The prospective Hemophilia Inhibitor PUP Study reveals distinct antibody signatures prior to FVIII inhibitor development

Abstract

Preventing factor VIII (FVIII) inhibitors following replacement therapies with FVIII products in patients with hemophilia A remains an unmet medical need. Better understanding of the early events of evolving FVIII inhibitors is essential for risk identification and the design of novel strategies to prevent inhibitor development. The Hemophilia Inhibitor Previously Untreated Patients (PUPs) Study (HIPS; www.clinicaltrials.gov #NCT01652027) is the first prospective cohort study to evaluate comprehensive changes in the immune system during the first 50 exposure days (EDs) to FVIII in patients with severe hemophilia A. HIPS participants were enrolled prior to their first exposure to FVIII or blood products (“true PUPs”) and were evaluated for different immunological and clinical parameters at specified time points during their first 50 EDs to a single source of recombinant FVIII. Longitudinal antibody data resulting from this study indicate that there are 4 subgroups of patients expressing distinct signatures of FVIII-binding antibodies. Subgroup 1 did not develop any detectable FVIII-binding immunoglobulin G (IgG) antibodies. Subgroup 2 developed nonneutralizing, FVIII-binding IgG1 antibodies, but other FVIII-binding IgG subclasses were not observed. Subgroup 3 developed transient FVIII inhibitors associated with FVIII-binding IgG1 antibodies, similar to subgroup 2. Subgroup 4 developed persistent FVIII inhibitors associated with an initial development of high-affinity, FVIII-binding IgG1 antibodies, followed by IgG3 and IgG4 antibodies. Appearance of FVIII-binding IgG3 was always associated with persistent FVIII inhibitors and the subsequent development of FVIII-binding IgG4. Some of the antibody signatures identified in HIPS could serve as candidates for early biomarkers of FVIII inhibitor development.

Emicizumab in tolerized patients with hemophilia A with inhibitors: A single‐institution pediatric cohort assessing inhibitor status

“…Detection of IgG1 antibodies in hemophilia A inhibitors samples by FLI demonstrated that IgG1 antibodies preceded a positive inhibitor titer in five of seven patients who ultimately developed inhibitors, but IgG4 antibodies were detected in patients with negative inhibitor titers and those without an inhibitor history 11,18 . Similarly, the prospective observational Hemophilia Inhibitor PUP study evaluating biomarkers in inhibitor development among previously untreated patients with severe hemophilia A detected high‐affinity IgG1 antibodies before detection of an inhibitor along with high‐affinity IgG3 and IgG4 antibodies; however, IgG4 was not detected in subjects with LTIs, FVIII‐specific non‐neutralizing antibodies, or subjects without FVIII inhibitors or antibodies 21 . A limitation of this current study is the lack of routine evaluation of FVIII antibody profiling over time prior to patients switching to emicizumab for comparison.…”

“…affinity IgG3 and IgG4 antibodies; however, IgG4 was not detected in subjects with LTIs, FVIII-specific non-neutralizing antibodies, or subjects without FVIII inhibitors or antibodies 21. A limitation of this current study is the lack of routine evaluation of FVIII antibody profiling over time prior to patients switching to emicizumab for comparison.TA B L E 2…”

B cell–activating factor modulates the factor VIII immune response in hemophilia A

“…Our study does have some limitations. First, there are likely differences in the established, longer immune response seen in adults versus pediatric inhibitor patients whose immune responses may still be evolving, as noted in the HIPS study ( 34 ). However, the continued elevation of BAFF in the adult HA inhibitor population (which otherwise should fall) supports the hypothesis that BAFF modulates the FVIII immune response.…”

“…Long-lived PCs (LLPCs) ultimately settle in the bone marrow (BM), sustaining the humoral response for extended periods. In humans, the antibody response against FVIII consists of both neutralizing (high affinity, IgG 4 ) and nonneutralizing (low affinity, IgG 1 ) antibodies ( 34 ), likely from B cell receptor (BCR) rearrangement driving affinity maturation. In mice, it is generally thought that IgG 1 most closely mimics human IgG 4 ( 35 ), but inhibitory α-FVIII antibodies of many subclasses have been described ( 36 , 37 ).…”

Reappearance of inhibitor in a tolerized patient with severe haemophilia A during FVIII‐free emicizumab therapy

“…Recurrence of the FVIII inhibitor was accompanied by an increase in levels of both anti-FVIII IgG1 and IgG4, which parallels the situation in previously untreated patients who develop FVIII inhibitors during FVIII prophylaxis. 7 Interestingly, in our patient, IgG1 was very low, but still detectable, before emicizumab initiation, while the inhibitor was undetectable for several years before. This may explain the difference with patients reported by Batsuli et al who remained negative in both Bethesda and ELISA for the IgG1 isotype.…”