Phenotype and Functional Characterization of B-Cells, T-Cells and Monocytes Following Alemtuzumab Treatment in Patients with Multiple Sclerosis.

Background: Alemtuzumab is a highly effective treatment for relapsing-remitting multiple sclerosis. It selectively targets the CD52 antigen to induce profound T and B lymphocyte depletion, followed by recovery of T cells and B cells with regulatory phenotypes. We previously found that regulatory T cell function is restored with cellular repletion, but there is little known about the functional capacity of regulatory B-cells and peripheral blood monocytes during the repletion phase.

Objectives: The goal of this study was to assess long-term functionality of B-cell and monocyte subsets during recovery from Alemtuzumab-induced peripheral blood cell depletion.

Methods: The study was a single center, cross-sectional cohort analysis of differences in leukocyte subsets collected from cohorts of different MS patients prior to (0), or 6, 12, 18, 24 or 36 months after their first course of treatment with Alemtuzumab. There were up to 20 patients at each time point and all patients in the 18, 24 and 36 month cohorts had two courses of treatment. Absolute numbers and proportions of leukocyte subsets were assessed in whole blood by FACS analysis and functional assays were performed in B cells and monocytes isolated from PBMC.

Results: The absolute number and percentage of cells with a regulatory B cell phenotype were significantly higher in most Alemtuzumab-treated patient cohorts and were positivity correlated with regulatory T cells. B-cells from treated patients secreted higher levels of IL-10 and BDNF, and inhibited the proliferation of autologous CD4⁺ CD25^- T cell targets. There was little change in number or percentage of monocytes, or the ratio of monocyte subsets. However, following the second course of treatment at 12 months, CD14⁺ monocytes demonstrated a significant anti-inflammatory bias in cytokine secretion patterns. There were no correlations between any of the monocyte measures (phenotype or function) and any of the measures of regulatory T cells or B cells.

Conclusions: Our results confirm that the immune system in Alemtuzumab-treated patients is biased in favor of a more regulatory milieu that involves expansion and increased functionality of multiple regulatory populations including B cells, T cells and monocytes. These findings justify the study of all regulatory cell types following Alemtuzumab treatment so as to reveal further insights into mechanisms of drug action, and to identify key immunological predictors of durable clinical efficacy in Alemtuzumab treated patients.