Lymphocyte subsets show different response patterns to in vivo bound natalizumab--a flow cytometric study on patients with multiple sclerosis.

Natalizumab is an effective monoclonal antibody therapy for the treatment of relapsing-remitting multiple sclerosis (RRMS) and interferes with immune cell migration into the central nervous system by blocking the α(4) subunit of very-late activation antigen-4 (VLA-4). Although well tolerated and very effective, some patients still suffer from relapses in spite of natalizumab therapy or from unwanted side effects like progressive multifocal leukoencephalopathy (PML). In search of a routine-qualified biomarker on the effectiveness of natalizumab therapy we applied flow cytometry and analyzed natalizumab binding to α(4) and α(4) integrin surface levels on T-cells, B-cells, natural killer (NK) cells, and NKT cells from 26 RRMS patients under up to 72 weeks of therapy. Four-weekly infusions of natalizumab resulted in a significant and sustained increase of lymphocyte-bound natalizumab (p