (95% CI 0.909-1.00). This suggested that the %-patient NK cells ROC model had the highest potential to correctly classify engraftment vs graft failure.

This study showed the potential for using %-patient NK cells to predict graft failure at day 28 in SCD patients after haploidentical HCT, however, the sample size was limited. To independently validate these findings, analyses of specimens from a multisite clinical trial are being performed.

Bollard:BMS: Consultancy; Pfizer: Consultancy; Roche: Consultancy; SOBI: Other: DSMB member; Cellmedica: Patents & Royalties; Cabalatte Bio: Membership on an entity's Board of Directors or advisory committees; Mana Therapeutics: Current equity holder in private company, Current holder of stock options in a privately-held company, Other: scientific ci-founder and SAB member, Patents & Royalties: VSTs/TAA-T; Catamaran Bio: Current equity holder in private company, Current holder of stock options in a privately-held company, Other: scientific ci-founder and SAB member; Neximmune: Current equity holder in private company; Repertoire Immune Medicines: Current equity holder in private company.