Specific Patterns of Immune Cell Dynamics May Explain the Early Onset and Prolonged Efficacy of Cladribine Tablets

Discussion

The primary analysis of the MAGNIFY-MS study, reported elsewhere, was conducted to determine the onset of action of cladribine tablets by observing changes in MRI lesion counts.¹⁹ In this study, we studied dynamic changes of various cellular and noncellular immune components. Cladribine induces a specific pattern of early and sustained PBMC subtype dynamics in the absence of relevant Ig changes: While total B cells were reduced dramatically, T cells were affected significantly less. While naive B cells recovered toward baseline, naive CD4 and CD8 T cells did not and memory B cells remained reduced until month 12. The results from this substudy help to explain the unique immune depletion and repopulation architecture regarding the onset of action and durability of effects of cladribine tablets while largely maintaining immune competence. The immunologic durability of cladribine tablets beyond this first year is still to be assessed as part of the full 2-year study findings of MAGNIFY-MS. Such results will add to previous studies that have reported on dynamics of major cell types during treatment with cladribine tablets 3.5 mg/kg cumulative dose over 2 years.^17,18

While we observed a fast and profound reduction in B-cell subtype counts with cladribine tablets (months 1 and 2), these cell counts recovered well in the following months. By contrast, memory B cells remained close to their lower nadir level to month 12. This confirms the findings from a small observational study of Italian patients treated with cladribine tablets²¹ and those from a cross-sectional study of people with MS treated with a different preparation of cladribine.²² The strong association between the time course of B-cell subtype depletion and the apparent inhibition of new lesion evolution on MRI demonstrated in MAGNIFY-MS, as well as the sustained and significant reduction in memory B cells over at least 12 months, suggest an important role of this specific B-cell subset for sustained disease control in people with MS.^5,23 Monoclonal B-cell depleting antibodies such as alemtuzumab,²⁴ ocrelizumab,²⁵ or rituximab²⁶ (although the latter is not licensed, it is commonly used off-label in people with MS)²⁷ lead to similar patterns of rapid and general B-cell depletion followed by reconstitution of B-cell subsets, except for memory B cells. Even with alemtuzumab, although targeting a broad range of T and B lymphocyte subsets, a distinct pattern of B-cell lysis followed by reconstitution (again, with the exception of memory B cells) can be observed.²⁴

The PBMC subtype dynamics identified in the MAGNIFY-MS study may not only explain how cladribine tablets effectively reduce MS disease activity and disability accrual but also indicate why patients treated with cladribine tablets largely retain immune competence and thus may be able to mount a relevant vaccine response, such as that seen in response to COVID-19 mRNA vaccines.²⁸ While there was a decrease in B-cell subtypes as early as month 1 after initiating treatment with cladribine tablets, naive B cells started to recover from month 2 and reached near baseline levels at month 12. It is these naive B cells that are associated with early immune responses and the generation of antibodies in response to new immune challenges (e.g., infections, vaccination). Indeed, another substudy of the MAGNIFY-MS population has reported a favorable humoral response to varicella zoster virus and influenza vaccines, including patients vaccinated against influenza shortly after initiating treatment with cladribine tablets.²⁹

The results also show that cladribine is able to directly affect and reduce plasmablasts, which may be associated with disease pathology, by means of autoantibody production and direct cellular toxicity within the CSF.³⁰ Most current DMTs for the treatment of MS do not target the plasmablast and plasma cell subtypes directly (including CD20 B cell depleting agents),²⁵ a cell subset potentially indicative of more severe MS disease.³¹

Despite the sustained decrease of memory B cells and a decrease in CD38⁺ plasmablasts in the first 12 months of MAGNIFY-MS, serum IgG and IgM levels remained within the normal range over this period. Atacicept, which was shown to target long-lived plasma cells, shows a fast onset of Ig reduction at 4 weeks.³² Consequently, it is likely that long-lived CD138⁺ plasma cells would not be affected by treatment with cladribine tablets given the unchanged profiles of total serum IgG and IgM up to 12 months, although this subtype was not evaluated in our substudy.

B_reg and B_trans cell subtypes contribute to the maintenance of immune tolerance and the modulation of immune responses.^33,34 A variety of B-cell subtypes have been documented as B_reg cells, with CD19⁺CD24^hiCD38^hi B cells being identified as an interleukin (IL)-10–expressing subtype that may participate in suppressing the increased autoimmune responses observed in MS. B_reg cells have not yet been studied for all MS DMTs; however, some recent data have shown that after alemtuzumab treatment levels of CD19⁺CD24^hiCD38^hi, B_reg cells are reduced after severe relapses and subsequently repopulate during recovery.^35,36 In this substudy, B_trans cells showed a similar repopulation profile to B_reg cells. These B_trans cells represent one of the B_reg cell subpopulations in healthy individuals, but the frequency of such cells in circulation may be altered in individuals with MS.³⁴ B_trans cells can also produce IL-10, as well as transforming growth factor-β, and can regulate CD4⁺ T-cell proliferation and differentiation toward Th effector cells. Given the sustained decrease of mature B-cell subtypes, especially memory B cells, along with an increase in B_reg and B_trans cells in the first year after treatment with cladribine tablets, it is believed that an increased B_reg/B cell (B_trans/B cell) or B_reg/B memory (B_trans/B memory) ratio might contribute to the prolonged clinical efficacy of cladribine tablets. Future analyses may determine the intracellular cytokine expression and the B_reg vs B effector subtype ratios and their correlation with clinical efficacy end points.

