Lutz Hamann

TLR4 polymorphisms, infectious diseases, and evolutionary pressure during migration of modern humans.

Infectious diseases exert a constant evolutionary pressure on the genetic makeup of our innate immune system. Polymorphisms in Toll-like receptor 4 (TLR4) have been related to susceptibility to Gram-negative infections and septic shock. Here we show that two polymorphisms of TLR4, Asp299Gly and Thr399Ile, have unique distributions in populations from Africa, Asia, and Europe. Genetic and functional studies are compatible with a model in which the nonsynonymous polymorphism Asp299Gly has evolved as a protective allele against malaria, explaining its high prevalence in subSaharan Africa. However, the same allele could have been disadvantageous after migration of modern humans into Eurasia, putatively because of increased susceptibility to severe bacterial infections. In contrast, the Asp299Gly allele, when present in cosegregation with Thr399Ile to form the Asp299Gly/Thr399Ile haplotype, shows selective neutrality. Polymorphisms in TLR4 exemplify how the interaction between our innate immune system and the infectious pressures in particular environments may have shaped the genetic variations and function of our immune system during the out-of-Africa migration of modern humans.

Cutting Edge: A Common Polymorphism Impairs Cell Surface Trafficking and Functional Responses of TLR1 but Protects against Leprosy

TLRs constitute an essential family of pattern recognition molecules that, through direct recognition of conserved microbial components, initiate inflammatory responses following infection. In this role, TLR1 enables host responses to a variety of bacteria, including pathogenic species of mycobacteria. In this study, we report that I602S, a common single nucleotide polymorphism within TLR1, is associated with aberrant trafficking of the receptor to the cell surface and diminished responses of blood monocytes to bacterial agonists. When expressed in heterologous systems, the TLR1 602S variant, but not the TLR1 602I variant, exhibits the expected deficiencies in trafficking and responsiveness. Among white Europeans, the 602S allele represents the most common single nucleotide polymorphism affecting TLR function identified to date. Surprisingly, the 602S allele is associated with a decreased incidence of leprosy, suggesting that Mycobacterium leprae subverts the TLR system as a mechanism of immune evasion.

Lipopolysaccharide Binding Protein Binds to Triacylated and Diacylated Lipopeptides and Mediates Innate Immune Responses

LPS binding protein (LBP) is an acute-phase protein synthesized predominantly in the liver of the mammalian host. It was first described to bind LPS of Gram-negative bacteria and transfer it via a CD14-enhanced mechanism to a receptor complex including TLR-4 and MD-2, initiating a signal transduction cascade leading to the release of proinflammatory cytokines. In recent studies, we found that LBP also mediates cytokine induction caused by compounds derived from Gram-positive bacteria, including lipoteichoic acid and peptidoglycan fragments. Lipoproteins and lipopeptides have repeatedly been shown to act as potent cytokine inducers, interacting with TLR-2, in synergy with TLR-1 or -6. In this study, we show that these compounds also interact with LBP and CD14. We used triacylated lipopeptides, corresponding to lipoproteins of Borrelia burgdorferi, mycobacteria, and Escherichia coli, as well as diacylated lipopeptides, corresponding to, e.g., 2-kDa macrophage activating lipopeptide of Mycoplasma spp. Activation of Chinese hamster ovary cells transfected with TLR-2 by both lipopeptides was enhanced by cotransfection of CD14. Responsiveness of human mononuclear cells to these compounds was greatly enhanced in the presence of human LBP. Binding of lipopeptides to LBP as well as competitive inhibition of this interaction by LPS was demonstrated in a microplate assay. Furthermore, we were able to show that LBP transfers lipopeptides to CD14 on human monocytes using FACS analysis. These results support that LBP is a pattern recognition receptor transferring a variety of bacterial ligands including the two major types of lipopeptides to CD14 present in different receptor complexes.

Common Polymorphisms of Toll-Like Receptors 4 and 9 Are Associated with the Clinical Manifestation of Malaria during Pregnancy

Toll-like receptors (TLRs) are involved in recognition of and response to Plasmodium falciparum. In 304 primiparous Ghanaian women, we examined whether common TLR4 and TLR9 polymorphisms influence susceptibility to and manifestation of malaria during pregnancy. The TLR variants did not affect P. falciparum prevalence or parasite density. However, in P. falciparum-infected women, both the TLR4 Asp299Gly and the TLR9 T-1486C polymorphisms increased the risk of low birth weight in term infants 6-fold, and, additionally, TLR4 Asp299Gly increased the risk of maternal anemia 5-fold; preterm delivery was not associated with these TLR variants. These findings suggest that TLR4 and TLR9 play a role in the manifestation of malaria during pregnancy.

High frequency of polymorphism Arg753Gln of the Toll-like receptor-2 gene detected by a novel allele-specific PCR

The recently described family of Toll-like receptors (TLRs) plays a major role in innate immunity by mediating inflammatory reactions against a wide array of pathogens. TLR-2 is reported to interact with various bacterial partial structures including lipoproteins, peptidoglycan, and lipoteichoic acid. Two polymorphisms of the TLR-2 gene have recently been described: Arg753Gln, correlated with the incidence of sepsis in a white population, and Arg677Trp, correlated with the incidence of lepromatous leprosy in an Asian population. Both polymorphisms, when inserted into expression vectors encoding for human TLR-2, reduced stimulation of Chinese hamster ovary cells by synthetic lipopeptides. We furthermore developed a rapid and inexpensive method for the detection of both single nucleotide polymorphisms based on restriction fragment length polymorphism. While no individuals carrying the Arg677Trp SNP were identified in a large group of whites, 9.4% of the study population were found to be heterozygous for the Arg753Gln polymorphism. This ratio is significantly higher than previously reported, and therefore detection of this polymorphism among patients may yield important information for the assessment of risk profiles regarding susceptibility to bacterial infections.

