Strategies that modulate antigen delivery are being tested to reverse autoimmunity
By Bana Jabri and Valerie Abadi
Autoimmune disorders encompass a wide range of immunological diseases that affect 4 to 10% of the population worldwide. They occur when immunological tolerance toward self-antigens is broken, resulting in immune responses against cells, tissues, or organs that lead to tissue dysfunction and/or destruction. The goal of autoimmune disease therapy is to restore tolerance to the self-antigen that causes the pathology by targeting autoreactive T cells while preserving immune competence to prevent infections and malignancies. The main challenge of this endeavor resides in choosing the mode by which the disease-driving antigen is delivered so that it can initiate the removal or reprogramming of the autoreactive T cells or the induction and/or expansion of antigen-specific regulatory T (Treg) cells to suppress autoreactive T cells. Although the potential of antigenbased immunotherapy approaches to restore tolerance have been demonstrated for the treatment of immunoglobulin E (IgE) – mediated peanut allergy (1), antigen therapies for autoimmune disorders are still being developed.
There are several key hurdles that a successful autoimmune disease antigen therapy needs to overcome. For most autoimmune disorders, there is absent or incomplete knowledge of the disease-driving autoantigen. In addition, although one particular disease-causing antigen triggers and drives the autoimmune process, additional antigens are exposed as tissue destruction takes place, which causes a phenomenon called epitope spreading, whereby the pathogenic T cell response widens to a diversity of antigens (2). Furthermore, a successful therapy needs to target functionally relevant subsets of both tissue-resident and circulating pathogenic memory T cells. This is not easily achieved owing to the incomplete understanding of the regulation of tissue-resident T cells and given that there are important distinctions in their mode of regulation (3) and transcriptional program (4). Moreover, even if Treg cells are induced, they need to be functional in an inflammatory environment. This is, for example, problematic for forkhead box P3–expressing (FOXP3+) Treg cells that fail to control pathogenic effector T cells that have been stimulated by the inflammatory cytokines interleukin-15 (IL-15), IL-21, or IL-7 that are present in the autoimmune tissue environment (3).
To attempt to circumvent at least some of these issues, diverse approaches have been developed with the common goal of restoring immune tolerance by fine-tuning antigen delivery to tolerogenic or resting antigenpresenting cells (APCs) that will induce Treg cell differentiation, T cell deletion, or anergy (a state of hyporesponsiveness). The rerouting of the T cell response involves two possible mechanisms. One results in pathogenic T cells acquiring tolerogenic properties, such as the ability to produce the immunosuppressive cytokine IL-10. The other depends on the expansion and/or de novo generation of inducible FOXP3+ Treg cells or type 1 regulatory T (Tr1) cells from naïve T cells. The definition of Tr1 cells remains ambiguous because they can coproduce IL-10 and inflammatory cytokines or produce IL-10 only (5, 6). Furthermore, it is important to note that the beneficial effects of IL-10 are context dependent because it can have detrimental effects in certain autoimmune diseases by promoting extrafollicular B cell responses and the production of autoantibodies (7).
Several approaches are being tested in clinical trials (see the table). One strategy is based on delivering the disease-driving antigens in their “naked” form (in the absence of any vehicle), leading to antigen presentation in a tolerogenic environment. This may be achieved by giving the antigen orally, a strategy that has proven successful in the context of peanut allergy, so that the antigen is presented by intestinal tolerogenic dendritic cells that promote the differentiation of FOXP3+ Treg cells (oral tolerance). Alternatively, the disease-driving antigen can be delivered outside of the site of the inflamed tissue to resting dendritic cells that lack costimulatory molecules by injecting immunodominant peptides subcutaneously or intramuscularly. Activating memory T cells and naïve T cells through the T cell receptor in the absence of costimulation by CD28 and CD40 ligand leads to the deletion (and anergy) of the antigen-specific T cells (8).
