Best known for his innovative research involving immunosuppressive therapies, Dr. Bromberg has devoted his career to investigating the role of immunology in tolerance, with a current focus on the effects of chemokines and cell migration on the immune response. A committed clinician with an active practice, he also has been named among New York Magazine’s New York's Best Doctors for five years in a row.
Over the course of 20 years, Dr. Bromberg's work has been continuously supported by the NIH. Since 1999, he has received more than $11 million in federal funding, including $1.4 million in 2008 alone, and more than $1.5 million support in non-federal funding. Significant research accomplishments include:
After completing both his M.D. and PhD from Harvard, Dr. Bromberg completed a surgical residency at the University of Washington. His training continued with a transplantation fellowship at the University of Pennsylvania. Prior to his arrival at Maryland, he held academic titles at the University of Michigan, where he was Professor of General Surgery in the Division of Transplantation and Professor of Microbiology and Immunology, and at the University of Pennsylvania, the Mount Sinai School of Medicine, and the Medical University of South Carolina.
Dr. Bromberg has authored more than 230 publications, of which includes 193 peer-reviewed publications. Additionally he has been an invited speaker for numerous presentations and has received several national honors, including most recently the Joel J. Roslyn Commemorative Lecture (New Considerations in Tolerization, Society of University Surgeons), the 2002 Andrew Lazarovits Lecture (Canadian Society of Transplantation), and the 2005 Alfred and Florence Gross Professor of Surgery.
Honors and Awards
1975-Edwards-Whitaker Award; 1975,76,77-John Harvard Award; 1976-Phi Beta Kappa; 1977-Detur Book Prize; 1977-Summa Cum Laude, Biology; 1979-83-Medical Scientist Training Program Fellowship; 1983-James Tolbert Shipley Prize; 1988-90-Sandoz Award, American Society of Transplant Surgeons; 1992-94-American Surgical Association Foundation Fellowship Award; 1992-Thomas A. and Shirley W. Roe Foundation Award; 1997-Excellence in reviewing, Journal of Surgical Research; 1998-ASTS Roche Presidential Travel Award; 1992-present: NIAID, NIDCR, SAT, and SBIR study sections; 2001-Roslyn Commemorative Lecture, Society of University Surgeons; 2000-2014-American Journal of Transplantation, Associate Editor, Deputy Editor, Section Editor for Literature Watch; 2000-2005-Journal of Immunology, Section Editor; 2002–Lazarovits Commemorative Lecture, Canadian Society of Transplantation; 2013-AST Basic Science Established Investigator Award; 2014-present-Transplantation, Clinical Sciences Executive Editor; 2014-NKF of Maryland, Kidney Champion Award
Recent Grant Review Committees and Boards
Contributions to Science
1. Major questions in the field of organ transplant are where does tolerance take place and what processes determine the choice between tolerance and immunity. Using a variety of pharmacologic and genetic approaches in both cardiac and islet transplant models, my lab demonstrated that normal lymph node functions and structures are required for tolerance induction and maintenance. We demonstrated the requirement for CD4+ T cell migration from blood into lymph nodes, regulated by a variety of selectins, integrins, and chemokines, that determine T cell anergy, apoptosis, and regulatory T cell induction and suppression. In addition, plasmacytoid dendritic cells (pDC) are also required to migrate into lymph nodes and present alloantigen to T cells. These studies provided novel evidence for active roles of the lymph node in determining the fate of T cells and the immune response.
2. There has been a great deal of interest in understanding the induction, stimulation, maintenance, and activity of FoxP3+ CD4+ suppressive regulatory T cells (Treg). My laboratory was one of the first to demonstrate that TGFb is required for Treg induction, and that inflammatory stimuli and cytokines can inhibit Foxp3 induction or stability. Epigenetic regulation of the Foxp3 gene is critical for Treg activity, and Foxp3 gene expression and structure can be manipulated with T cell receptor and costimulatory signals, cytokine and TLR signals, and methyltransferase inhibitors. These results were extended to the generation of human Tregs in vitro for therapeutic use in vivo. We also demonstrated critical roles for IL10, TGFb, and the induction of myeloid derived suppressor cells in the mechanisms of Treg suppression and tolerance. These studies defined important pharmacologic modulators of Treg that can be translated into clinically relevant approaches for therapy.
