Mitochondrial phenotypes in purified human immune cell subtypes and cell mixtures

Abstract

How mitochondria functionally differ between immune cell subtypes, between the sexes, across ages, and whether they dynamically change over time has not been characterized. Here we deploy a high-throughput mitochondrial phenotyping platform to define cell-type specific mitochondrial features in circulating immune cell subtypes. In women and men spanning four decades of life, we find that mitochondrial content, mitochondrial DNA copy number (mtDNAcn), and respiratory chain enzymatic activities vary by up to 3.5-fold between neutrophils, monocytes, B and T lymphocyte subtypes. Within individuals, mitochondrial content and mtDNAcn, and to a lesser degree respiratory chain function, are strongly correlated with each other across immune cell types, suggesting their systemic harmonization. Moreover, repeated weekly measurements of mitochondrial features in the same individual reveal remarkable variation in mitochondrial content and respiratory chain function over time. This suggests that immune cell mitochondria exhibit dynamic state properties, which we find to be partially correlated with circulating biomarkers. We also define multivariate mitochondrial phenotypes - mitotypes - that distinguish lymphoid from myeloid cell types, naïve-to-memory lymphocyte states, and moderately differ between women and men. Finally, we compare mitochondrial features of purified cell subtypes to peripheral blood mononuclear cells (PBMCs) and determine the influence of contaminating platelets and of variable cell type composition. Our results invite caution in using cell type mixtures to infer person-level mitochondrial behavior. Together, these findings identify dynamic cell-type specific variation in mitochondrial biology among circulating leukocytes and provide foundational knowledge to develop interpretable blood-based assays of mitochondrial health.

Age-Dependent Decline in Neuron Growth Potential and Mitochondria Functions in Cortical Neurons

“…This increase was linked to oxidative stress responses and suggested as a possible mechanism to compensate for defects in mitochondria retaining mutated mtDNA [ 38 ]. Other previous studies have consistently reported an age-related decline in mtDNA copy number (mtDNAcn) [ 39 , 40 , 41 , 42 ], but these results are confounded by mixed cell samples and contamination from platelets, which inflate mtDNAcn and decline in number with age [ 43 ]. A study specifically examining peripheral blood mononuclear cells observed a positive correlation between mtDNAcn and age [ 43 ].…”

“…Other previous studies have consistently reported an age-related decline in mtDNA copy number (mtDNAcn) [ 39 , 40 , 41 , 42 ], but these results are confounded by mixed cell samples and contamination from platelets, which inflate mtDNAcn and decline in number with age [ 43 ]. A study specifically examining peripheral blood mononuclear cells observed a positive correlation between mtDNAcn and age [ 43 ]. This may be used by cells as a compensatory upregulation to counteract the loss of functionally intact mitochondria with age [ 44 , 45 ].…”

“…However, this study examined whole CNS tissue, as opposed to the cortical neuron cell population examined in the current paper. There have been several previous studies that have shown an increase in mtDNAcn but not necessarily a corresponding increase in mitochondrial function, both in disease [ 48 , 49 , 50 ] and aging [ 43 ]. A recent study in polymerase gamma (POLG) mutator mice showed elevated mtDNA copy number but decreased OXPHOS function, despite having normal OXPHOS complex levels [ 49 ].…”

Characterization of mitochondrial DNA quantity and quality in the human aged and Alzheimer’s disease brain

“…The mtDNAcn has been suggested as a biomarker for aging because several studies of peripheral blood have reported an inverse correlation between age and mtDNAcn (Fries et al, 2017; Mengel-From et al, 2014). However, more recent studies demonstrated that the inverse correlation is likely caused by unaccounted age-related changes of cell type proportions in the blood (Moore et al, 2018; Rausser et al, 2021). In our study, we found no association between mtDNAcn and age or sex in the studied brain regions when we adjusted for pathologies.…”

Utilization of Human Samples for Assessment of Mitochondrial Bioenergetics: Gold Standards, Limitations, and Future Perspectives

“…An important consideration for blood-based bioenergetics is selecting an appropriate cell type to use for respirometry experiments and whether mixed cell populations, such as PBMCs, are suitable samples for respirometry. Rausser et al used a high-throughput mitochondrial phenotyping platform to compare PBMCs to leukocyte subtypes from the same individual [135]. They show large functional differences in both mitochondrial content and respiratory chain enzymatic activity between immune cell subtypes.…”