Background: Primary Sjögren’s Syndrome (pSS) shares with SLE and RA a variety of features including their predominance in women, immunological characteristics and genetic risk background. For these reasons, to dissect the heterogeneity across these IMIDs is paramount to design precision medicine strategies. Single cell transcriptomics allows the interrogation of patients’ samples at great resolution to inform on both common and specific pathogenic molecular mechanisms that drive disease. This is particularly relevant not only for the discovery of potentially actionable targets, but also to identify which patients could benefit from such novel therapeutic interventions. Despite pathogenic mechanisms can be organ-specific, immune cells are mobilized via peripheral blood and many inflammatory responses are initiated by blood cells. In that respect, the profiling of peripheral blood mononuclear cells (PBMCs) is important since PBMCs may show cell-type signatures that can define disease specific pathogenesis, with the advantage of being an easily accessible tissue.

Objectives: To study peripheral blood cells on pSS, SLE and RA patients one cell at the time, and to decipher the immune cell landscape in the peripheral blood underlying disease.

Methods: PBMCs from 8 pSS and 5 healthy (H) donors were isolated in Ficoll gradient, to generate a suspension of viable single cells that were processed with Chromium Single Cell 3’ kit v3 (10X Genomics). A median of 8,008 cells per sample were profiled. Raw sequences were demultiplexed and mapped against the genome using Cellranger. Downstream analyses, including QC filtering, normalization, and integration were done with Seurat. Immune cell types were automatically annotated using Azimuth. Single cell data from 6 RA patients (71,224 cells) and 8 SLE patients (71,224 cells), was obtained from the DoCTIS Consortium project.

Results: A total of 110,378 single cells (67,192 from pSS and 43,186 from HC) passed QC and were analyzed. Differential cell abundance analysis among the 4 groups (H, pSS, SLE and RA) showed a significant increment in the CD8 lymphocyte population with CD8 _TEM features, that is specific to pSS. In turn, reduced numbers of circulating CD8 naïve T cells were observed in pSS compared to healthy individuals, a cell abundance change that is also found in RA but not in SLE. Gene expression analysis revealed that CD14+ monocytes are the cell type that shows the highest number of differentially expressed genes (DEGs) in pSS compared to controls. Analysis of these DEGs in CD14+ monocytes from SLE and RA patients showed a significant antagonism, with many genes showing a differential expression in the opposite direction. Second to monocytes, CD4 _TCM lymphocytes showed also a large number of significant DEGs between pSS patients and controls. In this case, no significant antagonism was found in CD4 _TCM from RA or SLE. Pathway analysis of the genes exclusive to pSS (i.e. differentially expressed in pSS but not in SLE or RA), identified an association with the regulation of type I interferon and tumor necrosis factor production. Among the former, the family of genes encoding 2′–5′ oligoadenylate synthetases ( OAS1 to OAS3 ) were found to be significantly overexpressed in CD14+ monocytes. OAS1 is a well-known risk gene for pSS associated at a genome-wide level (Thorlacius et al).

Conclusion: We have generated a comprehensive map of cells from pSS PBMCs and compared it with healthy donors as well as SLE and RA, covering a total of 241,841 cells. This study provides a valuable resource for further in-depth analysis and reveals relevant disorder and cell-specific transcriptomic changes such as the upregulation of OAS family of IFNI response genes in CD14+ monocytes providing further clues into the pathological mechanisms driving the disease.

REFERENCES: [1] Thorlacius, Gudny Ella, Albin Björk, and Marie Wahren-Herlenius. “Genetics and epigenetics of primary Sjögren syndrome: implications for future therapies.” Nature Reviews Rheumatology 19.5 (2023): 288-306.

Acknowledgements: NIL.

Disclosure of Interests: None declared.