Signalling mechanisms underlying subversion of the immune response by the filarial nematode secreted product ES-62

Abstract

Secretion of immunomodulatory molecules is a key strategy employed by pathogens to enable their survival in host organisms. For example, arthropod-transmitted filarial nematodes, which achieve longevity within the infected host by suppressing and modulating the host immune response, produce excretory-secretory (ES) products that have been demonstrated to possess immunomodulatory properties. In this review we discuss the immunomodulatory effects of the phosphorylcholine-containing filarial nematode-secreted glycoprotein ES-62 and describe the intracellular signal transduction pathways it targets to achieve these effects.

Figure 1

ES-62 structural studies. (a) The location of key residues within the ES-62 sequence, including N-glycosylation sites, a possible site for interaction with PC donors, leucine-rich regions (likely to be involved in protein–protein interaction), and regions containing subcellular targeting motifs. (b) Prediction of a tertiary structure for the ES-62 monomer, obtained using dragon indicating α-helices and β-strands, as well as glycosylation sites, leucine-rich regions and residues involved in metal ion co-ordination (ES-62 shows homology with aminopeptidases that contain a divalent cation in their active site17). (c) A low-resolution dummy atom model of the ES-62 tetramer, which is likely to be slightly elongated, obtained using dammin under three symmetry conditions (P1, no symmetry; P2, two-point symmetry; P222, 222-point symmetry).

Figure 2

Immunomodulation by ES-62. ES-62 targets multiple cells of the immune system [black arrows (+) and T bars (–)] to achieve immunomodulation, broadly biasing the immune response to a Th2/anti-inflammatory response characterized by the production of low levels of IL-12, IFN-γ, TNF-α and IL-6 (red arrows), secretion of IL-4 (green arrow) by Th2 cells and production of the Th2-associated antibody isotypes IgG1 (mouse) and IgG4 (human). IL-10 production by B1 cells (green arrow) also contributes to this response. ES-62 alters costimulatory molecule expression on DCs and targets the signalling pathways triggered following cross-linking of the B- and T-cell antigen receptors (BCR and TCR), hence disrupting the responses of these cells to specific antigen.

Figure 3

BCR signalling in untreated (a) and ES-62-exposed (b) B cells. Following ligation of the B-cell antigen receptor (BCR) of untreated B cells (a) the kinase, Lyn, tyrosine phosphorylates the immunoreceptor tyrosine activation motifs (ITAMs) on the accessory transducing molecules Ig-α and Ig-β resulting in the recruitment and activation of the PI-3-K and PLC-γ-signalling pathways. Whilst PI-3-K activation results in the activation of atypical PKC isoforms (aPKC), PLC-γ activation induces inositol trisphosphate (IP₃) and diacylglycerol (DAG) generation, ultimately resulting in activation of classical (cPKC) and novel (nPKC) PKC isoforms. Binding of the adaptor proteins Shc and BLNK to the phosphorylated ITAMs leads to the recruitment of the Grb2Sos complexes (Grb2 is an adaptor protein which binds Sos, a guanine nucleotide exchange factor) required for activation of the GTPase, Ras. Active Ras initiates the Erk MAP kinase cascade by binding and activating the ser/thr kinase, Raf leading to stimulation of the thr/tyr kinase MEK and consequent activation and nuclear translocation of the ser/thr kinase Erk. ES-62/PC signalling (b) disrupts BCR coupling to the PI-3-K cascade as well as targeting major negative regulatory sites in the control of the Erk, p38 and JNK MAP kinase cascades. First, ES-62 signalling promotes the BCR-activation of SHP-1 tyrosine phosphatase to prevent initiation of BCR signalling by maintaining the ITAMs in a resting, dephosphorylated state and hence prevents recruitment of the ShcGrb2Sos complexes required to activate the Ras- and Rac-MAP kinase cascades. Second, ES-62 signalling promotes the BCR-mediated recruitment of RasGAP to terminate ongoing Ras signals. In addition, ES-62 is also likely to target MAP kinase activation by down-regulating PKC isoform expression. Finally, ES-62-signalling promotes the BCR-driven association of the nuclear MAP kinase dual (thr/tyr) phosphatase, Pac-1 with Erk to terminate any ongoing Erk signals. This multipronged mechanism results in a rapid and profound desensitization of BCR coupling to the MAP kinase cascades.

Figure 4

Alignment of Acanthocheilonema viteae ES-62 with homologues from Brugia malayi and Brugia pahangi. Alignment of A. viteae and B. malayi cDNA sequences and an incomplete sequence derived from two B. pahangi PCR products demonstrates significant homology between these filarial nematodes. Lower case letters represent the signal peptide cleavage sites. Putative N-glycosylation sites are coloured red and underlined. Per cent homology of the sequences was calculated for the regions indicated between the asterisks.