Rapid and Safe T Cell Immune Reconstitution By T Cell Progenitor Injection Following Haploidentical Transplantation for Severe Combined Immunodeficiency (SCID)
Authors: Moshous Despina , Magrin Elisa , Winter Sarah , Fournier Benjamin , Castelle Martin , Vendrame Laurence , Chatenoud Lucienne , Picard Capucine , Neven Bénédicte , Magnani Alessandra , Gaudeaux Pierre , Soheili Tayebeh-Shabi , Andre Isabelle , Simons Laura , Cavazzana Marina 
Published: November 2021

Abstract
Severe Combined Immunodeficiencies (SCID) are defined by a complete absence of T lymphocytes in the blood and lymphoid organs, with variable defects in other WBC subsets depending on the gene defect. From a clinical perspective SCIDs are characterized by early development of life-threatening infections accounting for early death if untreated. The treatment of choice is allogeneic HSCT with very high success rates if a HLA identical sibling (MRD) or unrelated donor (MUD) is used. However, due to the scarcity of matched-related donors, SCID can benefit from haploidentical HSCT. In contrast to the continuous improvement of HLA compatible donor transplantations, no significant improvements have been obtained over the last twenty years for haploidentical HSCT. The profound immunodeficiency during the first months following haploidentical HSCT exposes patients to opportunistic viral, bacterial and fungal infections, which account for approximately 30-40% of the transplant related mortality (TRM). The rapid restoration of the T-cell compartment is the main aim of stem cell therapy in this setting. To this end we have recently set up a phase I/II clinical trial (ClinicalTrials.gov Identifier: NCT03879876) aiming to accelerate the immune reconstitution by injection of ex vivo generated Human T lymphoid progenitors (ProTcell TM) following haploidentical HSCT. T cell progenitors in this trial are generated in vitro within 7 days from mobilized peripheral blood (mPB) CD34 + hematopoietic stem and precursor cells (HSPCs) using our Notch ligand Delta-like 4 GMP culture platform so called SMART Immune's SMART101 product.
This open-label, non-randomized study evaluates safety and efficacy of the SMART101 injection following CD34 + selected, haploidentical HSCT in SCID patients and is designed as a dose-escalation study comprising 6 doses of the SMART101 product obtained from the patient's haploidentical stem cell graft. The aim of this protocol is to define the highest efficacy dose without any toxicity.
The conditioning regimen is based on Busulfan and Fludarabine according to IE-WP/EBMT guidelines with upfront administration of ATG to prevent graft rejection. Tight monitoring of ATG serum levels is applied in order to assure injection of SMART101 when ATG is below the lymphotoxicity threshold.
Here we report the results of the first two SCID patients. P1 presented a homozygous Artemis deficiency. At diagnosis he had an ALC of 341/µl, with complete absence of T cells (CD3 + &lt; 4/µl, CD4 + &lt; 1/µl, CD8 + &lt; 2/µl) and B cells (CD19 + 0/µl). NK cells were present in the normal range for age (CD16 +CD56 + 331/µl).
In the absence of an HLA compatible donor, the patient`s father was chosen as haploidentical stem cell donor.
P1 received upfront ATG (5 mg /kg total dose), Busulfan (AUC of 16058 microM.min) and Fludarabine (160 mg/m²). He received Defibrotide prophylaxis from D0 until D+21, as well as Ursodeoxycholic acid until D+80. The CD34 + immunoselected graft contained 1.04 x 10 8 nucleated cells/kg with 24.15 x10 6 CD34 + cells/kg and 4000 CD3 + cells/kg on D0. After ATG monitoring 0.12x10 6 Smart101 cells were administered at D+14 post- HSCT.
In the follow-up P1 didn't develop any acute or chronic SAEs, no acute or chronic GVHD, and no infection. He was discharged at D+121 post HSCT.
The day +100 post transplantation CD4 + cell count/microliter exceeded 10 times the CD4 + count of our historical cohort of RAG1/2 or Artemis deficient patients transplanted with haploidentical HSCT alone following the same conditioning regimen.
At 6 months post HSCT this difference remains important (851 versus 300 CD4 + cells/µl); Ig replacement therapy could be stopped as early as 9 months post transplantation and vaccinations have been started. At last follow up; almost 14 months post HSCT P1 is alive and well.
P2 had an undefined molecular SCID diagnosis. She has been treated with the same conditioning regimen and received the second dose of 0.2x10 6 CD7 + cells, but unfortunately died from severe VOD emphasizing the need to replace chemotherapy with less toxic myeloablative agents.
The preliminary results obtained after injection of Human T lymphoid progenitors in P1 are encouraging. While deserving confirmation in larger numbers of patients they could represent an important step forward in improving the outcome of haploidentical HSCT for SCID.

Disclosures
Cavazzana: Smart Immune: Other: co-founder.