Immunological Studies on Schistosomasis (Schistosoma Haematobium) Resistant Individuals in an Endemic Rural Community in Ghana

Abstract

Schistosoma haematobium is the least studied amongst the three major human schistosomes (S. haematobium, S. mansoni and S. japonicum). This is so because of the ease with which S. mansoni and S. japonicum are maintained in the laboratory as against the difficulty in maintaining the life cycle of S. haematobium in the laboratory. Therefore, work on identification of potential protective antigens, which could serve as, vaccine candidates continues to lag behind. The work reported in this thesis was aimed at identifying S. haematobium resistant individuals from an endemic community, and to determine whether they produce certain immunoglobulin classes against specific schistosome parasite antigen(s) that have the capacity to protect against disease or serve as vaccine candidate(s). From studies reported in this thesis, following water contact activity index (WCAI >300), eleven putatively resistant individuals were identified in the endemic community. Fifteen (15) water contact sites were identified in the study community. A total of 346 individuals responded to having water contact, at three major sites; the Ayensu River 95.4% (330/346), the irrigation dam 74.9% and the irrigation fields 70.5% (244/346). None of the responded indicated having water contact at the remaining twelve minor well and ponds. Snail survey showed 10 B. truncatus snails and 25 B. pfeifferi at the irrigation dam, 52 B. pfeifferi snails at the irrigation fields. Out of 346 individuals, 115 subjects (33.2%), were found have S. haematobium eggs by microscopy using the standard 10ml urine filtration, although 10 (5.7%) of them could only be revealed by sediment obtained from excess urine. Prevalence of S. haematobium was low among females of all ages compared to the males. The disease prevalence was highest at age groups (10-14yrs) for both sexes, and decreased continuously with age. The highest intensity of infections were also found in the age groups with the highest prevalence as demonstrated by the egg range and mean egg counts. A high percentage (53.1%) of individuals in the S. haematobium egg positive group had high water contact (WCAI >300), whilst a smaller percentage (44.9%) of the S. haematobium egg negatives were in this high risk category. Repeated screening of 96 S. haematobium egg negative individuals for antigen by monoclonal antibody dipstick to confirm their uninfected status revealed that, 14 out of 25 who were able to provide urine on three or more occasions tested positive at least once for antigen. Eleven individuals (11/25) 44.0% remained negative for S. haematobium antigen and were selected as putative S. haematobium resistant candidates. Resistant, susceptible and control serum samples were used in Western immunoblot analysis to reveal anti-schistosome IgG, IgA, IgE and IgM against crude adult worm and egg antigens and to detect protein bands which could have potential of being use as a vaccine candidate. SDS-PAGE analysis of S. haematobium worm and egg antigens showed more than 14 polypeptide bands with molecular weight ranging from 115kDa to 16KDa for crude worm protein extract, where egg antigen revealed only four polypeptide bands ranging from (105 - 52). Two polypeptide bands (105 and 78) kDa were common to both S. haematobium worm and egg antigens. Sera from S. haematobium resistant and susceptible individuals analysed in the western immunoblot revealed 2 bands (102 and 104) kDa that were detected by both S. haematobium susceptible and resistant sera analyzed, as well as the normal controls. These two protein bands were revealed by all the different classes of immunoglobulins (anti-schistosome IgG, IgA, IgE and IgM) that were tested. Immunoglobulins IgA, IgE and IgM in each of the sera analyzed reacted with a 32kDa antigen band, which was not detected by IgG antibodies. However, anti-schistosome IgG was the only immunoglobulin isotype that detected two higher molecular weight antigens (112 and 110). Comparatively, anti-schistosome IgG detected distinct and more protein bands in the sera analysed. A 100 kDa was detected in all the resistant sera except one by only anti-schistosome IgG. From egg antigen, anti-schistosome schistosome IgA and anti-schistosome IgG detected only one band each 42kDa and 58kDa. Respectively. IgE and IgM failed to detect any antigens in the egg extracts using sera of resistant, susceptible and controls. The extensive reactivity of anti-schistosome IgG with worm antigens, suggests that it may be important in immunity during infection. However, IgG alone may not confer complete protection. The identification of a unique band detected by resistant individual sera may also suggest possible involvement of the antigens in the band in protection against the disease. However, more work is needed to determine the potential of this protein band as vaccine candidate in schistosomiasis. In conclusion, therefore, this study shows that anti- schistosome IgG may be involved in protection against S. heamatobium infection.