Platelet gene therapy induces robust immune tolerance even in a primed model via peripheral clonal deletion of antigen-specific T cells

Platelet-targeted gene therapy induces immune tolerance in hemophilia and beyond

Blood platelets have unique storage and delivery capabilities. Platelets play fundamental roles in hemostasis, inflammatory reactions, and immune responses. Beyond their functions, platelets have been used as a target for gene therapy. Platelet-targeted gene therapy aims to deliver a sustained expression of neo-protein in vivo by genetically modifying the target cells, resulting in a cure for the disease. Even though there has been substantial progress in the field of gene therapy, the potential development of immune responses to transgene products or vectors remains a significant concern. Of note, multiple preclinical studies using platelet-specific lentiviral gene delivery to hematopoietic stem cells in hemophilia have demonstrated promising results with therapeutic levels of neo-protein that rescue the hemorrhagic bleeding phenotype and induce antigen-specific immune tolerance. Further studies using ovalbumin as a surrogate protein for platelet gene therapy have shown robust antigen-specific immune tolerance induced via peripheral clonal deletions of antigen-specific CD4- and CD8-T effector cells and induction of antigen-specific regulatory T (Treg) cells. This review discusses platelet-targeted gene therapy, focusing on immune tolerance induction.

Pre-existing anti-factor VIII immunity alters therapeutic platelet-targeted factor VIII engraftment following busulfan conditioning through cytotoxic CD8 T cells

BACKGROUND

We previously demonstrated that busulfan preconditioning enabled sustained therapeutic platelet-derived factor VIII (FVIII) expression in naïve FVIII^null mice transplanted with 2bF8-transduced Sca-1⁺ cells. However, in mice with pre-existing inhibitors, platelet-FVIII expression was lost.

OBJECTIVE

In this study, we aimed to describe the mechanism of this platelet-FVIII loss.

METHODS

We monitored platelet-FVIII expression in FVIII^null mice that were immunized with rhFVIII to induce inhibitors and subsequently conditioned with busulfan before whole bone marrow transplantation or Sca-1⁺ hematopoietic stem cell transplantation (HSCT) from 2bF8 transgenic (2bF8^Tg) mice. Busulfan with or without antithymocyte globulin or anti-CD8 antibody was employed before 2bF8^Tg HSCT. Interferon gamma-ELISpot assay was used to assess which subset of cells was the target in platelet-FVIII loss. B-cell-deficient homozygous mutant mice were used to determine whether platelet-FVIII loss in FVIII-primed mice was mediated by antibody-dependent cellular cytotoxicity.

RESULTS

Platelet-FVIII expression was sustained in 2bF8^Tg bone marrow transplantation but not in 2bF8^Tg HSCT recipients. CD8 T-cell depletion in addition to busulfan preconditioning restored platelet-FVIII expression in 2bF8^Tg-HSCT recipients. ELISpot analyses showed that FVIII-primed CD8 T cells were efficiently restimulated by 2bF8^Tg-Sca-1⁺ cells and secreted interferon gamma, but were not stimulated by 2bF8^Tg platelets/megakaryocytes, suggesting that 2bF8^Tg-Sca-1⁺ cells are targets for FVIII-primed CD8 T cells. When 2bF8^Tg-Sca-1⁺ cells were transplanted into FVIII-primed homozygous mutant mice preconditioned with busulfan, no FVIII expression was detected, suggesting that antibody-dependent cellular cytotoxicity was not the mechanism of platelet-FVIII loss in FVIII-primed mice.

CONCLUSION

Pre-existng immunity can alter the engraftment of 2bF8^Tg-Sca-1⁺ cells through the cytotoxic CD8 T-cell-mediated pathway. Sufficient eradication of FVIII-primed CD8 T cells is critical for the success of platelet gene therapy in hemophilia A with inhibitors.

Targeting transmembrane-domain-less MOG expression to platelets prevents disease development in experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is a chronic inflammatory autoimmune disease of the central nervous system with no cure yet. Here, we report genetic engineering of hematopoietic stem cells (HSCs) to express myelin oligodendrocyte glycoprotein (MOG), specifically in platelets, as a means of intervention to induce immune tolerance in experimental autoimmune encephalomyelitis (EAE), the mouse model of MS. The platelet-specific αIIb promoter was used to drive either a full-length or truncated MOG expression cassette. Platelet-MOG expression was introduced by lentivirus transduction of HSCs followed by transplantation. MOG protein was detected on the cell surface of platelets only in full-length MOG-transduced recipients, but MOG was detected in transmembrane-domain-less MOG_1-157-transduced platelets intracellularly. We found that targeting MOG expression to platelets could prevent EAE development and attenuate disease severity, including the loss of bladder control in transduced recipients. Elimination of the transmembrane domains of MOG significantly enhanced the clinical efficacy in preventing the onset and development of the disease and induced CD4⁺Foxp3⁺ Treg cells in the EAE model. Together, our data demonstrated that targeting transmembrane domain-deleted MOG expression to platelets is an effective strategy to induce immune tolerance in EAE, which could be a promising approach for the treatment of patients with MS autoimmune disease.

Th1 immune maturation effects of

Nocardia rubra cell-wall skeleton (Nr-CWS) is reported as an external immunotherapeutic enhancer with the advantage of antitumor effect on human cancers. However, the immune regulatory role of Nr-CWS is not fully illustrated. We studied mouse CD4+ T lymphocytes isolated from mice spleen were induced by Nr-CWS and observed that the differentiation of Th1 CD4+ T cells and the cytokines of IL-2, TNF-α, IFN-γ were all enhanced by Nr-CWS. Furthermore, RNA sequencing was conducted to investigate the different mRNA profiling induced by Nr-CWS. We observed that paired box 8 (PAX8) was significantly up-regulated in Nr-CWS-treated Th1 cells compared to control. As a transcription factor, chromatin immunoprecipitation sequencing was carried out to study the genome-wide distribution of PAX8. Interestingly, we found that the binding domain of PAX8 was elevated by Nr-CWS, and the target genes associated with these binding sites showed a positive correlation between their transcription and PAX8 binding strength. Finally, we determined that Nr-CWS could enhance the activity of the PI3 K/Akt signaling pathway. Akt agonist could mimic the effect of Nr-CWS for PAX8 up-regulation, while Akt inhibitor compromised the expression of PAX8. Taken together, we determined a novel role of Nr-CWS in boosting the activity of Th1 maturation via the PI3 K/Akt/PAX8 axis.