Targeting FVIII Expression to Platelets Induces Immune Tolerance in Hemophilia A Mice with or without Pre-Existing Anti-FVIII Immunity,
Authors: Chen Yingyu , Kuether Erin L. , Schroeder Jocelyn A. , Montgomery Robert R. , Scott David W. , Shi Qizhen 
Published: October 2019

Abstract
Abstract 4170
Our previous studies have shown that targeting FVIII expression to platelets (2bF8) can correct murine hemophilia A phenotype even in the presence of inhibitory antibodies. In the present study, we wanted to explore 1) whether platelets containing FVIII can act as an immunogen; and 2) whether platelet-derived FVIII can induce immune tolerance in a hemophilia A mouse model. To investigate whether platelets containing FVIII can act as an immunogen in hemophilia A mice, we infused transgenic mouse platelets with a level of platelet-FVIII of 6 mU/108 platelets to naïve FVIIInull mice weekly for 8 weeks. These platelets were between 30 to 50% of total platelets upon infusion and the levels of platelet-FVIII in the infused animals were 0.11 ± 0.01 mU/108 platelets (n = 6) one week after infusion. No anti-FVIII inhibitory antibodies were detected in the infused mice during the study course. All animals developed inhibitors following further challenged with recombinant human FVIII (rhFVIII) at a dose of 50 U/kg by intravenous injection weekly for 4 weeks, indicating that infusion of platelets containing FVIII does not trigger immune response in hemophilia A mice.
To explore whether platelet-derived FVIII will act as an immunogen in the presence of primed spleen cells (from mice already producing inhibitory antibody), we co-transplanted splenocytes from highly immunized FVIIInull mice and bone marrow (BM) cells from 2bF8 transgenic mice into 400 cGy sub-lethal irradiated FVIIInull recipients. We monitored the levels of inhibitory antibodies in recipients for up to 8 weeks and found that inhibitor titers declined with time after transplantation. We then challenged co-transplantation recipients with rhFVIII and found that inhibitor titers in the control group co-transplantat of FVIIInull BM cells increased 103.55 ± 64.83 fold (n = 4), which was significantly more than the group receiving 2bF8 transgenic BM cells (14.34 ± 18.48, n = 5) (P <.05). The inhibitor titers decreased to undetectable in 40% of 2bF8 transgenic BM cells co-transplantation recipients even after rhFVIII challenge, indicating immune tolerance was induced in these recipients.
To further explore the immune response in the lentivirus-mediated platelet-derived FVIII gene therapy of hemophilia A mice, we transduced hematopoietic stem cells from pre-immunized FVIIInull mice with 2bF8 lentivirus (LV) followed by syngeneic transplantation into pre-immunized lethally irradiated FVIIInull recipients and monitored the levels of inhibitor titers in recipients. After full BM reconstitution, platelet-FVIII expression was sustained (1.56 ± 0.56 mU/108platelets, n = 10), but inhibitor titers declined with time, indicating that platelet-derived FVIII does not provoke a memory response in FVIIInullmice that had previously mounted an immune response to rhFVIII. The t1/2 of inhibitor disappearance in 2bF8 LV-transduced recipients (33.65 ± 11.12 days, n = 10) was significantly shorter than in untransduced controls (66.43 ± 22.24 days, n = 4) (P <.01). We also transplanted 2bF8 LV-transduced pre-immunized HSCs into 660 cGy sub-lethal irradiated naïve FVIIInull mice. After BM reconstituted, recipients were assessed by platelet lysate FVIII:C assay and tail clip survival test to confirm the success of genetic therapy. Animals were then challenged with rhFVIII. Only 2 of 7 2bF8 LV-transduced recipients developed inhibitory antibodies (55 and 87 BU/ml), while all untransduced control developed high titer of inhibitors (735.50 ± 94.65 BU/ml, n = 4).
In conclusion, our results demonstrate that 1) platelets containing FVIII are not immunogenic in hemophilia A mice; and 2) platelet-derived FVIII may induce immune tolerance in hemophilia A mice with or without pre-existing inhibitory antibodies. This tolerance induction would add an additional significant benefit to patients with platelet-derived FVIII gene therapy strategy because protein infusion could be administered in some special situations (e.g. surgery in which a greater levels of FVIII may be required) with minimized risk of inhibitor development.

Disclosures:
No relevant conflicts of interest to declare.