Treatment with a single dose of Ocrevus (ocrelizumab) depleted a subset of immune T-cells within two weeks in patients with relapsing multiple sclerosis (MS) or primary progressive MS (PPMS), according to a study.

The study, “Ocrelizumab Depletes CD20+ T Cells in Multiple Sclerosis Patients,” was published in the journal Cells.

Autoreactive immune T-cells, which attack the body’s own tissues, have been regarded as the primary mediator of MS; however, this view has been challenged by the effectiveness of therapies targeting immune B-cells that contain the CD20 cell surface protein in reducing disease activity.

One such therapy is Genentech’s Ocrevus, an anti-CD20 monoclonal antibody, which was first approved in the U.S. in 2017 for patients with relapsing MS or PPMS.

Because CD20 is mainly expressed by B-cell precursors and mature B-cells, Ocrevus is often considered to selectively deplete CD20-containing B-cells. However, CD20 is also expressed by highly activated T-cells with the CD3 protein marker, characterized by the increased production of proinflammatory molecules, or cytokines.

These T-cells are found in the blood, cerebrospinal fluid — the liquid surrounding the brain and spinal cord — and chronic brain lesions of MS patients, and show an elevated expression of the CD8 and CD45 markers.

Off-label use of rituximab (marketed as Rituxan in the U.S. and MabThera in Europe), a lymphoma and rheumatoid arthritis treatment that also targets CD20, has been associated with the depletion of CD20-containing T-cells in MS patients. Therefore, targeting this T-cell subtype has been hypothesized as an additional mechanism for rituximab’s clinical effectiveness.

However, scientists did not know whether Ocrevus, which is different from rituximab in terms of CD20 binding and cell toxicity, also depletes CD20-positive T-cells.

To address this unknown, a team from Hannover Medical School in Germany analyzed blood samples of MS patients through a technique called multicolor flow cytometry prior to the first dose of Ocrevus and after two weeks, immediately before the second dose.

They intended to evaluate the characteristics of the patients’ peripheral blood mononuclear cells, which include T-cells, B-cells, monocytes, and macrophages.

A total of 21 patients (13 women) were included, with a median age of 43 years (range 22-65 years). Of the participants, 17 had the relapsing form of the disease for a median of 14.6 years, while four had PPMS for a median of 5.6 years.

The analysis found T-cells containing CD20 and CD3 in all patients. These cells accounted for 2.4% of all CD45-expressing lymphocytes — white blood cells that include T- and B-cells — and for a significant proportion (18.4%) of all CD20 cells.

Evaluation of the cells’ fluorescence intensity revealed that CD20 levels were significantly lower on T-cells than on B-cells also expressing this marker.

Treatment with one dose of Ocrevus substantially lowered the levels of CD20-positive T- and B-cells within two weeks, reflected by a frequency of 0.04% and an absolute cell count decrease from 224.9 to 0.57/microliter.

“Our results demonstrate that treatment with [Ocrevus] does not exclusively target B-cells, but also CD20+ T-cells, which account for a substantial amount of CD20-expressing cells,” the researchers wrote.

“These findings suggest that CD20+ T-cells might play a pivotal role in the pathogenesis of MS, and we speculate that depletion of CD3+CD20+ cells by anti-CD20 monoclonal antibodies might contribute to the efficacy of anti-CD20 therapy,” they added.

However, they also emphasized that the findings need to be confirmed in studies with larger groups of MS patients.