The immune system uses many sensors to detect and report microbial invaders. Most of these sensors are associated with immune cells, but the extracellular matrix also seems to be essential for this sentinel duty.
The ability to recognize and respond to microbial invaders is central to the body's immune response. The immune system has devised many ways to recognize, neutralize and destroy pathogens. The front line of this defense includes proteins called pattern-recognition receptors (PRRs) that bind to microbes or microbial pathogens, which triggers immune cells to ingest and destroy the pathogen. PRRs are found outside the cell in the extracellular matrix or serum and on the cell surface, as well as inside cells. The coordinated actions of these different PRRs result in an effective immune response to the invaders. Recent work in this area has focused mainly on PRRs expressed on the cell surface or intracellularly1,2,3,4. In this issue of Nature Immunology, He et al.5 remind us that PRRs in the extracellular matrix are also an essential line of defense against bacteria.
Most PRRs recognize conserved motifs found in microbes, called pathogen-associated molecular patterns (PAMPs). PAMPs include components of the microbial cell wall such as lipopolysaccharide (LPS) and lipoteichoic acid, as well as internal structures like bacterial DNA (CpG). PRRs can act to enhance the binding of PAMPs to cell surface receptors, to activate inflammatory responses or to induce phagocytosis of the microbe, or to effect a combination of these actions1,2,3,4. Some PRRs recognize very specific PAMPs, such as the recognition of flagellin by Toll-like receptor 5 (TLR5), whereas others have a broad specificity of recognition, such as the binding of scavenger receptors to many different proteoglycan and lipid-containing PAMPs.
Extracellular proteins involved in microbe recognition include the serum proteins mannose-binding lectin and C-reactive protein, as well as the extracellular matrix components fibronectin and collagen. Cell surface PRRs include proteins that stimulate the phagocytosis of microbes (such as scavenger receptors) and receptors involved in generating an inflammatory response (such as TLRs), as well as receptors that are involved in both processes (such as Fc receptors)1,2,3. Intracellular PRRs called Nods have been described; these are involved in generating an inflammatory response to specific components of the bacterial cell wall4. These PRRs provide the body with the ability to detect and respond to a wide variety of bacteria found in various compartments of the body.
PRRs do not work in isolation, but instead are often found acting in conjunction with other molecules. For example, TLR4 was discovered to be an essential receptor for LPS responses through analysis of C3H/HeJ mice that are unresponsive to LPS6,7. Since then, several proteins have been found to be required for the production of inflammatory cytokines in cooperation with TLR4. These proteins include LPB, CD14 and MD-2 (refs. 1,2). MD-2 is a secreted protein that binds to the extracellular region of TLR4, enhancing responsiveness to LPS. LPB is a lipid-binding protein found in serum and binds to the lipid portion of LPS, assisting in the binding of LPS to a cell surface receptor CD14. The CD14 receptor lacks a transmembrane domain and is unable to transmit a signal to the interior of the cell by itself. To signal the recognition of LPS, the CD14-LPS complex interacts with TLR4 and MD-2, sending a signal to the cell to produce inflammatory cytokines and activate the immune response. Mutations in any one of these components of this TLR4 signaling complex results in an inability of the cell to effectively detect and respond to LPS.
The complexity of LPS and bacterial recognition increased another notch with the identification of another PRR by He et al.5. This extracellular matrix PRR mindin is required for both efficient production of inflammatory cytokines and phagocytosis of several bacteria. Mindin was originally characterized in zebrafish as a component of the basal lamina and was subsequently shown to be involved in the outgrowth of hippocampal neurons8,9. Mindin is a secreted protein expressed in several tissues, including the spleen and lymph nodes, where it forms a diffuse network in these organs and binds to the surfaces of macrophages. Because of its abundant expression in lymphoid tissues (much higher than in brain), the authors hypothesized that this protein may have additional functions, possibly in host defense.
To test this hypothesis, the authors generated mice deficient in mindin. Although these mice are normal in appearance, immune system development and lifespan, they have severe defects in many of their immune responses. Like many mice deficient in specific PRRs1,2,3, mindin-deficient mice are resistant to LPS-induced shock. This resistance seems to be because of an inability of the macrophages and mast cells to produce sufficient amounts of interleukin 6 and tumor necrosis factor in response to LPS. However, recombinant mindin protein restored the production of inflammatory cytokines to wild-type amounts, indicating that the presence of mindin protein on the surface of cells is required for the generation of an inflammatory response to LPS.
The mindin-deficient cells are hyporesponsive to many PAMPs, including lipotechoic acid, peptidoglycan, zymosan, mannan and CpG DNA. What do all these nonstimulatory PAMPs have in common? The answer may lie in the sugar contained within their structures. Indeed, the authors show that mindin binds to both LPS and lipoteichoic acid through their sugar moieties, as this binding can be competed with certain monosaccharides. This binding to 'sweet PAMPs' seems to be a specific function for mindin, as cells deficient in mindin are still responsive to a 'nonsweet PAMP' (lipoprotein).
The specific sugar moieties that mindin re-cognizes remain unclear. However, there is some specificity in binding, as mindin binds to and agglutinates many but not all strains of bacteria tested. This binding of bacteria by mindin seems to be essential to its function, as it correlates with the ability of macrophages to respond to these bacteria, both by secretion of inflammatory cytokines as well as by phagocytosis. This correlation extends to the response of the mindin-deficient mice to systemic infection. Mindin-deficient mice are more resistant to bacteria-induced shock when systemically infected with bacteria that are unable to induce inflammatory cytokine production in mindin-deficient cells. Mindin-deficient mice are more susceptible to bacteria-induced shock when infected with bacteria that are poorly bound by mindin. These bacteria could induce the production of inflammatory cytokines but were ineffective in phagocytosis of the bacteria. These results indicate that mindin may have two separate functions: stimulating production of inflammatory cytokines and regulating phagocytosis of the bacterium (Fig. 1).
He et al. demonstrate that extracellular matrix components are an essential line of defense to microbial invaders. The question remains, however, as to how mindin produces these antibacterial effects. It is apparent that mindin works extracellularly, binding to both the bacterial invader as well as the host cell, but the specific sugar moieties it recognizes and how it binds to the cell surface is unknown. It is also unclear whether mindin functions as a coreceptor for several PRRs (some involved in inflammatory signaling (TLR4) and others required for phagocytosis), or if it binds to a specific cell surface receptor to generate a costimulatory signal required to prime an effective immune response. TLR4-deficient cells are unresponsive to LPS, even in the presence of additional recombinant mindin protein, indicating a requirement for TLR4 in mindin's antibacterial effects in response to LPS. However, the activation of known signaling pathways in response to LPS is largely unaffected in mindin-deficient cells, indicating that mindin may bind to a distinct receptor and activate other signal transduction pathways that synergize with TLR4 signals. Further investigation into the mindin receptor and the types of intracellular signaling generated by mindin will be essential for understanding the mechanism of mindin's actions in the immune system. These findings emphasize the active nature of extracellular matrix proteins in host defense and point to an additional essential mechanism for detecting and responding to pathogens.
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About this article
The extracellular matrix protein mindin as a novel adjuvant elicits stronger immune responses for rBAG1, rSRS4 and rSRS9 antigens of Toxoplasma gondiiin BALB/c mice
BMC Infectious Diseases (2014)
Journal of Molecular Medicine (2012)
The EMBO Journal (2009)