Amid the ongoing COVID-19 pandemic, efforts to actively vaccinate the general population against the SARS-CoV-2 virus in the context of poorly neutralizing and waning immunity have renewed interest in the phenomenon of antibody-dependent enhancement (ADE). This property of antibodies attributes enhanced disease pathogenesis in specific instances of viral infection to the presence of sub-neutralizing titres of antiviral host antibodies. In cases of ADE, rather than contributing to antiviral immunity, pre-existing antibodies facilitate viral entry and subsequent infection of host cells, leading to both increased infectivity and virulence. ADE was first clearly described in dengue virus (DENV) infection by Halstead et al. in 1973 (refs 1,2), although earlier epidemiological evidence had identified that two specific Thai patient populations, specifically first-time infected infants born to immune mothers and children suffering from secondary infection, were associated with an increased incidence of dengue haemor rhagic fever and dengue shock syndrometwo patient groups that ostensibly had pre-existing antibodies to DENV 3. It was not until years later that studies proposed models of ADE, identified optimal conditions for in vitro ADE and quantified antibody titres permissive for ADE 4-9. Numerous studies have since identified Fcγ receptors (FcγRs), surface receptors on immune cells that recognize the Fc portion of IgG and trigger a wide array of downstream effector functions, as the key mediators of ADE in dengue pathogenesis, as they allow for the internalization of multimeric virus-bound IgG and subsequent productive infection.

The microbiome contributes to the development and maturation of the immune system. In response to commensal bacteria, intestinal CD4 + T lymphocytes differentiate into functional subtypes with regulatory or effector functions. The development of small intestine intraepithelial lymphocytes that coexpress CD4 and CD8αα homodimers (CD4IELs) depends on the microbiota. However, the identity of the microbial antigens recognized by CD4 + T cells that can differentiate into CD4IELs remains unknown. We identified β-hexosaminidase, a conserved enzyme across commensals of the Bacteroidetes phylum, as a driver of CD4IEL differentiation. In a mouse model of colitis, β-hexosaminidase–specific lymphocytes protected against intestinal inflammation. Thus, T cells of a single specificity can recognize a variety of abundant commensals and elicit a regulatory immune response at the intestinal mucosa.

Protein glycosylation is a crucial mediator of biological functions and is tightly regulated in health and disease. However, interrogating complex protein glycoforms is challenging, as current lectin tools are limited by cross-reactivity while mass spectrometry typically requires biochemical purification and isolation of the target protein. Here, we describe a method to identify and characterize a class of nanobodies that can distinguish glycoforms without reactivity to off-target glycoproteins or glycans. We apply this technology to immunoglobulin G (IgG) Fc glycoforms and define nanobodies that specifically recognize either IgG lacking its core-fucose or IgG bearing terminal sialic acid residues. By adapting these tools to standard biochemical methods, we can clinically stratify dengue virus and SARS-CoV-2 infected individuals based on their IgG glycan profile, selectively disrupt IgG–Fcγ receptor binding both in vitro and in vivo, and interrogate the B cell receptor (BCR) glycan structure on living cells. Ultimately, we provide a strategy for the development of reagents to identify and manipulate IgG Fc glycoforms.

The microbiome contributes to the development and maturation of the immune system1–3 In response to commensal bacteria, CD4+ T cells can differentiate into distinct functional subtypes with regulatory or effector functions along the intestine. Peripherally-induced Foxp3+-regulatory T cells (pTregs) maintain immune homeostasis at the intestinal mucosa by regulating effector T cell responses against dietary antigens and microbes4. Similarly to pTregs, a subset of small intestine intraepithelial lymphocytes CD4+CD8αα+ (CD4IELS) exhibit regulatory properties and promote tolerance against dietary antigens5. Development of CD4IELS from conventional CD4+ T cells or from Treg precursors depends on the microbiota5,6. However, the identity of the microbial antigens recognized by CD4IELs remains unknown. We identified species belonging to the Bacteroidetes phylum as commensal bacteria capable of generating CD4IEL from naïve CD4+ T cells expressing the pTreg transnuclear (TN) monoclonal TCR6 as well as from polyclonal WT T cells. We found that β-hexosaminidase, a widely conserved carbohydrate-metabolizing enzyme in the Bacteroidetes phylum, is recognized by TN T cells, which share their TCR specificity with CD4+ T cells found in the intraepithelial compartment of polyclonal specific-pathogen-free (SPF) mice. In a mouse model of colitis, β-hexosaminidase-specific CD4IELs provided protection from ulceration of the colon and weight loss. Thus, a single T cell clone can recognize a variety of abundant commensal bacteria and elicit a regulatory immune response at the intestinal epithelial surface.

Significance Species differences in IgG Fc–Fcγ receptor (FcγR) interactions have made humanized mouse models an attractive strategy to evaluate the efficacy and toxicity of human antibodies. We previously published a humanized FcγR mouse model that fully recapitulates the expression and function of these receptors in vivo. However, the immunogenicity of exogenous human IgG has made long-term assessment of antibody function challenging, since endogenous mouse anti-human IgG responses limit the duration and success of these studies. Here, we present a mouse strain that expresses human IgG1 and FcγRs, thereby conferring tolerance to chronic administration of human IgG and enabling functional assessment of antibodies. Because this strain is appropriate for chronic disease models, we expect that researchers will benefit from its use.