Entamoeba histolytica Develops Resistance to Complement Deposition and Lysis after Acquisition of Human Complement-Regulatory Proteins through Trogocytosis.

Entamoeba histolytica is the cause of amoebiasis. The trophozoite (amoeba) form of this parasite is capable of invading the intestine and can disseminate through the bloodstream to other organs. The mechanisms that allow amoebae to evade complement deposition during dissemination have not been well characterized. We previously discovered a novel complement-evasion mechanism employed by E. histolytica. E. histolytica ingests small bites of living human cells in a process termed trogocytosis. We demonstrated that amoebae were protected from lysis by human serum following trogocytosis of human cells and that amoebae acquired and displayed human membrane proteins from the cells they ingested. Here, we aimed to define how amoebae are protected from complement lysis after performing trogocytosis. We found that amoebae were protected from complement lysis after ingestion of both human Jurkat T cells and red blood cells and that the level of protection correlated with the amount of material ingested. Trogocytosis of human cells led to a reduction in deposition of C3b on the surface of amoebae. We asked whether display of human complement regulators is involved in amoebic protection, and found that CD59 was displayed by amoebae after trogocytosis. Deletion of a single complement-regulatory protein, CD59 or CD46, from Jurkat cells was not sufficient to alter amoebic protection from lysis, suggesting that multiple, redundant complement regulators mediate amoebic protection. However, exogeneous expression of CD46 or CD55 in amoebae was sufficient to confer protection from lysis. These studies shed light on a novel strategy for immune evasion by a pathogen. IMPORTANCE Entamoeba histolytica is the cause of amoebiasis, a diarrheal disease of global importance. While infection is often asymptomatic, the trophozoite (amoeba) form of this parasite is capable of invading and ulcerating the intestine and can disseminate through the bloodstream to other organs. Understanding how E. histolytica evades the complement system during dissemination is of great interest. Here, we demonstrate for the first time that amoebae that have performed trogocytosis (nibbling of human cells) resist deposition of the complement protein C3b. Amoebae that have performed trogocytosis display the complement-regulatory protein CD59. Overall, our studies suggest that acquisition and display of multiple, redundant complement regulators is involved in amoebic protection from complement lysis. These findings shed light on a novel strategy for immune evasion by a pathogen. Since other parasites use trogocytosis for cell killing, our findings may apply to the pathogenesis of other infections.

Establishment of quantitative RNAi-based forward genetics in Entamoeba histolytica and identification of genes required for growth.

While Entamoeba histolytica remains a globally important pathogen, it is dramatically understudied. The tractability of E. histolytica has historically been limited, which is largely due to challenging features of its genome. To enable forward genetics, we constructed and validated the first genome-wide E. histolytica RNAi knockdown mutant library. This library allows for Illumina deep sequencing analysis for quantitative identification of mutants that are enriched or depleted after selection. We developed a novel analysis pipeline to precisely define and quantify gene fragments. We used the library to perform the first RNAi screen in E. histolytica and identified slow growth (SG) mutants. Among genes targeted in SG mutants, many had annotated functions consistent with roles in cellular growth or metabolic pathways. Some targeted genes were annotated as hypothetical or lacked annotated domains, supporting the power of forward genetics in uncovering functional information that cannot be gleaned from databases. While the localization of neither of the proteins targeted in SG1 nor SG2 mutants could be predicted by sequence analysis, we showed experimentally that SG1 localized to the cytoplasm and cell surface, while SG2 localized to the cytoplasm. Overexpression of SG1 led to increased growth, while expression of a truncation mutant did not lead to increased growth, and thus aided in defining functional domains in this protein. Finally, in addition to establishing forward genetics, we uncovered new details of the unusual E. histolytica RNAi pathway. These studies dramatically improve the tractability of E. histolytica and open up the possibility of applying genetics to improve understanding of this important pathogen.

Biting Off What Can Be Chewed: Trogocytosis in Health, Infection, and Disease.

Trogocytosis is part of an emerging, exciting theme of cell-cell interactions both within and between species, and it is relevant to host-pathogen interactions in many different contexts. Trogocytosis is a process in which one cell physically extracts and ingests "bites" of cellular material from another cell. It was first described in eukaryotic microbes, where it was uncovered as a mechanism by which amoebae kill cells. Trogocytosis is potentially a fundamental form of eukaryotic cell-cell interaction, since it also occurs in multicellular organisms, where it has functions in the immune system, in the central nervous system, and during development. There are numerous scenarios in which trogocytosis occurs and an ever-evolving list of functions associated with this process. Many aspects of trogocytosis are relevant to microbial pathogenesis. It was recently discovered that immune cells perform trogocytosis to kill Trichomonas vaginalis parasites. Additionally, through trogocytosis, Entamoeba histolytica acquires and displays human cell membrane proteins, enabling immune evasion. Intracellular bacteria seem to exploit host cell trogocytosis, since they can use it to spread from cell to cell. Thus, a picture is emerging in which trogocytosis plays critical roles in normal physiology, infection, and disease.

