Donald J. Davidson

The Human Antimicrobial Peptide LL-37 Is a Multifunctional Modulator of Innate Immune Responses

The role of LL-37, a human cationic antimicrobial peptide, in the immune system is not yet clearly understood. It is a widely expressed peptide that can be up-regulated during an immune response. In this report, we demonstrate that LL-37 is a potent antisepsis agent with the ability to inhibit macrophage stimulation by bacterial components such as LPS, lipoteichoic acid, and noncapped lipoarabinomannan. We also demonstrate that LL-37 protects mice against lethal endotoxemia. In addition to preventing macrophage activation by bacterial components, we hypothesized the LL-37 may also have direct effects on macrophage function. We therefore used gene expression profiling to identify macrophage functions that might be modulated by LL-37. These studies revealed that LL-37 directly up-regulates 29 genes and down-regulated another 20 genes. Among the genes predicted to be up-regulated by LL-37 were those encoding chemokines and chemokine receptors. Consistent with this, LL-37 up-regulated the expression of chemokines in macrophages and the mouse lung (monocyte chemoattractant protein 1), human A549 epithelial cells (IL-8), and whole human blood (monocyte chemoattractant protein 1 and IL-8), without stimulating the proinflammatory cytokine, TNFα. LL-37 also up-regulated the chemokine receptors CXCR-4, CCR2, and IL-8RB. These findings indicate that LL-37 may contribute to the immune response by limiting the damage caused by bacterial products and by recruiting immune cells to the site of infection so that they can clear the infection.

The Cationic Antimicrobial Peptide LL-37 Modulates Dendritic Cell Differentiation and Dendritic Cell-Induced T Cell Polarization

Dendritic cells (DC) are instrumental in orchestrating an appropriately polarized Th cell response to pathogens. DC exhibit considerable phenotypic and functional plasticity, influenced by lineage, Ag engagement, and the environment in which they develop and mature. In this study, we identify the human cationic peptide LL-37, found in abundance at sites of inflammation, as a potent modifier of DC differentiation, bridging innate and adaptive immune responses. LL-37-derived DC displayed significantly up-regulated endocytic capacity, modified phagocytic receptor expression and function, up-regulated costimulatory molecule expression, enhanced secretion of Th-1 inducing cytokines, and promoted Th1 responses in vitro. LL-37 may be an attractive therapeutic candidate for manipulating T cell polarization by DC.

Immunomodulatory Properties of Defensins and Cathelicidins

Host defence peptides are a conserved component of the innate immune response in all complex life forms. In humans, the major classes of host defence peptides include the α- and β-defensins and the cathelicidin, hCAP-18/LL-37. These peptides are expressed in the granules of neutrophils and by a wide variety of tissue types. They have many roles in the immune response including both indirect and direct antimicrobial activity, the ability to act as chemokines as well as induce chemokine production leading to recruitment of leukocytes to the site of infection, the promotion of wound healing and an ability to modulate adaptive immunity. It appears that many of these properties are mediated though direct interaction of peptides with the cells of the innate immune response including monocytes, dendritic cells, T cells and epithelial cells. The importance of these peptides in immune responses has been demonstrated since animals defective in the expression of certain host defence peptides showgreater susceptibility to bacterial infections. In the very few instances in which human patients have been demonstrated to have defective host defence peptide expression, these individuals suffer from frequent infections. Although studies of the immunomodulatory properties of these peptides are in their infancy, there is a growing body of evidence suggesting that the immunomodulatory properties of these small, naturally occurring molecules might be harnessed for development as novel therapeutic agents.

The human cationic host defense peptide LL‐37 mediates contrasting effects on apoptotic pathways in different primary cells of the innate immune system

The human cathelicidin LL‐37 is a cationic host defense peptide (antimicrobial peptide) expressed primarily by neutrophils and epithelial cells. This peptide, up‐regulated under conditions of inflammation, has immunomodulatory and antimicrobial functions. We demonstrate that LL‐37 is a potent inhibitor of human neutrophil apoptosis, signaling through P2X7 receptors and G‐protein‐coupled receptors other than the formyl peptide receptor‐like‐1 molecule. This process involved modulation of Mcl‐1 expression, inhibition of BID and procaspase‐3 cleavage, and the activation of phosphatidylinositol‐3 kinase but not the extracellular signal‐regulated kinase 1/2 mitogen‐activated protein kinase pathway. In contrast to the inhibition of neutrophil apoptosis, LL‐37 induced apoptosis in primary airway epithelial cells, demonstrating alternate consequences of LL‐37‐mediated modulation of apoptotic pathways in different human primary cells. We propose that these novel immunomodulatory properties of LL‐37 contribute to peptide‐mediated enhancement of innate host defenses against acute infection and are of considerable significance in the development of such peptides and their synthetic analogs as potential therapeutics for use against multiple antibiotic‐resistant infectious diseases.

Mouse models of cystic fibrosis.

The development of mouse models for cystic fibrosis has provided the opportunity to dissect disease pathogenesis, correlate genotype and phenotype, study disease-modifying genes and develop novel therapeutics. This review discusses the successes and the challenges encountered in characterizing and optimizing these models.

IRAK-4 Mutation (Q293X): Rapid Detection and Characterization of Defective Post-Transcriptional TLR/IL-1R Responses in Human Myeloid and Non-Myeloid Cells

Innate immunodeficiency has recently been reported as resulting from the Q293X IRAK-4 mutation with consequent defective TLR/IL-1R signaling. In this study we report a method for the rapid allele-specific detection of this mutation and demonstrate both cell type specificity and ligand specificity in defective IL-1R-associated kinase (IRAK)-4-deficient cellular responses, indicating differential roles for this protein in human PBMCs and primary dermal fibroblasts and in LPS, IL-1β, and TNF-α signaling. We demonstrate transcriptional and post-transcriptional defects despite NF-κB signaling and intact MyD88-independent signaling and propose that dysfunctional complex 1 (IRAK1/TRAF6/TAK1) signaling, as a consequence of IRAK-4 deficiency, generates specific defects in MAPK activation that could underpin this patient’s innate immunodeficiency. These studies demonstrate the importance of studying primary human cells bearing a clinically relevant mutation; they underscore the complexity of innate immune signaling and illuminate novel roles for IRAK-4 and the fundamental importance of accessory proinflammatory signaling to normal human innate immune responses and immunodeficiencies.

Pseudomonas Aeruginosa Infections in Individuals with Cystic Fibrosis

In Canada and the United States, more than 30,000 people suffer from cystic fibrosis (CF); it is therefore the most common potentially fatal inherited disease among Caucasians in those North American countries. Infection of the lung is the leading cause of death in patients with CF, and Pseudomonas aeruginosa is the predominant infectious agent. Whereas CF patients may become infected with other organisms (such as Staphylococcus aureus, Haemophilus influenzae, Burkholderia cepacia, and atypical mycobacteria),P. aeruginosa is the predominant pathogen, ultimately infecting approximately 80% of patients. Once infected with P. aeruginosa, patients usually retain the organism for the duration of their lives. This host-microbe interaction is unique and raises many important questions about mode of acquisition, microbial virulence, host susceptibility and strategies for control of infection. This brief chapter will review the latest information on the host-microbial detente in the CF lung, focusing on new observations from a North American perspective. Chapter 6 will reflect on observations from Europe.