Anti-B- cell-directed immunotherapy (rituximab) in the treatment of refractory pemphigus – an update

ARTICLE

Auteur(s) :, Meral J Arin, Nicolas Hunzelmann^*

Department of Dermatology University of Cologne, 50924 Cologne, Germany

accepté le 4 Mai 2005

Rituximab – mode of action

Since the murine CD20-binding Fab regions are retained and a human Fc portion is used, the half-life of the antibody is prolonged and with the human Fc a more effective complement activation and attraction of cytotoxic cells can be achieved [9]. Presence of complement and its activation are important factors for rituximab efficacy, but also seem to be related to the toxicity of adverse reactions [10]. Several mechanisms have been demonstrated by which rituximab depletes B cells, including complement activation, antibody-dependent cytotoxicity, apoptosis induction, and antiproliferative effects. It has been suggested that the key event of rituximab action is binding to the Fc receptor which leads to complement activation [11]. Three Fc receptor classes exist (FcγRI, II and III) and polymorphisms are related to sensitivity to infectious and autoimmune diseases [12]. Interestingly, the response to rituximab was also associated with Fc receptor polymorphisms and it was demonstrated that a dimorphism exists in which residue 158 of FcγRIIIA can be either valine with higher affinity to IgG or phenylalanine with weaker binding. These polymorphisms correspond with differences in activation of antibody-dependent cellular cytotoxicity by rituximab [13, 14]. The importance of FcγRIIIA in rituximab activity was also demonstrated in lupus patients with a diminished response to rituximab that were shown to harbour a homozygous phenylalanine in position 158 [15]. These data suggest that the response to rituximab can be predicted and has implications for the development of new antibody constructs to overcome resistance.

Resistance to rituximab has also been attributed to either rapid metabolism of rituximab due to alterations in host antibody metabolism or high numbers of accessible CD20 molecules. Moreover, anti-chimeric antibodies (HACA) which are directed against the human Fc portion may play a role. It has been reported in less than 1% of patients with non-Hodgkin lymphoma thus being an uncommon finding in clinical trials [16]. However, patients exist where the development of HACA impairs the function of rituximab. Just recently it was demonstrated in a patient with systemic lupus erythematosus that in the presence of host antibodies against the chimeric anti-CD20 antibody rituximab, a humanized monoclonal anti-CD20 antibody (hCD20) can be effective [17]. This antibody is currently in phase I trials for NHL and trials for autoimmune diseases are planned.

Because CD20 is not expressed on stem cells and plasma cells, depletion of the CD20 bearing B cell subpopulation is transient and does not affect immunoglobulin synthesis. Usually, normal peripheral B cells are replenished by hematopoetic stem cells in most patients 3-12 months after therapy [18]. Depletion affects B lymphocytes which are not secreting antibodies and it occurs in every patient regardless of the clinical response. It could be argued that after depletion of parent cells the antibody producing plasma cells are not longer present, but this would not explain why the immunoglobulin levels do not lower proportionally in some patients despite clinical improvement. It has therefore been suggested that long-lived plasma cells exist in the bone marrow that secrete auto-antibodies and that the therapeutic effect is not solely due to destruction of B cells. For instance, in rheumatoid arthritis it has been postulated that the cross-talk between T and B cells is a central event and autoreactive T cell clones depend on antigen presentation by B lymphocytes to maintain their activation. Therefore it was speculated that the removal of autoreactive B lymphocytes induces the collapse of a vicious cycle in which auto-antibodies drive their own production [19].

Current experience with the use of rituximab in autoimmune diseases

The rationale for the use of rituximab in autoimmune disease is long-term depletion of disease causing B cells. Its success in antibody-mediated disorders such as idiopathic thrombocytopenic purpura (ITP) and autoimmune haemolytic anaemia (table 1( Table 1 )) has further substantiated the importance of auto-antibodies in the pathogenesis of autoimmune diseases. In disorders where auto-antibody production is not obviously the underlying pathogenetic factor, targeting of CD20 allowed to gain insights into the mechanisms that lead to the development of autoimmunity. To date, various disorders such as rheumatoid arthritis, myasthenia gravis, Wegener’s granulomatosis, systemic lupus erythematosus, dermatomyositis, Sjögren syndrome, multiple sclerosis, cold agglutinin disease, Goodpasture’s syndrome, glomerulonephritis, antiphospholipid syndrome, and IgM associated neuropathies have been treated with rituximab.