Trogocytosis by Entamoeba histolytica Mediates Acquisition and Display of Human Cell Membrane Proteins and Evasion of Lysis by Human Serum.

We previously showed that Entamoeba histolytica kills human cells through a mechanism that we termed trogocytosis ("trogo-" means "nibble"), due to its resemblance to trogocytosis in other organisms. In microbial eukaryotes like E. histolytica, trogocytosis is used to kill host cells. In multicellular eukaryotes, trogocytosis is used for cell killing and cell-cell communication in a variety of contexts. Thus, nibbling is an emerging theme in cell-cell interactions both within and between species. When trogocytosis occurs between mammalian immune cells, cell membrane proteins from the nibbled cell are acquired and displayed by the recipient cell. In this study, we tested the hypothesis that through trogocytosis, amoebae acquire and display human cell membrane proteins. We demonstrate that E. histolytica acquires and displays human cell membrane proteins through trogocytosis and that this leads to protection from lysis by human serum. Protection from human serum occurs only after amoebae have undergone trogocytosis of live cells but not phagocytosis of dead cells. Likewise, mutant amoebae defective in phagocytosis, but unaltered in their capacity to perform trogocytosis, are protected from human serum. Our studies are the first to reveal that amoebae can display human cell membrane proteins and suggest that the acquisition and display of membrane proteins is a general feature of trogocytosis. These studies have major implications for interactions between E. histolytica and the immune system and also reveal a novel strategy for immune evasion by a pathogen. Since other microbial eukaryotes use trogocytosis for cell killing, our findings may apply to the pathogenesis of other infections.IMPORTANCEEntamoeba histolytica causes amoebiasis, a potentially fatal diarrheal disease. Abscesses in organs such as the liver can occur when amoebae are able to breach the intestinal wall and travel through the bloodstream to other areas of the body. Therefore, understanding how E. histolytica evades immune detection is of great interest. Here, we demonstrate for the first time that E. histolytica acquires and displays human cell membrane proteins by taking "bites" of human cell material in a process named trogocytosis ("trogo-" means "nibble"), and that this allows amoebae to survive in human serum. Display of acquired proteins through trogocytosis has been previously characterized only in mammalian immune cells. Our study suggests that this is a more general feature of trogocytosis not restricted to immune cells and broadens our knowledge of eukaryotic biology. These findings also reveal a novel strategy for immune evasion by a pathogen and may apply to the pathogenesis of other infections.

Controlled Human Malaria Infection Leads to Long-Lasting Changes in Innate and Innate-like Lymphocyte Populations.

Animal model studies highlight the role of innate-like lymphocyte populations in the early inflammatory response and subsequent parasite control following Plasmodium infection. IFN-γ production by these lymphocytes likely plays a key role in the early control of the parasite and disease severity. Analyzing human innate-like T cell and NK cell responses following infection with Plasmodium has been challenging because the early stages of infection are clinically silent. To overcome this limitation, we examined blood samples from a controlled human malaria infection (CHMI) study in a Tanzanian cohort, in which volunteers underwent CHMI with a low or high dose of Plasmodium falciparum sporozoites. The CHMI differentially affected NK, NKT (invariant NKT), and mucosal-associated invariant T cell populations in a dose-dependent manner, resulting in an altered composition of this innate-like lymphocyte compartment. Although these innate-like responses are typically thought of as short-lived, we found that changes persisted for months after the infection was cleared, leading to significantly increased frequencies of mucosal-associated invariant T cells 6 mo postinfection. We used single-cell RNA sequencing and TCR αß-chain usage analysis to define potential mechanisms for this expansion. These single-cell data suggest that this increase was mediated by homeostatic expansion-like mechanisms. Together, these data demonstrate that CHMI leads to previously unappreciated long-lasting alterations in the human innate-like lymphocyte compartment. We discuss the consequences of these changes for recurrent parasite infection and infection-associated pathologies and highlight the importance of considering host immunity and infection history for vaccine design.

Silymarin suppresses basal and stimulus-induced activation, exhaustion, differentiation, and inflammatory markers in primary human immune cells.

Silymarin (SM), and its flavonolignan components, alter cellular metabolism and inhibit inflammatory status in human liver and T cell lines. In this study, we hypothesized that SM suppresses both acute and chronic immune activation (CIA), including in the context of HIV infection. SM treatment suppressed the expression of T cell activation and exhaustion markers on CD4+ and CD8+ T cells from chronically-infected, HIV-positive subjects. SM also showed a trend towards modifying CD4+ T cell memory subsets from HIV+ subjects. In the HIV-negative setting, SM treatment showed trends towards suppressing pro-inflammatory cytokines from non-activated and pathogen-associated molecular pattern (PAMP)-activated primary human monocytes, and non-activated and cytokine- and T cell receptor (TCR)-activated mucosal-associated invariant T (MAIT) cells. The data suggest that SM elicits broad anti-inflammatory and immunoregulatory activity in primary human immune cells. By using novel compounds to alter cellular inflammatory status, it may be possible to regulate inflammation in both non-disease and disease states.

