Abstract
Recent evidence supports the involvement of inducible, highly diverse lectin-like recognition molecules in snail hemocyte-mediated responses to larval Schistosoma mansoni. Because host lectins likely are involved in initial parasite recognition, we sought to identify specific carbohydrate structures (glycans) shared between larval S. mansoni and its host Biomphalaria glabrata to address possible mechanisms of immune avoidance through mimicry of elements associated with the host immunoreactivity. A panel of monoclonal antibodies (mABs) to specific S. mansoni glycans was used to identify the distribution and abundance of shared glycan epitopes (glycotopes) on plasma glycoproteins from B. glabrata strains that differ in their susceptibilities to infection by S. mansoni. In addition, a major aim of this study was to determine if larval transformation products (LTPs) could bind to plasma proteins, and thereby alter the glycotopes exposed on plasma proteins in a snail strain-specific fashion. Plasma fractions (<100 kDa/>100 kDa) from susceptible (NMRI) and resistant (BS-90) snail strains were subjected to SDS-PAGE and immunoblot analyses using mAB to LacdiNAc (LDN), fucosylated LDN variants, Lewis X and trimannosyl core glycans. Results confirmed a high degree of glycan sharing, with NMRI plasma exhibiting a greater distribution/abundance of LDN, F-LDN and F-LDN-F than BS-90 plasma (<100 kDa fraction). Pretreatment of blotted proteins with LTPs significantly altered the reactivity of specific mABs to shared glycotopes on blots, mainly through the binding of LTPs to plasma proteins resulting in either glycotope blocking or increased glycotope attachment to plasma. Many LTP-mediated changes in shared glycans were snail-strain specific, especially those in the <100 kDa fraction for NMRI plasma proteins, and for BS-90, mainly those in the >100 kDa fraction. Our data suggest that differential binding of S. mansoni LTPs to plasma proteins of susceptible and resistant B. glabrata strains may significantly impact early anti-larval immune reactivity, and in turn, compatibility, in this parasite-host system.
Highlights
Glycans are complex carbohydrate (CHO) chains normally covalently bound to polypeptides, lipids or other carrier molecules
A popular notion that recently has gained traction in the Biomphalaria glabrata-Schistosoma mansoni system poses that larval glycans and/or their associated glycoconjugates may be serving as pathogen-associated molecular patterns (PAMPs) that interact with lectin-like pathogen recognition receptors (PRRs), thereby mediating innate immune responses to invading miracidia
Since host lectins likely are involved in early immunorecognition events, the recent identification of specific carbohydrate structures shared between larval stages of S. mansoni and its host Biomphalaria glabrata suggests that larvae may be avoiding immune recognition through a molecular mimicry mechanism mediated by lectin-reactive glycans
Summary
Glycans are complex carbohydrate (CHO) chains normally covalently bound to polypeptides, lipids or other carrier molecules. A popular notion that recently has gained traction in the Biomphalaria glabrata-Schistosoma mansoni system poses that larval glycans and/or their associated glycoconjugates may be serving as pathogen-associated molecular patterns (PAMPs) that interact with lectin-like pathogen recognition receptors (PRRs), thereby mediating innate immune responses to invading miracidia (see reviews [10,11,12,13]) This concept has been incorporated into a proposed mechanism, termed compatibility polymorphism [14], in which it is hypothesized that high molecular diversity in relevant PAMP and PRR systems can provide the necessary variation in receptorligand interactions to account for differences in infection rates seen in different snail-schistosome strain combinations [15]. Recent studies have reported the selective ability of SmPoMuc to form complexes with Freps from snail plasma [18], as well as the demonstration of a direct linkage between expression of one B. glabrata Frep (Frep 3) and resistance to trematode infection [19], Larval Products Alter Shared Snail Glycotopes
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