CHICAGO—In the basic science session, Beyond NETs: Alternative Roles of Neutrophils in Spondyloarthritis, neutrophils are explored as a common pathophysiologic thread among the spondyloarthropathies. In this two-part talk, Dennis McGonagle, PhD, professor of investigative rheumatology at the Leeds Institute of Rheumatic and Musculoskeletal Medicine at St James Hospital, Leeds, England, opens by examining the role of neutrophil-derived interleukin (IL) 23 in spondyloarthropathy spectrum disorders. To close, Nigil Haroon, MD, PhD, MBA, professor and division head in rheumatology at the University of Toronto, details a series of experiments unraveling the MIF-HIF (macrophage migration inhibitory factor; hypoxia-inducible factor) axis, how neutrophils may be implicated and its role in spondyloarthritis pathogenesis.
The Role of Neutrophil IL-23 in Spondyloarthropathy Spectrum Disorders
Dr. McGonagle’s presentation, based on his 2023 Lancet Rheumatology paper by the same name, explored the pathologic role of neutrophil-derived IL-23 in the spondyloarthropathies, including axial spondyloarthritis, psoriatic arthritis and inflammatory bowel disease.1 He began by reviewing basic neutrophil biology, emphasizing their ubiquity, short half-lives and role as the first line of defense against microorganisms.
He highlighted the overlapping clinical features in the spondyloarthropathies and the numerous clinicopathologic findings that suggest a neutrophilic fingerprint, including Munro’s abscesses in the skin, crypt abscesses in the gut, neutrophilic inflammation in enthesitis, and hypopyon in anterior uveitis, as well as pyoderma gangrenosum and hidradenitis suppurativa. Driving this inflammation, he highlighted the IL-23/17 pathway, a T cell-neutrophil axis active in spondyloarthritis, in which neutrophil-derived IL-23 activates T cells, leading to production of IL-17A, IL-17F and tumor necrosis factor (TNF) alpha, with subsequent amplification of neutrophilic inflammation. In other words, rather than simply acting as foot soldiers, neutrophil derived IL-23 may be able to regulate T cell responses in some tissue compartments, which may help explain the lack of effectiveness of IL-23 inhibitors in ankylosing spondylitis.
Dr. McGonagle described the spondyloarthropathies as a “mixed pattern diseases”, meaning that they have MCH class I associations as well as autoinflammatory components – a sort of “hybrid state between pure innate and autoimmunity.”2 He introduced the term MHC-I-opathy, proposing that target tissues in spondyloarthritis exposed to environmental and mechanical stress trigger secondary adaptive immunity via CD8 positive T-cells, leading to IL-17 production and prominent neutrophilic inflammation.3
Considering the role neutrophils play in managing extracellular infections, it seems logical that they would mediate inflammation in the gut and skin, but what is their role at the enthesis?
He highlighted the work of Raimondo et al., demonstrating that IL-17 inhibition with sekukinumab leads to reduction of entheseal granulocytes.4 Likewise, in a mouse model for spondyloarthritis, the addition of IL-23 to IL-23 positive receptor cells led to upregulation of IL-17A and F, resulting in an abundance of myeloid cells and clinical enthesitis.5 Bridgewood et al. demonstrated that resident macrophages in the marrow of entheses make IL-23, and when examining the human enthesis at the spine, they found abundant inducible IL-23 production from neutrophils.6 In an SKG mouse model of spondyloarthritis, histological examination of the enthesitis demonstrated early appearance of MPO positive neutrophils and examination of gene expression identified numerous active neutrophil related genes and pathways.7
Dr. McGonagle then addressed one of the major therapeutic paradoxes in the field: why do IL-23 inhibitors work in psoriasis and peripheral arthritis, but fail in axial disease? The axial skeleton is enriched in neutrophils and type 17 T cells due to local myelopoiesis, meaning there is greater IL-23 production, as well as IL-17 production independent of neutrophil derived IL-23. Studies have demonstrated a relative paucity of Type 17 T cells at entheses, meaning that IL-17 production is IL-23 dependent, and thus can be halted by IL-23 inhibition alone.7 Variations in the cell types of these tissue compartments may explain the inefficacy of IL-23 inhibitors or the potential need for higher dose IL-23 inhibition in axial disease.1
In summary, neutrophils play a central role in spondyloarthritis and enthesis immunity. Their interactions with IL-23 provide a mechanistic explanation for enthesitis, skin and gut inflammation, and the failure of IL-23 inhibition in axial disease. Dr. McGonagle’s research demonstrates the importance of understanding neutrophil function to guide precise therapeutic strategies in spondyloarthritis.
The HIF-MIF Axis in a Murine Spondylarthritis Model
Dr. Haroon shifted gears to detailing the MIF-HIX axis and the role of neutrophils in the pathogenesis of spondyloarthritis using a pre-clinical spondyloarthritis murine model.
The interest in MIF, also known as macrophage inhibitory factor, came about after the report of anti-CD74 antibodies in axial spondyloarthritis. CD74 is a surface receptor for MIF, which is a protein involved in innate immunity. Under normal circumstances, MIF binds to CD74, leading to inflammation and bone proliferation: both cardinal features of spondyloarthritis.
