Pathogenic mitochondrial DNA point mutations in the immune system : ‘A tale of two cities’

Study I: The Effects of High Heteroplasmic Pathogenic mt-tRNA Mutations on Immunity. High levels of heteroplasmic pathogenic mtDNA mutations in mitochondrial tRNA (mt-tRNA) genes can severely disrupt mitochondrial translation and oxidative phosphorylation, leading to multisystemic disorders. However, the detailed impact of these mutations on the immune system at a molecular level is not fully understood.

In this study, we investigated the immune systems of mice with the m.5024 C>T mutation in mt-tRNA^Ala and human patients with MELAS syndrome, or carriers of the m.3243 A>G mutation in mt-tRNA^Leu. Our findings reveal that myeloid cells tend to maintain a higher mutation burden compared to lymphoid cells, despite the impairment of macrophage mitochondrial translation and oxidative phosphorylation by the m.5024 C>T mutation. Additionally, we observed that naturally accumulated memory T and B cells carry lower burdens of pathogenic mttRNA mutations compared to their naive counterparts. This difference was further highlighted during well-controlled immune challenges with vaccines, which confirmed the disparity in an antigen-specific manner.

Mechanistically, our study uncovered that antigen receptor activation induces proliferation in T and B cells, which rapidly dilutes the burden of the m.5024 C>T mutation. This selection process is further accelerated under conditions of metabolic stress and most likely executed at the cell population level. Moreover, post-activation, the high burden m.5024 C>T mutation disrupts CD8+ T cell metabolic remodelling and interferon-gamma (IFN-_) production. These findings suggest that the generation of memory lymphocytes modifies the mtDNA mutation landscape, impacting the regulation of immune responses by pathogenic mt-tRNA variants.

Study II: Investigating the Immunological Impact of the Homoplasmic m.11778 G>A Mutation in mt-ND4 of Complex I. The m.11778 G>A mutation in MT-ND4 leads to a substitution of arginine with histidine at position 340, mildly impairing Complex I activity and altering reactive oxygen species (ROS) production. This mutation is the primary cause of Leber's Hereditary Optic Neuropathy (LHON), a condition characterized by degeneration of retinal ganglion cells, mainly impacting the vision of young adult males.

Building on insights from Study I, which highlighted how high levels of mt-tRNA mutations can impair mitochondrial protein synthesisŃgiven that mtDNA encodes seven critical subunits of Complex IŃthis study further investigates the impact of such mutations, specifically in LHON. The homoplasmic nature of the m.11778 G>A mutation, found in about 85% of LHON patients, provides an ideal cohort disease to study the influence of mtDNA mutations in complex I on systemic immunity.

In Study II, we examined the immune profiles of 45 LHON patients or carriers, comparing them to 45 healthy individuals from the Stockholm region. We observed a tendency for LHON patients or carriers to accumulate more effector/memory cells, suggesting a shift towards an immunosenescence profile in the overall immune cell composition. Additionally, the homoplasmic m.11778 G>A mutation was found to disrupt T cell activation at the transcriptome level during short-term stimulation.

Mechanistically, this study delves into the enigmatic hyperactivation and 'memory' phenotype tendency of LHON T cells by exploring: 1. The intricate patterns of calcium influx triggered by antigen receptor-mediated T cell activation; 2. The dynamic phosphorylation signaling cascades, both proximal and distal, initiated by T cell receptor activation; and 3. The short and long-term proliferation of T cells and the potential metabolic rewiring that accompanies this process.

Study III: Role of I_BNS and Oxidative Phosphorylation in B Cell Activation and Plasma Cell Differentiation. In Study III, we investigate the role of I_BNS and oxidative phosphorylation in B cell activation and plasma cell differentiation using an I_BNS-deficient mouse model. I_BNS is a nuclear immune related gene, its deficiency offers valuable insights into metabolic processes influencing immune cell function. LPS-stimulated I_BNSdeficient B cells exhibited increased mTOR activation, higher OXPHOS activity, greater mitochondrial biogenesis or accumulation, and elevated mitochondrial reactive oxygen species production compared to wildtype B cells, along with reduced autophagic capacity. Accelerated Blimp-1 upregulation in these cells correlated with more pre-plasmablasts and plasmablasts but lower CD138 levels and impaired PC differentiation. These findings highlight I_BNS's crucial role in B cell metabolism and differentiation, emphasizing the impact of metabolic dysregulation on the immune system.

List of scientific papers

I. Zhang J, Koolmeister C, Han J, Filograna R, Hanke L, Ëdori M, Sheward DJ, Teifel S, Gopalakrishna S, Shao Q, Liu Y, Zhu K, Harris RA, McInerney G, Murrell B, Aoun M, BŠckdahl L, Holmdahl R, Pekalski M, Wedell A, Engvall M, Wredenberg A, Karlsson Hedestam GB, Castro Dopico X*, Rorbach J*. Antigen receptor stimulation induces purifying selection against pathogenic mitochondrial tRNA mutations. JCI Insight. 2023 Sep 8;8(17): e167656. *Co-senior authors.
https://doi.org/10.1172/jci.insight.167656

II. Jingdian Zhang, Qi Chen, Jinming Han, Qiuya Shao, Lakshmikanth Tadepally, Constantin Habimana Mugabo, Hugo Barcenilla, Marcin Pekalski, Anna Wredenberg, Martin Engvall, Petter Brodin , Xaquin Castro Dopico*, Joanna Rorbach*. Ancestral mitochondrial complex I mutations drive T cell exhaustion. *Co-senior authors. [Manuscript]

III. Erikson E, çdori M, Khoenkhoen S, Zhang J, Rorbach J, Castro Dopico X, Karlsson Hedestam GB. Impaired plasma cell differentiation associates with increased oxidative metabolism in I_BNS-deficient B cells. Cell Immunol. 2022 May;375:104516. (In this study, I contributed to the molecular metabolic characterization of I_BNS-deficient B cells following activation and proliferation).
https://doi.org/10.1016/j.cellimm.2022.104516