Recent insights into innate and adaptive immune responses to Giardia

A core UPS molecular complement implicates unique endocytic compartments at the parasite-host interface in Giardia lamblia

Unconventional protein secretion (UPS) plays important roles in cell physiology. In contrast to canonical secretory routes, UPS does not generally require secretory signal sequences and often bypasses secretory compartments such as the ER and the Golgi apparatus. Giardia lamblia is a protist parasite with reduced subcellular complexity which releases several proteins, some of them virulence factors, without canonical secretory signals. This implicates UPS at the parasite-host interface. No dedicated machinery nor mechanism(s) for UPS in Giardia are currently known, although speculations on the involvement of endocytic organelles called PV/PECs, have been put forth. To begin to address the question of whether PV/PECs are implicated in virulence-associated UPS and to define the composition of molecular machinery involved in protein release, we employed affinity purification and mass spectrometry, coupled to microscopy-based subcellular localization and signal correlation quantification to investigate the interactomes of 11 reported unconventionally secreted proteins, all predicted to be cytosolic. A subset of these are associated with PV/PECs. Extended and validated interactomes point to a core PV/PECs-associated UPS machinery, which includes uncharacterized and Giardia-specific coiled-coil proteins and NEK kinases. Finally, a subset of the alpha-giardin protein family was enriched in all PV/PECs-associated protein interactomes, highlighting a previously unappreciated role for these proteins at PV/PECs and in UPS. Taken together, our results provide the first characterization of a virulence-associated UPS protein complex in Giardia lamblia at PV/PECs, suggesting a novel link between these primarily endocytic and feeding organelles and UPS at the parasite-host interface.

Epidemiology of giardiasis and assemblages A and B and effects on diarrhea and growth trajectories during the first 8 years of life: Analysis of a birth cohort in a rural district in tropical Ecuador

BACKGROUND

There are limited longitudinal data on the acquisition of Giardia lamblia infections in childhood using molecular assays to detect and type assemblages, and measure effects of infections on diarrhea risk and childhood growth.

METHODS

We analysed stool samples from a surveillance sample within a birth cohort in a rural district in tropical Ecuador. The cohort was followed to 8 years of age for the presence of G. lamblia in stools by quantitative PCR and A and B assemblages by Taqman assay or Sanger sequencing. We explored risk factors associated with infection using generalized estimating equations applied to longitudinal binary outcomes, and longitudinal panel data analysis to estimate effects of infection on diarrhea and growth trajectories.

RESULTS

2,812 stool samples collected between 1 month and 8 years of age from 498 children were analyzed and showed high rates of infection: 79.7% were infected at least once with peak prevalence (53.9%) at 5 years. Assemblage B was accounted for 56.8% of genotyped infections. Risk factors for infection included male sex (P = 0.001), daycare attendance (P<0.001), having a household latrine (P = 0.04), childhood (P<0.001) and maternal soil-transmitted helminth (P = 0.029) infections, and exposures to donkeys (age interaction P = 0.034). G. lamblia was associated with increased risk of diarrhea (per episode, RR 1.03, 95% CI 1.01-1.06, P = 0.011) during the first 3 years of life and a transient impairment of weight (age interaction P = 0.017) and height-for-age (age interaction P = 0.025) trajectories between 1 and 4 years of age. There was no increased risk of either assemblage being associated with outcomes.

CONCLUSION

Our data show a relatively high edemicity of G. lamblia transmission during childhood in coastal Ecuador, and evidence that infection is associated with a transiently increased risk of diarrhea during the first 3 years of life and impairment of weight and height between 1 and 4 years.

Giardia VSPAS7 protein attenuates Giardia intestinalis-induced host macrophage pyroptosis

BACKGROUND

The unicellular protozoan parasite Giardia intestinalis, which primarily infects humans and animals such as cattle and sheep, is having a major negative impact on public health. Giardia is able to evade the recognition and elimination of the host immune system because of the trophozoite surface and extracellular vesicles (EVs) covered by variant-specific surface proteins (VSPs). As key proteins for immune evasion, whether VSPs can regulate Giardia-induced pyroptosis and promote Giardia evasion of host immune responses has not been reported.

