Efficacy and safety of anti-CD45-Saporin as conditioning agent for RAG deficiency

ABSTRACT Background Mutations in the RAG genes cause severe immunodeficiency, with a spectrum of phenotypes ranging from severe combined immunodeficiency to immune dysregulation. Hematopoietic stem cell transplantation is the only curative option, but a high risk of graft failure and poor immune reconstitution have been observed in the absence of myeloablation. Objectives To improve multi-lineage engraftment, we tested non-genotoxic conditioning with anti-CD45-Saporin (CD45-SAP). Methods Rag1-KO and Rag1-F971L mice, which represent models of SCID and of combined immune deficiency with immune dysregulation, respectively, were conditioned with CD45-SAP, CD45-SAP + 2Gy, 2Gy, 8Gy or no conditioning, and transplanted with Lineage-negative bone marrow cells from wild-type mice. Flow cytometry and immunohistochemistry were used to assess engraftment and immune reconstitution. Antibody responses to TNP-KLH were measured by ELISA, and presence of autoantibody was detected by microarray. Results Conditioning with CD45-SAP enabled high levels of multi-lineage engraftment in both Rag1 mutant models, allowed overcoming of B and T cell differentiation blocks and of thymic epithelial cell defects and induced robust cellular and humoral immunity in the periphery. Conclusions Conditioning with CD45-SAP allows multilineage engraftment and robust immune reconstitution in mice with either null or hypomorphic Rag mutations, while preserving thymic epithelial cell homeostasis. Clinical implications Conditioning with anti-CD45 antibody-drug conjugates may represent a novel and safe conditioning regimen for patients with RAG deficiency and other inborn errors of immunity.