Purpose of review: To provide neurologists with an update on the proposed mechanisms of action (MOAs) of disease-modifying therapies (DMTs) for the treatment of relapsing MS, and their effect on peripheral blood leukocytes, in order to inform treatment decisions.
Recent findings: DMTs have vastly differing MOAs, including effects on peripheral blood leukocyte counts, particularly lymphocytes. The clinical implications of changes in lymphocyte counts need to be understood in the context of the underlying MOAs of each respective DMT, with treatment tailored to individual patient needs.
Summary: DMTs can alter lymphocyte counts, subsets, activation, and distribution, and thus can influence immune surveillance. Serial monitoring of total leukocytes and absolute lymphocyte counts (ALCs) is advisable in patients receiving DMTs. ALCs should be interpreted regarding expected immunologic changes and individual patient characteristics. Any decision to switch DMTs should consider these factors, along with drug efficacy, safety, and effect on quality of life.