Transient transfection of Echinococcus multilocularis primary cells and complete in vitro regeneration of metacestode vesicles

Abstract

A major limitation in studying molecular interactions between parasitic helminths and their hosts is the lack of suitable in vitro cultivation systems for helminth cells and larvae. Here we present a method for long-term in vitro cultivation of larval cells of the tapeworm Echinococcus multilocularis, the causative agent of alveolar echinococcosis. Primary cells isolated from cultivated metacestode vesicles in vitro showed a morphology typical of Echinococcus germinal cells, displayed an Echinococcus-specific gene expression profile and a cestode-like DNA content of ∼300 Mbp. When kept under reducing conditions in the presence of Echinococcus vesicle fluid, the primary cells could be maintained in vitro for several months and proliferated. Most interestingly, upon co-cultivation with host hepatocytes in a trans-well system, mitotically active Echinococcus cells formed cell aggregates that subsequently developed central cavities, surrounded by germinal cells. After 4 weeks, the cell aggregates gave rise to young metacestode vesicles lacking an outer laminated layer. This layer was formed after 6 weeks of cultivation indicating the complete in vitro regeneration of metacestode larvae. As an initial step toward the creation of a fully transgenic strain, we carried out transient transfection of Echinococcus primary cells using plasmids and obtained heterologous expression of a reporter gene. Furthermore, we successfully carried out targeted infection of Echinococcus cells with the facultatively intracellular bacterium Listeria monocytogenes, a DNA delivery system for genetic manipulation of mammalian cells. Taken together, the methods presented herein constitute important new tools for molecular investigations on host–parasite interactions in alveolar echinococcosis and on the roles of totipotent germinal cells in parasite regeneration and metastasis formation. Moreover, they enable the development of fully transgenic techniques in this group of helminth parasites for the first time.

Introduction

The ability to cultivate and to genetically manipulate infectious agents in vitro, such as bacteria, viruses, fungi or protozoa, has contributed decisively to our understanding of the molecular mechanisms of host–parasite/pathogen interactions. Unfortunately, comparable techniques for parasitic helminths, which infect more than two billion people world-wide, are lagging far behind those that have been established for unicellular pathogens. Principle reasons for this are the highly complex life-cycles and nutritional requirements of metazoan parasites that make the application of appropriate growth conditions in the laboratory difficult. As a consequence, in vitro cultivation systems that support the entire life-cycle of helminth parasites, long-term cultivation of helminth cells or the establishment of helminth cell lines have only been achieved in very rare cases (Coustau and Yoshino, 2000, Coyne and Brake, 2001). Furthermore, current techniques to genetically manipulate parasitic helminths are still in their infancy despite several promising approaches using RNA interference (RNAi), virus-based transfection systems or biolistics (Boyle and Yoshino, 2003, Grevelding, 2006, Brindley and Pearce, 2007, Geldhof et al., 2007). Due to the steadily increasing availability of genetic information for model organisms of all three major groups of parasitic helminths (nematodes, trematodes and cestodes), molecular helminthology is now at the transition from descriptive to functional investigations, which underscores the need to develop methods for in vitro cultivation and genetic manipulation.

One of the few helminth systems for which in vitro cultivation has been relatively successfully carried out in the past is the larval stage of the tapeworm Echinococcus multilocularis, the causative agent of alveolar echinococcosis in humans (Hemphill et al., 2002, Brehm et al., 2006). Adult stages of the cestode reside in the small intestine of foxes and other carnivores and produce embryonated eggs containing oncospheres. Infection of the intermediate host (small rodents and, occasionally, humans) results from ingestion of infective eggs, followed by penetration of the intestinal epithelium by the hatched oncosphere. Metacestodes then develop almost exclusively within the liver of the intermediate host. The metacestode vesicles are bladder-like structures with an inner, cellular and proliferative ‘germinal layer’ and an outer, acellular laminated layer that is mainly composed of high molecular mass glycans. Through continuous vesicle proliferation and exogenous budding, E. multilocularis metacestodes infiltrate the surrounding host tissue, eventually resulting in organ failure (Craig, 2003, Eckert and Deplazes, 2004, Brehm et al., 2006).

The enormous regenerative capacity of the E. multilocularis metacestode is demonstrated by the fact that metacestode tissue can be kept for years to decades in the laboratory through serial passages in the peritoneum of suitable intermediate hosts (e.g., Mongolian jirds) (Siles-Lucas and Hemphill, 2002). Such regenerative capacity is a hallmark of flatworms and has been attributed to a population of totipotent stem cells called ‘neoblasts’ in the case of free-living flatworms such as planarians or ‘germinal cells’ in case of the obligately parasitic groups (Reuter and Kreshchenko, 2004, Sanchez-Alvarado and Kang, 2005). Neoblasts (or germinal cells) are regarded to be the only flatworm cell type that is mitotically active (Reuter and Kreshchenko, 2004) and, in E. multilocularis, germinal cells are thought to contribute crucially to metacestode growth via fusion with the tegument as well as to metastasis during chronic infection (Eckert et al., 1983, Mehlhorn et al., 1983). Furthermore, evidence has been obtained that post-oncospheral development and differentiation to the metacestode stage in cestodes originates from mitotically active germinal cells that are present in the oncosphere (Sakamoto and Sugimura, 1970, Freeman, 1973, Slais, 1973, Swiderski, 1983).

Due to their decisive role in parasite development, several attempts to cultivate germinal cells in vitro from larval material of E. multilocularis or from the closely related tapeworm Echinococcus granulosus were undertaken decades ago (Fiori et al., 1988, Furuya, 1991). However, in these studies the cells which had been isolated from parasite larvae cultivated in vivo were ill-defined with respect to their origin and closely resembled host fibroblasts (Fiori et al., 1988, Howell and Matthaei, 1988, Furuya, 1991). The most successful attempt so far has been reported by Yamashita et al. (1997) who managed to maintain isolated E. multilocularis germinal cells in culture for 28 days, whereupon the parasite cells rapidly degenerated. Although a complete regeneration of metacestode tissue was observed when isolated cestode germinal cells were injected i.p. into laboratory mice (Dieckmann and Frank, 1988, Toledo et al., 1997), no such regeneration has yet been achieved in vitro. As a consequence, no method for the long-term cultivation of Echinococcus primary cells is currently available, nor have Echinococcus cell lines or transgenic techniques for this parasite been established.

As pointed out by Yamashita et al. (1997), the most challenging problems in Echinococcus in vitro cell cultivation are to find suitable conditions that promote cell proliferation whilst minimising or eliminating contamination with host cells. We have recently introduced an axenic cultivation system that allows the cultivation of metacestode vesicles that are essentially free of host cells and we have shown that E. multilocularis larvae are highly sensitive to reactive oxygen species that are formed during conventional cell culture (Spiliotis et al., 2004a). Based on this axenic cultivation system, we have now developed a method for long-term cultivation of Echinococcus primary cells that, in the presence of host hepatocytes, develop viable and infective metacestode vesicles in vitro. We also report the first successful attempt to transiently transfect Echinococcus cells and show that they can be infected by the intracellular bacterium Listeria monocytogenes, a nucleic acid carrier system used for genetic manipulation in other metazoan groups.