Localisation of host-protective antigens of taeniid cestodes

© 2010 Dr. Abdul JabbarA number of recombinant vaccines have been developed against taeniid cestodes of veterinary and medical importance. These vaccine antigens are associated with the oncospheral life cycle stage but almost nothing is known about the location of these antigens in the oncosphere. The aim of this thesis was to use immunohistochemical methods to locate host-protective antigens in developing and mature oncospheres and early developmental stages of metacestodes. For this purpose, comprehensive investigations were undertaken with Taenia ovis and additional studies were also performed on other taeniid species. Immunohistochemistry (immunoperoxidase and immunofluorescence) in light microscopy revealed that the antigens of T. ovis (To16, To18 and To45W) were initially seen as diffuse staining in the developing oncospheres but in mature oncospheres the cytoplasm of four distinct cells stained positively for the antigens. Confocal immunofluorescence microscopy revealed that each of the antigens co-localises within the same cells in the oncosphere. In situ hybridisation also showed that the location of transcripts of genes encoding three antigens of T. ovis was in four distinct cells. During development of the parasites in in vitro culture, surface localisation of the proteins was seen in parasites after three (To18) or seven (To16 and To45) days in culture. All three antigens were found to be completely absent in parasites by 15 days of culture. Based on serial sections, ultrastructural reconstruction of the T. ovis non-activated oncosphere identified two novel cell types which were named as a median mesophoric gland cell and a pair of median somatophoric cells. In addition, two types of penetration gland (PG1 and PG2) cells were identified in T. ovis oncospheres. Ultrastructural localisation revealed that the all three antigens of T. ovis were present predominantly in the secretory granules and cytoplasm of four cell bodies of the PG1 and PG2 cells. In addition, this method revealed that the three antigens of T. ovis were found in the penetration gland secretions following the activation of the oncospheres. Like T. ovis, the EG95 vaccine antigen was also found to be present predominantly in the secretory granules and cytoplasm of the PG1 cell of Echinococcus granulosus oncospheres. However, a variation in the location of host-protective antigens was seen in Taenia saginata and Taenia solium oncospheres where some antigens were predominantly present in the PG2 cell. No positive staining was observed on the surface of non-activated or recently activated oncospheres of all the studied taeniid species. The findings of this thesis suggest that initially the invading oncospheres are not susceptible to vaccine-induced antibody and complement mediated attack, but that as the parasites mature, the host-protective antigens are associated with the parasite’s surface, rendering them susceptible to immune attack. Further studies are required to find out when precisely the host-protective antigens become associated with the oncospheral surface of different taeniid species and what role these antigens play in the biology of oncospheres