Background/Purpose: Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by the production of pathogenic anti-nuclear antibodies (ANA) by plasma cells (PC).  Importantly, long-lived plasma cells (LLPC) residing in the bone marrow (BM) remain a challenging therapeutic target. We previously reported that the BM PC are less susceptible to therapies such as selective inhibitors of nuclear export (SINE) compared to their blood-derived counterparts, plasmablasts (PB).  We have also shown that bulk transcriptomic analysis of BM PC reveals an interferon (IFN) gene signature and that these cells remain responsive to type I IFN as determined by the phosphorylation of STAT1.  Anifrolumab is a monoclonal antibody against type I IFN receptor approved for therapeutic use in SLE. Here, we explore the targeting of BM PC by anifrolumab using in vitro techniques.

Methods: Matched SLE and healthy control (HC) human peripheral blood (PB) and BM mononuclear cells (MC) were phenotyped by multiparameter flow cytometry (HC: n = 7 and SLE: n = 9). Human IFN-α was quantitated by an all-subtype ELISA and ANA titer was determined by immunofluorescence in BM supernatant and blood serum in SLE (n=9) and HC (n=7) subjects. IFN response gene mRNA normalized counts were quantified by bulk RNA-sequencing from BM PC and blood PB. Anifrolumab was labeled with Alexa Fluor-647 for use in flow cytometry and immunofluorescence microscopy.  The binding of labeled anifrolumab to PBMC and BM B cell subsets and PC was quantified as median fluorescent intensity (MFI) in age-, race-, and sex-matched HC and SLE BM and PBMC samples (n=4 HC, n=4 SLE).

Results: BM supernatant and blood serum IFN-α levels were highly correlated (spearman correlation of 1 with p< 0.0001, simple linear regression coefficient of 0.998). Donors with detectable IFN-α had higher normalized counts of IFN response genes MX1 or IFI27 in blood PB and BM PC. BM CD19+ PC from SLE had significantly higher normalized counts of IFIT1 (p = 0.009), IFI27 (p=0.009), and IFI44 (p=0.03) than BM CD19+ PC from HC. ANA positivity in serum correlated with ANA positivity in BM supernatants. Labeled-anifrolumab bound to canonical B cell subsets and PC in the blood and BM, with higher binding detected in the blood. In healthy controls, less anifrolumab bound to activated naïve B cells compared to resting naïve B cells, as well as IgD- CD27- DN1 and DN2 populations, suggesting that IFNAR1 expression is downregulated in activated B cells.

Conclusion: Our data show a strong correlation between IFN-α levels in BM and blood and highlight the differential expression of IFN response genes in SLE PC. However, the lower binding of anifrolumab to BM PC in SLE patients, compared to its higher binding capacity in the blood, suggests that while anifrolumab effectively targets circulating B cell subsets, its efficacy in targeting BM-resident PC may be more limited.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/type-i-interferon-and-anifrolumab-effects-on-bone-marrow-and-blood-plasma-cells-in-systemic-lupus-erythematosus/