METHODS

To examine the role of Giardia VSPAS7 on Giardia-induced activation of the signaling pathway, secretion of pro-inflammatory cytokines, pyroptosis and the mechanism involved, we constructed the pcDNA3.1-vspas7 expression plasmid and transfected this plasmid into mouse macrophages. Key proteins for pyroptosis, IL-1β secretion and LDH release were detected in pcDNA3.1-vspas7-transfected wild-type (WT) cells and NLRP3-deficient cells by western blot, ELISA and LDH assays, respectively. The interactions of Giardia VSPAS7 and mouse NLRP3 were examined using immunofluorescence assays (IFA), co-immunoprecipitation (Co-IP) and bimolecular fluorescence complementation (BiFC) assays.

RESULTS

VSPAS7 could decrease the levels of phosphorylated-p65 (P-p65), P-IκBα and P-ERK caused by Giardia and reduce the production levels of Giardia-induced pro-inflammatory cytokine IL-6, IL-12 p40 and TNF-α. The results showed that VSPAS7 inhibited Giardia-mediated activation of NF-κB, ERK/MAPK signaling and secretion of pro-inflammatory cytokines. Furthermore, VSPAS7 suppressed Giardia-induced macrophage pyroptosis by reducing GSDMD cleavage, caspase-1 activation, IL-1β secretion and LDH release. We further found that VSPAS7 could interact with mouse NLRP3 directly, and in NLRP3-deficient cells the suppression of Giardia-induced macrophage pyroptosis by VSPAS7 was significantly attenuated.

CONCLUSIONS

Overall, VSPAS7 could inhibit Giardia-induced activation of signaling pathways and pyroptosis in host macrophages, allowing Giardia evasion of host immune responses. Studies on Giardia VSP-mediated immune evasion provide an important theoretical basis for in-depth studies on Giardia pathogenicity.

Gut Microbiota Crosstalk with Resident Macrophages and Their Role in Invasive Amebic Colitis and Giardiasis-Review

The innate immune response is highly dependent on the action of macrophages. They are abundant in the intestine subepithelial lamina propria of the mucosa, where they deploy multiple tasks and play a critical role. The balance between the gut microbiota and M2 macrophages is critical for gut health and homeostasis. Gut microbiota has the power to change macrophage phenotype and replenish the resident macrophage niche during and post infection. As far as the extracellular enteric parasitic infections invasive amebic colitis and giardiasis are concerned, a change of macrophages phenotype to a pro-inflammatory state is dependent on direct contact of the protozoan parasites with host cells. Macrophages induce strong pro-inflammatory response by inflammasome activation and secretion of interleukin IL-1β. Inflammasomes play a key role in the response to cellular stress and microbe attacks. The balance between gut mucosal homeostasis and infection is dependent on the crosstalk between microbiota and resident macrophages. Parasitic infections involve NLRP1 and NLRP3 inflammasome activation. For Entamoeba histolytica and Giardia duodenalis infections, inflammasome NLRP3 activation is crucial to promote the host defenses. More studies are needed to further elucidate possible therapeutic and protective strategies against these protozoan enteric parasites' invasive infections in humans.