In haematological disorders such as autoimmune haemolytic anaemia, mixed cryoglobulinaemia and idiopathic thrombocytopenic purpura, overall response rates of up to 84% can be achieved (table 1). In ITP, the time to response varied widely between patients and it was suggested that an early response might be due to rapid activation of inhibitory receptors whereas a delayed response is related to gradual elimination of auto-antibody producing cells [21]. In patients with immune hemolytic anemia, ITP and myasthenia gravis that were treated with rituximab the clinical improvement has been reported to occur very rapidly, indicating that elimination of B cells and subsequently a drop in the autoantibody level cannot be the only explanation and that other factors secondary to T cell de-regulation might play a role [22].

In patients with lupus erythematosus, a phase I/II open-label, dose-escalating trial has been recently completed. Ten out of 16 patients treated with rituximab showed a good depletion of B cells. Depletion persisted for 12 months and correlated well with the clinical improvement [23]. Several cases of myasthenia gravis, Wegener’s granulomatosis, Sjögren syndrome and dermatomyositis have been reported that were treated with varying doses and schedules of rituximab (table 1). Complete responses were demonstrated in about half of the cases.

Most studies are open-label but recently a double-blind, placebo-controlled trial has demonstrated the effectiveness of rituximab in 161 patients with rheumatoid factor positive- rheumatoid arthritis [2]. This study showed that all groups treated with rituximab had a significantly higher proportion of patients with a 20% reduction in disease symptoms. The combination of rituximab with methotrexate was most effective, leading to a 50% improvement of disease symptoms in 43% of the patients. The majority of adverse events were infusion related, serious infections occurred in four patients (3.3%) compared to 1 patient in the control group (2.5%). A single course of two infusions of rituximab showed a long-term improvement of the clinical disease course and some patients were in remission one year after the last infusion.
Table 1 Overview of autoimmune diseases treated with rituximab

Rituximab in refractory pemphigus

Although the exact mechanisms are still unclear, B and T lymphocytes and the interaction between these cell types are involved in the pathogenesis. The critical role of auto-antibodies produced by B cells stems from the observation that passive transfer of anti-desmoglein 3 antibodies into newborn mice causes a bullous disease resembling pemphigus [26, 27]. In addition, B cells function as antigen-presenting cells and stimulate Dsg 3-specific CD4+ T cells to secrete cytokines such as Il-4, IL-6 and IL-10 which are required for proliferation of memory B cells and differentiation to antibody-producing plasma cells, respectively. Recognition of distinct epitopes of the desmoglein molecule by T cells seems therefore crucial for the initiation and maintenance of auto-antibody production by B cells [28-30]. The concept of self-perpetuating B cells with activation of T cells and generation of plasma cells that secrete immunoglobulins capable of forming immune complexes that promote survival of parent B cells was also proposed in rheumatoid arthritis. By depleting B cells, not only would the production of auto-antibodies be reduced but also the “disease memory” be removed which is held in mutated immunoglobulin genes in specific autoreactive clones [31]. The interplay between B and T cells is further emphasized by the finding that in vitro stimulation of peripheral blood mononuclear cells with Dsg3 does not result in auto-antibody production when CD4+ T cells are depleted [32].

Several case reports using rituximab in refractory or life-threatening cases of pemphigus have been reported and showed promising results (table 2)( Table 2 ). The overall response rates were good with remissions lasting several months. Treatment was well tolerated and adverse events mainly consisted of infusion-related side effects such as nausea, fever and chills. However, two case series of three patients each reported serious infections such as pneumonia and septic arthritis [33] and fatal pneumocystis carinii pneumonia [34]. An open-label study of rituximab in five patients with refractory disease courses of pemphigus vulgaris and pemphigus foliaceus has recently been completed [35]. Significant B cell depletion in the peripheral blood correlated with clinical improvement (assessed by the disease activity score). However, the clinical improvement ( (figure 1) ) was not accompanied by a decrease of auto-antibody titres in all patients suggesting that auto-antibody producing plasma cells with longer life spans exist. Previously, it was thought that auto-antibody producing plasma cells are continuously generated. Recent data indicate that plasma cells which are non dividing and have down regulated B cell markers such as CD20, may reside in the bone marrow for years [36-39]. This might explain persistent auto-antibody and immunoglobulin levels such as anti-tetanus toxoid IgG even with prolonged depletion of memory B cells using cytotoxic agents or rituximab [40]. Moreover, different classes of auto-antibodies may respond differently to cytotoxic agents. This was recently suggested in patients with lupus erythematosus where anti-dsDNA IgG were very responsive to treatment and disappeared fast whereas antinuclear or anti-cardiolipin antibodies persisted and remained elevated even in absence of overt clinical disease [40]. Besides, auto-antibodies reactive with a broader range of epitopes, also known as epitope spreading, may account for the lack of antibody titre reduction in pemphigus, since these epitopes are not differentiated by standard analysis of pemphigus antibody titre [25, 41]. The pathogenic heterogeneity among anti-Dsg3 antibodies due to their epitopes has been demonstrated in pemphigus vulgaris, where antibodies directed against the amino terminus of desmoglein 3 are most pathogenic [42]. Thus, IgG4 directed against a particular extracellular domain (EC2 domain) of desmoglein 3 is the main acantholytic antibody while IgG4 against EC1 may act as a facilitator or enhancer of this process [28]. Moreover, it has been shown recently that in endemic pemphigus foliaceus (fogo selvagem) the anti-desmoglein 1 response is initially raised against nonpathogenic epitopes located in the EC5 domain whereas a pathogenic response involves the EC1 and EC2 domains of the molecule [43, 44].

