Fluid biomarkers for microglial activation and axonal injury in multiple sclerosis.

Although it is clear that the immune system is an important disease driver in multiple sclerosis (MS), it is presently unknown what initiates the process. Infections have been mentioned as potential triggers, which is specifically dealt with in other articles of this volume. Here, I give an overview of two fluid biomarkers that reflect key elements of the MS process: microglial activation (cerebrospinal fluid [CSF] sTREM2) and axonal injury (CSF and serum/plasma neurofilament light). I review recent data on how these markers are altered in MS, how they change in relation to disease progression and treatment and, finally, how they can be used as tools in MS research.