Chronic fatigue and depression due to multiple sclerosis: Immune-inflammatory pathways, tryptophan catabolites and the gut-brain axis as possible shared pathways

•Chronic fatigue and major depression (MDD)-like symptoms are common manifestations of multiple sclerosis (MS), both with huge impact on quality of life.•It seems likely that proinflammatory cytokines, tryptophan catabolites (the KYN pathway) and the gut-brain axis are involved in the mechanisms causing chronic fatigue and MDD-like symptoms in MS.•In future research, MS-related fatigue and depression should be examined using a longitudinal design and both immune-inflammatory and KYN pathway biomarkers should be measured, relevant clinical characteristics judiciously registered, and self-report instruments for both fatigue and depression should be used.