The question why patients relapse has not been resolved. It is possible that B cell clones have not been fully ablated or that continued production of auto-antibodies by plasma cells provides survival signals for re-emerging autoreactive B cells [45]. The remission following rituximab usually lasts several months and coincides with the drop of B cells. Recovery of B cells usually begins at 6-9 months and the values return to normal after 9-12 months. Interestingly, one patient with pemphigus vulgaris has been reported with a low CD 19 count three years after the last application of rituximab which correlated well with clinical remission [35]. Whether FcγRIIIA polymorphisms are the only explanation for this finding remains to be elucidated.
Table 2 Overview of pemphigus patients treated with rituximab to date

Dosage and adverse reactions

Systemic infusion reactions are frequently observed in lymphoma patients most probably due to a high load of abnormal B cells [48]. Approximately 50% of patients treated with rituximab experience infusion-related adverse reactions, including cytokine release syndrome. These are accompanied by hypotension and bronchospasm in about 10% of patients. Severe cytokine release syndrome has been reported to occur mostly in lymphoma patients, however a few cases have been described that were treated for indications other than lymphoma. A common feature was development of severe reactions during the first infusion, particularly dyspnoe, severe bronchospasm, and/or hypoxia [49]. In autoimmune diseases, these adverse reactions seem less of a problem. A pricking sensation in the throat that occurs 30 to 60 minutes after the start of the infusion is a common feature and has been interpreted as penetration of rituximab into the Waldeyer’s ring [45].

The absence of normal B cells for several months has not been associated with a significant increase in infectious risk [16]. Total IgG levels and anti-tetanus titres are unaffected whereas IgM levels are reduced to the lower end of the normal range [50]. In patients with chronic lymphocytic leukaemia receiving fludarabine and rituximab, the incidence of pneumocystis carinii pneumonia was calculated 1-2%, reactivation of herpes simplex and varicella zoster virus 5-10% whereas reactivation of cytomegalo virus seems a rare event [51]. In patients with rheumatoid arthritis, there is now some evidence that there is an increased risk of lower respiratory infection [2].

In pemphigus patients treated with rituximab, treatment is generally well tolerated, most adverse reactions occur during the first infusion and consist of nausea, fever, chills, occasional hypotension and dyspnoea. Usually, these reactions diminish with subsequent infusions and can be well controlled by pre-medication with paracetamol and antihistamines. However, single reports exist in pemphigus patients that describe serious side effects, such as pneumonia, septic arthritis [33] and fatal pneumocystis carinii pneumonia [34].

Contraindications

Conclusions and outlook

Clinical remissions are usually retained over several months, sometimes prolonged responses are observed. The treatment is well tolerated and infusion-related adverse events can be readily controlled. Although infections do not seem to be a major problem in autoimmune diseases, severe and fatal outcomes have been reported. In case of relapse different protocols are used and often a second course is applied. However, the long-lasting reduction of circulating B cells in some patients raises the question of long-term effects of repeated cycles on the complete maintenance of the cellular and humoral immunity. New protocols using a reduced dosage or a single infusion of rituximab in the case of relapse have been used in some patients and might be sufficient to achieve remission.

These data are based on small case series and should therefore be interpreted with caution. Further controlled studies are needed to optimise treatment regimes and to investigate the safety profile. Whether rituximab will have its place as first-line therapy in selected cases awaits further investigation.

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