Immune Defects of B- and T-Cell Compartments in Natalizumab-Induced Progressive Multifocal Leukoencephalopathy,

Progressive multifocal leukoencephalopathy (PML) is a rare, but often fatal demyelinating brain disease caused by the JC virus, which usually occurs in immunosuppressed patients, including those with hematological malignancies or receiving monoclonal antibodies-based immunotherapies. PML is likely the result of a complex combination of several pathogenic mechanisms, such as alterations of peripheral cell-mediated immunity and mobilization of JC virus-carrying CD34⁺-hematopoetic stem cells and pre-B-cells. Taking advantage of the availability of samples from a multiple sclerosis (MS) patient treated with the anti-α4β1 monoclonal antibody natalizumab who developed PML, which was monitored for 35 months since before therapy initiation, we investigated the role of B and T lymphocytes in PML onset.

Real-Time PCR was used to measure the release of T and B cells from the production sites by means of T-cell receptor excision circles (TRECs) and K-deleting excision circles (KRECs) analysis and to quantify transcripts for immature hematopoietic cells such as terminal deoxynucleotidyl transferase, CD34, and pre-B lymphocyte gene 1. Naïve and mature T- and B-cell subsets were identified by flow cytometry, T-cell heterogeneity was quantified by spectratyping and IgA, IgG and IgM by turbidimetric assay. Data were compared to those of untreated and natalizumab-treated MS patients and healthy donors.

After 34 months of natalizumab therapy, a 42 years old female developed PML, diagnosed on the basis of magnetic resonance imaging and JC virus positivity in cerebrospinal fluid. Before therapy, her thymus and bone marrow produced a significant low number of TRECs⁺ and KRECs⁺ cells. While TRECs remained low during all therapy period, KRECs and transcripts for pre-B lymphocyte gene 1, which is selectively expressed in pre-B cells, peaked after 6 months of therapy, remained high at 12 and 15 months of treatment, and then decreased at the moment of PML onset. Flow cytometry confirmed a deficient production of CD4⁺CD45RA⁺CCR7⁺CD31⁺ recent T emigrants, counterbalanced by an increased number of CD8⁺CCR7^–CD45RA⁺ TEMRA cells for all observation period, but showed a modification of peripheral CD4 and CD8 cell number only at the moment of PML. While the percentage of naïve B cells increased by about 70% after 6 months of therapy, the number of B lymphocytes within each B-cell subpopulations remained low for the entire treatment period. T-cell repertoire and immunoglobulin production were altered.

Although performed in a single patient, data all agree in demonstrating that a deficient production of new TRECs⁺ T lymphocytes, together with an increase of newly produced KRECs⁺ B cells just few months after therapy beginning may predispose to PML. These findings encourage further researches on the utility of the TRECs/KRECs assay as a potential tool for the identification of patients at risk of developing PML after monoclonal antibodies-based therapies.

No relevant conflicts of interest to declare.