Tfh Cell Plasticity and Heterogeneity in Shaping Immune Responses to Allergy, Infection, and Tolerance

National Institute Of Allergy And Infectious Diseases

Investigators

Abstract

T follicular helper (Tfh) cells are a subset of CD4⁺ T cells that support B cells within germinal centers (GCs)—specialized structures in lymphoid organs where B cells refine their antibodies and generate long-lived plasma cells and memory B cells. By providing key signals through molecules such as CD40L and cytokines like IL-21, Tfh cells drive effective GC responses and promote protection against infection. However, dysregulated Tfh activity contributes to disease. Excessive or overactive Tfh cells fuel autoantibody production in autoimmune disorders such as lupus and rheumatoid arthritis, while heightened Tfh responses in allergies promote IgE production and allergic sensitization. Tfh cells can also function as long-term reservoirs of effector T cell (Teff) precursors—specialized T cells that produce cytokines, kill infected cells, or drive inflammation. By serving as a source of Teff precursors, Tfh cells can be reactivated upon antigen re-exposure to replenish the Teff compartment. This reservoir function has pathogenic implications, as it can perpetuate chronic autoimmune and allergic diseases by continuously replenishing pathogenic T cells and sustaining long-lasting IgE responses. Thus, Tfh cells can be either protective or pathogenic, underscoring the importance of understanding the mechanisms that govern their differentiation, plasticity, and effector functions. Given the pathogenic roles of Tfh cells in allergy and autoimmunity, in previous years we investigated the therapeutic potential of depleting Tfh cells to prevent Tfh-dependent pathology. A major limitation when we initiated this work was the lack of therapies capable of selectively eliminating Tfh cells in vivo. Notably, our previous studies demonstrated that IL-2 is a potent inhibitor of Tfh differentiation, as it suppresses expression of Bcl6, the master transcription factor required for Tfh development (Science Immunology 2019, Nature Immunology 2017, Immunity 2012). Thus, we hypothesized that an IL-2–based immunotherapy could serve as a mechanism-driven approach to selectively deplete autoreactive Tfh cells in vivo, thereby preventing pathogenic antibody responses and providing a new therapeutic strategy for autoimmune and allergic diseases. Supporting this view, low-dose recombinant IL-2 (rIL-2) has already shown promise in preventing immunopathology in both lupus-prone mice and SLE patients. However, because IL-2 enhances Treg activity, it has long been assumed that the therapeutic effects of rIL-2 are mediated primarily through Treg-dependent immunosuppression. Indeed, numerous IL-2–based therapies are being developed to selectively expand Tregs, including IL-2 muteins, PEGylated IL-2, and IL-2/antibody complexes. Based on our data showing that IL-2 directly inhibits Tfh differentiation, we hypothesized that suppression of Tfh responses represents an additional and underappreciated mechanism underlying rIL-2 therapy. A major obstacle to demonstrate our hypothesis, however, has been the lack of adequate experimental tools to distinguish between Treg-dependent and Treg-independent mechanisms of immunosuppression in response to low-dose rIL-2. To overcome this limitation, we developed novel lupus-prone mouse models that enabled us to separate Treg-dependent from Treg-independent effects of rIL-2. Using these tools, we demonstrated that low-dose rIL-2 efficiently depletes autoreactive Tfh cells in a Treg-independent manner in lupus-prone mice. Furthermore, Tfh depletion alone was sufficient to prevent pathogenic B cells responses and autoantibody production. Together, these findings reveal a novel Treg-independent mechanism of immunosuppression mediated by rIL-2 and establish a direct causal link between IL-2 treatment, Tfh depletion and inhibition of pathogenic B cell responses. This knowledge provides a foundation for the rational design of mechanism-based immunotherapies that target Tfh cells to prevent antibody-driven pathology in autoimmunity and allergy. Extending this concept to allergic disease, we assessed the therapeutic potential of IL-2–mediated depletion of Tfh cells in the context of respiratory allergies. Our FY2025 studies revealed that if Tfh type 2 (Tfh2) cells are generated during the initial allergen encounter, they persist as stem cell–like memory precursors in the lung-draining lymph nodes. Upon subsequent allergen exposures, these memory Tfh2 cells rapidly expand and dominate the recall response, regardless of the context of rechallenge, effectively locking the system into a pathogenic trajectory. This makes it extremely difficult to reprogram allergic immunity once it has been imprinted, representing a major barrier to the success of current desensitization and tolerance-induction strategies. This phenomenon closely resembles the “B cell original antigenic sin” model—classically described in B cell viral immunology—in which memory B cells formed during the first antigen encounter dictates future B cell responses and limits adaptability. We term this phenomenon “allergen–original antigenic sin” (allergen-OAS) to highlight its relevance in allergic disease. In this setting, allergen-OAS implies that early generation of Tfh2 cells establishes a long-term pathogenic bias by sustaining IgE production and perpetuating Th2-driven inflammation. By selectively targeting the stem cell–like Tfh2 memory reservoir in lung-draining lymph nodes, IL-2–based interventions may not only block pathogenic recall responses but also overcome the constraints of allergen-OAS, offering a path toward durable immune reprogramming and long-term disease modification in respiratory allergic disorders. While our studies demonstrate the pathogenic potential of Tfh cells in systemic and respiratory disease, in FY2025 we also uncovered a distinct protective role for these cells in the gut. Specifically, we found that IL-2 produced by Tfh cells is required to sustain microbiota-specific Tregs and preserve gut tolerance. Using conditional knockout models and state-of-the-art molecular and cellular approaches to selectively ablate IL-2 in Tfh cells, our ongoing work demonstrates that IL-2 production by Tfh cells in gut-associated lymphoid tissues (GALT) is essential for maintaining the stability and function of microbiota-specific regulatory T cells. Mechanistically, gut-associated Tregs cluster near Tfh cells in specialized B cell rich microenvironments within GALT, where they rely on Tfh-derived IL-2 for survival and function. Because Tfh cells are the predominant source of IL-2 in these B cell–rich niches, their failure to produce IL-2 deprives local Tregs of this critical support, leading to their destabilization, loss of suppressive function, and impaired immune regulation. This breakdown of Treg homeostasis disrupts mucosal tolerance, triggers colitis, and drives features of systemic autoimmunity. We are now extending these studies to single-cell analyses to define the molecular consequences of this disruption and to map the potential lineage relationships between distinct populations of Tregs, Tfh cells, and pathogenic Teff cells. Collectively, these findings point to a critical role for Tfh cells—beyond their classical function as B cell helpers—in preserving intestinal immune balance, with important implications for inflammatory bowel disease, food allergy, and systemic autoimmunity. Finally, as part of this project, in FY2025 we initiated a collaboration with Kristi Briggs (NIDDK) and Gulbu Uzel (NIAID) to dissect the role of CTLA-4 in Tfh cell biology. Preliminary findings in CTLA-4–deficient patients suggest that loss of CTLA-4 in Tfh cells disrupts IL-2 production. Based on our FY2025 studies, we propose a model in which impaired CTLA-4 signaling in Tfh cells reduces IL-2 availability, destabilizes local Tregs, and promotes mucosal immune dysregulation. Through a combination of clinical data and preclinical models, we aim to define a previously unrecognized cell-intrinsic role of CTLA-4 in Tfh cell biology and uncover new mechanisms linking Tfh cell dysfunction with impaired gastrointestinal tolerance. By addressing a critical knowledge gap in how CTLA-4 deficiency drives mucosal immune dysregulation in humans, this work is expected to inform the development of more effective and targeted therapies for CTLA-4 deficiency, immune-mediated colitis, and food allergy.