Molecular Detection and Multilocus Genotyping of Giardia duodenalis in Pigs in Fujian Province, Southeastern China

Giardia duodenalis, an intestinal parasite, is widely distributed in humans and various animals, such as pigs, cattle and cats. The clinical symptoms of giardiasis are characterized as including abdominal pain, acute or chronic diarrhea, and bloating and weight loss in humans and animals, leading to public and veterinary health problems worldwide. However, the prevalence and genotypes of G. duodenalis in pigs in Fujian Province, southeastern China, have not been reported. In the present study, 725 fecal samples were collected from six cities (Fuqing, Putian, Nanping, Longyan, Sanming, Zhangzhou) in Fujian Province and analyzed for G. duodenalis prevalence and genotypes using nested PCR targeting the beta-giardin (bg), glutamate dehydrogenase (gdh) and triosephosphate isomerase (tpi) genes. The results shown that total occurrence rate of G. duodenalis was 26.9% (195/725) in pigs, with significant differences in the prevalence among different regions (χ2 = 86.508, p < 0.05) and groups (χ2 = 12.748, p < 0.05). 195, 11 and 6 samples were detected at the bg, tpi and gdh loci, respectively. Each one belonged to a subtype of assemblage E and was analyzed using sequences obtained in this study. Based on phylogenetic analyses of sequences from the three genetic loci, only one MLG E1 was found. The results indicated that pigs may present a potential zoonotic risk of spreading G. duodenalis infection from animals to humans in this area. The findings of the present study also provide basic data for the prevention and control of G. duodenalis infection in pigs and humans in China.

The Giardial Arginine Deiminase Participates in Giardia-Host Immunomodulation in a Structure-Dependent Fashion via Toll-like Receptors

Beyond the problem in public health that protist-generated diseases represent, understanding the variety of mechanisms used by these parasites to interact with the human immune system is of biological and medical relevance. Giardia lamblia is an early divergent eukaryotic microorganism showing remarkable pathogenic strategies for evading the immune system of vertebrates. Among various multifunctional proteins in Giardia, arginine deiminase is considered an enzyme that plays multiple regulatory roles during the life cycle of this parasite. One of its most important roles is the crosstalk between the parasite and host. Such a molecular "chat" is mediated in human cells by membrane receptors called Toll-like receptors (TLRs). Here, we studied the importance of the 3D structure of giardial arginine deiminase (GlADI) to immunomodulate the human immune response through TLRs. We demonstrated the direct effect of GlADI on human TLR signaling. We predicted its mode of interaction with TLRs two and four by using the AlphaFold-predicted structure of GlADI and molecular docking. Furthermore, we showed that the immunomodulatory capacity of this virulent factor of Giardia depends on the maintenance of its 3D structure. Finally, we also showed the influence of this enzyme to exert specific responses on infant-like dendritic cells.

Intestinal immune responses to commensal and pathogenic protozoa

The physical barrier of the intestine and associated mucosal immunity maintains a delicate homeostatic balance between the host and the external environment by regulating immune responses to commensals, as well as functioning as the first line of defense against pathogenic microorganisms. Understanding the orchestration and characteristics of the intestinal mucosal immune response during commensal or pathological conditions may provide novel insights into the mechanisms underlying microbe-induced immunological tolerance, protection, and/or pathogenesis. Over the last decade, our knowledge about the interface between the host intestinal mucosa and the gut microbiome has been dominated by studies focused on bacterial communities, helminth parasites, and intestinal viruses. In contrast, specifically how commensal and pathogenic protozoa regulate intestinal immunity is less well studied. In this review, we provide an overview of mucosal immune responses induced by intestinal protozoa, with a major focus on the role of different cell types and immune mediators triggered by commensal (Blastocystis spp. and Tritrichomonas spp.) and pathogenic (Toxoplasma gondii, Giardia intestinalis, Cryptosporidium parvum) protozoa. We will discuss how these various protozoa modulate innate and adaptive immune responses induced in experimental models of infection that benefit or harm the host.

