Scientists Target FVIII Inhibitor Formation in Hemophilia A

Scientists found the immune signaling protein BAFF can promote the formation of neutralizing antibodies, or inhibitors, against blood clotting factor VIII (FVIII), which can lower the effectiveness of FVIII replacement therapies used to treat people with hemophilia A. 

They also discovered that hemophilia A patients who developed anti-FVIII inhibitors had higher BAFF levels in their bloodstream.

According to researchers, the findings also support further investigation into the potential treatment combination of rituximab, a B-cell-depleting therapy to lower the production of inhibitors, and belimumab (sold as Benlysta), an approved anti-BAFF medication, to improve the clinical outcomes of patients with hemophilia A. 

“Given that an FDA-approved anti-BAFF antibody is currently used to suppress the immune response in autoimmune diseases, future research should explore the use of this treatment in combination with rituximab to achieve better outcomes for hemophilia A patients resistant to FVIII protein replacement therapy,” Valder Arruda, MD, PhD, co-senior author of the study, said in a press release.

The study, “B cell-activating factor modulates the factor VIII immune response in hemophilia A,” was published in the Journal of Clinical Investigation. 

FVIII replacement therapy is a standard therapy used to treat hemophilia A that works by supplying patients with FVIII, the clotting factor they are missing.

However, up to 30% of patients end up developing neutralizing antibodies against the FVIII protein supplied by these treatments, lowering their effectiveness and ultimately rendering them useless. Still, the reason why patients develop these inhibitors is not well-understood. 

Patients with FVIII inhibitors can receive immune tolerance induction (ITI), in which high doses of FVIII are administered for one to two years to build tolerance and avoid inhibitor production. Yet, ITI is invasive and costly, poses a significant burden to patients, and is not consistently effective. 

A protein called B-cell-activating factor (BAFF) is responsible for stimulating and promoting the survival of B-cells, the immune cells that produce antibodies to fight against infections, but also the neutralizing antibodies targeting FVIII. 

Elevated BAFF levels in the bloodstream have been implicated in several autoimmune diseases, and also led to the development of belimumab, an FDA-approved biologic therapy designed to block BAFF that is used mainly to treat systemic lupus erythematosus, an autoimmune disorder.

Researchers at the Children’s Hospital of Philadelphia (CHOP) wondered if BAFF might also be involved in the development of FVIII inhibitors in patients with hemophilia A.

To answer this question, they conducted a series of experiments using  samples collected from adults and children with hemophilia A, as well as disease-related mouse models. 

First, they collected blood samples from 69 pediatric patients, 24 (34.8%) of whom had FVIII inhibitors, and 45 (65.2%) did not. There was no difference in age, race, or disease severity between those with and without inhibitors. Of the 69 participants, only two were female and neither had inhibitors.

Analyses showed that BAFF levels were higher in children with inhibitors than in those without inhibitors — 1.30 vs. 0.99 nanograms per milliliter (ng/mL).

Moreover, in patients who underwent ITI and achieved tolerance, BAFF levels decreased from 1.43 ng/mL at baseline (prior to beginning treatment) to 0.81 ng/mL following treatment. In contrast, BAFF levels remained high in those who did not achieve tolerance. 

Similarly, in a group of 46 adults with hemophilia A, 22 (47.8%) of whom had inhibitors, and 24 (52.2%) who did not, BAFF levels were higher in those who had inhibitors (1.14 vs. 1.03 ng/mL). 

In addition to ITI to induce tolerance, some patients receive rituximab, a biologic therapy designed to deplete antibody-producing B-cells. Still, the results have been limited and for reasons that are not clear. 

The team measured BAFF levels in blood samples obtained from 17 hemophilia A patients with inhibitors. From these, nine were enrolled in a clinical study evaluating rituximab alone in those who failed to achieve ITI-based tolerance, while the remaining eight participated in a different study in which they were given the same dose of rituximab combined with ITI.

Three of the 17 patients (17%) achieved tolerance — one after being treated with rituximab alone and two after receiving a combination of rituximab and ITI.

In the 14 patients who failed to achieve tolerance, BAFF levels increased threefold from baseline (0.89 to 2.66 ng/mL). Although BAFF levels also increased in those who achieved tolerance (0.78 to 20.60 ng/mL), the low number of cases limited statistical comparison.

Next, the team tested whether blocking BAFF with an anti-BAFF antibody (similar to belimumab) could prevent or eliminate the production of FVIII inhibitors in two mouse models of hemophilia A. 

One hemophilia A mouse strain was given an anti-BAFF antibody, or a control, before immunization with human FVIII. Only three of 14 mice treated with anti-BAFF developed inhibitors compared with nine of 10 animals in the control group, indicating that anti-BAFF therapy lowered the risk of inhibitor formation by 23%. 

Long-term tolerance was tested by repeatedly injecting these mice with human FVIII 22 weeks following anti-BAFF treatment. Half of the mice developed high-level (titer) inhibitors, whereas the remaining mice had low inhibitor levels. 

“Thus, a single dose of [anti-BAFF antibody] was sufficient to prevent the formation of high-titer inhibitors in HA [hemophilia A] mice, with a sustained effect (more than 22 weeks) beyond the relative short initial period of reduction of BAFF levels (four weeks),” the team wrote. 

A second hemophilia A mouse model was used to explore the effects of anti-BAFF therapy in animals with inhibitors. The mice were treated with an anti-CD20 antibody alone (a B-cell-depleting agent similar to rituximab), anti-BAFF alone, or with both. 

Analyses showed that anti-CD20 antibody-treated mice had a threefold increase in BAFF levels, and this rise was mitigated by the addition of anti-BAFF, which was similar to anti-BAFF alone.

Notably, only the combination therapy resulted in a substantial decrease in inhibitor levels compared with controls, even after repeated injections with human FVIII. B-cells specific to FVIII also were partially reduced in treated animals. 

“Our data suggest that BAFF may regulate the generation and maintenance of FVIII inhibitors, as well as anti-FVIII B cells,” said Arruda, who also is a researcher at CHOP and director of CHOP’s Center for the Investigation of Factor VIII Immunogenicity. 

“In summary, our data establish the potential to use [anti-BAFF] therapy in conjunction with [anti-CD20] therapy for eradication of FVIII inhibitors in patients with [hemophilia A],” the team concluded.