Therapeutic factor VIII is highly immunogenic. Despite intensive research in the last decades, the reasons why 5-30% of patients with hemophilia A (of all severities) develop inhibitory anti-factor VIII antibodies (inhibitors) following replacement therapy remain an enigma. Under physiological conditions, endogenous factor VIII is recognized by the immune system. Likewise, numerous observations indicate that, in hemophilia A patients without inhibitors, exogenous therapeutic factor VIII is immunologically assessed and tolerated. A large part of the research on the immunogenicity of therapeutic factor VIII is attempting to identify the ‘danger signals’ that act as adjuvants to the deleterious anti-factor VIII immune responses. However, several of the inflammatory assaults concomitant to factor VIII administration initially hypothesized as potential sources of danger signals (e.g., bleeding, infection, and vaccination) have been disproved to be such signals. Conversely, recent evidence suggests that cells from inhibitor-negative patients are able to activate anti-inflammatory and tolerogenic mechanisms required to suppress deleterious immune responses, while cells from inhibitor-positive patients are not.
The immune response to therapeutic FVIII is believed to be a classical immune response against an exogenous antigen, wherein some of the intravenously administered FVIII transiently accumulates in secondary lymphoid organs, as observed in spleens of FVIII-deficient mice, is internalized by antigen-presenting cells and presented to naïve FVIII-specific CD4+ T cells. Upon activation, FVIII-specific T cells proliferate and provide help to naïve FVIII-specific B cells that differentiate into memory B cells or plasmocytes secreting inhibitory anti-FVIII IgG.
The dose of administered FVIII is one of the few parameters that has shown consistent association with the development of an anti-FVIII immune response. Seminal work in FVIII-deficient mice clearly demonstrated that increasing the dose of intravenously injected FVIII results in a proportional increase in the intensity of the immune response.
The experimental data were confirmed by the RODIN study in which the intensity of FVIII treatment and mean dose of administered FVIII correlated with the incidence of inhibitor development in patients with severe hemophilia. Conversely, the reduction in the amount of FVIII internalized by antigen-presenting cells and, consequently, presented to CD4+ T cells has been hypothesized as a mechanism by which von Willebrand factor may play an immuno-protective role towards therapeutic FVIII.
Major bleeds, surgery, infections or vaccination at the time of FVIII administration have been proposed as potential inflammatory assaults that could predispose hemophilia A patients to develop allo-antibodies to exogenous FVIII.
The experimental data were confirmed by retrospective clinical observations from the PEDNET registry showing that pediatric vaccination given in close proximity to the administration of FVIII is not associated with an increased risk of FVIII inhibitor development. Interestingly, influenza vaccination concurrent to FVIII treatment significantly reduced the incidence of anti-FVIII immune responses in mice.
Chronic inflammation associated with recurrent bleeding as well as acute hemarthrosis following knee injury in FVIII-deficient mice failed to increase the immune response to exogenous FVIII.
Taken together, the large majority of the investigations performed over the last 10 years failed to identify candidate danger signals the control of which would unequivocally reduce the immunogenicity of therapeutic FVIII, reflecting the possibility that danger signals may not be as important as first thought for the initiation of the anti- FVIII immune response. Alternatively, the background level of activation of the innate/adaptive immune system may be sufficient to allow activation of naïve FVIII-specific T cells without the need for overt danger signals, provided that a sufficient amount of FVIII is internalized, processed by antigen-presenting cells and presented to T cells.
The recognition of self, and in particular FVIII, is part of normal immune homeostasis. It is clear today that both B and T lymphocytes undergo positive and negative selection processes, in the bone marrow and thymus, respectively, wherein highly autoreactive lymphocytes are eliminated and poorly autoreactive ones are retained. In the case of T lymphocytes, thymic T cells with intermediate affinities for self-antigens may differentiate into natural regulatory T cells that suppress autoreactive T-cell responses.
FVIII inhibitors develop in 5 to 30% of patients depending on the severity of hemophilia A. The abnormality in the F8 gene responsible for the disease is the strongest predictor of alloimunization against therapeutic FVIII. In particular, patients with severe hemophilia A, and among them patients lacking circulating FVIII:Ag, have the highest incidence of FVIII inhibitor development. Conversely, patients with missense mutations have the lowest risk of developing FVIII inhibitors. This correlation illustrates the importance of the degree of education, either centrally in the thymus or at the periphery, of the immune system of the patients towards endogenous FVIII: the more the endogenously produced FVIII resembles the exogenously administered therapeutic FVIII, the lower the risk of developing a neutralizing immune response.
Observations over the years demonstrate that endogenous FVIII is not ignored by the immune system under physiological conditions but is immunologically assessed, leading to its homeostatic recognition based on counteracting reactive and tolerogenic adaptive immune effectors. Nonneutralizing FVIII-reactive antibodies are found in virtually all patients with hemophilia A; clinically irrelevant inhibitory antibodies are transiently detected in a substantial number of patients after their first exposures to therapeutic FVIII; and FVIII inhibitors may arise at an age when immune regulatory mechanisms are compromised. Furthermore, the development of FVIII inhibitors is associated with a hampered ability of the organism to activate HO-1 or IDO1-dependent anti-inflammatory/tolerogenic mechanisms.
Collectively, the accumulated evidence favors the hypothesis that all hemophilia A patients treated with FVIII develop an immune response to therapeutic FVIII: mounting an immune response to exogenous FVIII is part of the normal recognition and assessment of an innocuous antigen. In this context, the immunological assessment of therapeutic FVIII does not require the presence of overt danger signals.
Based on the available observations, we propose a model in which all hemophilia A patients develop anti-factor VIII immune responses during replacement therapy irrespective of associated danger signals. We further postulate that the onset of clinically relevant factor VIII inhibitors results from an inability to develop counteractive tolerogenic responses to exogenous factor VIII rather than from an exacerbated activation of the immune system at the time of factor VIII administration.
A better understanding of the pathogenesis of neutralizing anti-factor VIII antibodies will have repercussions on the clinical management of patients and highlight new strategies to achieve active immune tolerance to therapeutic factor VIII.
Pathogenic immune response to therapeutic factor VIII: exacerbated response or failed induction of tolerance?
Varthaman A and Lacroix-Desmazes S.