The mammalian immune system has innate and adaptive components, which cooperate to protect the host against microbial infections. The innate immune system consists of functionally distinct 'modules' that evolved to provide different forms of protection against pathogens. It senses pathogens through pattern-recognition receptors, which trigger the activation of antimicrobial defences and stimulate the adaptive immune response. The adaptive immune system, in turn, activates innate effector mechanisms in an antigen-specific manner. The connections between the various immune components are not fully understood, but recent progress brings us closer to an integrated view of the immune system and its function in host defence.

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Recognition of microorganisms and activation of the immune response.

Helper T cells coordinate immune responses through the production of cytokines Th2 cells express the closely linked Il4, Il13, and Il5 cytokine genes, whereas these same genes are silenced in the Th1 lineage The Th1/Th2 lineage choice has become a textbook example for the regulation of cell differentiation, and recent discoveries have further refined and expanded our understanding of how Th2 differentiation is initiated and reinforced by signals from antigen-presenting cells and cytokine-driven feedback loops Epigenetic changes that stabilize the active or silent state of the Il4 locus in differentiating helper T cells have been a major focus of recent research Overall, the field is progressing toward an integrated model of the signaling and transcription factor networks, cis-regulatory elements, epigenetic modifications, and RNA interference mechanisms that converge to determine the lineage fate and gene expression patterns of differentiating helper T cells

REGULATION OF TH2 DIFFERENTIATION AND Il4 LOCUS ACCESSIBILITY

Cell differentiation involves activation and silencing of lineage-specific genes. Here we show that the transcription factor Runx3 is induced in T helper type 1 (T(H)1) cells in a T-bet-dependent manner, and that both transcription factors T-bet and Runx3 are required for maximal production of interferon-gamma (IFN-gamma) and silencing of the gene encoding interleukin 4 (Il4) in T(H)1 cells. T-bet does not repress Il4 in Runx3-deficient T(H)2 cells, but coexpression of Runx3 and T-bet induces potent repression in those cells. Both T-bet and Runx3 bind to the Ifng promoter and the Il4 silencer, and deletion of the silencer decreases the sensitivity of Il4 to repression by either factor. Our data indicate that cytokine gene expression in T(H)1 cells may be controlled by a feed-forward regulatory circuit in which T-bet induces Runx3 and then 'partners' with Runx3 to direct lineage-specific gene activation and silencing.

Transcription factors T-bet and Runx3 cooperate to activate Ifng and silence Il4 in T helper type 1 cells

The gram-positive bacterium Staphylococcus aureus is a major pathogen responsible for a variety of diseases ranging from minor skin infections to life-threatening conditions such as sepsis. Cell wall-associated and secreted proteins (e.g., protein A, hemolysins, and phenol-soluble modulin) and cell wall components (e.g., peptidoglycan and alanylated lipoteichoic acid) have been shown to be inflammatory, and these staphylococcal components may contribute to sepsis. On the host side, many host factors have been implicated in the innate detection of staphylococcal components. One class of pattern recognition molecules, Toll-like receptor 2, has been shown to function as the transmembrane component involved in the detection of staphylococcal lipoteichoic acid and phenol-soluble modulin and is involved in the synthesis of inflammatory cytokines by monocytes/macrophages in response to these components. Nod2 (nucleotide-binding oligomerization domain 2) is the intracellular sensor for muramyl dipeptide, the minimal bioactive structure of peptidoglycan, and it may contribute to the innate immune defense against S. aureus. The staphylococcal virulence factor protein A was recently shown to interact directly with tumor necrosis factor receptor 1 in airway epithelium and to reproduce the effects of tumor necrosis factor alpha. Finally, peptidoglycan recognition protein L is an amidase that inactivates the proinflammatory activities of peptidoglycan. However, peptidoglycan recognition protein L probably plays a minor role in the innate immune response to S. aureus. Thus, several innate immunity receptors may be implicated in host defense against S. aureus.

https://cmr.asm.org/content/cmr/18/3/521.full.pdf

Recognition of Staphylococcus aureus by the Innate Immune System

The Drosophila Amidase PGRP-LB Modulates the Immune Response to Bacterial Infection

The ability of the host to distinguish between self and nonself remains a central hallmark of innate immunity. Microbial organisms express distinct cell surface molecules, such as peptidoglycan (PGN) and lipoteichoic acid (LTA) on gram-positive bacteria and lipopolysaccharide (LPS) on gram-negative bacteria, that are structurally indispensable components of the cell wall. Recognition of these pathogen-associated molecular patterns, or PAMPs (14), is achieved predominantly by the vertebrate Toll-like receptor (TLR) family proteins, which collectively mediate induction of antimicrobial defensins, cytokines, chemokines, and dendritic cell maturation important to innate and adaptive immune responses.

PGN recognition proteins (PGRPs) were first identified by their ability to bind PGN and complement the prophenoloxidase cascade that induces melanization around pathogens in insects (37). Thirteen PGRP family members are present in Drosophila (36), and each contains a conserved carboxyl-terminal region with homology to bacterial T7 lysozyme (11). Some family members, such as PGRP-SC1b, have enzymatic activity and can digest PGN by hydrolyzing the amide bond between N-acetylmuramic acid and l-alanine (20). The N-acetylmuramyl-l-alanine amidase activity may function to reduce the immunostimulatory effects of PGN (12).

