Mitochondrial DNA Mismatches in Thoracic Transplantation: Potential Triggers of Allograft Immunogenicity

Abstract

Purpose Organ transplant (Tx) recipients of non-European ancestry experience higher rejection rates. Stem cell research shows that single nucleotide variant mismatches (SNVMs) in mitochondria DNA (mtDNA) of the donor and host cells trigger alloimmune responses. However, no prior studies have addressed mtDNA SNVMs in organ transplantation. We hypothesized that SNVMs between donor-recipient (D-R) mtDNA sequences contribute to allograft immunogenicity. Our purpose was to conduct a proof-of-concept study examining the relationship between mtDNA SNVMs and immunogenicity in human thoracic Tx recipients. Methods We assayed 364 mtDNAs in pre-Tx peripheral blood mononuclear cells (PBMCs) samples from 182 Tx recipients (19 heart and 163 lung) and their respective donors, using customized standard techniques for mtDNA next-generation sequencing to a mean depth of 9750x. MtDNA sequence reads for D-R pairs were compared to a reference: rCRS database. SNVMs were identified and analyzed for relationships to donor and recipient self-reported sex and race. To determine if nonsynonymous SNVMs trigger allo-specific immunity, we constructed peptides with and without SNVMs and ran Elispot assays to quantify Interferon gamma (INFγ) release from recipients’ re-activated PBMCs collected 1 year post-transplant. Results We identified on average 46 (SD=20) mtDNA SNVMs per D-R pair, 13 (SD=7) of which were nonsynonymous and could form allo-specific immunogenic proteins. Nonsynonymous SNVMs did not differ significantly in D-R sex-matched (N=133, M=13, SD=6) and sex-mismatched pairs (N=46, M=14, SD=7), t(72)=0.87, p = 0.386. In contrast, nonsynonymous SNVMs were significantly higher in D-R race-discordant transplant pairs (N=50, M=16, SD=9) compared to race-concordant pairs (N=100, M=12, SD=5); t(64)=3.07, p=0.003. This difference persisted even after excluding 69 mtDNA haplogroups hot spots: race-discordant pairs (N=50, M=11, SD=7) and race-concordant pairs (N=100, M=8, SD=4); t(68)=2.14, p=0.029. Elispot data showed 30-200 fold increase in INFγ release from peptides with SNVM compared to control, p Conclusion The higher frequency of D-R mtDNA SNVMs in race-discordant transplant pairs, and the immunogenicity of peptides generated from patients with mtDNA SNVMs supports the hypothesis that SNVMs may contribute to allograft immunogenicity.