Changes in Systemic Regulatory T Cells, Effector T Cells, and Monocyte Populations Associated With Early-Life Stunting

Stunting and environmental enteric dysfunction (EED) may be responsible for altered gut and systemic immune responses. However, their impact on circulating immune cell populations remains poorly characterized during early life. A detailed flow cytometry analysis of major systemic immune cell populations in 53 stunted and 52 non-stunted (2 to 5 years old) children living in Antananarivo (Madagascar) was performed. Compared to age-matched non-stunted controls, stunted children aged 2-3 years old had a significantly lower relative proportion of classical monocytes. No significant associations were found between stunting and the percentages of effector T helper cell populations (Th1, Th2, Th17, Th1Th17, and cTfh). However, we found that HLA-DR expression (MFI) on all memory CD4⁺ or CD8⁺ T cell subsets was significantly lower in stunted children compared to non-stunted controls. Interestingly, in stunted children compared to the same age-matched non-stunted controls, we observed statistically significant age-specific differences in regulatory T cells (Treg) subsets. Indeed, in 2- to 3-year-old stunted children, a significantly higher percentage of memory Treg, whilst a significantly lower percentage of naive Treg, was found. Our results revealed that both innate and adaptive systemic cell percentages, as well as activation status, were impacted in an age-related manner during stunting. Our study provides valuable insights into the understanding of systemic immune system changes in stunted children.

Conserved metabolic enzymes as vaccine antigens for giardiasis

Giardia lamblia is a leading protozoal cause of diarrheal disease worldwide. Infection is associated with abdominal pain, malabsorption and weight loss, and protracted post-infectious syndromes. A human vaccine is not available against G. lamblia. Prior studies with human and murine immune sera have identified several parasite antigens, including surface proteins and metabolic enzymes with intracellular functions. While surface proteins have demonstrated vaccine potential, they can exhibit significant variation between G. lamblia strains. By comparison, metabolic enzymes show greater conservation but their vaccine potential has not been established. To determine whether such proteins can serve as vaccine candidates, we focused on two enzymes, α-enolase (ENO) and ornithine carbamoyl transferase (OCT), which are involved in glycolysis and arginine metabolism, respectively. We show in a cohort of patients with confirmed giardiasis that both enzymes are immunogenic. Intranasal immunization with either enzyme antigen in mice induced strong systemic IgG1 and IgG2b responses and modest mucosal IgA responses, and a marked 100- to 1,000-fold reduction in peak trophozoite load upon oral G. lamblia challenge. ENO immunization also reduced the extent and duration of cyst excretion. Examination of 44 cytokines showed only minimal intestinal changes in immunized mice, although a modest increase of CCL22 was observed in ENO-immunized mice. Spectral flow cytometry revealed increased numbers and activation state of CD4 T cells in the small intestine and an increase in α4β7-expressing CD4 T cells in mesenteric lymph nodes of ENO-immunized mice. Consistent with a key role of CD4 T cells, immunization of CD4-deficient and Rag-2 deficient mice failed to induce protection, whereas mice lacking IgA were fully protected by immunization, indicating that immunity was CD4 T cell-dependent but IgA-independent. These results demonstrate that conserved metabolic enzymes can be effective vaccine antigens for protection against G. lamblia infection, thereby expanding the repertoire of candidate antigens beyond primary surface proteins.

Giardia duodenalis and Its Secreted PPIB Trigger Inflammasome Activation and Pyroptosis in Macrophages through TLR4-Induced ROS Signaling and A20-Mediated NLRP3 Deubiquitination

The extracellular protozoan parasite Giardia duodenalis is a well-known and important causative agent of diarrhea on a global scale. Macrophage pyroptosis has been recognized as an important innate immune effector mechanism against intracellular pathogens. Yet, the effects of noninvasive Giardia infection on macrophage pyroptosis and the associated molecular triggers and regulators remain poorly defined. Here we initially observed that NLRP3 inflammasome-mediated pyroptosis was activated in Giardia-treated macrophages, and inhibition of ROS, NLRP3, or caspase-1 could block GSDMD cleavage, IL-1β, IL-18 and LDH release, and the cell viability reduction. We also confirmed that Giardia-induced NLRP3 inflammasome activation was involved in its K63 deubiquitination. Thus, six candidate deubiquitinases were screened, among which A20 was identified as an effective regulator. We then screened TLRs on macrophage membranes and found that upon stimulation TLR4 was tightly correlated to ROS enhancement, A20-mediated NLRP3 deubiquitination, and pyroptotic signaling. In addition, several Giardia-secreted proteins were predicted as trigger factors via secretome analysis, of which peptidyl-prolyl cis-trans isomerase B (PPIB) independently induced macrophage pyroptosis. This was similar to the findings from the trophozoite treatment, and also led to the TLR4-mediated activation of NLRP3 through K63 deubiquitination by A20. Collectively, the results of this study have significant implications for expanding our understanding of host defense mechanisms after infection with G. duodenalis.

