Praziquantel drug modulation of human immune responses: a tool for schistosome vaccine discovery and immunotherapy
Item statusRestricted Access
Embargo end date31/07/2022
Eyoh, EnwonoAbasi Bassey
Schistosomiasis is a devastating parasitic disease caused by infection with flatworms from the genus Schistosoma. Schistosoma haematobium is the most prevalent species in Africa and the Middle East and causes urogenital schistosomiasis. Chronic schistosome infection causes morbidities such as portal hypertension, hepatosplenic, ureter fibrosis, anaemia, and bladder cancer. Moreover, an estimated 1.5 million disability adjusted life years (DALYs) are lost as a result of schistosome infections. Treatment of schistosomiasis with the anthelmintic drug, praziquantel (PZQ) alters schistosome specific immune responses that can confer a level of (protective) immunity against reinfection in some individuals. Previous studies have shown that these immunological changes are due to the killing and removal of the immunosuppressive parasite and the exposure of parasite antigens to the host’s immune system. However, little is known of the direct effect of PZQ on host immune cells, specifically CD4+ T-cells and type 1 regulatory T-cells (Tr1), which are particularly important due to their protective role in schistosome infection and pathology. Thus, the objectives of this study were to use the knowledge of the effects/impact of PZQ treatment on potentially protective immune responses against S. haematobium, to identify and characterise potential S. haematobium vaccines and, to determine the effects of PZQ on human cellular immune responses in the absence of exposure to, or infection with schistosomes. To address the first objective, a bioinformatic pipeline was developed and validated for the ability to identify schistosome antigens using an in silico approach that queried parasite protein databases with an a priori selection criteria. The bioinformatic tool identified intrinsically disorder protein (IUP) domains on parasite tegument proteins. An IUP domain lacks a secondary or tertiary structure and is therefore linear. Analysis of licensed human, veterinary, and experimental parasite vaccines validated this domain as an antigenic target. Schistosoma. haematobium tegument proteins with B-cell epitopes on an IUP were selected. Peptides from this protein were screened by post-treatment schistosome-specific IgG antibodies to identify schistosome antigens for vaccine discovery. Once validated, the bioinformatic pipeline was used to identify potential vaccine candidates using sera from people putatively resistant to schistosome infection following PZQ treatment. Antigen (s) that showed preferential IgG reactivity in post-treatment DIR serum samples were selected as schistosome vaccine antigens. These antigen(s) were then evaluated in serum samples from people exposed to schistosome infection in different populations of Zimbabwe (n=268). One antigenic peptide (SNYQGHEHYDDFGHYSE) identified was from an S. haematobium band 7 protein by the bioinformatic model, and showed the highest preferential IgG reactivity in post-treatment samples of individuals who were resistant to reinfection. The antigen induced an immune profile consistent with a partial protective immune response seen in people exposed to schistosome in Zimbabwe. Immuno-epidemiological studies in schistosome endemic populations have shown antibody responses that are associated with partial protection from schistosomes. In these populations, young children with rising infections predominately produce IgA responses against schistosome antigens. These IgA responses switch to IgG1 responses as cumulative exposure to parasite antigens increases in late childhood, and to a predominately IgG1 response in adulthood as infections decline. The selected peptide antigen induced a similar antibody profile; the IgA response was highest in younger children, exhibited the same switch to an IgG1 response in the age group with the highest cumulative exposure and the IgG1 response continued to increase in adults who had developed an acquired immunity. To study the direct effects of PZQ on immune cells, the drug was used to treat CD4+ T-cells and type 1 regulatory T-cells (Tr1) from healthy volunteers unexposed to schistosome infection. Proliferation and cytokine production (IFNγ, IL-17, GM-CSF and IL-10) of CD4+ T-cells, and the effector response of Tr1 cells were measured after treatment with PZQ. PZQ treatment showed an immunomodulatory role; the drug promoted the proliferation of CD4+ T-cells and increased production of IL-17 and IL-10. PZQ also promoted Tr1 differentiation and the effector response of these cells. The inference and interpretation of the results presented in this thesis describe a novel schistosome vaccine candidate, which validates the bioinformatic pipeline developed to identify new targets, and in addition demonstrated the immunomodulatory role of PZQ, providing possible insight into the potential of the drug as an immune adjuvant or therapy for diseases such as multiple sclerosis.