3. A major issue concerning Treg suppressive and tolerogenic competence is to discover how to deliver these cells to the right place at the right time. My lab was the first to demonstrate that Treg not only must be induced in lymph nodes, but also must migration from tissues through afferent lymphatics into lymph nodes in order to fully suppress inflammation and immunity and prolong islet allograft survival. Lymphatic migration is regulated by a number of integrins, selectins, chemokines, and sphingosine 1-phospate receptors (S1PR) on the T cell. Treg interact with endothelial cells, parenchymal cells, and antigen presenting cells during their migration, effecting distinct suppressive activities required for graft survival and required for the induction and maintenance of Treg activation and suppressive function. These studies defined novel aspects of Treg function that point toward therapeutically important implications for manipulating immunity and suppression.
4. The structure and function of lymphatic vessels are poorly understood, in large part due to the difficulty of isolating these cells for in vitro work and manipulating and imaging these structures in vivo. Our studies on Treg migration led to more general studies of lymphatic function. We defined a stable lymphatic endothelial cell (LEC) line that recapitulates LEC function in vitro, allowing ablumenal-to-lumenal migration to a chemokine gradient, but not the reverse migration. In contrast, blood endothelial cells permit migration in both directions. A sphingosine 1-phosphate (S1P) gradient promotes transendothelial migration across LEC, while a high concentration of S1P, such as occurs in acute inflammation, inhibits afferent lymphatic migration, retaining immune cells in tissues. Lymphangiogenesis not only occurs in the presence of inflammation, but also promotes inflammation and can be targeted to prevent allograft rejection. These studies defined new tools for lymphatic research and defined potential novel therapeutic approaches to modulating inflammation.
5. The recognition that lymph nodes are required for tolerance and that there are distinct domains within the lymph node committed to different aspects of immunity, led my lab to investigate other discrete cells and structures, their regulation, and their roles in immunity and tolerance. During tolerization we noted that alloantigen specific Treg and pDC presenting specific alloantigen were concentration around the cortical ridge, an area that encompasses the high endothelial venules and is a site for trafficking into the lymph node and between cortex and medulla. During tolerance there is increased laminin a4 and decreased laminin a5 in the cortical ridge, while during immunity the ratios are reversed. There is a role for fibroblastic reticular cells in regulating lymph node structure and cytokines, antigen presentation, and tolerance. Other stromal fibers, such as ERTR-7, also dictate CD4+ T cell, Treg, and pDC movements and the choice between tolerance and immunity. These studies defined novel roles for stromal fibers, stromal cells, and the cortical ridge in tolerance.
6. The discovery of the role of S1PR in leukocyte-transendothelial migration has recently opened up new areas of investigation to uncover the role of S1P and S1PR in diverse aspects of immunity and inflammation. We assessed the role of the major T cell S1PR1 receptor in migration, immunity, and tolerance. We uncovered novel activities for the S1PR agonist/antagonist FTY20 in modulating lymph node versus splenic migration, and immunity versus tolerance. My lab discovered that S1PR signaling involves a complex cascade, engaging multidrug transporters and cysteinyl leukotriene synthesis and transport to fully effect changes in lymphocyte migration. S1P acts as both a chemotactic cytokine and as an inhibitor of migration, depending on concentration and gradients. Targeting S1PR promotes graft survival and tolerance. These studies defined novel aspects of S1P and S1PR metabolism and function and shed new light on how activators and inhibitors may have highly complex effects in vivo.
CD4+CD25+Foxp3+ regulatory T cells (Treg) are important regulators for virtually all immune responses, and are key elements for establishing and maintaining antigen specific tolerance. While Treg and Foxp3 expression have been the focus of much research, basic aspects of Treg physiology and function remain incompletely understood. In particular, it is not known where and when Treg or their precursors precisely interact with antigen, become activated, and deploy their suppressive regulatory mechanisms. The cellular and molecular mechanisms important for each of these steps are poorly delineated, and the migration and trafficking mechanisms essential for priming at one site and effector function in another site remain largely unknown. In the previous funding period, we published a number of important studies revealing that during tolerization, naïve T cells migrate to lymph nodes (LN), where they are stimulated in the cortical area by specific alloantigen presenting plasmacytoid dendritic cells (pDC), next to the abluminal surface of the high endothelial venules (HEV), to generate adaptive or induced Treg (aTreg). We elucidated several important molecular mechanisms in these interactions including roles for IL-2, TGF-beta, T cell receptor (TCR), chemokines (CCL17, CCL19, CCL21), chemokine receptors (CCR4, CCR7), sphingosine-1-phosphate (S1P), and S1P receptor-1 (S1P1).