Although promising results with these approaches have been reported in animal models of autoimmunity, they have not yet given rise to an efficacious antigen-specific immunotherapy that provides a significant clinical benefit. This may be for several reasons, including the degradation of antigens before they reach their target, a failure to delete pathogenic tissue-resident T cells, and the emergence of de novo disease-driving antigens. To improve these approaches by promoting the differentiation of antigen-specific Treg cells, delivery of the antigen by bacteria that are genetically manipulated to secrete regulatory cytokines, such as IL-10, has been investigated. This improved the mucosal delivery of the antigen and induced more immunosuppressive Treg cells in recent-onset nonobese diabetic (NOD) mice (9). The expansion of FOXP3+ Treg cells can also be augmented by coadministration of IL-2 muteins, which are IL-2 variants specifically designed to interact exclusively with the high-affinity IL-2 receptor. This approach demonstrated prolonged control of autoimmunity in the NOD mouse model (10). However, if IL-2 muteins are provided alone, Treg cells expand in a non–antigen-specific manner. Furthermore, they might not be able to suppress effector T cell responses because of the inflammatory nature of the tissue environment targeted by the autoimmune condition.
Another approach involves bioengineering that relies on either conjugating the antigen to polymers (11) or nanotechnology (6) to target different subsets of APCs . For example, to better harness the tolerogenic environment of the liver, antigens can be conjugated to linear polymeric glycosylations, such as N-acetylgalactosamine and N-acetylglucosamine, that are recognized by C-type lectin receptors expressed by hepatic dendritic cells, Kupffer cells, liver sinusoidal endothelial cells, and hepatocytes. This results in increased uptake and presentation of these synthetically glycosylated antigens by liver APCs, which are poised to maintain tolerance. This approach provided encouraging results in the BDC2.5 T cell adoptivetransfer mouse model of type 1 diabetes (11). Intravenous administration of antigens coupled to erythrocytes, which undergo sustained cell death called eryptosis, also leads to continuous uptake of the coupled antigens by splenic dendritic cells, hepatocytes, and hepatic stellate cells. In animal models, these approaches were shown to lead to deletion of autoreactive T cells, de novo induction of Treg cells, or reprogramming of autoreactive T cells. Although phase 1 and 2 clinical trials in patients with multiple sclerosis and celiac disease are ongoing (2022-000801-28 and NCT05574010, respectively), the efficacy of these approaches in humans remains to be demonstrated.
Modes of antigen delivery Systemic or mucosal administration of “naked” antigens targets resting or tolerogenic dendritic cells (DCs) to promote the deletion or induction of forkhead box P3–expressing (FOXP3+) regulatory T (Treg) cells, respectively. To target additional types of antigen-presenting cells (APCs), several bioengineering approaches have been developed. Antigens can be conjugated or delivered in nanoparticles. Nanoparticles can not only deliver antigens to many different types of APCs at different locations but also deliver therapeutic cargo. Bioengineering approaches have the potential to promote more wide-ranging mechanisms of tolerance by inducing regulatory type 1 and type 1–like cells and reprogramming pathogenic autoreactive T cells.
The use of nanoparticles offers distinct ways to manipulate the immune system because they can be engineered to carry ligands that target specific cell-surface receptors (e.g., DEC-205, which is highly expressed in dendritic cells) and therapeutic cargos (e.g., tolerance-promoting drugs, such as rapamycin, or small interfering RNA) (6). Modulating the amount of antigen delivered is also more easily achieved with nanoparticles than with naked antigen delivery. Delivering higher doses of antigen may enhance tolerance induction by promoting the gradual establishment of a regulatory program in Tr1 cells that is characterized by the expression of specific negative costimulatory molecules and transcription factors—such as MAF, aryl hydrocarbon receptor (AHR), and nuclear factor interleukin-3– regulated protein (NFIL3)—in addition to IL-10 production and in the absence of inflammatory cytokine production (12). Activation of an immunosuppressive network that involves IL-10–producing Tr1 cells might be critical for establishing bystander suppression owing to the ability of IL-10 to downregulate the expression of costimulatory molecules and major histocompatibility complex class II (MHCII) molecules on the surface of APCs (5). Furthermore, the size, shape, and surface chemistry of nanoparticles can be manipulated to influence trafficking and the functional program that is activated in the targeted APCs. For instance, it is possible to increase the presentation of disease-driving antigens by tolerogenic phagocytes and dendritic cells. This has been experimentally achieved in mice by using negatively charged microparticles engineered with polymeric organic materials such as polylactide-co-glycolic acid (PLGA) that were passively taken up by splenic marginal zone macrophages expressing the scavenger receptor MARCO (6).