Giardia duodenalis: Biology and Pathogenesis

Giardia duodenalis captured the attention of Leeuwenhoek in 1681 while he was examining his own diarrheal stool, but, ironically, it did not really gain attention as a human pathogen until the 1960s, when outbreaks were reported. Key technological advances, including in vitro cultivation, genomic and proteomic databases, and advances in microscopic and molecular approaches, have led to an understanding that this is a eukaryotic organism with a reduced genome rather than a truly premitochondriate eukaryote. This has included the discovery of mitosomes (vestiges of mitochondria), a transport system with many of the features of the Golgi apparatus, and even evidence for a sexual or parasexual cycle. Cell biology approaches have led to a better understanding of how Giardia survives with two nuclei and how it goes through its life cycle as a noninvasive organism in the hostile environment of the lumen of the host intestine. Studies of its immunology and pathogenesis have moved past the general understanding of the importance of the antibody response in controlling infection to determining the key role of the Th17 response. This work has led to understanding of the requirement for a balanced host immune response that avoids the extremes of an excessive response with collateral damage or one that is unable to clear the organism. This understanding is especially important in view of the remarkable ranges of early manifestations, which range from asymptomatic to persistent diarrhea and weight loss, and longer-term sequelae that include growth stunting in children who had no obvious symptoms and a high frequency of postinfectious irritable bowel syndrome (IBS).

First case of community acquired giardiasis in Nuuk, Greenland

We report a case of community acquired giardiasis, in Nuuk, Greenland. Likely source of infection being consumption of untreated water from a local reservoir, alternatively through contact with sewage. Giardia is widespread worldwide but has not commonly been considered a cause of gastrointestinal distress in patients in Greenland, without relevant travel history. This may be due to under diagnosis, or historically low prevalence of Giardia in the region. Climate change with increasing temperatures, growing tourism and pet travel may influence the presence of Giardia in the region. This case highlights the need to include giardiasis as a differential diagnosis in patients presenting with suspected infectious gastroenteritis in Greenland.

Serglycin-Deficiency Causes Reduced Weight Gain and Changed Intestinal Cytokine Responses in Mice Infected With Giardia intestinalis

The proteoglycan serglycin (SG) is expressed by different innate and adaptive immune cells, e.g. mast cells, macrophages, neutrophils, and cytotoxic T lymphocytes, where SG contributes to correct granule storage and extracellular activity of inflammatory mediators. Here the serglycin-deficient (SG^-/-) mouse strain was used to investigate the impact of SG on intestinal immune responses during infection with the non-invasive protozoan parasite Giardia intestinalis. Young (≈11 weeks old) oral gavage-infected congenic SG^-/- mice showed reduced weight gain as compared with the infected SG^+/+ littermate mice and the PBS-challenged SG^-/- and SG^+/+ littermate mice. The infection caused no major morphological changes in the small intestine. However, a SG-independent increased goblet cell and granulocyte cell count was observed, which did not correlate with an increased myeloperoxidase or neutrophil elastase activity. Furthermore, infected mice showed increased serum IL-6 levels, with significantly reduced serum IL-6 levels in infected SG-deficient mice and decreased intestinal expression levels of IL-6 in the infected SG-deficient mice. In infected mice the qPCR analysis of alarmins, chemokines, cytokines, and nitric oxide synthases (NOS), showed that the SG-deficiency caused reduced intestinal expression levels of TNF-α and CXCL2, and increased IFN-γ, CXCL1, and NOS1 levels as compared with SG-competent mice. This study shows that SG plays a regulatory role in intestinal immune responses, reflected by changes in chemokine and cytokine expression levels and a delayed weight gain in young SG^-/- mice infected with G. intestinalis.

