Fasciola gigantica: studies of the tegument as a basis for the developments of immunodiagnosis and vaccine.

The tegument of bile-dwelling Fasciola gigantica is the interfacing layer that helps the parasite to maintain its homeostasis, and evade the hostile environment, including the host's immune attacks. The tegument is a syncytial layer about 10 mm thick, that is formed by the fusion of cytoplasmic processes of tegument cells, whose soma lie underneath the two muscle layers. The surface of the tegument is highly folded and invaginated into numerous ridges, pits and spines, which help to increase the surface area of the tegument for the absorption and exchanging of molecules, as well as for attachment. The outer membrane covering the tegument is a trilaminate sheet about 12 nm thick, and coated with a carbohydrate-rich glycocalyx layer that also bears high negative charges. Some host molecules may also be adsorbed onto this layer. These unique characteristics enable the parasite to evade the antibody-dependent cell-mediated cytotoxicity (ADCC) reaction exerted by the host. The outer membrane and glycocalyx is continuously replaced by the reserved membrane synthesized and stored in secretory granules of tegument cells, that are transported via cell processes towards the tegument by microtubules. The basal membrane of the tegument is trilaminate and invaginated to form membrane infoldings with closely aligned mitochondria. The tegument cytoskeleton is composed of a highly cross-linked network of 4-6 nm knobby microtrabecular fibers, bundles of intermediate filaments, microtubules that splay out from the tegument cells' processes. Major proteins of the cytoskeleton are actin, paramyosin and tubulin. The flukes' antigens that can elicit strong immunological responses in animal hosts are synthesized and released mainly from the tegument and the cecum. The majority of antigens derived from the surface membrane and the tegument are of MW 97, 66, 58, 54, 47 and 14 kDa, while those released from the cecum are cysteine proteases of MW 27, 26 kDa. Monoclonal antibodies have been raised against some of these antigens, and have been employed in immunodiagnosis of the infection. From the protection conferred to animal models and the in vitro killing assays of young parasites by specific antibodies, candidate vaccines could be selected from these antigens, such as, an antioxidant enzyme, glutathione-S-transferase, the digestive enzyme cysteine proteases, the surface-tegument proteins, such as fatty acid binding protein (14 kDa), membrane proteins (at 66 kDa), as well as muscle protein paramyosin, and hemoprotein. Ongoing research have been directed at deciphering the genetic codes and the syntheses of some of these antigens by recombinant DNA technology.