Immunotherapy for Pulmonary TB
Immunotherapy for Pulmonary TB
AMPs have been shown to act as chemoattractants for cells of innate and adaptive immunity, and in fact, many authors consider them as a bridge between innate and adaptive immunity. One of the most studied peptides has been human cathelicidin LL-37 because of its regulatory functions and being the only member of the cathelicidin family present in humans. LL-37 is capable of attracting neutrophils, monocytes, T cells and mast cells using formyl peptide receptor-like 1, and a distinct Gi-coupled receptor at nanomolar concentrations. Besides the remarkable chemotactic activity, it also induces several other responses in leukocytes and epithelial cells, modifying gene expression in order to improve or modulate immune response. It has been reported that stimulation of primary human monocytes and macrophages with LL-37 led to the induction of a wide range of chemokines, chemokine receptors and other genes involved in cell adhesion, communication and motility. In fact, most of the encounters between pathogenic microorganisms and cells lead to LL-37 production, which in turn promotes proinflammatory chemokine production. Along with cathelicidin, defensins have a wide range of immunoregulatory activities owing to their ability to engage several cell surface receptors promoting chemotaxis, such as the recruitment of immature dendritic cells (DCs) and T lymphocytes after hβD-2 engagement to the CCR6 receptors. Moreover, hβD-3 can downregulate CXCR4, which in certain conditions, such as HIV-1 infection, may contribute to avoiding virus infection. In addition, APCs undergo maturation in the presence of hβD-3 via Toll-like receptors (TLRs)-1 and -2. Furthermore, murine β-defensin and hβD-3 have been shown to act directly on immature DCs as an endogenous ligand for TLR-4, -2 and -3, inducing upregulation of costimulatory molecules and DC maturation, triggering robust Th1 polarized adaptive immune responses in vivo. hβD-3 also has a high affinity for interaction with CCR2 on myeloid cells resulting in chemoattraction in the absence of the natural ligand-1/CCL2. Finally, hβD-3 can compete with melanocyte-stimulating hormone α, the natural ligand of melanocortin (MC1R) in myeloid cells, which suggests that hβD-3 may inhibit anti-inflammatory activity promoted by melanocyte-stimulating hormone α since this ligand has been shown to induce IL-10 in cells expressing the ligand of melanocortin. Moreover, some peptides are capable of activating cells of the immune system through TLRs or inducing the production of chemokines such as IL-8. In fact, our group, based on this important characteristic, has found that β-defensin-2 binds to DCs during early stages of infection and promotes IFN-γ production in experimental TB, which has led us to design vaccines using defensins as an adjuvant and applying this vaccine to boost BCG with promising results. These peptides with anti- and pro-inflammatory properties, such as β defensins, can induce the release of prostaglandin D2 and histamine in mast cells, which causes vascular permeability triggering the inflammatory process.
Although many of the immunoregulatory effects of AMPs are related to proinflammatory responses, in the past few years the anti-inflammatory aspect of AMPs has been demonstrated. For instance, defensins have powerful anti-inflammatory effects on human monocytes, human monocyte-derived macrophages and human myeloid DCs. In fact, after phagocytic cells were treated with HNPs after exposure to lipopolysaccharides (LPS), it was observed that HNP-1 blocked the release of IL-1β from LPS-activated monocytes, but not the expression and release of TNF-α. It has been demonstrated that apoptotic and necrotic neutrophils inhibit the secretion of proinflammatory cytokines from macrophages by releasing HNPs in the presence of both live and dead whole bacteria; thus, HNPs inhibit the LPS-mediated activation of macrophages without affecting the release of proinflammatory cytokines by macrophages. Pingel et al. found that hβD-3 attenuates the IL-6, IL-10, GM-CSF and TNF-α response of human myeloid DCs.
Similarly, LL-37 modulates host cell responses to stimuli and also affects the action of endogenous immune mediators: IL-1β and GM-CSF. This activity is dependent on the cell type and activation status, timing of exposure and the microenvironment. In certain cells, such as monocytes, macrophages, DCs and B lymphocytes, it inhibits cellular responses to interferon, showing suppression of cell activation and proliferation, and production of proinflammatory and Th1-polarizing cytokines and antibodies. It was further demonstrated in monocytes that the suppressive effects of LL-37 were mediated through inhibition of STAT1-independent signaling events, involving both the p65 subunit of NF-κB and p38 MAPK.
Another anti-inflammatory function of AMPs is mediated by their ability to bind antigenic molecules preventing the activation of immune responses. One clear example of this is the binding of cathelicidins and defensins to LPS to prevent TNF-α secretion. Additionally, HNP-1 binds to Bacillus anthracis lethal factor, inducing conformational changes that prevents enzymatic conversion and protects mice from B. anthracis lethal factor intoxication and death. HNP-1, HNP-3 and HD-5 bind to toxin B from Clostridium inhibiting glycosylation in vitro of Rho guanosine triphosphatases. Figure 1 outlines the different mechanisms of action described for AMPs.
(Enlarge Image)
Figure 1.
Diverse mechanisms of action of antimicrobial peptides.
(A) The different bactericidal effects observed in AMPs. Left: extracellular action; right: intracellular targets. (B & C) The inflammatory effects demonstrated by AMPs.
AMP: Antimicrobial peptide; DC: Dendritic cell.
