The Influence of Genetic Variation in Toll-like Receptors on Susceptibility to Tuberculosis and the Immune Response to BCG Vaccination

Thomas Hawn, M.D., Ph.D.

University of Washington

Funded in December, 2005: $600000 for 3 years
LAY SUMMARY . ABSTRACT . BIOGRAPHY . SELECTED PUBLICATIONS .

LAY SUMMARY

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What Innate Immune System Gene Variations Make Some People Susceptible to TB Infection?

This consortium will try to determine how genetic variations in human innate immune responses influence an individual's susceptibility to tuberculosis (TB) infection and protection from the current TB vaccine.

Even though only about ten percent of people infected with the TB bacteria called mycobacterium tuberculosis develop active disease, TB is a leading cause of death worldwide. TB treatment is problematic. It takes a long time and is becoming drug resistant, and the existing vaccine, called "BCG," only confers protection in some people. The researchers are exploring genetic variations in people's immune responses to TB infection to answer three questions:

  • Which immune responses are associated with susceptibility or resistance to TB?
  • Why does TB infection spread to the central nervous system in some people—causing TB meningitis—while remaining localized in the lungs in most people?
  • What accounts for the wide variation in BCG vaccine efficacy? Consortium researchers hypothesize that each of these questions is influenced by genetic variation of critical immune response genes.

The answers may emanate from further studies of initial findings by the University of Washington researchers. They have found the existence of different chemical forms of genes that cause a deficient inflammatory response, and also are associated with susceptibility to infections. These genes are called "Toll-like" receptors. Ordinarily, Toll-like receptors coordinate a three-pronged immune attack of harmful bacteria. This involves innate immune cells' recognition of the bacteria, followed by the activation of "adaptive" immune cells to attack the bacteria, followed by activation of immune "macrophages," which eliminate the bacteria from the body. Now, the University of Washington researchers will work with consortium members studying TB in Cape Town and Vietnam to see if Toll-like receptor differences are found in TB-prone populations. In Cape Town, where BCG vaccination is required in infancy, researchers will determine whether Toll-like receptors differ among children who have developed TB and were not protected by the vaccine, in comparison to children who are TB-free after vaccination. In Vietnam, the investigators will try to identify whether variations in the Toll-like receptor pathway are involved in the development of TB meningitis. The consortium also will see whether immune macrophages respond differently to TB in these populations.

Significance: If this consortium identifies Toll-like receptor changes that produce susceptibility to TB infection and lack of responsiveness to the TB vaccine, the research could lead to development of improved vaccines and treatment.

ABSTRACT

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The Influence of Genetic Variation in Toll-like Receptors on Susceptibility to Tuberculosis and the Immune Response to BCG Vaccination

Developing a highly efficacious vaccine against Mycobacterium tuberculosis (MTb) is a high worldwide priority. Success depends on a thorough understanding of the host response to infection. While a series of studies over the past 50 years suggest that variation in host genes influences susceptibility to tuberculosis, a comprehensive understanding of these genetic factors remains an elusive and important goal. The discovery of Toll-like receptors (TLRs) precipitated a major advance in understanding the molecular mechanisms of both MTb recognition and the inflammation that results from activation of the innate immune response.

Although we and others have recently shown that common TLR-pathway polymorphisms are associated with deficient cellular inflammatory responses and the development of clinical disease, it is not known if these polymorphisms affect the adaptive immune response, microbial killing mechanisms, or vaccine efficacy. We propose to examine these possibilities by examining polymorphisms in the TLR pathway and their influence on each of these aspects of the immune response. By identifying individuals with TLR pathway signaling deficiencies, we have a unique tool to understand the immunopathogenesis of TLRs in human health and to gain insight into methods of modulating TLR pathway signaling.

We hypothesize that genetically regulated variation of the innate immune response is associated with altered protection to vaccination against MTb with the Bacille-Calmette-Guerin (BCG) vaccine and to altered susceptibility to tuberculosis. We further hypothesize that this genetic variation is associated with altered innate and adaptive cellular immune responses to MTb. To test these hypotheses, we propose to examine 2 case-control studies:

1. In South Africa, we recently completed a large trial to identify infants who are protected against Tb after newborn BCG vaccination (n=450, among 5,675 initially enrolled), and infants who are not protected against the disease by the vaccine (n=130). We propose to examine whether variants in TLRs are associated with protection against MTb, by comparing the 2 participant groups. We are also comprehensively evaluating the innate and adaptive cellular immune responses associated with vaccination-induced protection against Tb. We will determine whether the TLR pathway variants are associated with altered cellular immune responses.

2. In Vietnam, we have completed enrollment of a large case-control study of individuals with meningeal Tb (n=175), pulmonary Tb (n=183), or population cord blood controls (n=392). We propose to examine whether polymorphisms in TLR-pathway genes are associated with susceptibility to meningeal or pulmonary tuberculosis. We will also examine whether these polymorphisms are associated with altered immune responses, including measurements of cytokine production in ex vivo whole blood assays as well as the cerebrospinal fluid in subjects with Tb meningitis.