Distinct activation thresholds of human conventional and innate-like memory T cells.

Conventional memory CD8+ T cells and mucosal-associated invariant T cells (MAIT cells) are found in blood, liver, and mucosal tissues and have similar effector potential following activation, specifically expression of IFN-γ and granzyme B. To better understand each subset's unique contributions to immunity and pathology, we interrogated inflammation- and TCR-driven activation requirements using human memory CD8+ T and MAIT cells isolated from blood and mucosal tissue biopsies in ex vivo functional assays and single cell gene expression experiments. We found that MAIT cells had a robust IFN-γ and granzyme B response to inflammatory signals but limited responsiveness when stimulated directly via their TCR. Importantly, this is not due to an overall hyporesponsiveness to TCR signals. When delivered together, TCR and inflammatory signals synergize to elicit potent effector function in MAIT cells. This unique control of effector function allows MAIT cells to respond to the same TCR signal in a dichotomous and situation-specific manner. We propose that this could serve to prevent responses to antigen in noninflamed healthy mucosal tissue, while maintaining responsiveness and great sensitivity to inflammation-eliciting infections. We discuss the implications of these findings in context of inflammation-inducing damage to tissues such as BM transplant conditioning or HIV infection.

MAST: a flexible statistical framework for assessing transcriptional changes and characterizing heterogeneity in single-cell RNA sequencing data.

Single-cell transcriptomics reveals gene expression heterogeneity but suffers from stochastic dropout and characteristic bimodal expression distributions in which expression is either strongly non-zero or non-detectable. We propose a two-part, generalized linear model for such bimodal data that parameterizes both of these features. We argue that the cellular detection rate, the fraction of genes expressed in a cell, should be adjusted for as a source of nuisance variation. Our model provides gene set enrichment analysis tailored to single-cell data. It provides insights into how networks of co-expressed genes evolve across an experimental treatment. MAST is available at https://github.com/RGLab/MAST .

Lentivirus-mediated Gene Transfer in Hematopoietic Stem Cells Is Impaired in SHIV-infected, ART-treated Nonhuman Primates.

Recent studies have demonstrated that genetically modified hematopoietic stem cells (HSCs) can reduce HIV viremia. We have developed an HIV/AIDS-patient model in Simian/human immunodeficiency virus (SHIV)-infected pigtailed macaques that are stably suppressed on antiretroviral therapy (ART: raltegravir, emtricitabine and tenofovir). Following SHIV infection and ART, animals undergo autologous HSC transplantation (HSCT) with lentivirally transduced cluster of differentiation (CD)34(+) cells expressing the mC46 anti-HIV fusion protein. We show that SHIV(+), ART-treated animals had very low gene marking levels after HSCT. Pretransduction CD34(+) cells contained detectable levels of all three ART drugs, likely contributing to the low gene transfer efficiency. Following HSCT recovery and the cessation of ART, plasma viremia rebounded, indicating that myeloablative total body irradiation cannot completely eliminate viral reservoirs after autologous HSCT. The kinetics of recovery following autologous HSCT in SHIV(+), ART-treated macaques paralleled those observed following transplantation of control animals. However, T-cell subset analyses demonstrated a high percentage of C-C chemokine receptor 5 (CCR5)-expressing CD4(+) T-cells after HSCT. These data suggest that an extended ART interruption time may be required for more efficient lentiviral transduction. To avoid complications associated with ART interruption in the context of high percentages of CD4(+)CCR5(+)T-cells after HSCT, the use of vector systems not impaired by the presence of residual ART may also be beneficial.

Robust suppression of env-SHIV viremia in Macaca nemestrina by 3-drug ART is independent of timing of initiation during chronic infection.

BACKGROUND: Nonhuman primates (NHPs) are an important model organism for studies of HIV pathogenesis and preclinical evaluation of anti-HIV therapies. The successful translation of NHP-derived data to clinically relevant anti-HIV studies will require better understanding of the viral strains and NHP species used and their responses to existing antiretroviral therapies (ART). METHODS: Five pigtailed macaques (Macaca nemestrina) were productively infected with the SIV/HIV chimeric virus SHIV-1157 ipd3N4 following intravenous challenge. After 8 or 27 weeks, ART (PMPA, FTC, raltegravir) was initiated. Viral load, T-cell counts, and production of SHIV-specific antibodies were monitored throughout the course of infection and ART. RESULTS: ART led to a rapid and sustained decrease in plasma viral load. Suppression of plasma viremia by ART was independent of the timing of initiation during chronic infection. CONCLUSIONS: We present a new NHP model of HIV infection on antiretroviral therapy, which should prove applicable to multiple clinically relevant anti-HIV approaches.