Immune Regulatory Functions
Proinflammatory
AMPs have been shown to act as chemoattractants for cells of innate and adaptive immunity, and in fact, many authors consider them as a bridge between innate and adaptive immunity. One of the most studied peptides has been human cathelicidin LL-37 because of its regulatory functions and being the only member of the cathelicidin family present in humans. LL-37 is capable of attracting neutrophils, monocytes, T cells and mast cells using formyl peptide receptor-like 1, and a distinct Gi-coupled receptor at nanomolar concentrations. Besides the remarkable chemotactic activity, it also induces several other responses in leukocytes and epithelial cells, modifying gene expression in order to improve or modulate immune response. It has been reported that stimulation of primary human monocytes and macrophages with LL-37 led to the induction of a wide range of chemokines, chemokine receptors and other genes involved in cell adhesion, communication and motility. In fact, most of the encounters between pathogenic microorganisms and cells lead to LL-37 production, which in turn promotes proinflammatory chemokine production. Along with cathelicidin, defensins have a wide range of immunoregulatory activities owing to their ability to engage several cell surface receptors promoting chemotaxis, such as the recruitment of immature dendritic cells (DCs) and T lymphocytes after hβD-2 engagement to the CCR6 receptors. Moreover, hβD-3 can downregulate CXCR4, which in certain conditions, such as HIV-1 infection, may contribute to avoiding virus infection. In addition, APCs undergo maturation in the presence of hβD-3 via Toll-like receptors (TLRs)-1 and -2. Furthermore, murine β-defensin and hβD-3 have been shown to act directly on immature DCs as an endogenous ligand for TLR-4, -2 and -3, inducing upregulation of costimulatory molecules and DC maturation, triggering robust Th1 polarized adaptive immune responses in vivo. hβD-3 also has a high affinity for interaction with CCR2 on myeloid cells resulting in chemoattraction in the absence of the natural ligand-1/CCL2. Finally, hβD-3 can compete with melanocyte-stimulating hormone α, the natural ligand of melanocortin (MC1R) in myeloid cells, which suggests that hβD-3 may inhibit anti-inflammatory activity promoted by melanocyte-stimulating hormone α since this ligand has been shown to induce IL-10 in cells expressing the ligand of melanocortin. Moreover, some peptides are capable of activating cells of the immune system through TLRs or inducing the production of chemokines such as IL-8. In fact, our group, based on this important characteristic, has found that β-defensin-2 binds to DCs during early stages of infection and promotes IFN-γ production in experimental TB, which has led us to design vaccines using defensins as an adjuvant and applying this vaccine to boost BCG with promising results. These peptides with anti- and pro-inflammatory properties, such as β defensins, can induce the release of prostaglandin D2 and histamine in mast cells, which causes vascular permeability triggering the inflammatory process.
Anti-inflammatory
Although many of the immunoregulatory effects of AMPs are related to proinflammatory responses, in the past few years the anti-inflammatory aspect of AMPs has been demonstrated. For instance, defensins have powerful anti-inflammatory effects on human monocytes, human monocyte-derived macrophages and human myeloid DCs. In fact, after phagocytic cells were treated with HNPs after exposure to lipopolysaccharides (LPS), it was observed that HNP-1 blocked the release of IL-1β from LPS-activated monocytes, but not the expression and release of TNF-α. It has been demonstrated that apoptotic and necrotic neutrophils inhibit the secretion of proinflammatory cytokines from macrophages by releasing HNPs in the presence of both live and dead whole bacteria; thus, HNPs inhibit the LPS-mediated activation of macrophages without affecting the release of proinflammatory cytokines by macrophages. Pingel et al. found that hβD-3 attenuates the IL-6, IL-10, GM-CSF and TNF-α response of human myeloid DCs.
Similarly, LL-37 modulates host cell responses to stimuli and also affects the action of endogenous immune mediators: IL-1β and GM-CSF. This activity is dependent on the cell type and activation status, timing of exposure and the microenvironment. In certain cells, such as monocytes, macrophages, DCs and B lymphocytes, it inhibits cellular responses to interferon, showing suppression of cell activation and proliferation, and production of proinflammatory and Th1-polarizing cytokines and antibodies. It was further demonstrated in monocytes that the suppressive effects of LL-37 were mediated through inhibition of STAT1-independent signaling events, involving both the p65 subunit of NF-κB and p38 MAPK.
Another anti-inflammatory function of AMPs is mediated by their ability to bind antigenic molecules preventing the activation of immune responses. One clear example of this is the binding of cathelicidins and defensins to LPS to prevent TNF-α secretion. Additionally, HNP-1 binds to Bacillus anthracis lethal factor, inducing conformational changes that prevents enzymatic conversion and protects mice from B. anthracis lethal factor intoxication and death. HNP-1, HNP-3 and HD-5 bind to toxin B from Clostridium inhibiting glycosylation in vitro of Rho guanosine triphosphatases. Figure 1 outlines the different mechanisms of action described for AMPs.
(Enlarge Image)
Figure 1.
Diverse mechanisms of action of antimicrobial peptides.
(A) The different bactericidal effects observed in AMPs. Left: extracellular action; right: intracellular targets. (B & C) The inflammatory effects demonstrated by AMPs.
AMP: Antimicrobial peptide; DC: Dendritic cell.