We will complement these human genetic studies with an investigation of the cellular consequences of this gene variation. We have recently identified variants in TIRAP and TLR1 that are associated with deficient innate immune responses to MTb. We will examine the cellular mechanisms of how inflammation is altered in TIRAP and/or TLR1-deficient individuals. These studies will generate insight into novel ways of modulating innate immune responses that may have implications for the design of adjuvants in vaccine formulations. Together, these genetic, immunologic, and cellular studies of the host response to tuberculosis could lead to novel insights for vaccine development, as well as diagnostic advances to target treatment to those who are at risk for developing active disease.

INVESTIGATOR BIOGRAPHIES

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Thomas Hawn, M.D., Ph.D.

Thomas Hawn obtained his M.D. and Ph.D. degrees at Johns Hopkins University School of Medicine. He then moved to the University of Washington, where he trained in an Internal Medicine residency and then specialized further with a fellowship in Infectious Diseases. Dr. Hawn is currently an Assistant Professor in the Division of Allergy and Infectious Diseases in the Department of Medicine at the University of Washington School of Medicine.

His laboratory investigates molecular, cellular, and immunologic mechanisms of disease pathogenesis with an emphasis on genetic studies of the innate immune response. These studies include investigations of why individuals have variable susceptibility to tuberculosis and the BCG vaccine to prevent it. The overall goal is to understand why individuals have different susceptibility to infections and whether these insights can lead to novel treatment and vaccine strategies.

Dr. Willem Hanekom qualified as a pediatrician at the University of Cape Town in South Africa. He then completed a clinical pediatric infectious diseases fellowship at Northwestern University in Chicago. While a research associate at Rockefeller University in New York, he completed studies of dendritic cell interactions with Mycobacterium tuberculosis. During this time he also developed international projects aimed at characterizing host immune responses induced by BCG.

Dr. Hanekom has continued to work on these projects while an Assistant Professor in Clinical Pediatrics, Microbiology and Immunology at the University of Miami. He also holds an appointment at the University of Cape Town, South Africa, where he will take part in novel TB vaccine trials.

Sarah Dunstan obtained her Ph.D at the Department of Microbiology and Immunology of the University of Melbourne, Australia. She then completed post doctoral training in the Department of Biochemistry at Imperial College of Science Technology and Medicine in London.

Dr. Dunstan currently works at the Oxford University Clinical Research Unit in Ho Chi Minh City where she heads the Human Genetics research group and has recently been appointed Vice Director. The Human Genetics group at OUCRU investigates the human host's genetic susceptibility to infectious diseases prevalent in Vietnam, namely tuberculosis (focusing on tuberculous meninigitis), malaria, typhoid, and dengue. The overall aim is to obtain a greater understanding of the interaction between the host and pathogen and ultimately to understand what protects an individual from specific diseases, in the hope to identify potential targets for novel therapeutics.

Jeremy Farrar, B.Sc., MBBS, FRCP, D.Phil OBE, undertook his medical training at University College and the Westminster Hospital London and subsequently trained in Neurology in Edinburgh, Melbourne, San Francisco, and Oxford. He has a Ph.D. in Immunology from Oxford.

His research interests include malaria, dengue, typhoid, TB, encephalitis, tetanus, diphtheria, and recently "Bird Flu" (Avian Influenza). He is a Wellcome Trust Senior Clinical Research Fellow and has been Director of the Wellcome Trust funded Clinical Research Unit in Vietnam for the last ten years. In 2004, he was awarded the Ho Chi Minh Medal for contributions to Clinical Medicine in Vietnam 2004 and an OBE for services to health care, particularly tropical diseases, in Vietnam 2005. He was appointed to Professor of International Health at the London School of Tropical Medicine and Hygiene in December 2005.

SELECTED PUBLICATIONS

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Hawn T.R., Dunstan S.J., Thwaites G.E., Simmons C.P., Thuong N.T., Lan N.T., Quy H.T., Chau T.T., Hieu N.T., Rodrigues S., Janer M., Zhao L.P., Hien T.T., Farrar J.J., and Aderem A.  A polymorphism in Toll-interleukin 1 receptor domain containing adaptor protein is associated with susceptibility to meningeal tuberculosis.  J Infect Dis. 2006 Oct 15;194(8):1127-34.

Hawn T.R., Misch E.A., Dunstan S.J., Thwaites G.E., Lan N.T., Quy H.T., Chau T.T., Rodrigues S., Nachman A., Janer M., Hien T.T., Farrar J.J., and Aderem A.  A common human TLR1 polymorphism regulates the innate immune response to lipopeptides. Eur J Immunol. 2007 Aug;37(8):2280-9.

Thuong N.T., Hawn T.R., Thwaites G.E., Chau T.T., Lan N.T., Quy H.T., Hieu N.T., Aderem A., Hien T.T., Farrar J.J., and Dunstan S.J. A polymorphism in human TLR2 is associated with increased susceptibility to tuberculous meningitis. Genes Immun. 2007 Jul;8(5):422-8.