Regulation of immune tolerance and anti-tumor immunity
Research Interests
The goal of our research program is to understand the cellular and molecular mechanisms regulating immune tolerance and the immune response to cancer. Current major projects include:
1. Biology of self-specific memory-phenotype CD8+ T cells
Classically, memory T cells arise from naive T cells after an immune response to a foreign pathogen in the periphery, and are poised to respond more rapidly upon repeated challenge. However, in mice that have never encountered pathogens, there is a substantial population of “memory-phenotype” CD8+ T cells (CD8-MP cells) that exhibit hallmarks of activation and innate-like functional properties. Due to the lack of markers to distinguish CD8-MP cells from conventional CD8+ memory T cells, fundamental aspects of CD8-MP cell biology remain incompletely defined, including the developmental forces coordinating their differentiation, the nature of antigenic ligands, and the function of CD8-MP cells in health and disease. In a recent study (Miller et al. Nature Immunology 2020), we used a clonal approach pairing complete T cell receptor profiling with in-depth developmental analysis of individual CD8-MP clones. Our findings reveal the unexpected discovery that many facets of CD8-MP differentiation parallel the development of Foxp3+ regulatory T cells, which are crucial for the prevention of autoimmunity. Specifically, CD8-MP differentiation is a TCR-instructed process that is triggered by self-ligand recognition in the thymus, occurs optimally at low clonal frequencies, and involves a two-step process marked by an initial TCR-dependent triggering step followed by a second phase of consolidation. These findings challenge the common notion that CD8-MP cells represent “lymphopenia-induced memory” cells that differentiate from naive T cells in the periphery, and suggest the existence of dedicated mechanisms driving TCR-dependent differentiation in the thymus. Additionally, we used cell transfer and comparative TCR profiling to demonstrate that CD8-MP cells readily infiltrate mouse prostate tumors and express high densities of PD-1, suggesting a previously unanticipated role for CD8-MP cells in the tumor context. We hypothesize that CD8-MP cells may account for many of the PD-1-expressing T cells of undefined antigen specificity found in many human cancers, and may be critical determinants of PD-1 checkpoint blockade therapies. In ongoing work, we aim to define the immunological mechanisms directing CD8-MP differentiation in the thymus, identify natural self-ligands recognized by CD8-MP clones, and define the functional role of tumor-infiltrating CD8-MP cells in endogenous anti-tumor immunity and the response to immune-based therapies.
2. Development and function of tumor-associated regulatory T cells
Foxp3+ regulatory T (Treg) cells are critical for the suppression of autoimmunity and the regulation of immune homeostasis, and are often prevalent in human cancers. Many emerging therapeutic strategies for the treatment of cancer have focused on the modulation or depletion of Tregs concomitant with vaccination or cell transfer, in order to stimulate effective anti-tumor immune responses. Yet despite this intense interest in modulating Tregs in the context of cancer, fundamental questions regarding the biology of tumor-associated Tregs remain unanswered. Specifically, the developmental origins, antigen specificity, and in situ function of tumor-infiltrating Tregs are not well understood. Using mouse models of prostate cancer (Malchow et al Science 2013) and carcinogen-induced head-and-neck squamous cell carcinoma, our goal is to elucidate the fundamental rules by which Tregs function in the context of cancer. In essence, we aim to understand the “life cycle” of a tumor-infiltrating Treg, starting from its development in the thymus or periphery, its circulation throughout the body, its activation and recruitment into a developing neoplasm, and the functional role that the cell plays in shaping tumor development and metastasis.
3. Antigen specificity of thymus-derived Treg cells
A large body of indirect evidence suggests that thymus-derived Treg (tTreg) cells recognize autologous antigens. However, the major self-antigens recognized by Treg cells have remained largely undefined, representing a major barrier to the understanding of immune regulation. Recently, in collaboration with Dr. Erin Adams at the University of Chicago, we identified natural Treg cell ligands in mice (Leonard, Gilmore, et al. Immunity 2017). We found that two recurrent Treg cell clones, one prevalent in prostate tumors and the second associated with prostatic autoimmune lesions, recognized distinct non-overlapping MHC class-II-restricted peptides derived from the same prostate-specific protein. Notably, this protein is frequently targeted by autoantibodies in experimental models of prostatic autoimmunity. Based on these findings, we propose a model in which Treg cell responses at peripheral sites converge on those self proteins that are most susceptible to autoimmune attack, and we suggest that this link may be exploited as a generalizable strategy to identify the Treg cell antigens relevant to human autoimmunity. Moving forward, we are using this model system to define the role of cognate antigen in coordinating Treg development and peripheral homeostasis, to characterize endogenous antigen-specific Treg cell populations at steady state and in disease contexts using pMHC tetramers, and to understand the molecular basis of ligand recognition by tTreg cells.
4. Aire and the establishment of immune tolerance
The promiscuous expression of tissue-restricted antigens in the thymus, driven in part by Autoimmune Regulator (Aire), is essential for the protection of peripheral tissues from autoimmune attack. Aire-dependent processes are thought to promote both clonal deletion and the development of Foxp3+ Treg cells (Malchow et al. Science 2013), suggesting that autoimmunity associated with Aire deficiency results from two failed tolerance mechanisms. In recent work (Malchow et al. Immunity 2016), our examination of autoimmune lesions in Aire-/-mice revealed an unexpected third possibility. We found that the predominant conventional T cell clones infiltrating target lesions express antigen receptors that are preferentially expressed by Foxp3+ Treg cells in Aire+/+mice. Our results reveal that a primary mechanism by which Aire functions is to ensure that distinct autoreactive T cell specificities differentiate into the Treg cell lineage. Dysregulation of this process results in the emergence of “T-rogues” – Treg-biased specificities that are mis-directed into the T conventional subset and “go rogue” in the absence of Aire.
5. Role of dendritic cells in Treg cell development and function
The recognition of self antigen is critical for many aspects of Treg cell biology, including development, homeostasis, anatomical distribution, and function. However, little is known about the identity of the cell types that present self antigen for recognition by Treg cells. The identity of the “dance partners” that interface with Treg cells at various anatomical sites is likely to reveal new insights into Treg cell biology and immune regulation. In a recent study, we identified a pivotal role for dendritic cells (DCs) in coordinating the development and homeostasis of an archetypal population of Aire-dependent organ-specific Treg cells (Leventhal et al., Immunity 2016). The thymic development of this Treg population required antigen presentation and co-stimulatory signals provided by DCs, implying that Aire-dependent antigen must be transferred from medullary thymic epithelial cells to DCs. In the periphery, the activation and enrichment of organ-specific Treg cells in the organ-draining lymph nodes required CCR7-dependent migratory DCs. Our results demonstrate that the development and peripheral regulation of organ-specific Treg cells are dependent on antigen presentation by DCs, implicating DCs as key mediators of organ-specific immune tolerance.