The winner of our Bright Sparks PhD session at the BSI Congress in 2017 was Tomas Castro from the University of Cambridge with his talk entitled ‘Anticommensal IgG augments intestinal inflammation in ulcerative colitis via IL1beta-dependent Th17 immunity’. Here, Tomas tells us more about his research on unravelling the role of IgG-mediated inflammation in patients with ulcerative colitis.
A healthy lifestyle is all about balance; be it professional, emotional, nutritional or immunological. A vigorous immune response is required to fight malignancies and the multitude of microbes that seek to invade us, but these responses are tightly regulated to suppress overt immunity and return us to homeostasis. This immunological Goldilocks zone is exemplified in the dual nature of many (if not most) immune cell subsets, from effector versus regulatory T cells to ‘M1’ versus ‘M2’ macrophages, all acting in concert to mount appropriate immune responses. In the gastrointestinal tract, maintaining immunological balance is particularly key.
The gut is colonised by trillions of commensal microorganisms (our microbiome), with essential roles in nutrient salvage, pathogen exclusion and immune education. However, the gut represents a major mucosal entry point for pathogens, requiring the immune system to be primed to respond to invasive microorganisms that breach the intestinal epithelium.
IBD – dysregulation of the immune response
In certain individuals, a complex interaction of environmental and genetic factors results in the breakdown of the rules governing intestinal health. Exacerbated immune responses towards components of the microbiome lead to a spectrum of disorders collectively known as inflammatory bowel disease (IBD). Incidence of the two major subtypes of IBD – Crohn’s disease (CD) and ulcerative colitis (UC) – is increasing globally alongside Western lifestyles and causes significant morbidity and cancer-associated mortality.
A dysregulated immune response is at the heart of IBD. Intense research has focused on pathogenic T cells, informed by seminal animal and genetic studies that implicate the IL-23–Th17 axis as a common contributor to both CD and UC susceptibility. In this setting, innate immune cells, such as macrophages and dendritic cells, integrate poorly defined triggers that drive IFNγ- and IL-17producing T cells through the production of key cytokines, including IL-23 and IL-1β. These discoveries have driven a clinical revolution, with monoclonal antibodies targeting these cytokines showing therapeutic promise in IBD.
In contrast to T cells, whether and how B cells and the humoral immune response are altered in IBD is poorly understood. Immunoglobulin (Ig)A is a well-known immunological cornerstone of intestinal health. IgA, produced by intestinal plasma cells, is a predominantly non-inflammatory class of antibody secreted into the gut lumen where it binds to commensal microbes and keeps them away from the epithelium (a process known as ‘immune exclusion’). This function is essential in preventing intestinal invasion by opportunistic microorganisms. However, the assumption that intestinal humoral immunity is all about IgA has been called into question by genetic studies implicating a variant of FcγRIIA, an activating receptor for IgG, with susceptibility to UC.
Potency of IgG antibodies
Unlike IgA, IgG antibodies are potently inflammatory – they exhibit complement fixing activity and can engage cell surface Fcγ receptors (FcγRs), leading to immune cell activation. This leads us to two unsurprising observations: 1) IgG-FcγR signalling is associated with numerous inflammatory and autoimmune disorders, such as rheumatoid arthritis and systemic lupus erythematosus, and 2) IgG is almost entirely excluded from the healthy adult gut. How could this receptor influence disease susceptibility in an IgA-dominated organ?
This was the question I began trying to answer in 2014 when I joined Dr Menna Clatworthy’s lab (University of Cambridge) as a PhD student, the result of which I was fortunate to present at the 2017 BSI Congress in Brighton. We had a simple hypothesis – that immune dysregulation in IBD leads to the emergence of pathogenic IgG that can drive disease. We began by profiling IgA and IgG binding to commensal microbes in stool samples, and made the rather remarkable observation that up to 80% of intestinal microbes from UC patients were coated in IgG. This was most evident in individuals with severe disease, where the levels of IgG and IgA binding were equivalent, whereas IgG was almost entirely absent in healthy controls.
