Innate Immune Mechanisms That Regulate Disease Flare in Systemic Lupus Erythematosus

Abstract

Systemic lupus erythematosus (SLE) is a heterogenous autoimmune disease characterized by dysfunction within the innate and adaptive immune systems. The breakdown in immune tolerance that accompanies this promotes aberrant B cell activation, production of autoantibodies targeting self-nucleic acids, overproduction of pro-inflammatory cytokines, and widespread tissue damage. While some patients experience persistently active disease, most SLE patients follow a relapsing-remitting course with periods of clinical quiescence interspersed with unpredictable disease flares. Delineating the underlying mechanisms that contribute to pathogenesis of SLE as well as those that trigger flares will aid in development of novel therapeutics to improve patient outcomes. Several cytokines have been implicated in SLE including type I interferons (IFNs) and the IL (interleukin)-1 superfamily. Type I IFNs are overexpressed in both the circulation and non-lesional skin of SLE patients compared to controls. Levels of type I IFNs correlate with disease activity and severity and promote increased death of SLE keratinocytes after exposure to ultraviolet (UV) light. This likely contributes to photosensitivity, or enhanced sensitivity to UV light, which is experienced by up to 90% of patients. The IL-1 family of cytokines includes IL-1beta (IL-1β) which is activated by the inflammasome, a component of the innate immune system that detects infectious or endogenous danger signals. Inflammasome inhibition in lupus-prone mice improves several disease manifestations including nephritis. The specific mechanisms by which cutaneous type I IFNs contribute to photosensitivity and IL-1β influences lupus pathogenesis are currently unknown. The primary focus of this dissertation is to examine the role of type I IFNs in the skin and IL-1β in the kidney. The first objective of this study was to explore the activation and regulation of cell death pathways in keratinocytes exposed to type I IFNs and UV irradiation. Immortalized human keratinocytes were treated with IFN-α and UVB light in combination with apoptosis, necroptosis, and pyroptosis inhibitors or neutralizing antibodies. Cell death was measured by Annexin V and propidium iodide or cleaved caspase-3 staining. We identified enhanced activation of caspase-8-dependent apoptosis in IFN-primed keratinocytes after UVB exposure. This apoptosis occurred independently of known caspase-8-activating death ligands but was dependent on interferon regulatory factor 1 (IRF1). Lastly, we observed increased UVB-induced keratinocyte apoptosis in mice that overexpressed epidermal IFN-к compared to wild-type mice. Together, these data suggest that photosensitivity exhibited by SLE patients may result from IFN priming of keratinocytes that drives IRF1 expression and sensitizes cells to undergo increased apoptosis after minimal exposures to UVB. Continued investigation into mechanisms by which this occurs will provide prophylactic options to prevent SLE flares. The second part of our study involved investigating how loss of IL-1β affects disease severity in the lupus-prone NZM2328 mouse model. We generated a knockout strain (NZM-Il1b-/-) and examined common manifestations of disease. Surprisingly, loss of IL-1β did not affect overall survival, autoantibody generation, or renal immune cell infiltration; however, it did worsen renal immune complex deposition and proteinuria, specifically in female mice. IL-17 and TNF signaling pathways were enriched in the kidneys of female NZM-Il1b-/- mice, suggesting a potential mechanism by which nephritis is aggravated. Female patients with lupus nephritis also demonstrate upregulation of these signaling pathways, suggesting they may be of clinical relevance. Together, these data suggest that blocking IL-1β may need to be approached with caution in SLE patients, especially those with nephritis, to prevent potential exacerbation of disease.