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.