The NLRP1 inflammasome as a driver of systemic lupus erythematosus pathogenesis

Systemic lupus erythematosus (SLE) is a chronic autoimmune disease. The genetic, molecular, and cellular events that cause SLE are complex, resulting in varied disease manifestations. Among these are skin inflammatory syndromes, including the characteristic malar “butterfly” rash and more severe outcomes. Although inappropriate immune responses to nucleic acids and the aberrant activation of host defense pathways (e.g., interferon) are established drivers of SLE disease, treatments targeting these pathways have been only partially effective. Thus, a more complete understanding of other mechanisms that initiate and/or modulate SLE inflammation of the skin remains a critical goal towards the development of more effective treatments for people living with SLE.

Here we propose a new link between the NLRP1 inflammasome and SLE inflammation of the skin. Inflammasomes are an integral arm of our immune response to infectious threats, but can also drive autoinflammatory disease when inappropriately activated. Inflammasome activation initiates inflammation by producing signals (i.e., cytokines) like interleukin (IL)-1β, which is known to be elevated in people living with SLE. Inflammasome activation also induces a form of lytic cell death called pyroptosis, which can cause tissue damage. In humans, NLRP1 is expressed in the epithelial cells of the skin called keratinocytes. Genome-wide association studies have implicated NLRP1 in several human autoinflammatory diseases with skin pathology including SLE. However, a mechanism connecting NLRP1 to SLE has remained enigmatic.

The NLRP1 inflammasome can be activated by many stimuli, including ultraviolet B (UVB) irradiation and double-stranded RNA (dsRNA), which are both known to initiate or enhance SLE disease. Excitingly, we have now defined the pathways linking these activating stimuli to the NLRP1 inflammasome, representing a new and exciting opportunity to determine how NLRP1 contributes to SLE skin inflammation and disease.

We propose to determine the importance of NLRP1 in SLE using multiple approaches: For example, we will further dissect the pathway connecting lupus-relevant activating stimuli to the NLRP1 inflammasome. Additional mechanistic insights will help guide future studies developing drugs that target these pathways. We will also use physiologic models of human keratinocytes, including 3D ‘skin’ that we grow in the lab. These models allow us to study skin inflammation in the presence of lupus-associated stimuli (i.e., UVB and dsRNA) and perform precise genetic and mechanistic studies to interrogate the role of NLRP1 in skin inflammation.

Taken together, our studies will resolve important gaps in our understanding by uncovering new drivers of skin inflammation, which we envision will inform novel therapeutic strategies to improve treatments for people living with SLE.

Systemic lupus erythematosus (SLE) is a chronic, multisystem autoimmune disease. SLE etiology is complex with heterogenous clinical manifestations. Among these are mucocutaneous pathologies, including the characteristic malar “butterfly” rash and more severe manifestations in discoid lupus erythematosus and subacute cutaneous lupus erythematosus. Although the loss of tolerance to nucleic acids and the activation of type I interferon (IFN) signaling are established drivers of SLE pathogenesis, treatments targeting these pathways have been only partially effective. Thus, mechanisms that initiate and/or modulate SLE inflammatory syndromes of the skin remains a critical gap in our understanding.

Here we propose that the NLRP1 inflammasome is central to SLE pathogenesis. Inflammasomes are cytosolic immune complexes that are essential for host defense. Inflammasome activation initiates inflammatory signaling via interleukin (IL)-1β and IL-18 – cytokines elevated in people living with SLE – and a form of lytic cell death called pyroptosis. In humans, the NLRP1 inflammasome is primarily expressed in epithelia including keratinocytes, and disease-associated NLRP1 gain-of-function variants are associated with rare monogenic skin disorders including multiple self-healing palmoplantar carcinoma and autoinflammation with arthritis and dyskeratosis. Genome-wide association studies have also implicated NLRP1 in several human autoinflammatory diseases with skin pathology including SLE. However, a mechanistic basis linking the NLRP1 inflammasome to SLE has been enigmatic.

Diverse pathogen- and stress-associated stimuli activate the NLRP1 inflammasome, all of which culminate in the release and assembly of the bioactive NLRP1 C-terminal domains – a mechanism that we previously described. This mechanistic understanding contributed to the discovery that the NLRP1 inflammasome can be triggered by ultraviolet B (UVB) irradiation and double-stranded RNA (dsRNA). NLRP1 inflammasome activation by UVB occurs downstream of the ribotoxic stress response (RSR), mediated by a ZAKα- and p38-dependent signal transduction cascade that phosphorylates the NLRP1 N-terminus leading to inflammasome activation. Our findings now demonstrate that dsRNA-triggered NLRP1 inflammasome activation also occurs via p38-dependent phosphorylation. We find that NLRP1 is integrated into the canonical dsRNA innate immune sensing pathways RIG-I/MDA5–MAVS and TLR3–TRIF, which drive the production of type I IFN – a known driver of SLE. These dsRNA sensing pathways converge on the kinase TAK1, which results in p38-dependent NLRP1 inflammasome activation. For example, we find that genetic deletion of MAP3K7 (TAK1) prevents NLRP1 inflammasome activation in human keratinocytes triggered by the synthetic dsRNA analog poly(I:C). Thus, the NLRP1 inflammasome is activated by stimuli with well-documented links to SLE: (1) UVB exposure, which can cause photosensitivity-driven enhancement of symptoms, and (2) dsRNA and the type I IFN response.

In this proposal, we seek to (1) define the mechanism by which SLE-relevant activating stimuli trigger NLRP1 inflammasome activation, and (2) characterize SLE-associated genetic variants and environmental (i.e., UVB) activating stimuli of NLRP1 in human keratinocytes and 3D organotypic skin models of SLE.