Background/Purpose: Pathogen-associated transient antiphospholipid antibodies suggest a microbial trigger for antiphospholipid syndrome (APS). We hypothesized that the gut microbiota could represent a chronic stimulus in APS patients and defined the fecal microbiome and IgA-coated microbiota in APS patients over time and correlated them with autoantigen-related responses.

Methods: Anti-β₂-glycoprotein I (β₂GPI)-positive APS patients, patients with non-autoimmune thrombophilic states, and healthy donors were sampled at 3 time points (0, 4 and 8 weeks). Sixty stool samples of 22 APS patients, 13 samples of 6 controls, and 49 samples of 19 healthy donors were collected to date including dietary and medication history as well as HLA-DR53 genotype. PBMC proliferation to β₂GPI was determined by [³H]-thymidine incorporation and autoantibodies against β₂GPIdomain I using a standardized kit (Inova). Stool DNA was PCR-amplified using barcoded primers targeting the V4 region of the 16S rRNA gene. Samples were sequenced using 2x250bp paired-end reads on an Illumina MiSeq instrument and analyzed using Quantitative Insights Into Microbial Ecology (QIIME) and Linear Discriminant Analysis Effect Size (LEfSe) methods.

Results: PBMCs from HLA-DR53+ APS patients responded preferentially to β₂GPI compared to controls. APS versus control fecal microbiomes showed a significant decrease in the genus Bilophila and an increase in the genus Slackia within the family of Coriobacteriaceae across multiple time points. 59% of APS patients but none of the control subjects were persistently positive for anti-domain I (DI) antibodies. Anti-DI IgG positivity correlated significantly with increased abundances of Slackia and a decrease in the butyrate-producing genus Butyricimonas (p<0.01 for each taxon).

Conclusion: High-throughput sequencing of APS gut microbiomes revealed a persistent enrichment of the genus Slackia over time, particularly among the anti-DI IgG+ APS patients. Slackia is capable of producing phospholipids including cardiolipin. We speculate that gut commensal-derived cardiolipin contributes to a conformational change of β₂GPI exposing DI, thereby promoting autoreactivity against the major B cell epitope. Reduction of the butyrate producer Butyricimonas might associate with less regulatory T cells allowing for stronger proliferation towards β₂GPI as butyrate is known to directly induce this cell type. In summary, this ongoing microbiome study represents the first 16S rRNA community profiling in APS patients and uncovered an altered microbial community with potential implications for the pathogenesis of APS that will be tested in follow-up studies.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/cardiolipin-producing-candidate-commensals-in-the-gut-microbiome-of-antiphospholipid-syndrome-patients/