Giardia lamblia: Laboratory Maintenance, Lifecycle Induction, and Infection of Murine Models

Giardia lamblia is a protozoan parasite that is found ubiquitously throughout the world and is a major contributor to diarrheal disease. Giardia exhibits a biphasic lifestyle existing as either a dormant cyst or a vegetative trophozoite. Infections are typically initiated through the consumption of cyst-contaminated water or food. Giardia was first axenized in the 1970s and can be readily maintained in a laboratory setting. Additionally, Giardia is one of the few protozoans that can be induced to complete its complete lifecycle using laboratory methods. In this article, we outline protocols to maintain Giardia and induce passage through its lifecycle. We also provide protocols for infecting and quantifying parasites in an animal infection model. © 2020 Wiley Periodicals LLC. Basic Protocol 1: In vitro maintenance and growth of Giardia trophozoites Basic Protocol 2: In vitro encystation of Giardia cysts Basic Protocol 3: In vivo infections using Giardia trophozoites.

Giardia spp. and the Gut Microbiota: Dangerous Liaisons

Alteration of the intestinal microbiome by enteropathogens is commonly associated with gastrointestinal diseases and disorders and has far-reaching consequences for overall health. Significant advances have been made in understanding the role of microbial dysbiosis during intestinal infections, including infection with the protozoan parasite Giardia duodenalis, one of the most prevalent gut protozoa. Altered species composition and diversity, functional changes in the commensal microbiota, and changes to intestinal bacterial biofilm structure have all been demonstrated during the course of Giardia infection and have been implicated in Giardia pathogenesis. Conversely, the gut microbiota has been found to regulate parasite colonization and establishment and plays a critical role in immune modulation during mono and polymicrobial infections. These disruptions to the commensal microbiome may contribute to a number of acute, chronic, and post-infectious clinical manifestations of giardiasis and may account for variations in disease presentation within and between infected populations. This review discusses recent advances in characterizing Giardia-induced bacterial dysbiosis in the gut and the roles of dysbiosis in Giardia pathogenesis.

Update on Giardia: Highlights from the seventh International Giardia and Cryptosporidium Conference

Although Giardia duodenalis is recognized as one of the leading causes of parasitic human diarrhea in the world, knowledge of the mechanisms of infection is limited, as the pathophysiological consequences of infection remain incompletely elucidated. Similarly, the reason for and consequences of the very specific genome-organization in this parasite with 2 active nuclei is only partially known. Consistent with its tradition, the 7th International Giardia and Cryptosporidium Conference (IGCC 2019) was held from June 23 to 26, 2019, at the Faculty of Medicine and Pharmacy of the University of Rouen-Normandie, France, to discuss current research perspectives in the field. This renowned event brought together an international delegation of researchers to present and debate recent advances and identify the main research themes and knowledge gaps. The program for this interdisciplinary conference included all aspects of host-parasite relationships, from basic research to applications in human and veterinary medicine, as well as the environmental issues raised by water-borne parasites and their epidemiological consequences. With regard to Giardia and giardiasis, the main areas of research for which new findings and the most impressive communications were presented and discussed included: parasite ecology and epidemiology of giardiasis, Giardia-host interactions, and cell biology of Giardia, genomes and genomic evolution. The high-quality presentations discussed at the Conference noted breakthroughs and identified new opportunities that will inspire researchers and funding agencies to stimulate future research in a “one health” approach to improve basic knowledge and clinical and public health management of zoonotic giardiasis.