Immune correlates of protection against re-infection of schistosomiasis mansoni in school children before and after mass drug administration

Abstract

The areas around Lake Victoria in Western Kenya are endemic for schistosomiasis mansoni,a disease that infects more than 200 million people worldwide, majority of whom are children. Kenya and other countries where the disease is endemic are currently carrying out mass drug administration using praziquantel targeting school children for morbidity control and reduction in prevalence. Prevalence and intensity of infection of this disease in endemic areas tends to increase with age, plateauing and eventually falling from early adulthood. It is hypothesised that this age associated prevalence and intensity of infection is a reflection of a similar age-dependent development of protective anti-schistosome immune responses. The immunological age profile of children in western Kenya infected with Schistosoma mansoni has never been determined. Mass drug administration (MDA) with praziquantel was hypothesised to alter this profile. This study therefore sought to profile immune responses associated with protection against re-infection of schistosomiasis in 6-17 year old school children, and the effect of one round of mass drug administration (MDA) with praziquantel on these responses in pre- and early teenage years. It was a repeated cross-sectional study in which a total of 387 participants were recruited, 288 at baseline and 99 1 year later from all classes of primary and secondary schools in Asembo area of Rarieda Sub-county of Siaya County in Western Kenya. Each consenting participant provided 3 consecutive stool samples for diagnosis of xv schistosomiasis by Kato-Katz and about 10 ml of blood for immunological assays. Anti-schistosome crude soluble worm antigen preparation (SWAP) and soluble egg antigen (SEA) and recombinant tegument allergen-like (TAL) antibodies were assayed by ELISA. Parasite antigen-specific cytokines were also assayed from whole blood culture supernatants by ELISA, while CD19+CD23+ B lymphocytes were determined by flow cytometry.Data was analysed using GraphPad Prism software, version 5. Parametric data was analysed by ANOVA and Tukey’s multiple comparison post-test. Kruskal Wallis test and Mann Whitney U tests were used for non-parametric data. While most of the anti-SWAP and anti-SEA antibodies did not differ significantly across the different age-groups, anti-SEA IgE and total IgG were significantly different across the age-groups (P<0.0001). Subsequent post-hoc analysis showed that the 9-11 and 12-14 year old age groups had higher anti-SEA IgE (median, 433.8 arbitrary units (AU); Interquartile range, IQR, 692.0 AU and median, 268.1; IQR 497.9 AU respectively) compared to the 15-17 year olds (median, 90.3; IQR 160.0 AU; P<0.0001. The pattern for anti-SEA IgG was quite the opposite, with 9-11 year olds producing less IgG (median, 20.6; IQR, 120.2 AU) than 15-17 year olds (median, 201.2; IQR, 488.6 AU; P<0.0001). Moreover, 12-14 year olds had less IgG antibody (median, 54.9; IQR, 177.1 AU) than 15-17 year olds (median, 201.2; IQR 488.6 AU; P<0.0001). The rest of the antibodies assayed, CD23+ B cell levels did not differ significantly with age (P>0.05). Anti-Schistosoma mansoni-Tegument Allergen-like (SmTAL) antibodies similarly did not differ significantly across the age groups (P>0.05). In post-treatment comparisons, anti-SEA IgE levels were higher in the combined post-treatment group in contrast to anti-SEA IgG. In addition, the posttreatment schistosomiasis egg positive group had higher anti-SEA IgG4 levels (median, 117.4; IQR, 553.2 AU; P<0.0001) compared to the schistosomiasis negative xvi group (median, 3.7; IQR, 37.2 AU). The same IgG4 levels against SmTAL-2 antigen was higher in the pre-treatment group (median, 16.0; IQR, 183.6 AU) relative to schistosomiasis egg negative post-treatment group (median 0.0; IQR, 1.6 AU; P<0.05). Anti-SmTAL-5 IgE was similarly reduced post-treatment. A decline-posttreatment was also observed with the proportion of CD23+ B cell levels posttreatment (mean, 61.7; SEM, 1.2) relative to pre-treatment proportions (mean, 67.4; SEM, 0.8 %; P<0.05) . For cytokines, post-hoc analysis for IL-5 showed that the schistosomiasis negative post-treatment group had higher mean levels of this cytokine (mean, 84.9; SEM, 32.1 pg) compared to mean baseline levels (mean, 6.4; SEM, 2.1 pg; P<0.0001) and schistosomiasis positive post-treatment group (mean, 21.7; SEM, 7.3 pg; P<0.001). Soluble egg antigen stimulated mean IL-13 levels were higher in the combined post-treatment group (mean, 137.8; SEM 41.7 pg) compared to mean baseline levels (mean, 19.9; SEM, 4.0 pg; P<0.05). Similarly, mean IL-13 levels produced in response to SWAP were higher in the combined follow-up group (mean, 127.0; SEM, 28.3 pg; P<0.001) and in the schistosomiasis positive post-treatment group (mean, 134.6; SEM, 42.3 pg; P< 0.05) relative to mean baseline levels (mean, 29.3; SEM, 6.2 pg) Taken together, these results are indicative of the complex nature of immune responses in individuals of comparable demographic and epidemiological conditions. It is clear from these results that a single treatment of schistosomiasis with praziquantel may not result in augmented anti-schistosome immune responses that correlate to resistance to reinfection. It remains possible, however, that multiple rounds of annual mass drug administration may have beneficial effects leading to the development of resistance in children, necessitating further longitudinal research in this area.