Background/Purpose: Systemic lupus erythematosus (SLE) is complex autoimmune disorders characterized by B cell hyperactivity resulting in autoantibody and cytokine production. Approximately 90% of patients are female. We have produced data an X-chromosome gene dose effect increases susceptibility. Therefore, our objective is to functionally describe an X-linked protein which escapes X-inactivation, Chromosome X open reading frame 21 (CXorf21), to uncover any role this protein may have in the pathogenesis or susceptibility to SLE. Publicly available data predict CXorf21 is a dehydrogenase/reductase expressed almost exclusively in monocytes, B cells, and and other antigen-presenting cells. Additional studies show that CXorf21, a SLE risk allele, directly interacts with another SS/SLE-associated risk allele, Slc15a4. SLC15a4, a lysosomal proton-oligopeptide co-transporter, is necessary for endolysosomal antigen processing, TLR7- and NOD1-mediated cytokine as well as antibody production in dendritic cells and B cells.

Methods: We used quantitative real-time PCR, Western blot protein analysis, Bio-plex cytokine immunoassay, and pHrodo™ assay, as well as, in vitro CRISPR-Cas9 knockdown experiments to examine the role of CXorf21 in monocytes and B cell immunity.

Results: Our data show that CXorf21 basal gene and protein expression is elevated in female primary monocytes and B cells compared to male cells. We also found CXorf21 mRNA expression was higher in both male and female SLE-affected EBV-transformed B cells and female primary Sjogren’s Syndrome patient’s PBMC subsets compared to healthy male controls. Additionally, we found that following activation by TLR7 (Imiquimod) and NOD1 (iE-DAP) agonists CXorf21, IFN-alpha, and NFkappaB expression increases, in addition to an increase IL-6 and TNF-alpha cytokine production. This response is exaggerated in a female-specific manner. Successful knockdown of CXorf21, using CXorf21-specific gRNA (CRISPR-Cas9), abrogated both the expression levels and cytokine response in the female samples, but had not affect in male subjects. pHrodo™ lysosomal pH experiments revealed that knockdown of CXorf21 protein in healthy female monocytes resulted in an increased lysosomal pH. As result of the increase from acidic to a more alkaline lysosomal environment in the female samples (auto)antigen processing is perceived to be disrupted.

Conclusion: CXorf21 is over-expressed in female immune cells compared to male cells and is involved in a sex-dependent dimorphic response to activation through TLR7 and NOD1. We propose that CXorf21 via interaction with the lysosome proton cotransporter, SLC15a4, maintains the lysosomal pH gradient necessary for monocyte and B cell immune response. Thus, sexual dimorphic expression of CXorf21, based on escape of X chromosome inactivation, skews (auto)antigen processing and immune response by women compared to men. CXorf21 may be a major contributor to TLR7 disease pathogenesis, and sex bias of the diseases based on an X chromosome dosage effect.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/sex-specific-expression-of-cxorf21-provide-molecular-explanation-for-the-fundamental-difference-in-male-and-female-immune-response-an-explanation-for-female-bias-sle-pathogenesis/