Spatiotemporal regulation of peripheral T cell tolerance

Early life imprinting of intestinal immune tolerance and tissue homeostasis

Besides its canonical role in protecting the host from pathogens, the immune system plays an arguably equally important role in maintaining tissue homeostasis. Within barrier tissues that interface with the external microenvironment, induction of immune tolerance to innocuous antigens, such as commensal, dietary, and environmental antigens, is key to establishing immune homeostasis. The early postnatal period represents a critical window of opportunity in which parallel development of the tissue, immune cells, and microbiota allows for reciprocal regulation that shapes the long-term immunological tone of the tissue and subsequent risk of immune-mediated diseases. During early infancy, the immune system appears to sacrifice pro-inflammatory functions, prioritizing the establishment of tissue tolerance. In this review, we discuss mechanisms underlying early life windows for intestinal tolerance with a focus on newly identified RORγt+ antigen-presenting cells-Thetis cells-and highlight the role of the intestinal microenvironment in shaping intestinal immune system development and tolerance.

Therapeutic induction of antigen-specific immune tolerance

The development of therapeutic approaches for the induction of robust, long-lasting and antigen-specific immune tolerance remains an important unmet clinical need for the management of autoimmunity, allergy, organ transplantation and gene therapy. Recent breakthroughs in our understanding of immune tolerance mechanisms have opened new research avenues and therapeutic opportunities in this area. Here, we review mechanisms of immune tolerance and novel methods for its therapeutic induction.

Principles and therapeutic applications of adaptive immunity

Adaptive immunity provides protection against infectious and malignant diseases. These effects are mediated by lymphocytes that sense and respond with targeted precision to perturbations induced by pathogens and tissue damage. Here, we review key principles underlying adaptive immunity orchestrated by distinct T cell and B cell populations and their extensions to disease therapies. We discuss the intracellular and intercellular processes shaping antigen specificity and recognition in immune activation and lymphocyte functions in mediating effector and memory responses. We also describe how lymphocytes balance protective immunity against autoimmunity and immunopathology, including during immune tolerance, response to chronic antigen stimulation, and adaptation to non-lymphoid tissues in coordinating tissue immunity and homeostasis. Finally, we discuss extracellular signals and cell-intrinsic programs underpinning adaptive immunity and conclude by summarizing key advances in vaccination and engineering adaptive immune responses for therapeutic interventions. A deeper understanding of these principles holds promise for uncovering new means to improve human health.

Cellular and molecular signaling towards T cell immunological self-tolerance

The binding of a cognate antigen to T cell receptor (TCR) complex triggers a series of intracellular events controlling T cell activation, proliferation, and differentiation. Upon TCR engagement, different negative regulatory feedback mechanisms are rapidly activated to counterbalance T cell activation, thus preventing excessive signal propagation and promoting the induction of immunological self-tolerance. Both positive and negative regulatory processes are tightly controlled to ensure the effective elimination of foreign antigens while limiting surrounding tissue damage and autoimmunity. In this context, signals deriving from co-stimulatory molecules (i.e., CD80, CD86), co-inhibitory receptors (PD-1, CTLA-4), the tyrosine phosphatase CD45 and cytokines such as IL-2 synergize with TCR-derived signals to guide T cell fate and differentiation. The balance of these mechanisms is also crucial for the generation of CD4+ Foxp3+ regulatory T cells, a cellular subset involved in the control of immunological self-tolerance. This review provides an overview of the most relevant pathways induced by TCR activation combined with those derived from co-stimulatory and co-inhibitory molecules implicated in the cell-intrinsic modulation of T cell activation. In addition to the latter, we dissected mechanisms responsible for T cell-mediated suppression of immune cell activation through regulatory T cell generation, homeostasis, and effector functions. We also discuss how imbalanced signaling derived from TCR and accessory molecules can contribute to autoimmune disease pathogenesis.

How does the microbiota control systemic innate immunity?

The intestinal microbiota has a pervasive influence on mammalian innate immunity fortifying defenses to infection in tissues throughout the host. How intestinal microbes control innate defenses in systemic tissues is, however, poorly defined. In our opinion, there are three core challenges that need addressing to advance our understanding of how the intestinal microbiota controls innate immunity systemically: first, deciphering how signals from intestinal microbes are transmitted to distal tissues; second, unraveling how intestinal microbes prime systemic innate immunity without inducing widespread immunopathology; and third, identifying which intestinal microbes control systemic immunity. Here, we propose answers to these problems which provide a framework for understanding how microbes in the intestine can regulate innate immunity systemically.

Rational Engineering of Islet Tolerance via Biomaterial-Mediated Immune Modulation

Type 1 diabetes (T1D) onset is characterized by an autoimmune attack on β islet cells within the pancreas, preventing the insulin secretion required to maintain glucose homeostasis. Targeted modulation of key immunoregulatory cell populations is a promising strategy to restore tolerance to β cells. This strategy can be used to prevent T1D onset or reverse T1D with transplanted islets. To this end, drug delivery systems can be employed to transport immunomodulatory cargo to specific cell populations that inhibit autoreactive T cell-mediated destruction of the β cell mass. The rational engineering of biomaterials into nanoscale and microscale drug carriers can facilitate targeted interactions with immune cells. The physicochemical properties of the biomaterial, the delivered immunomodulatory agent, and the target cell populations are critical variables in the design of these delivery systems. In this review, we discuss recent biomaterials-based drug delivery approaches to induce islet tolerance and the need to consider both immune and metabolic markers of disease progression.

Eras of designer Tregs: Harnessing synthetic biology for immune suppression

Since their discovery, CD4+ CD25hi FOXP3hi regulatory T cells (Tregs) have been firmly established as a critical cell type for regulating immune homeostasis through a plethora of mechanisms. Due to their immunoregulatory power, delivery of polyclonal Tregs has been explored as a therapy to dampen inflammation in the settings of transplantation and autoimmunity. Evidence shows that Treg therapy is safe and well-tolerated, but efficacy remains undefined and could be limited by poor persistence in vivo and lack of antigen specificity. With the advent of new genetic engineering tools, it is now possible to create bespoke "designer" Tregs that not only overcome possible limitations of polyclonal Tregs but also introduce new features. Here, we review the development of designer Tregs through the perspective of three 'eras': (1) the era of FOXP3 engineering, in which breakthroughs in the biological understanding of this transcription factor enabled the conversion of conventional T cells to Tregs; (2) the antigen-specificity era, in which transgenic T-cell receptors and chimeric antigen receptors were introduced to create more potent and directed Treg therapies; and (3) the current era, which is harnessing advanced genome-editing techniques to introduce and refine existing and new engineering approaches. The year 2022 marked the entry of "designer" Tregs into the clinic, with exciting potential for application and efficacy in a wide variety of immune-mediated diseases.

TGF-β signaling in health and disease

The TGF-β regulatory system plays crucial roles in the preservation of organismal integrity. TGF-β signaling controls metazoan embryo development, tissue homeostasis, and injury repair through coordinated effects on cell proliferation, phenotypic plasticity, migration, metabolic adaptation, and immune surveillance of multiple cell types in shared ecosystems. Defects of TGF-β signaling, particularly in epithelial cells, tissue fibroblasts, and immune cells, disrupt immune tolerance, promote inflammation, underlie the pathogenesis of fibrosis and cancer, and contribute to the resistance of these diseases to treatment. Here, we review how TGF-β coordinates multicellular response programs in health and disease and how this knowledge can be leveraged to develop treatments for diseases of the TGF-β system.