Other approaches consist of using liposomes that contain peptide antigen and phosphatidylserine to facilitate their phagocytosis by macrophages. By adding therapeutic cargo that is delivered to different cell types and in distinct anatomical locations, nanoparticles have the capacity to induce all types of immune modulation and hence be adapted to a given organ-specific autoimmune disorder. This may be critical because each organ and autoimmune disease has a particular immunological and tissue signature that requires different modes of tolerance induction. For example, KAN-101, a glycosylation signature conjugated to deamidated gliadin peptides that is specifically designed for liver targeting, is now advancing into phase 2 clinical trials for the treatment of celiac disease (13). Delivering antigens to anatomically distinct sites can also be achieved by altering the nanoparticle chemistry. Selective organ targeting has been developed, whereby manipulating multiple classes of lipid nanoparticles allows tissue-specific delivery of diverse cargos, including mRNA, Cas9 mRNA–single guide RNA (sgRNA), and Cas9 ribonucleoprotein complexes, which allows CRISPRCas– mediated gene editing in therapeutically relevant cell types (14). Nevertheless, encapsulating many antigens might be necessary to overcome epitope spreading.
Although nanoparticle technology is attractive because of the flexibility and holistic nature of its approach, mRNA therapeutics may transform immune regulatory therapy by enabling efficient delivery of antigen as well as easier development and manufacturing. However, the inherent adjuvanticity of mRNA-based vaccines that can activate innate immune receptors may also lead to the unwanted activation of APCs and the enhancement of autoreactive immune responses. To tackle this issue, some modified mRNA vaccines have been developed by introducing methylpseudouridine in place of uridine. This nucleoside modification results in an anti-inflammatory mRNA vaccine that has reduced innate immune activation properties because it fails to activate Toll-like receptor 7 (TLR7) (15).
Despite the impressive development of new technologies that offer the promise of antigen-specific tolerogenic therapies, the problems of epitope spreading and immune regulation in an inflammatory tissue environment remain to be solved. Approaches that can control both pathogenic circulating central memory and tissue-resident effector memory T cells also need to be identified. This will likely only be achieved through combination therapies that are adapted to each tissue and disease. As technologies for antigen delivery and therapeutic strategies with broader and stronger tolerogenic potential are developed, the risk of nonspecific immunosuppression also increases. Another critical issue to consider is establishing the criteria for authorizing human trials and evaluating the feasibility of acquiring pertinent preclinical data. To ensure the future success of these approaches, it will be crucial to incorporate comprehensive and unbiased immunological and multiomics investigations in clinical trials. Although animal models are valuable fo uncovering and defining general immune concepts, pathways, and molecules, disparities exist between humans and animals in terms of immune cell subsets, innate receptors, and exposure to environmental factors, such as diet, that might affect autoimmune pathogenesis. Consequently, reliably predicting the response to antigen-delivery therapy remains a challenge, even when using animal models that offer a higher degree of physiological relevance to humans than laboratory mice.
Successfully navigating the complexities of an individual’s immune system requires a comprehensive approach, with the effectiveness of strategies being contingent upon their alignment with the disease stage. This challenge intensifies as tissue destruction advances, which introduces intricacies to tolerance restoration. Amid the myriad of technical and knowledge advances, it is crucial to recognize the complexity of reinstating tolerance through antigen delivery. Consequently, celiac disease, an autoimmune disorder that targets the intestine, emerges as an exemplary human disease model. It provides a vital platform for exploring the mechanisms that underlie tolerance reinstatement through antigen delivery and understanding the factors that influence therapeutic success or failure. Notably, the strict human leukocyte antigen (HLA) restriction of celiac disease coupled with the knowledge of the disease-driving antigen (gluten) and accessibility of the tissue targeted by the pathogenic immune response make it a valuable focal point for advancing therapeutic interventions aimed at tolerance induction through antigen delivery.
Acknowledgments
The authors thank C. Khosla and J. A. Hubbell for insightful comments. The work of B.J. and V.A. is supported by grants from the National Institutes of Health (NIH): R01 DK067180 to B.J., R01 DK128352 to V.A., and Digestive Diseases Research Core Center P30 DK42086 and R01 DK063158 to B.J. and V.A.
Source: Science.org
