Toll-like receptors: critical proteins linking innate and acquired immunity

Drosophila MyD88 is required for the response to fungal and Gram-positive bacterial infections

We report here the identification and functional characterization of DmMyD88, a gene encoding the Drosophila homolog of mammalian MyD88. DmMyD88 combines a Toll-IL-1R homology (TIR) domain and a death domain. Overexpression of DmMyD88 was sufficient to induce expression of the antifungal peptide Drosomycin, and induction of Drosomycin was markedly reduced in DmMyD88-mutant flies. DmMyD88 interacted with Toll through its TIR domain and required the death domain proteins Tube and Pelle to activate expression of Drs, which encodes Drosomycin. DmMyD88-mutant flies were highly susceptible to infection by fungi and Gram-positive bacteria, but resisted Gram-negative bacterial infection much as did wild-type flies. Phenotypic comparison of DmMyD88-mutant flies and MyD88-deficient mice showed essential differences in the control of Gram-negative infection in insects and mammals.

MIF regulates innate immune responses through modulation of Toll-like receptor 4

Macrophages are pivotal effector cells of the innate immune system, which is vital for recognizing and eliminating invasive microbial pathogens. When microbial products bind to pathogen-recognition receptors, macrophages become activated and release a broad array of cytokines that orchestrate the host innate and adaptive immune responses. Initially identified as a T-cell cytokine, macrophage migration inhibitory factor (MIF) is also a macrophage cytokine and an important mediator of inflammation and sepsis. Here we report that MIF is an essential regulator of macrophage responses to endotoxin (lipopolysaccharide) and Gram-negative bacteria. Compared with wild-type cells, MIF-deficient macrophages are hyporesponsive to lipopolysaccharide and Gram-negative bacteria, as shown by a profound reduction in the activity of NF-kappaB and the production of tumour-necrosis factor-alpha. This reduction is due to a downregulation of Toll-like receptor 4 (TLR4), the signal-transducing molecule of the lipopolysaccharide receptor complex, and is associated with decreased activity of transcription factor PU.1, which is required for optimal expression of the Tlr4 gene in myeloid cells. These findings identify an important role for MIF in innate immunity and provide a molecular basis for the resistance of MIF-deficient mice to endotoxic shock.

Drosophila Toll is activated by Gram-positive bacteria through a circulating peptidoglycan recognition protein

Microbial infection activates two distinct intracellular signalling cascades in the immune-responsive fat body of Drosophila. Gram-positive bacteria and fungi predominantly induce the Toll signalling pathway, whereas Gram-negative bacteria activate the Imd pathway. Loss-of-function mutants in either pathway reduce the resistance to corresponding infections. Genetic screens have identified a range of genes involved in these intracellular signalling cascades, but how they are activated by microbial infection is largely unknown. Activation of the transmembrane receptor Toll requires a proteolytically cleaved form of an extracellular cytokine-like polypeptide, Spätzle, suggesting that Toll does not itself function as a bona fide recognition receptor of microbial patterns. This is in apparent contrast with the mammalian Toll-like receptors and raises the question of which host molecules actually recognize microbial patterns to activate Toll through Spätzle. Here we present a mutation that blocks Toll activation by Gram-positive bacteria and significantly decreases resistance to this type of infection. The mutation semmelweis (seml) inactivates the gene encoding a peptidoglycan recognition protein (PGRP-SA). Interestingly, seml does not affect Toll activation by fungal infection, indicating the existence of a distinct recognition system for fungi to activate the Toll pathway.

Genetics of susceptibility to human infectious disease

Before Robert Koch's work in the late nineteenth century, diseases such as tuberculosis and leprosy were widely believed to be inherited disorders. Heritability of susceptibility to several infectious diseases has been confirmed by studies in the twentieth century. Infectious diseases, old and new, continue to be an important cause of mortality worldwide. A greater understanding of disease processes is needed if more effective therapies and more useful vaccines are to be produced. As part of this effort, developments in genetics have allowed a more systematic study of the impact that the human genome and infectious disease have on each other.

Evolutionary perspective on innate immune recognition

Analysis of human and Drosophila genomes demonstrates an ancient origin of innate immunity and the diversity of the mechanisms of innate immune recognition.

Toll-like receptors and innate immunity

Toll-like receptors have a crucial role in the detection of microbial infection in mammals and insects. In mammals, these receptors have evolved to recognize conserved products unique to microbial metabolism. This specificity allows the Toll proteins to detect the presence of infection and to induce activation of inflammatory and antimicrobial innate immune responses. Recognition of microbial products by Toll-like receptors expressed on dendritic cells triggers functional maturation of dendritic cells and leads to initiation of antigen-specific adaptive immune responses.

Man the barrier! Strategic defences in the intestinal mucosa

Immunologists typically study the immune responses induced in the spleen or peripheral lymph nodes after parenteral immunization with antigen and poorly defined experimental adjuvants. However, most antigens enter the body through mucosal surfaces. It is now clear that the microenvironment in these mucosal barriers has a marked influence on the immune response that ultimately ensues. Nowhere is the microenvironment more influential than in the gut-associated lymphoid tissue (GALT). The GALT must constantly distinguish harmless antigens that are present in food or on commensal bacteria from pathogenic assault by microbes. It is perhaps not surprising, then, that the GALT contains more lymphocytes than all of the secondary lymphoid organs combined.