In ongoing studies, we have made several additional key observations. First, investigation of molecular factors that control Foxp3 expression revealed an upstream CpG island enhancer that is epigenetically regulated by DNA methylation, and that is completely methylated in aTreg versus being unmethylated in thymus derived natural Treg (nTreg). Manipulation of this enhancer by using DNA methyltransferase (DNMT) inhibitors drives Foxp3 expression, allowing us to generate Treg subsets that resemble either aTreg or nTreg. Second, we demonstrated that nTreg migrate sequentially from the blood through microvascular endothelium into the site of tissue inflammation, and then subsequently from tissue into afferent lymphatics and then the draining LN (dLN), in order to become activated and display suppressor functions for islet allograft survival. Sequential tissue and dLN migration are key components of nTreg function, and migration is coupled to Treg developmental and differentiative steps. In contrast, aTreg precursors migrate first to the LN, and then to sites of inflammation. Third, Treg specifically express increased levels of lymphotoxin-alpha (LT-alpha) and LT-beta. In contrast, endothelial cells, including lymphatic endothelial cells, express LT-beta receptor (LT-beta-R). Inhibition of the LTalpha-beta-LT-beta-R receptor-ligand interaction prevents Treg translymphatic endothelial migration (nTreg more so than aTreg), and inhibits tolerance to islet and cardiac allografts. Together these novel observations demonstrate important differences in the physiology and function of aTreg versus nTreg. We hypothesize that aTreg and nTreg have distinct requirements for induction, migration, trafficking and suppressive function, and that these distinctions are critically important for their separate roles in immune suppression and regulation. To investigate this hypothesis, we propose the following three specific aims:
Aim 1. Elucidate mechanisms of Treg migration from blood to tissues to dLN, and how sequential migration regulates Treg differentiation and suppressive function. Using adoptive transfer and islet transplant models, we will investigate the separate steps of migration of aTreg and nTreg from blood into tissues and then tissues into dLN. The experiments will determine the molecular and cellular mechanisms that regulate Treg migration at each step, determine how these steps regulate Treg activation, and determine how these Treg responses then regulate immunity. aTreg will be compared to nTreg, as the Preliminary data demonstrate that distinct mechanisms of action and molecular signatures distinguish between these Treg subsets.
Aim 2. Define mechanisms of LT-alpha-beta-LT-beta-R regulation of Treg migration and suppressor function. Using adoptive transfer, islet transplant and inflammatory models, and a novel in vitro model of afferent lymphatic migration, we will investigate the functional interaction of LT-alpha-beta on Treg with LT-beta-R on lymphatic vascular endothelial cells and antigen presenting cells, and how this interaction regulates Treg induction, proliferation, suppressor function, and migration.
Aim 3. Determine how Treg regulate lymphatic migration. Using in vitro models and adoptive transfer with islet transplant and inflammatory models, we will investigate the interaction of Treg with afferent lymphatics and how this interaction alters lymphatic function and the migration of other leukocyte subsets, including B cells, CD4+ and CD8+ T cells, macrophages, and dendritic cells (DC). Molecular mechanisms involving chemokines, integrins, S1PRs, and LT-beta-R on these other leukocyte subsets will be investigated.
Despite the detailed understanding of signals 1, 2, and 3 and the critical function of T cells and dendritic cells (DC), clinical transplantation tolerance is almost never achieved. In transplantation, models of immune function, and interventions designed to promote immune regulation, have been based on simplified receptor-ligand and cell-cell interaction models. There has been a lack of studies that place immunological recognition within anatomic contexts and evaluate the critical role of anatomic microdomains in the regulation of the immune response. Our preliminary data now demonstrate that during tolerization there is alloantigen specific clustering of T cells and plasmacytoid DC (pDC) in the T cell areas of the lymph node (LN) near the abluminal surface of the high endothelial venules (HEV). Within these clusters T cells undergo either priming or development into de novo CD4+CD25+ regulatory T cells (Treg). Importantly, B cells presenting specific alloantigen are present in these cell clusters and contribute to Treg development.
