TLR1 I602S: trafficking dynamics and roles in resistance to mycobacterial infection

Hart, Bryan

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Description

Title

TLR1 I602S: trafficking dynamics and roles in resistance to mycobacterial infection

Author(s)

Hart, Bryan

Issue Date

2013-02-03T19:35:42Z

Director of Research (if dissertation) or Advisor (if thesis)

Tapping, Richard I.

Doctoral Committee Chair(s)

Tapping, Richard I.

Committee Member(s)

• Salyers, Abigail A.
• Shisler, Joanna L.
• Wang, Fei

Department of Study

Microbiology

Discipline

Microbiology

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Innate immunity
• Toll-like receptor
• mycobacteria
• polymorphism

Abstract

Innate immunity provides defense mechanisms required for protection against constant exposure to potentially pathogenic microorganisms. Essential to this role is the ability to rapidly recognize the presence of foreign components or organisms indicative of infection. Toll-like receptors (TLR), a ten-member family of innate immune proteins, serve as major pattern recognition receptors which sense conserved constituents of bacteria, fungi, and viruses. TLR activation results in the production of proinflammatory cytokines and co-stimulatory molecules required for the establishment of local inflammation and adaptive immunity. Toll-like receptor 1 (TLR1), in cooperation with TLR2, is essential to the innate immune sensing of bacterial cell wall components, such as triacylated lipoproteins, lipoglycans and, in particular, many mycobacterial-associated molecules. The other eight TLRs recognize a diverse array of microbial ligands, including lipopolysaccharide, flagellin, and nucleic acids. TLRs are expressed by various immune cells and can be both displayed on the cell surface or in intracellular compartments. Following protein synthesis, these receptors must traffic to appropriate subcellular locations in order to have maximum access to their particular microbial ligand. If improper distribution occurs, TLRs may be rendered nonfunctional due to the inability to sense the presence of these ligands. We have previously identified a frequent single nucleotide polymorphism in TLR1 (I602S) which greatly inhibits trafficking of the receptor to the cell surface. Individuals homozygous for the serine allele at amino acid position 602 have low to undetectable levels of plasma membrane TLR1. Conversely, the TLR1 602I variant can be readily detected on the cell surface. This striking phenotype is strictly caused by a trafficking defect and not a difference in total cellular expression of each receptor. The absence of surface TLR1 confers hyporesponsiveness of innate immune cells to TLR1 agonists whereby cells homozygous for TLR1 602S have deficient secretion of proinflammatory cytokines in response to extracellular ligands. It could be expected that individuals deficient in recognition of bacterial TLR1 ligands may be vulnerable to infection. Interestingly however, homozygosity for TLR1 602S protects against leprosy and tuberculosis: diseases caused by the intracellular pathogens Mycobacterium leprae and Mycobacterium tuberculosis, respectively. This dissertation describes our investigation of the TLR1 I602S polymorphism in the context of receptor trafficking and host defense against mycobacterial infection. My thesis project has sought to determine the mechanisms underlying the protective roles of TLR1 I602S in infection and immunity, and how differential trafficking and signaling contribute to these functions. To apply this phenotype more globally to TLR biology, we have also used the I602S polymorphism as a valuable model system for studying both TLR trafficking dynamics and the role of TLR1/2 in immune defense. Chapter 1 of this thesis serves as a general introduction to the field of immunology and in particular, mechanisms of innate immunity. The concept of pattern recognition receptors is discussed, with a major focus on the Toll-like receptor family of proteins including their structure, ligands, and signaling functions. Also in Chapter 1, the concept of how genetic variation within TLRs contributes to individual resistance or susceptibility to infectious disease, especially mycobacterial disease, is introduced. Here I highlight single nucleotide polymorphisms in TLR1, 2, 4, and 9 which affect resistance to these types of infections, and link TLR genotype to phenotypic effects on disease susceptibility. Since TLR1 and TLR2 are the primary sensors of mycobacterial products, I focus on SNPs in these receptors with particular focus on the I602S polymorphism. Chapter 2 describes the characterization of processes controlling TLR1 trafficking and attempts to shed light on the mechanisms responsible for the trafficking deficiency exhibited by TLR1 602S. Specifically it discusses how mutagenesis was used to identify a short trafficking motif within the TLR1 primary amino acid sequence, which is interrupted by the 602S polymorphism. In addition two endoplasmic reticulum chaperones, PRAT4Aand PRAT4B, were shown to differentially regulate TLR1 surface trafficking based upon the presence of the polymorphism. A number of studies have identified several mechanisms by which mycobacteria subvert TLR1/2 signaling to create immunosuppressed environments conducive for bacterial replication. I hypothesized that since these immunosuppressive effects are caused by mycobacterial stimulation of TLR1/2, then the lack of TLR1 602S on the cell surface would prevent these inhibitory signals. Chapter 3 goes on to discuss experiments based on this hypothesis and highlights several mechanisms by which homozygosity for TLR1 602S protects individuals against mycobacterial disease. Such mechanisms include differential resistance to inhibitory effects on macrophage activation and microbicidal functions. Finally, Chapter 4 serves as an overall summary of this research’s implications for the field of innate immunity and anti-mycobacterial defense. It describes the possible future directions this thesis project could take and proposes clinical significance of the work.

Graduation Semester

2012-12

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Copyright and License Information

Copyright 2012 Bryan Hart

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