Immune Cell Mitochondrial Phenotypes Are Largely Preserved in Mitochondrial Diseases and Do Not Reflect Disease Severity

Background and Objectives: The aim of this study was to profile immune cell mitochondrial phenotypes in mitochondrial diseases (MitoD) and evaluate how these phenotypes relate to disease manifestations or biomarkers. Methods: We profiled mitochondrial content and oxidative phosphorylation (OxPhos) enzymatic activities in isolated monocytes, lymphocytes, neutrophils, platelets, and mixed peripheral blood mononuclear cells (PBMCs) from 37 individuals with MitoD (m.3243A > G, n = 23; single, large-scale mitochondrial DNA (mtDNA) deletions, n = 14) and 68 healthy women and men from the Mitochondrial Stress, Brain Imaging, and Epigenetics study. Results: We first confirmed and quantified robust cell type differences in mitochondrial content; activities of OxPhos complexes I, II, and IV; and the mitochondrial respiratory capacity (MRC) index. In relation to MitoD, neither mitochondrial content nor OxPhos capacity was consistently affected, other than a mild monocyte-specific reduction in complex I (partially mtDNA encoded) relative to complex II (entirely nDNA encoded), consistent with the mtDNA defects examined. Relative to the large differences in cell type-specific mitochondrial phenotypes, differences in MitoD relative to controls were generally small (