Polymorphism N248S in the Human Toll‐Like Receptor 1 Gene Is Related to Leprosy and Leprosy Reactions

We investigated the association between a polymorphism of a key innate immunity receptor, Toll-like receptor 1 (TLR1) N248S, and susceptibility to leprosy and its clinical presentation. TLR1 N248S has been shown elsewhere to diminish TLR1 signaling and subsequent leprosy disease. The homozygous genotype SS was more frequent (P=.012) and the heterozygous SN genotype was less frequent (P=.015) in patients with leprosy than in control subjects. Additional observed differences in allelic frequency in patients who experienced reversal reactions and/or erythema nodosum leprosum reactions indicates that altered TLR1 function, or at least a TLR1 N248S-linked trait, may affect the progression from infection to disease as well as the disease course and the risk of debilitating reactional episodes in this population.

Functional and genetic evidence that the Mal/TIRAP allele variant 180L has been selected by providing protection against septic shock.

Adequate responses by our innate immune system toward invading pathogens were of vital importance for surviving infections, especially before the antibiotic era. Recently, a polymorphism in Mal (Ser180Leu, TIRAP rs8177374), an important adaptor protein downstream of the Toll-like receptor (TLR) 2 and 4 pathways, has been described to provide protection against a broad range of infectious pathogens. We assessed the functional effects of this polymorphism in human experimental endotoxemia, and we demonstrate that individuals bearing the TIRAP 180L allele display an increased, innate immune response to TLR4 and TLR2 ligands, but not to TLR9 stimulation. This phenotype has been related to an increased resistance to infection. However, an overshoot in the release of proinflammatory cytokines by TIRAP 180L homozygous individuals suggests a scenario of balanced evolution. We have also investigated the worldwide distribution of the Ser180Leu polymorphism in 14 populations around the globe to correlate the genetic makeup of TIRAP with the local infectious pressures. Based on the immunological, clinical, and genetic data, we propose that this mutation might have been selected in West Eurasia during the early settlement of this region after the out-of-Africa migration of modern Homo sapiens. This combination of functional and genetic data provides unique insights to our understanding of the pathogenesis of sepsis.

The biology of endotoxin.

Endotoxin (lipopolysaccharide, LPS) is the major component of the outer leaflet of Gram-negative bacteria and has profound immunostimulatory and inflammatory capacity. The septic shock syndrome caused by endotoxin still has an unacceptably high mortality rate and, owing to increasing numbers of resistant strains, remains an ongoing threat throughout the world. However, the past years have provided new insights especially into the receptors of the innate immune system that are involved into the recognition of LPS and the initial signal transduction pathways that are engaged after the primary recognition on the cell surface. The knowledge about the molecular basis for the responses to endotoxin may eventually lead to the development of new drugs to fight the fatal effects of bacterial infections.

Acute-Phase Concentrations of Lipopolysaccharide (LPS)-Binding Protein Inhibit Innate Immune Cell Activation by Different LPS Chemotypes via Different Mechanisms

ABSTRACT The chain length of bacterial lipopolysaccharide (LPS) is a crucial factor for host-pathogen interaction during bacterial infection. While rough (R)-type and smooth (S)-type LPSs have been shown to differ in their ability to interact with the bactericidal/permeability-increasing protein, little is known about the differential mode of interaction with the acute-phase reactant LPS-binding protein (LBP). At lower concentrations, LBP catalyzes the binding of LPS to CD14 and enhances LPS-induced cellular activation via Toll-like receptor 4. In humans, however, concentrations of LBP in serum increase during an acute-phase response, and these LBP concentrations exhibit inhibitory effects in terms of cellular activation. The mechanisms of inhibition of LPS effects by LBP are not completely understood. Here, we report that human high-dose LBP (hd-LBP) suppresses binding of both R-type and S-type LPS to CD14 and inhibits LPS-induced nuclear translocation of NF-κB, although cellular uptake of R-type LPS was found to be increased by hd-LBP. In contrast, we found that hd-LBP enhanced the binding and uptake of S-type LPS only under serum-free conditions, whereas in the presence of serum, hd-LBP inhibited cellular binding and uptake. This inhibitory effect of serum could be mimicked by the addition of purified high-density lipoprotein (HDL) to serum-free medium, indicating an LBP-mediated transfer of preferentially S-type LPS to plasma lipoproteins such as HDL. A complete understanding of the hosts mechanisms to modulate the proinflammatory effects of LPS will most likely help in the understanding of inflammation and infection and may lead to novel therapeutic intervention strategies.

Components of gut bacteria as immunomodulators

In 1885 Louis Pasteur was the first to propose that the human immune system may be influenced by microorganisms. A large body of data has since been accumulated proving this assumption to be correct. Bacteria constitute the main constituents of the microbial flora of the human digestive tract and compounds of the bacterial cell wall have been shown to play an important role in the interaction of microbes with higher organisms. These components include peptidoglycan (PG) and lipopolysaccharide (LPS) of gram-negative bacteria. Both types of molecules are potent activators of the human immune system and exert their activity through the induction of endogenous mediators which are endowed with biological activity. This review focuses on the structure and activity of LPS and PG and illustrates how these bacterial factors stimulate the immune cells resulting in desired physiological or dramatic pathophysiological responses of the host organism.