Is Strongyloides stercoralis hyperinfection induced by glucocorticoids a result of both suppressed host immunity and altered parasite genetics?

The gastrointestinal (GI) nematode Strongyloides stercoralis (S.s.) causes human strongyloidiasis, a potentially life-threatening disease that currently affects over 600 million people globally. The uniquely pernicious aspect of S.s. infection, as compared to all other GI nematodes, is its autoinfective larval stage (L3a) that maintains a low-grade chronic infection, allowing undetectable persistence for decades. Infected individuals who are administered glucocorticoid therapy can develop a rapid and often lethal hyperinfection syndrome within days. Hyperinfection patients often present with dramatic increases in first- and second-stage larvae and L3a in their GI tract, with L3a widely disseminating throughout host organs leading to sepsis. How glucocorticoid administration drives hyperinfection remains a critical unanswered question; specifically, it is unknown whether these steroids promote hyperinfection through eliminating essential host protective mechanisms and/or through dysregulating parasite development. This current deficiency in understanding is largely due to the previous absence of a genetically defined mouse model that would support all S.s. life-cycle stages and the lack of successful approaches for S.s. genetic manipulation. However, there are currently new possibilities through the recent demonstration that immunodeficient NOD.Cg-PrkdcscidIl2rgtm1Wjl/SzJ (NSG) mice support sub-clinical infections that can be transformed to lethal hyperinfection syndrome following glucocorticoid administration. This is coupled with advances in transcriptomics, transgenesis, and gene inactivation strategies that now allow rigorous scientific inquiry into S.s. biology. We propose that combining in vivo manipulation of host immunity and deep immunoprofiling strategies with the latest advances in S.s. transcriptomics, piggyBac transposon-mediated transgene insertion, and CRISPR/Cas-9-mediated gene inactivation will facilitate new insights into the mechanisms that could be targeted to block lethality in humans with S.s. hyperinfection.

Transcriptional profiles in Strongyloides stercoralis males reveal deviations from the Caenorhabditis sex determination model.

The human and canine parasitic nematode Strongyloides stercoralis utilizes an XX/XO sex determination system, with parasitic females reproducing by mitotic parthenogenesis and free-living males and females reproducing sexually. However, the genes controlling S. stercoralis sex determination and male development are unknown. We observed precocious development of rhabditiform males in permissive hosts treated with corticosteroids, suggesting that steroid hormones can regulate male development. To examine differences in transcript abundance between free-living adult males and other developmental stages, we utilized RNA-Seq. We found two clusters of S. stercoralis-specific genes encoding predicted transmembrane proteins that are only expressed in free-living males. We additionally identified homologs of several genes important for sex determination in Caenorhabditis species, including mab-3, tra-1, fem-2, and sex-1, which may have similar functions. However, we identified three paralogs of gld-1; Ss-qki-1 transcripts were highly abundant in adult males, while Ss-qki-2 and Ss-qki-3 transcripts were highly abundant in adult females. We also identified paralogs of pumilio domain-containing proteins with sex-specific transcripts. Intriguingly, her-1 appears to have been lost in several parasite lineages, and we were unable to identify homologs of tra-2 outside of Caenorhabditis species. Together, our data suggest that different mechanisms control male development in S. stercoralis and Caenorhabditis species.

Dafachronic acid and temperature regulate canonical dauer pathways during Nippostrongylus brasiliensis infectious larvae activation.

BACKGROUND: While immune responses to the murine hookworm Nippostrongylus brasiliensis have been investigated, signaling pathways regulating development of infectious larvae (iL3) are not well understood. We hypothesized that N. brasiliensis would use pathways similar to those controlling dauer development in the free-living nematode Caenorhabditis elegans, which is formally known as the "dauer hypothesis." METHODS: To investigate whether dafachronic acid activates the N. brasiliensis DAF-12 homolog, we utilized an in vitro reporter assay. We then utilized RNA-Seq and subsequent bioinformatic analyses to identify N. brasiliensis dauer pathway homologs and examine regulation of these genes during iL3 activation. RESULTS: In this study, we demonstrated that dafachronic acid activates the N. brasiliensis DAF-12 homolog. We then identified N. brasiliensis homologs for members in each of the four canonical dauer pathways and examined their regulation during iL3 activation by either temperature or dafachronic acid. Similar to C. elegans, we found that transcripts encoding antagonistic insulin-like peptides were significantly downregulated during iL3 activation, and that a transcript encoding a phylogenetic homolog of DAF-9 increased during iL3 activation, suggesting that both increased insulin-like and DAF-12 nuclear hormone receptor signaling accompanies iL3 activation. In contrast to C. elegans, we observed a significant decrease in transcripts encoding the dauer transforming growth factor beta ligand DAF-7 during iL3 activation, suggesting a different role for this pathway in parasitic nematode development. CONCLUSIONS: Our data suggest that canonical dauer pathways indeed regulate iL3 activation in the hookworm N. brasiliensis and that DAF-12 may be a therapeutic target in hookworm infections.