At month 12, the total B-cell count after treatment with cladribine tablets was generally more than 30% below baseline levels. At this same time point, an overshoot of the total B-cell count of about 30% above baseline has been reported in the pivotal studies of alemtuzumab.^24,37 However, our analysis of the MAGNIFY-MS data reported here revealed that while B memory cells remain depleted, B_reg and B_trans cells exceed their baseline values by month 3 and remain elevated above baseline at month 12 at +50% and +36%, respectively. Hence, rebound and overshoot of B-cell subsets alone may not sufficiently explain why secondary B-cell autoimmunities do not seem to occur with cladribine tablets, yet are so common after treatment with alemtuzumab. The reported secondary autoimmunities reported for alemtuzumab include glomerulonephritis, immune thrombocytopenia, sarcoidosis, thyroid autoimmunity, and vitiligo.³⁸

As previously suggested, the less pronounced effects of cladribine on T cells, particularly during the first few months after treatment, may be key, alongside the described immune cell dynamics of specific B-cell subsets, for the protection from secondary autoimmunity with cladribine tablets.²⁴ A recent in-depth immune phenotyping study³⁸ provided further evidence of alemtuzumab-induced changes to lymphocytes and the underlying immunologic processes involved in secondary autoimmunity. Autoimmunity after treatment with alemtuzumab is associated with homeostatic proliferation of T cells, and it has been suggested that this occurs in the context of a defective thymus.³⁹ However, antibody-mediated T-cell depletion typically leads to expansion of memory cells and is not associated with the development of B-cell autoimmunities.^24,40,41 It is therefore hypothesized that in the context of B-cell depletion and recovery with alemtuzumab, the associated depletion of T_reg cells, normally silencing autoreactive immature B cells exiting the bone marrow as they mature into naive B cells,^42,43 facilitates secondary autoimmunity.^24,44,45 Because autoantibody production is likely to be CD4⁺ T-cell–dependent, autoimmunity will require T-cell support, and thus, autoimmunity is unlikely to occur until CD4⁺ T-cell numbers recover, explaining the delay between the time of B-cell hyper-reactivity and the development of secondary autoimmunity.²⁴

The reduction and repopulation profile for CD8⁺ T cells was comparable with CD4⁺ T cells for both alemtuzumab and cladribine, respectively. A further, more recent study has also shown alemtuzumab to decrease CD4⁺ and CD8⁺ T-cell counts followed by a period of repopulation, with CD8⁺ T cells increasing above the lower limit of normal at 12 months after treatment.³⁶ For CD8⁺ T cells, there was a small percentage decrease from baseline to week 2, which was then maintained throughout the study period. In addition to the greater reduction of T cells with alemtuzumab, CD4⁺ effector memory and CD8⁺ TEMRA T cells have been shown to be substantially expanded during the first 6 months after treatment.³⁹ Such clonal expansion has not been observed with cladribine tablets.

Initial analysis of the full 2-year safety findings from the present MAGNIFY-MS study have not raised such concerns for cladribine tablets; indeed, no treatment-related serious AEs were reported (data on file; the healthcare business of Merck KGaA, Darmstadt, Germany). Regarding lymphopenia, very few patients experienced grade 4 events (0.7%), with 24.4% and 54.4% of patients, respectively, experiencing grade 3 and grade 1 or 2 lymphopenia (data on file; the healthcare business of Merck KGaA, Darmstadt, Germany).

While PBMC counts in this study were measured in peripheral blood samples, it should be remembered that cladribine penetrates into other tissues including the CNS shortly after administration,⁴⁶ raising the possibility of direct effects on CNS resident lymphocyte subtypes. Further studies to explore these potential effects are under way.⁴⁷

One shortcoming of this study is that we did not directly correlate the MRI findings with the PBMC subtype dynamics reported here. Our data, however, suggest an early and subsequent sustained effect of cladribine tablets in a specific pattern of depletion and reconstitution of B-cell and T-cell subtypes. The pronounced reduction of B-cell subsets from month 2 and the sustained depletion of memory B cells seem most closely associated with the effects of cladribine detected on MRI and clinical outcomes.^13,17 The contribution of moderate reductions across T-cell subtypes, as well as the sustained increase of regulatory and transitional B cells from month 3 onward, are less clear. While they might contribute to the long-term therapeutic effect of cladribine tablets, the limited reduction of T-cell subsets is likely advantageous for safety. In the context of safety, it should also be noted that, despite the sustained decrease of memory B cells and a decrease in CD38⁺ plasma cells, no relevant changes in serum IgG and IgM concentrations were observed over the 12-month study period.