Monitoring B-cell Depletion Therapy
B-cell depletion therapy has long been part of the management of autoimmune diseases characterized by autoantibody production. With the development of monoclonal antibody (mAb) therapy, B-cell depletion can be accomplished in a relatively specific manner. Rituximab is a chimeric (mouse / human) monoclonal antibody that targets CD20, exclusively expressed by human B cells. Pioneered for the treatment of B-cell malignancies, its therapeutic repertoire has expanded to include many other scenarios, including EBV-associated lymphoproliferative disease (post bone marrow or solid organ transplantation), rheumatologic disorders, autoimmune cytopenias, solid organ rejection, chronic graft-versus-host disease, vasculitic disorders, among others.
The next generation of B-cell specific monoclonal antibodies, including antibodies directed against CD19 and CD22, are already finding their way into the clinic. Their mechanisms of action, as well those of yet-to-be-developed mAbs may be similar to that of rituximab, but may also be fundamentally different.
These mechanisms determine if and how B cells are eliminated, and, conversely, how B cells, or specific B-cell populations, reconstitute / recover after depletion (and potentially cause relapse). Thus, an important component in the determination whether mAb-mediated B-cell depletion therapy is successful, is evaluating B-cell recovery. The DIL are exploring several approaches that together will form the B-cell reconstitution panel.
When fully implemented, this panel will consist of several components that include (longitudinal) immunophenotyping of B-cell subsets, measurement of constituents of the BAFF system, as well as evaluation of relevant genetic determinants.
In this edition, B-cell immunophenotyping will be touched upon, while a more detailed description of the BAFF system, as well as genetic determinants and their roles in B-cell reconstitution, will appear in a future edition. It stands to reason that some or more of these components can be used to monitor B-cell reconstitution following other modes of therapy (e.g. cyclophosphamide) as well.
The above figure shows a relatively well-recognized phenomenon: widely variable B-cell recovery kinetics. It has been observed that B-cell counts reach or exceed pre-rituximab levels after only a few months in some individuals, while in others it can take many months. In fact, in several individuals, no substantial recovery of B-cell counts has occurred in more than one year following completion of rituximab treatment.
Two possibilities that may operate in tandem, are considered. The first concerns the BAFF system; important for B-cell differentiation, following release of immature B cells from the bone marrow. The second relates to our genetic make-up with regard to single-nucleotide polymorphisms in the Fcgamma receptor genes (CD16 and CD32, respectively). Both are considered to become part of the panel (discussed in the next issue of the Newsletter).
Longitudinal determination of B-cell recovery kinetics will be tied into the phenotype of the recovering B-cell compartment, as it relates to the differentiation state of the B cells. It has been reported that the initial wave of B-cell recovery is characterized by release of naïve (transitional) recent bone marrow emigrants (as defined by expression of CD10, IgD, CD24, CD38 and lack of CD27 (identifying memory B cells). This stage in B-cell differentiation appears to be highly dependent on the BAFF system. It is possible that recovery of memory B cells is indicative of altered B-cell reconstitution; possibly predictive of relapse of the pre-rituximab condition.
CD5 is included in our panel because CD5 is an age-dependent B-cell marker, and because altered distribution of CD5+ versus CD5-negative B-cell subsets is often observed in B-cell derived disorders.
The above figures show 2 relatively similar kinetics but strikingly different profiles, as determined by CD10, CD5 and CD27 expression. The indication for rituximab therapy in patient 1 was EBVinduced lymphoproliferative disease; autoimmune cytopenias in patient 2. Despite variant immunophenotype, both patients have (so far) remained in remission from EBV disease and cytopenias, respectively.
