Human IgG1 Responses to Surface Localised Schistosoma mansoni Ly6 Family Members Drop following Praziquantel Treatment

Abstract

BackgroundThe heptalaminate-covered, syncytial tegument is an important anatomical adaptation that enables schistosome parasites to maintain long-term, intravascular residence in definitive hosts. Investigation of the proteins present in this surface layer and the immune responses elicited by them during infection is crucial to our understanding of host/parasite interactions. Recent studies have revealed a number of novel tegumental surface proteins including three (SmCD59a, SmCD59b and Sm29) containing uPAR/Ly6 domains (renamed SmLy6A SmLy6B and SmLy6D in this study). While vaccination with SmLy6A (SmCD59a) and SmLy6D (Sm29) induces protective immunity in experimental models, human immunoglobulin responses to representative SmLy6 family members have yet to be thoroughly explored.Methodology/Principal FindingsUsing a PSI-BLAST-based search, we present a comprehensive reanalysis of the Schistosoma mansoni Ly6 family (SmLy6A-K). Our examination extends the number of members to eleven (including three novel proteins) and provides strong evidence that the previously identified vaccine candidate Sm29 (renamed SmLy6D) is a unique double uPAR/Ly6 domain-containing representative. Presence of canonical cysteine residues, signal peptides and GPI-anchor sites strongly suggest that all SmLy6 proteins are cell surface-bound. To provide evidence that SmLy6 members are immunogenic in human populations, we report IgG1 (as well as IgG4 and IgE) responses against two surface-bound representatives (SmLy6A and SmLy6B) within a cohort of S. mansoni-infected Ugandan males before and after praziquantel treatment. While pre-treatment IgG1 prevalence for SmLy6A and SmLy6B differs amongst the studied population (7.4% and 25.3% of the cohort, respectively), these values are both higher than IgG1 prevalence (2.7%) for a sub-surface tegumental antigen, SmTAL1. Further, post-treatment IgG1 levels against surface-associated SmLy6A and SmLy6B significantly drop (p = 0.020 and p < 0.001, respectively) when compared to rising IgG1 levels against sub-surface SmTAL1.Conclusions/SignificanceCollectively, these results expand the number of SmLy6 proteins found within S. mansoni and specifically demonstrate that surface-associated SmLy6A and SmLy6B elicit immunological responses during infection in endemic communities.

Antibody responses to Schistosoma mansoni schistosomula antigens

“…On the other hand, we did observe an increase to SmLy6A. These data are in apparent contradiction to a previously published study showing that antibodies to SmLy6A decreased after treatment . Similar to the study by Chalmers et al, the effect of treatment was examined in participants with an antibody response above the cut‐off set using nonendemic European donors at baseline.…”

“…However, not all Schistosoma antigens induce an increase in antibodies following PZQ treatment. For example, IgG1 levels against rSmLy6A and rSmLy6B have been shown to drop following PZQ treatment among antibody responders . Similarly, we also found a drop in IgG1, IgG4 and IgE to rSmKK7, rSmLy6B and rSmTSP7, respectively, after PZQ treatment among antibody responders.…”

“…The antigens used in this study were S. mansoni adult worm (AWA) and egg antigens (SEA) and recombinant schistosomula antigens rSmKK7, rSmLy6A, rSmLy6B and rSmTSP7. These antigens were identified as highly expressed products in the schistosomula life cycle stage after screening the Schistosoma transcriptome using DNA microarrays, as previously described . Specifically, a >5‐fold increase in expression when comparing normalized expression averages from snail (egg, miracidia, mother sporocyst and daughter sporocyst) to schistosomula (3‐h, 24‐h, 3‐day and 6‐day schistosomula) life stages was observed .…”

“…These antigens were identified as highly expressed products in the schistosomula life cycle stage after screening the Schistosoma transcriptome using DNA microarrays, as previously described . Specifically, a >5‐fold increase in expression when comparing normalized expression averages from snail (egg, miracidia, mother sporocyst and daughter sporocyst) to schistosomula (3‐h, 24‐h, 3‐day and 6‐day schistosomula) life stages was observed . Recombinant antigens were expressed in Escherichia coli and purified using methods as previously described for rSmKK7 (smp_194830), rSmLy6A and rSmLy6B .…”

“…Indeed, DNA microarray‐based analysis of the S. mansoni life cycle has revealed schistosomula‐enriched gene products, such as those coding for SmKK7 (smp_194830), S. mansoni lymphocyte antigen 6 isoforms A and B (SmLy6A; smp_019350, SmLy6B; smp_105220) and S. mansoni tetraspanin 7 (SmTSP7; smp_099770) . SmLy6A and SmLy6B are probably glycophosphatidylinositol (GPI)‐anchored antigens, found in schistosomula, but also in adult tegumental and mesenchymal tissues . Although SmLy6A and SmLy6B are homologues of human CD59, which inhibits the formation of the complement membrane attack complex, they do not inhibit complement fixation and their function remains unknown .…”