The regulation and pharmacological modulation of immune complex induced type III IFN production by plasmacytoid dendritic cells [RNA-seq]

Objective: Patients with Systemic Lupus Erythematosus (SLE) have an ongoing interferon (IFN) production due to an activation of plasmacytoid dendritic cells (pDCs), which can be triggered to type I IFN synthesis by RNA containing immune complexes (RNA-IC). Considering emerging data suggesting a role of type III interferon (IFN) in the SLE disease process, we asked if RNA-IC can induce type III IFN production in pDC, and how this production can be regulated. Methods: Peripheral blood mononuclear cells (PBMCs) or immune cell subsets were isolated from healthy blood donors or SLE patients and stimulated with IC containing U1 snRNP and SLE-IgG (RNA-IC). Hydroxychloroquine (HCQ) and an interleukin receptor 1 associated kinase 4 inhibitor (IRAK4i) were added to cell cultures. Cytokine mRNA levels were determined with a microarray and protein levels with immunoassays. Single-cell RNA-sequencing of pDCs using ddSEQ technology was performed. Results: Type III IFN mRNA and protein was induced in RNA-IC stimulated pDC-NK and pDC-B cell co-cultures. A subset of activated pDCs (3%) expressed both type III and type I IFN mRNA. IFN-λ2, IFN-α2b, interleukin (IL)-3, IL-6 or granulocyte-macrophage colony stimulating factor (GM-CSF) enhanced IFN-λ1/3 production 2-5-fold. HCQ and an IRAK4i blocked the RNA-IC-triggered IFN-λ1/3 production (p<0.01). IFN-α2b and GM-CSF increased the proportion of SLE patients producing IFN-λ1/3 in response to RNA-IC from 11 to 33%. Conclusions: Type III IFN production is triggered by RNA-IC in pDCs in a TLR-MyD88-dependent manner, enhanced by NK and B cells as well as several pro-inflammatory cytokines. These results support a contributing role for both type I and type III IFNs in SLE, which needs to be considered when targeting the IFN system in this disease. PDCs from two healthy blood donors (50 x 103cells/well) were stimulated with RNA-IC together with IFN-α2b (500 IU/ml) and IL-3 (1 ng/ml). **Raw data not provided due to privacy concerns**