Sm16, ASchistosoma mansoniImmunomodulatory Protein, Fails to Elicit a Protective Immune Response and Does Not Have an Essential Role in Parasite Survival in the Definitive Host

Abstract

Sm16 is an immunomodulatory protein that seems to play a key role in the suppression of the cutaneous inflammatory response during Schistosoma mansoni penetration of the skin of definitive hosts. Therefore, Sm16 represents a potential target for protective immune responses induced by vaccination. In this work, we generated the recombinant protein rSm16 and produced polyclonal antibodies against this protein to evaluate its expression during different parasite life-cycle stages and its location on the surface of the parasite. In addition, we analyzed the immune responses elicited by immunization with rSm16 using two different vaccine formulations, as well as its ability to induce protection in Balb/c mice. In order to explore the biological function of Sm16 during the course of experimental infection, RNA interference was also employed. Our results demonstrated that Sm16 is expressed in cercaria and schistosomula and is located in the schistosomula surface. Despite humoral and cellular immune responses triggered by vaccination using rSm16 associated with either Freund's or alum adjuvants, immunized mice presented no reduction in either parasite burden or parasite egg laying. Knockdown of Sm16 gene expression in schistosomula resulted in decreased parasite size in vitro but had no effect on parasite survival or egg production in vivo. Thus, our findings demonstrate that although the vaccine formulations used in this study succeeded in activating immune responses, these failed to promote parasite elimination. Finally, we have shown that Sm16 is not vital for parasite survival in the definitive host and hence may not represent a suitable target for vaccine development.

Schistosoma mansoni immunomodulatory molecule Sm16/SPO-1/SmSLP is a member of the trematode-specific helminth defence molecules (HDMs)

“…Our finding that Sm16 is expressed in eggs disagrees with most earlier studies and the more recent report by Bernardes et al [12]. The discrepancy may be because we employed more sensitive methods of Western blots (chemiluminescence) and gene amplification by qPCR.…”

“…The molecule is stored in abundance within the acetabular glands and rapidly expelled from these during skin penetration [ 45 , 46 ]; however, this transient expression and secretion into the host tissues is clearly insufficient to induce detectable antibodies in the early weeks following a primary infection. Most reports agree that Sm16 is not expressed by adult worms [ 9 – 12 , 46 ]. Although we detected low levels of Sm16 expression in female adult and mixed-adult extracts by qPCR in this study, we presume that this is due to some residual presence of eggs in adult female worms as no expression was found in male worms.…”

“…Much of the research to date that has evaluated the function of Sm16 has been conducted using a recombinant formulation expressed in E. coli that either features a mutation with two alanine substitutions at positions 92 and 93 [13,14,44] or a truncation of the last 27 C-terminal residues [12]. These modifications were made due to the inability to express soluble recombinant Sm16 in this prokaryotic system, perhaps owing to the amphipathic nature of the C-terminal section of the protein.…”

“…pastoris and that this recombinant, as well as a synthetic version, bound to macrophages and was endocytosed into the endosomal/lysosomal system like other HDMs [ 36 ]. Bernardes et al [ 12 ] acknowledged that the failure of their recombinant Sm16 vaccine to promote parasite elimination could have been because it lacked the C-terminal 27 residues; therefore, a repeat of these trials with yeast-expressed or synthetic full-length Sm16 may be worthwhile.…”

“…japonicum suggested that the Sm16 homolog, Sj16, is enriched in eggs, miracidia, sporocysts, cercariae, and lung stage schistosomulae [ 11 ]. Recently, however, Bernardes et al [ 12 ] reported that Sm16 is expressed in cercariae and newly transformed schistosomulae but not in adults or eggs.…”