Introduction: Genetic variants of interferon regulatory factor 5 (IRF5) are associated with susceptibility to systemic lupus erythematosus (SLE). The IRF5 risk haplotype has been shown to associate with elevated IRF5 expression in SLE patients. However, this may have been a result of disease-associated factors rather than haplotype. To avoid interference from the disease, we examined IRF5 expression and function in healthy donors carrying the homozygous risk or non-risk IRF5 haplotype. The aim of this study was to identify the functional contribution of the IRF5 risk haplotype to SLE onset.
Methods: Genotyped healthy donors (n=5 homozygous risk, n=7 homozygous non-risk) were consented as part of the Feinstein Institute Genotype and Phenotype (GaP) Registry. IRF5 expression and cellular localization in PBMC subsets (CD19+ B cells, CD14+ monocytes, BDCA2+CD123+ pDC, and CD66b+ neutrophils) were measured using imaging flow cytometry; basal IRF5 activation was determined by DRAQ5 co-staining. Plasmablasts were characterized by CD19+CD27+CD38hiIgD- surface-staining. Anti-nuclear antibodies (ANA) were detected by Hep2-ANA, and spontaneous NETosis by CD66b, cit-H3 and MPO surface staining.
Results: Surprisingly, there was no difference in IRF5 expression in any immune cell subset examined between homozygous risk and non-risk donors; however, significantly elevated IRF5 activation was detected in pDC and no other immune cell type from risk donors. In addition, we detected significantly elevated circulating plasmablasts in risk donors and positive ANA staining. Spontaneous NETosis was also significantly elevated in risk donors.
Conclusions: Together, data presented here support a contribution of the IRF5 risk haplotype to early SLE disease onset, since plasmablasts are known to be elevated in SLE, and SLE neutrophils undergo spontaneous NETosis. Further, it has previously been shown that autoantibodies occur in SLE patients prior to disease onset. Our study is the first to identify a functional link between IRF5 genetic variation and SLE onset.