Having identified macrophages as the major source of intestinal FcγRIIA expression, we set out to define how IgG-mediated activation of these cells may contribute to IBD. For this, we are thankful that IBD research is greatly facilitated by the wealth of transcriptomic datasets deposited in free-to-access public repositories, aided by the relative ease with which clinicians can directly sample diseased tissue by endoscopic biopsy. Using this data as a starting point, we identified IL-1β as the dominant inflammatory cytokine elevated in inflamed UC mucosal biopsies, and found it to associate very closely to FcγRIIA, ultimately attributable to their co-expression within intestinal macrophages. Importantly, we were able to show that FcγRIIA signalling could drive NLRP3-dependent IL-1β production, an effect modulated by FcγRIIA genotype, potentially linking IgG and FcγRIIA to mucosal inflammation in IBD via IL-1β.
In vivo studies in DSS mouse model
As interventionist human studies are very difficult to perform, we turned to an in vivo mouse model of IBD, dextran sodium sulfate (DSS)-induced colitis, to investigate the contribution of our in vitro -defined mechanism to intestinal inflammation. Similar to IBD patients, DSS-exposed mice exhibited a local IgG response towards commensal microbes, and elevated IL-1β production by colonic macrophages. Importantly, passive transfer of anti-commensal IgG to naïve animals was enough to enhance disease activity and increase colonic macrophage IL-1β production, demonstrating a direct pathogenic role of IgG in the gut in vivo.
The FcγRIIA variant implicated in protection from UC encodes a receptor with reduced IgG affinity. To mimic the effect of this variant in vivo , we made use of transgenic mouse strains with graded activating FcγR signalling due to absent or elevated expression of the sole inhibitory FcγR, FcγRIIB, the latter specifically in macrophages. This was necessary as mice lack FcγRIIA itself but express other functionally homologous FcγRs.
Exacerbated FcγR signalling in FcγRIIB-deficient mice resulted in augmented intestinal disease activity and elevated macrophage IL-1β expression. Importantly, this augmented IL-1β response was pathogenic, as treatment with a blocking IL-1 receptor monoclonal antibody effectively suppressed intestinal inflammation. In contrast, macrophagespecific overexpression of FcγRIIB was disease-protective relative to control mice, directly implicating the FcγR-macrophageIL-1β axis as a major disease mediator.
Last piece of the puzzle
Given this strong IL-1β effect, the last piece of the puzzle was to define how this FcγR network influenced local T cell activity, a cell type heavily implicated in IBD susceptibility. We demonstrated an IL-1βdependent increase in IL-17A production by colonic CD4+ T cells and γδ T cells in FcγRIIB-deficient mice, while macrophagespecific FcγRIIB overexpression was enough to suppress this response. All together, our work delineated a mechanism whereby FcγR signalling in colonic macrophages induced a pathogenic IL-1β response, which in turn mediates activation of an IBD-associated T cell response, causing inflammation and disease.
We were pleased to have our study published recently1 but many unanswered questions remain. The most pertinent being whether we can target FcγR signalling therapeutically in IBD. Time and clinical trials will be required to test the potential of this pathway as a therapeutic target, but one thing is clear – IgG can pull the trIgGer in IBD.
Postdoctoral Researcher, Molecular Immunity Unit, Department of Medicine, University of Cambridge
REFERENCES 1. Castro-Dopico et al . 2019 Immunity 50 1099–1114 https://bit.ly/2IMk8nm
Could you be the next Bright Spark in Immunology?
Bright Sparks in Immunology is our event to showcase the work of PhD students and early career postdocs who are submitting an abstract for Congress. To enter your abstract, just tick the relevant box on the abstract submission form for BSI Congress. If your abstract is chosen, you will be invited to present your work at this year’s Bright Sparks session, which will take place on the afternoon of Monday 2 December at the BSI Congress in Liverpool.