We hypothesize these multicellular clustered interactions in the LN are key to the induction and maintenance of tolerance. Specifically, we hypothesize that the precise interaction of T-APC (pDC, B cells) determines the outcome of T cell migration, positioning, proliferation, and maturation, and ultimately whether rejection or tolerance are induced. This hypothesis integrates many of the known receptor-ligand and cell-cell interactions, and places these interactions in the context of secondary lymphoid organ structure. To investigate the role of these cellular and structural elements in LN clustered interactions, we propose the following specific aims:
Aim 1. What are the T-APC-LN interactions that are important for tolerance? Using a transplant model that allows the tracking of alloantigen specific T cells, specific alloantigen presenting APC, and the positioning of the cells with respect to HEV, we will characterize specific receptor-ligand interactions between T-HEV and pDC-HEV that regulate tolerance, and define the interactions between T-pDC and other DC that determine the generation of Treg and inhibition of effector T cells within the LN.
Aim 2. How is the priming of effector T cells altered in the LN during tolerance? This aim will study the specific interaction between effector T cells and Treg in the LN clusters during tolerization. We will determine how Treg-T effector interactions regulate effector T cell migration, proliferation, and differentiation. We will characterize specific receptor-ligand interactions between Treg and T effector cells that determine whether T cell priming and rejection versus suppression and tolerance predominate in the immune response.
Aim 3. Determine the role of B cells in tolerance in the LN We will investigate the roles of B cells in the LN during the T-APC clustered interaction that results in tolerization. Analyses will focus on B cell APC function, chemokine production, and immunoglobulin production.
Our currently funded investigations are focused on the fate and differentiation of Treg, on T cell migration and trafficking in tolerance, and on the role of T-pDC interactions that regulate both Treg and T cell trafficking. While T cell responses have a central role in transplantation tolerance, our new preliminary data suggest novel and important roles in the LN for B cells, stromal cells, stromal fibers, and other LN elements in the clustered interaction that leads to tolerization. To investigate the role of these novel cellular and structural elements in LN clustered interactions, we propose the following specific aims:
Aim 1. Determine the roles of B cells in LN structure and tolerance Using a transplant model that allows the specific tracking of antigen specific T cells and specific alloantigen presenting antigen presenting cells (APC), we will investigate the roles of B cells in the LN during the T-APC clustered interaction that results in tolerization. Analyses will focus on B cell APC function, chemokine production, and immunoglobulin production.
Aim 2. Determine the LN stromal structures and cells essential for tolerance We will investigate the role of stromal cells and stromal cell derived elements in the LN in the T-APC clustered interaction during tolerization.
Aim 3. Determine the role of invariant innate cellular immune responses in tolerance We will determine how NK1.1+ cells, that respond to innate and invariant signals, affect the T-APC clustered interaction, LN structure, and tolerization.
We hypothesize these multicellular clustered interactions in the LN are key to the induction and maintenance of tolerance to islet allografts. Specifically, we hypothesize that the precise interaction of T-pDC determines the outcome of T cell migration, positioning, proliferation, and maturation, and ultimately whether rejection or tolerance are induced. This novel hypothesis integrates many of the known receptor-ligand and cell-cell interactions, and places these interactions in the context of secondary lymphoid organ structure and function. To investigate the role of these cellular and structural interactions in LN clustered interactions, we propose the following specific aims:
Aim 1. Determine the T-DC-LN interactions that are required for tolerance Using a transplant model that allows the tracking of alloantigen specific T cells, specific alloantigen presenting antigen presenting cells (APC), and the positioning of the cells with respect to HEV, we will characterize specific receptor-ligand interactions between T-HEV and pDC-HEV that regulate tolerance, and define the interactions between T-pDC and other DC that determine the generation of Treg and inhibition of effector T cells within the LN.
Aim 2.Determine the LN stromal structures and cells essential for tolerance We will investigate the role of stromal cells and stromal cell derived elements in the LN in the T-DC clustered interaction during tolerization.
Kidney and pancreas transplantation
Anatomy of Tolerance: Lymph Node Migration and Induction of Treg, Transplant Grand Rounds, University of Wisconsin, Madison, WI, September 18, 2012.
Anatomy of Immunity (or Why Surgeons are so Important for Research), Surgery Grand Rounds, University of Wisconsin, Madison, WI, September 19, 2012.
Grant Award and Service:
Lymphoid Structure in Tolerance, NIH R56 AI072039, 08/01/12-07/31/13, $250,000. (PI)
Grant review for the Armed Forces Institute of Regenerative Medicine, September 2012