Definitive evidence indicating essential roles for PGRPs in Drosophila innate immunity was revealed in studies of mutants defective in Toll and Imd pathway-mediated antimicrobial responses. Infection of flies by gram-positive bacteria and fungi induces localized production of cleaved Spaetzle, the Toll ligand. Stimulation of Toll by ligand binding activates latent NF-κB-like transcription factors, resulting in expression of the antimicrobial peptide drosomycin. Infection by gram-negative bacteria results in activation of Imd, a Drosophila RIP-like adapter (5), and the Relish pathway, with expression of antibacterial peptides active against these bacteria, including diptericin, attacin, drosocin, and cecropins (11). Drosophila with a mutated PGRP-SA gene failed to activate drosomycin expression and was unable to survive challenge with gram-positive bacteria (21). In contrast, responses to gram-negative bacteria and fungi were not affected. A constitutively active Toll mutant could overcome the PGRP-SA mutation, suggesting that PGRP-SA functions upstream of Toll (21). The capacity of PGRP-SA to bind to PGN, likely in conjunction with gram-negative binding protein (6, 24), must therefore constitute a crucial and nonredundant recognition element upstream of Toll pathway activation in Drosophila.

A similar screen identified PGRP-LC upstream of the Imd pathway (1, 7, 27). Like Imd mutants, PGRP-LC mutants poorly induced antimicrobial peptides, particularly diptericin, and demonstrated increased susceptibility to gram-negative bacteria; responses to gram-positive bacteria and fungi were unaffected. As in the Toll pathway, a constitutively active Imd mutation could rescue antimicrobial peptide induction attenuated by the PGRP-LC mutation (1, 7). Therefore, PGRP-LC plays a crucial and nonredundant role in detecting gram-negative bacteria and leading to activation of the Imd/Relish pathway.

Although LPS is a major component of the gram-negative bacterial cell wall, PGN constitutes a shared cell wall element in both gram-positive and gram-negative bacteria (28). Structurally similar, these cell wall PGNs differ at the third amino acid of the peptide side chain, where lysine (Lys-PGN) and meso-diaminopimelic acid (Dap-PGN) are used in gram-positive and gram-negative organisms, respectively (28). Unlike LPS, Dap-PGN induced gram-negative-like Imd activation in flies, whereas Lys-PGN fully induced the gram-positive-like Toll pathway response (15, 18). Together, these studies suggest that different PGRP proteins in Drosophila mediate direct interactions with distinctive PGN moieties in bacterial cell walls as a proximal mechanism upstream of Toll and Imd activation. Indeed, PGRP-LE binds Dap-PGN but not Lys-PGN in vitro, consistent with this paradigm (31).

PGRP is conserved in mice and humans, where four PGRP proteins are generated from three genes (19). The longest, PGRP-L, is a predicted membrane protein expressed mainly in liver. The shortest, PGRP-S, is a predicted soluble protein expressed in neutrophils. The intermediately sized family members, PGRP-Iα and -Iβ, are splice products from the same gene. PGRP-Iα/Iβ are predicted membrane proteins that are expressed in the esophagus (19). Like the Drosophila PGRPs, mammalian PGRPs have conserved carboxyl-terminal PGRP domains homologous to lysozyme (19). Only PGRP-L, however, has N-acetylmuramyl-l-alanine-amidase activity that digests PGN. The remaining family members have critical amino acid substitutions that abrogate enzymatic activity (4, 35), but all four mammalian proteins retain the ability to bind PGN in vitro (19).

In contrast to the dramatic phenotype of PGRP-SA mutants in Drosophila, the PGRP-S null mouse displayed normal susceptibility to gram-negative and gram-positive challenge with Escherichia coli and Staphylococcus aureus. A nonpathogenic gram-positive Bacillus species was cleared less efficiently, however, and the killing and digestion of gram-positive bacteria by PGRP-SA-deficient neutrophils were compromised in vitro, although phagocytosis remained intact. Further, murine PGRP-S had no role in TLR-2-mediated cytokine induction (3). To define further the function of PGRPs in mammalian innate immunity, we disrupted the mouse PGRP-L gene and examined the role of this protein in host defense against bacterial and fungal challenges.

Innate Immune Responses in Peptidoglycan Recognition Protein L-Deficient Mice

Publisher Summary This chapter summarizes recent insights regarding the generation of T helper (Th) 2 cells and factors regulating expression of IL-4—the canonical cytokine secreted by these cells. Th2 cells orchestrate a stereotyped cell response characterized by eosinophil and basophil infiltration, mast cell hyperplasia, and changes in epithelial mucus and water secretion. Th2 cells have been demonstrated to mediate protection against not only a variety of intestinal helminthes but also ectoparasites that invade at epithelial barriers. When activated systemically, Th2 cells protect against the tissue-damaging proinflammatory actions of Th1 cells. When dysregulated, Th2 cells drive chronic atopic diseases of wide prevalence such as allergy and asthma. Basic insights into the understanding of the generation, activation, tissue localization, and turnover of Th2 cells will provide numerous opportunities for sustained benefits to human health.

Min Xu

Papers

Innate Immune Responses in Peptidoglycan Recognition Protein L-Deficient Mice

Th2 cells: orchestrating barrier immunity.