Immunomodulatory mechanisms of lactobacilli

Abstract

Over the past decade it has become clear that lactobacilli and other probiotic and commensal organisms can interact with mucosal immune cells or epithelial cells lining the mucosa to modulate specific functions of the mucosal immune system. The most well understood signalling mechanisms involve the innate pattern recognition receptors such as Toll-like receptors, nucleotide oligomerization domain-like receptors and C-type lectin receptors. Binding of microbe-associated molecular patterns with these receptors can activate antigen presenting cells and modulate their function through the expression of surface receptors, secreted cytokines and chemokines. In vitro the cytokine response of human peripheral blood mononuclear cells and dendritic cells to lactobacilli can be strikingly different depending on both the bacterial species and the strain. Several factors have been identified in lactobacilli that influence the immune response in vitro and in vivo including cell surface carbohydrates, enzymes modifying the structure of lipoteichoic acids and metabolites. In mice mechanistic studies point to a role for the homeostatic control of inducible T regulatory cells in the mucosal tissues as one possible immunomodulatory mechanism. Increasing evidence also suggests that induction of epithelial signalling by intestinal lactobacilli can modulate barrier functions, defensin production and regulate inflammatory signalling. Other probiotic mechanisms include modulation of the T cell effector subsets, enhancement of humoral immunity and interactions with the epithelial-associated dendritic cells and macrophages. A major challenge for the future will be to gain more knowledge about the interactions occurring between lactobacilli and the host in vivo and to understand the molecular basis of innate signalling in response to whole bacteria which trigger multiple signalling pathways.

Figure 1

Simplified scheme for TLR signaling in enterocytes. TLRs in the cytoplasmic membrane or subcellular compartments such as the endosome bind the MyD88 adapter protein to initiate signaling. MyD88 recruits TRAF6 and members of the IRAK family (IRAKs) which leads to activation of the TAK1 complex (TAK1, TAB1/2). The activated TAK complex then activates the IKK complex (IKKα and IKKβ) which phosphorylates the inhibitor of NF-κB (IKB) leading to its degradation and the translocation of NF-κB (typically p50 and p65 heterodimers) into the nucleus where it activates gene expression. The activated TAK1 complex simultaneously activates the MAPK pathway resulting in phosphorylation (P) and activation of the transcription factor AP1. The canonical pathway of NF-κB activation and can also be triggered by binding of TRAM and TRIF adaptor proteins to TLR4 (not shown). The adaptor protein TRIF which binds to TLR3 recruits TRAF3 which interacts with the TBK and IKKi kinases to promote phosphorylation (P) of IRF3 which translocates to the nucleus and activates transcription of type 1 interferons, especially IFN-β. The NOD1 and 2 receptros activate NK-κB via the serine threonine kinase RICK. This diagram is a modified version of Figure 1 published by Wells et al., 2010 [17].

Figure 2

TLR2 recognition of lipoproteins and LTA. Recognition of LTA and lipoproteins is mediated through binding of the lipid chains which anchor these molecules in the membrane. a). All lipoproteins possess a specific N-terminal lipoprotein signal which targets the protein for secretion and post-translational modification. In Gram-positive bacteria and Gram-negative bacteria the Lgt enzyme transfers a diacylglyceride group to the cysteine sulfhydryl group adjacent to the signal peptide cleavage site. Subsequently the signal peptide is cleaved just before the cysteine residue by LspA yielding a mature di-acylated lipoprotein. However, in Gram-negative bacteria and mycobacteria the Lnt enzyme attaches a third acyl group to the amino group of the N-terminal cysteine promoting its transport to the outer membrane. b). The diacyl lipid chains added by Lgt bind in a hydrophobic pocket in the extracellular domain of TLR2 and the head group of the peptide interacts with TLR6 to promote hetero-dimerization and signalling. In the case of tri-acylated lipoproteins the third lipid chain interacts with a hydrophobic channel in TLR1 to promote dimerization and signalling. This diagram is a modified version of Figure 1 published by Schenk et al., 2009 [101].

Figure 3

IL-10, IL-12p70 production and the IL-10/IL-12p70 ratio by monocyte-derived dendritic cells stimulated with 20 different L. plantarum strains. The graphs were produced using data from Meijerink et al., [69] based on one of the 5 representative donors. The different strains induce striking different amounts of these cytokines. Based on the IL-12 to IL-10 ratios it is clear that these cytokines can vary independently of each other resulting in strains with distinct pro-inflammatory and anti-inflammatory profiles.

Figure 4

Potential immunomodulatory mechanisms of probiotic lactobacilli (1). It is not yet clear precisely how oral administration of certain strains of probiotic lactobacilli can expand the Treg population in the mesenteric lymph nodes but it may involve direct activation of lamina propria (LP) CD103+ DC by MAMPs leading to up-regulation of MHC II and the co-stimulatory molecules needed for signaling and antigen presentation to naïve T cells in the lymphoid tissue. (2) Alternatively lactobacilli may be initially taken up by epithelial-associated CX3CR1⁺ DCs which could indirectly lead to maturation of the migratory CD103+ DC population. The oral administration of probiotic lactobacilli may also stimulate epithelial signaling and the production of cytokines such as TGFβ and TSLP as well as other factors which imprint a tolerogenic phenotype on resident CD103+ DC in the LP.