Dr. Haroon began by examining the serum of patients with axial spondyloarthritis and found elevated levels of MIF, which was also found to be an independent risk factor for disease progression.8 In the tissues, MIF was found in increased levels in Paneth cells in the gut. Likewise, there was greater MIF expression in spinal tissues in patients with axial spondyloarthritis relative to those with osteoarthritis.
With the pro-inflammatory effect of MIF in mind, they found that treatment of healthy peripheral blood monocytes with MIF led to greater TNF alpha production. Likewise, blocking intracellular disassociation of CD74, TNF alpha production was halted. They then took SaOS22 cell lines, induced them with MIF, and found greater osteoblastic activity.
Dr. Haroon then introduced the SKG mouse model, which was the basis for the remainder of his presentation. Curdlan-treated SKG mice develop a spondyloarthritis-like phenotype with weight loss, arthritis, dermatitis and blepharitis. In these mice, they demonstrated increased expression of MIF and its receptor CD74. Even in the ankle joint synovium, as well as in the sacroiliac joint, there was increased MIF and CD74 expression.10
To test whether MIF overexpression without Curdlan was sufficient to induce this phenotype, they used a plasmid to express MIF and found similar disease manifestations. Conversely, MIF knockout led to clinical improvements in weight, arthritis, dermatitis and blepharitis. Pharmacological inhibition of MIF with a MIF antagonist, MIF098, similarly prevented and reversed these spondyloarthritis-like manifestations.
Dr. Haroon posed the question: What is the cellular source of MIF? Using human peripheral blood cells, they found that almost all cell types produce MIF, but it is significantly higher in granulocytes and in Curdlan-injected SKG mice neutrophils.
With the neutrophil now in focus, they performed comparative total RNA-sequencing on neutrophils from MIF positive and MIF knock out mice and identified DTL (denticleless E3 ubiquitin protein ligase homolog), which is a negative inhibitor of IL-23/24 production. In MIF knockout mice, DTL expression was increased, leading to greater inhibition of IL-23. This led them to hypothesize that overexpression of DTL could be therapeutic in spondyloarthritis.
To demonstrate the importance of neutrophil mediated MIF production, they isolated neutrophils capable of MIF production and transferred them into MIF knockout mice, and which led to the development of spondyloarthritis-like manifestations.
Spondyloarthritis is unique for its tissue specificity, so they wondered which proteins MIF was interacting with at the tissue level. This inquiry led them to the discovery of HIF1alpha, which is significantly increased in production in Curdlan treated mice. Blocking of HIF1alpha (with PX-748) led to improvement of symptoms including arthritis, dermatitis and blepharitis, and decreased production of pro-inflammatory cytokines. Blocking HIF1alpha was also able to prevent new bone formation in Curdlan treated SKG mice.11
In summary, the MIF-HIF axis seems to be mediated through neutrophils and has an important role in symptom development in a pre-clinical axial spondyloarthritis model. Neutrophils are activated and expanded in the tissues, lead to production of inflammatory cytokines, such as MIF, IL-1 and IL-23, and warrant continued study to uncover novel therapeutic targets.
Michael Cammarata, MD, RhMSUS, is an assistant professor of medicine at the Johns Hopkins University School of Medicine, Baltimore.
References
- Macleod T, Bridgewood C, McGonagle D. Role of neutrophil interleukin-23 in spondyloarthropathy spectrum disorders. Lancet Rheumatol. 2023;5(1):e47–e57.
- McGonagle D, McDermott MF. A proposed classification of the immunological diseases. PLoS Med. 2006;3(8):e297.
- McGonagle D, Aydin SZ, Gül A, Mahr A, Direskeneli H. ‘MHC-I-opathy’-unified concept for spondyloarthritis and Behçet disease. Nat Rev Rheumatol. 2015;11(12):731–740.
- Raimondo MG, Mohammadian H, Rauber S, et al. Entheseal tissue signature in response to IL-17A inhibition in psoriatic arthritis: Results from the EBIO entheseal biopsy study. Ann Rheum Dis. 2025 Oct 16.
- Sherlock JP, Joyce-Shaikh B, Turner SP, et al. IL-23 induces spondyloarthropathy by acting on ROR-γt+ CD3+CD4-CD8- entheseal resident T cells. Nat Med. 2012;18(7):1069–1076.
- Bridgewood C, Watad A, Russell T, et al. Identification of myeloid cells in the human enthesis as the main source of local IL-23 production. Ann Rheum Dis. 2019;78(7):929–933.
- Stavre Z, Bridgewood C, Zhou Q, et al. A role for neutrophils in early enthesitis in spondyloarthritis. Arthritis Res Ther. 2022;24(1):24.
- Tonutti et al. Lancet Rheumatology. 2025; in press.
- Ranganathan V, Ciccia F, Zeng F, et al. Macrophage migration inhibitory factor induces inflammation and predicts spinal progression in ankylosing spondylitis. Arthritis Rheumatol. 2017;69(9):1796–1806.
- Nakamura A, Zeng F, Nakamura S, et al. Macrophage migration inhibitory factor drives pathology in a mouse model of spondyloarthritis and is associated with human disease. Sci Transl Med. 2021;13(616):eabg1210.
- Nakamura A, Jo S, Nakamura S, et al. HIF-1α and MIF enhance neutrophil-driven type 3 immunity and chondrogenesis in a murine spondyloarthritis model. Cell Mol Immunol. 2024;21(7):770–786.
