Autoimmune diseases are among the leading causes of mortality in young and middle-aged adults in the United States. They are chronic and progressive in nature, leading to organ destruction and death if left untreated. However, current treatment modalities are for the most part, symptomatic, limited to reducing inflammation and slowing disease progression. Furthermore, immune modulation strategies used to treat autoimmunity suffer from lack of precise discrimination between the autoreactive effectors and protective defense mechanisms, thus creating a hole in the body’s defense to infections and malignancies. An alternative approach is to restore peripheral tolerance via enriching the regulatory T cells (Treg) which are the immune system’s in-house combatants against self-reactivity. So far polyclonal Treg enrichment via cellular biotherapy and/or low dose IL-2 treatment strategies have been put into clinical trials. However, these can also impose broad immunosuppression and currently, there are no successful methods to fine tune Treg activity. These shortcomings mainly originate from the polyclonal nature of the enrichment strategies which are based on the longstanding conception in the field that Treg mediated suppression is largely antigen non-specific.
The immunosuppressive machinery of Tregs is activated by Treg antigen receptor stimulation. Early in vitro studies proposed that once Tregs receive activating stimuli, they suppress all effector T cells in the same microenvironment. This has been termed bystander suppression, and despite lack of in vivo evidence, bystander suppression has been accepted as the only major way Tregs suppress immune responses. We addressed the longstanding paradigm of bystander versus antigen-specific suppression in vivo and demonstrated that, in sharp contrast with in vitro observation of bystander suppression, inhibition of effector activity occurs when Tregs and helper T cells recognize the same antigen. We reported that this antigen-specific suppression is a direct outcome of highly specific Treg mediated depletion of antigen from the dendritic cell (DC) surface during Treg-DC contact. These findings are paradigm shifting in the way they show for the first time that Tregs mediate antigen-specific suppression in vivo by a cell contact dependent mechanism. Now, with this new understanding of Treg-DC contact dependent antigen-specific suppression, it is a propitious effort to elucidate the cellular processes that regulate antigen-specific supresssion with a view towards developing novel strategies to induce tolerance as a therapy for autoimmune diseases. Thus, our lab aims to identify unique cellular and molecular events that trigger antigen-specific Tregs to silence self-reactive T cells and following are the three main ongoing projects in our lab:
Project 1: Deciphering the antigen mediated interactions and suppression mechanisms of human Tregs. For this project we use antigen-specific Tregs derived from type 1 diabetes patients via insertion of genes that encode self-specific T cell receptors into polyclonal Tregs. With this unique tool, we investigate for the first time, whether human antigen-specific Tregs capture and deplete their cognate antigenic peptides bound to major histocompatibility complex class II (pMHCII) from autologous antigen presenting cells (APC) and if so, how does this depletion regulate Treg-mediated suppression. We are also in the process of devising a new antigen discovery platform for finding the antigenic epitopes that bona fide human Tregs derived from patients with type 1 diabetes react to. Findings from this project will reveal how self-antigen mediated interactions between human Tregs and APCs contribute to the suppression of autoreactive T cells and also identify valuable Treg clones that can be used for antigen-specific Treg therapies.
Project 2: Discovering the molecular machinery underlying Treg interactions and Treg-mediated suppression. This project aims to elucidate the signals provided to Tregs via antigen mediated contacts with DCs. We compare Tregs and helper T cells using conventional techniques such as phosphoflow and western blot while also performing an unbiased analysis of phosphoproteome, to decipher the pathways underlying pMHCII capture and antigen-specific suppression. We also delineate how LAG3, a molecule that we have shown to facilitate pMHCII capture in Tregs but not helper T cells, plays a role in antigen-specific suppression. Furthermore, we investigate which DC surface molecules contribute to optimal Treg activity and how professional antigen-presenting cells (APC) other than DCs interact with Tregs. Overall, this project will uncover the molecular basis of antigen-specific suppression performed by Tregs and identify key molecules that can potentially be targeted to fine-tune the Treg suppression activity in autoimmunity and cancer.
Project 3: Determining the antigen-presentation capability of antigen-specific Tregs post- primary immune synapse. In this project we explore whether pMHCII capture renders antigen-specific Tregs an atypical group of APCs that are capable of inducing T cell tolerance. We focus on the events after Treg-APC synapse and investigate whether capturing pMHCII containing DC membrane equips Tregs with the unique ability to target helper T cells during the physical absence of the DCs. We perform live confocal and intravital two-photon microscopy imaging as well as flow cytometry 1) to quantify direct contact between Tregs and helper T cells post-pMHCII acquisition 2) to determine modes of paracrine communication. Altogether, the findings from this project will describe how the Treg-T cell interactions shape peripheral tolerance and what mechanisms additional to pMHCII depletion are in place to prevent autoimmunity and also to promote tumor development.
The outcome of these studies will provide the basis for how self-reactivity is dampened by antigen-specific Tregs, thus constituting a springboard for the precision therapies against autoimmunity and cancer.