Cladribine treatment of multiple sclerosis is associated with depletion of memory B cells
Journal of Neurology
https://doi.org/10.1007/s00415
Cladribine treatment of multiple sclerosis is associated with depletion of memory B cells
Bryan Ceronie 0 1 2 3 4
Benjamin M. Jacobs 0 1 2 3 4
David Baker 0 1 2 3 4
Nicolas Dubuisson 0 1 2 3 4
Zhifeng Mao 0 1 2 3 4
Francesca Ammoscato 0 1 2 3 4
Helen Lock 0 1 2 3 4
Hilary J. Longhurst 0 1 2 3 4
Gavin Giovannoni 0 1 2 3 4
Klaus Schmierer 0 1 2 3 4
0 Haematology Unit, The Royal London Hospital, Barts Health NHS Trust , London , UK
1 Blizard Institute, Barts and the London School of Medicine and Dentistry, Queen Mary University of London , 4 Newark Street, London E1 2AT , UK
2 Bryan Ceronie , Benjamin M. Jacobs, and David Baker contributed equally
3 Abbreviations 5′NT 5′ Nucleotidase ADA Adenosine deaminase DCK Deoxycytidine kinase DMT Disease-modifying treatments EDSS Extended disability status scale EMA European Medicines Agency MS Multiple sclerosis PwMS People with MS , USA
4 Emergency Care and Acute Medicine Clinical Academic Group Neuroscience, The Royal London Hospital, Barts Health NHS Trust , London , UK
Background The mechanism of action of oral cladribine, recently licensed for relapsing multiple sclerosis, is unknown. Objective To determine whether cladribine depletes memory B cells consistent with our recent hypothesis that effective, disease-modifying treatments act by physical/functional depletion of memory B cells. Methods A cross-sectional study examined 40 people with multiple sclerosis at the end of the first cycle of alemtuzumab or injectable cladribine. The relative proportions and absolute numbers of peripheral blood B lymphocyte subsets were measured using flow cytometry. Cell-subtype expression of genes involved in cladribine metabolism was examined from data in public repositories. Results Cladribine markedly depleted class-switched and unswitched memory B cells to levels comparable with alemtuzumab, but without the associated initial lymphopenia. CD3+ T cell depletion was modest. The mRNA expression of metabolism genes varied between lymphocyte subsets. A high ratio of deoxycytidine kinase to group I cytosolic 5′ nucleotidase expression was present in B cells and was particularly high in mature, memory and notably germinal centre B cells, but not plasma cells. Conclusions Selective B cell cytotoxicity coupled with slow repopulation kinetics results in long-term, memory B cell depletion by cladribine. These may offer a new target, possibly with potential biomarker activity, for future drug development.
B cell; Cladribine; Disease-modifying treatment; Deoxycytidine kinase; Multiple sclerosis; Memory B cells
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Gavin Giovannoni and Klaus Schmierer are joint senior authors.
Introduction
Multiple sclerosis (MS) is a chronic-inflammatory disorder
of the central nervous system leading to accumulating
disability. Historically, myelin-directed, CD4+, Th17+ cells
have been considered to be the important mediators of the
inflammatory process [
1
]. This is largely based on animal
experimental autoimmune encephalitis models where T
cellmediated responses are key to pathogenesis [
2
]. However,
suppression of CD4+, Th17+ cells has only produced
modest benefits in clinical trials in MS [
3, 4
]. Therefore, other
mechanisms may play a crucial role in the disease process.
Whilst animal models and in vitro studies in MS provide
circumstantial evidence for the involvement of particular
pathways, positive and properly conducted, negative trials
provide invaluable information for understanding the
cellular mechanisms that drive disease activity. Alemtuzumab,
a CD52, T and B cell depleting antibody [
5
], ocrelizumab,
a CD20-B cell depleting antibody [
6
] and cladribine, a
lymphocyte depleting agent [
7
], are immune-reconstitution
therapies that are amongst the most efficacious treatments
for MS and can provide long-term benefit from few
treatment cycles [
4, 8, 9
]. However, there is no clear consensus
on their mode of action, particularly for cladribine [
7, 10
].
It is well known that lymphocytes produce many secreted
proteins, rapidly proliferate in response to infections and
rely on both de novo and importantly salvage pathways for
nucleotide synthesis. To accommodate this requirement,
lymphocytes express high levels of adenosine deaminase
(ADA) protein [
11, 12
]. This prevents the accumulation of
cytotoxic levels of deoxyadenosine triphosphate, by
catalysing the deamination of adenosines to inosines [12].
Cladribine is a chlorinated analogue of deoxyadenosine that is
partially resistant to ADA [
12, 13
]. This is phosphorylated
by deoxycytidine kinase (DCK) to create lymphocyte
cytotoxicity [12]. This action is countered by cytosolic 5′
nucleotidase (5′NT) enzymatic activity that dephosphorylates
phosphorylated-cladribine. Lymphocytes and lymphocytic
cancers, including hairy cell leukaemia for which
parenteral cladribine is licensed, have a high DCK-to-5′NT ratio
relative to other cell types and are susceptible to
cla (...truncated)