The agents that are highly-effective deplete memory B cells well and those that are not highly-effective don’t.
We didn’t have the data on whether cladribine depletes memory B cells, but as cladribine is highly effective in treating relapsing MS.
It suggests that they should be depleted.
The hypothesis can be tested, as we have been using cladribine off-label.
If you want to know the answer…read on
You can make proteins in two ways :
(2) Salvage pathway. You take proteins and break them down and re-use the amino acids/nucleosides to make new proteins.
This process is countered by cytoplasmic 5 prime nucleotidases (5′-NT) which dephophorylate (remove phosphate groups) from ATP, ADP and AMP. These are also present in mitochondria (5’NTM) and on the cell surface (5’NTE (ecto)).
The observation of marked depletion of memory B cells is consistent with the high efficacy of cladribine in relapsing MS.
It was nice to see the protein expression that we found matched the expression profile of the gene message that we found (see below).
Different types of B cells expressed varying amounts of the phosphorylating enzymes. It was found that naive and memory cells express a lot and make it likely that they are killed by cladribine, especially as they also have low levels of cytoplasmic 5’NT and adenosine diaminase.
Neutrophils have low deoxycytidine kinase and high levels of 5’NT and so are not killed very much. This is what we found when we looked in the blood of people treated with cladribine. This means that you can still fight infections.
It was found that germinal centre cells (dark green circles in the diagram above), which divide to form memory cells had loads of deoxycytidine kinase and so will be probably killed the most. This will kill the precursors of B memory cells, notably the centroblasts in the B cell follicles (round green areas in the diagram above) of lymphoid tissue. Plasma cells have lower levels of deoxycytidine kinase (see below) and so we predict that cladribine will not deplete antibody forming cells. This means that you shouldn’t loose your vaccine responses to childhood infections.
So after cladribine a number of different types of B cells get it.
However, we think the sensitivity to killing is not what causes a selective depletion of memory B cells. We believe that the major problem is that once depleted memory B cells repopulate very slowly from lymph glands and spleen. This can take many years for repopulation to occur. Maybe this is why the drugs (or HSCT) work for such a long time. This will cause long term inhibition of MS
This contrasts to the more rapid repopulation of immature and naive cells B cells coming from the bone marrow. This makes it look like CD19 cells come back to normal levels and masks the loss of the memory cells
We currently believe memory B cell depletion is a common mechanism of action of MS treatments. Memory B cells are a reservoir of Epstein Barr virus and therefore cladribine will indirectly get rid of EBV. EBV can make B cells become independent of T cell help so maybe you do not need T cells. At least we can put B cells at the top of the pile of important cells.
The thorn in the side is course…….evidence.
Depletion of CD4 T cells and depletion of Th1/TH17 does not do a great deal in MS. Yes this can do something in EAE but EAE is not MS and animals do not get infected with EBV, unless you are a non human primate.
Cladribine does not work in rodents because they have a different balance of deoxycytidine kinase, 5’NT and adenosine deaminase. So when you see claims in rodents, you know the study is probably pants.
Ocrelizumab (CD20-specific) is a B cell depleting agent that works well in MS, because I predict it depletes memory B cells very well.
You draw the same blank. It is all circumstantial.
It is hard to prove things categorically in humans. Just because EAE is a T cell-mediated problem is not sufficient argument.
Please feel free to send this link to your friends so that you can read the whole paper. This is relevant to the use of the oral variant too.
However, people should do the experiments to disprove the hypothesis for the central importance of (memory) B cells in MS. Disproving ideas is how science evolves.
Over the past year there have been a lot of attempts to and these have so far failed.
Ceronie B, Jacobs BM, Baker D, Dubuisson N, Mao Z, Ammoscato F, Lock H, Longhurst HJ, Giovannoni G, Schmierer K. Cladribine treatment of multiple sclerosis is associated with depletion of memory B cells. J Neurol 2018;
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.