Population differences in vaccine response: the role, reversibility and mediators of immunomodulation by chronic parasitic infections in the tropics

Infectious diseases continue to have major detrimental impacts on health and development in in low-income countries (LICs). They are also pose a global threat, as shown by recent Ebola and Zika epidemics. Vaccines are potent weapons against them, and a potential solution to emerging antibiotic resistance. However, some important vaccines have lower efficacy, or induce weaker immune responses, in tropical LICs and in rural, compared to urban, settings. An important example is BCG (used to protect against tuberculosis [TB]) which provides 80% protection in some temperate countries, but 0% in some tropical settings. New vaccines (including for TB, malaria and Ebola) seem also to be affected. Our goal is to understand why this is so.

Greater exposure to parasites, such as worms and malaria, is one possible explanation, and addressed in this proposal. Parasites have evolved over millennia to control host immune responses so that they can survive and reproduce, sometimes for decades, if left untreated. It has long been suggested that these mechanisms might spill-over to impair responses to vaccines and to unrelated infections, but it is not yet clear to what extent this is so. We plan to address this among adolescents in Uganda. Parasites are very common in this age group, which is also the target for school-based immunisation programmes.

First, we will compare vaccine responses between three groups: (1) urban-dwellers participating in our Entebbe Mother and Baby Study birth cohort [we have followed up these children from birth and know that they have low parasite exposure]; (2) island communities where over 80% have schistosomiasis [a worm infection transmitted through snails in the lake]; (3) rural communities with high malaria exposure, where over 50% of school-children have malaria infection, without knowing it.

Of course, differences between urban and rural people, other than parasite infections, probably influence vaccine response. So, to obtain stronger evidence that parasites have an effect, we will randomly select half the participants in each rural group to receive intensive treatment for the main parasite in their setting. If this alters the vaccine responses (we predict it will improve them), we can be sure that parasites are involved.

Some parasite effects may be indirect. As well as parasites (from the biological kingdom "Animalia"), humans host many bacteria and viruses. Interactions between these major life-forms, within the host, have been termed "transkingdom" effects. For example, immune suppression by parasites can activate dormant viruses which, in turn, may add immunological effects. Also, worms damage the intestinal lining. This causes leakage of bacterial products into the blood, stimulating the immune system. So, we will test whether parasite infections and their treatment change levels of viral replication and of bacterial products in the blood, and relate this, also, to vaccine responses.

Vaccines are given to deliver lasting protective immune responses against specific infections. Thus, parasite effects on vaccine responses must act via the host immune system. We will use immunological tools, including the cutting-edge method "mass cytometry" which can examine immune cell types in unprecedented detail, to investigate which cells and mediators are altered by parasites.

To bring all our work together, we will undertake a statistical approach called "causal mediation analysis" to explore how urban-rural environment, parasites, "transkingdom" effects and immune responses interrelate to determine vaccine responses.

This fundamental information will contribute to the development of suitable vaccines for populations living in low-income, tropical settings, where they are greatly needed; and help public health experts to know whether controlling parasites will also improve effectiveness of vaccine programmes: ultimately leading to better health (and greater wealth) for all.

We propose that parasite infections contribute substantially to population differences in vaccine response; and that their effects are mediated partly by "transkingdom" pathways (activation of herpesviruses; intestinal translocation of microbial products), and ultimately by pre-immunisation immune characteristics of the host.

We will test this hypothesis through four linked objectives among Ugandan adolescents. An immunisation programme comprising relevant live and inert vaccines will be given over one school year, with primary endpoints one month post-immunisation. A secondary endpoint at one year will assess response waning.

1. We will compare vaccine response profiles in urban adolescents (low parasite burden) with two rural cohorts, chosen for high schistosomiasis and high malaria prevalence.

2. We will establish whether current parasite infections have causal effects using individually-randomised, placebo-controlled interventions targeting the dominant infection in each rural cohort. Sample sizes will be robust, powered to detect vaccine response differences of 0.14log10 between study arms (modest effects compared to preliminary data).

3. We will assess herpesvirus-specific antibodies (Luminex), viral loads (droplet digital PCR) and cellular responses (ELISpot); and microbial translocation (MT; PCR for bacterial 16s ribosomal DNA, ELISA for lipopolysaccharide and other biomarkers). Markers of viral activation and MT will be related to parasite exposures and vaccine outcomes.

4. We will investigate pre-immunisation immunological characteristics using simple biomarkers and cell phenotyping, and with in-depth studies (including by mass cytometry) in smaller, representative groups; and link findings to both parasite exposures and vaccine outcomes.

Our data will be integrated using causal mediation analyses to determine how urban-rural environment, parasites, "transkingdom" effects and immune responses relate to determine vaccine responses.

The beneficiaries of this research will be
- Academics
- Research participants, their schools and communities
- The research team
- Public health policy makers
- Populations of low- and middle-income countries
- Populations of high-income countries

ACADEMICS, including parasitologists, vaccinologists and epidemiologists will obtain immediate benefit from the results which will answer current questions on the drivers and mediators of population differences in vaccine responses. As detailed under "Academic Beneficiaries" the results will provide evidence to guide vaccine development (in the medium term; with commercial implications in the longer term), and inform strategies for future testing of vaccines in LICs. As well, an important archive will be developed providing opportunities for additional studies.

RESEARCH PARTICIPANTS, including individual participants, schools and communities, will obtain immediate and lasting benefit from the provision of relevant interventions (treatment of parasites and vaccinations) only some of which are currently freely available through the Ministry of Health. As well, they will receive education about parasites and vaccines resulting in improved health awareness for individuals, families and communities.

THE RESEARCH TEAM will obtain immediate and lasting benefit from training opportunities, building on current skills and introducing new skills. Ugandan, as well as UK, staff will extend their knowledge and experience through conference attendance. A Ugandan post-doctoral scientist will travel to Leiden to learn the technique mass cytometry (CyToF: Cytometry Time of Flight), currently emerging internationally as a novel tool for in-depth investigation of cellular immune responses: he will learn to perform and analyse experiments, and will participate in evaluating its potential to address questions of importance to LICs.

PUBLIC HEALTH POLICY MAKERS, including the Ugandan Ministry of Health and Ministry of Education and internationally WHO, will obtain immediate benefit from results regarding the impact of current and past parasite infection on vaccine responses and the degree to which effects are reversed by intensive intervention in the short term. This will help to inform policy regarding combined vaccine, malaria and helminth intervention programmes for schools.

Our ultimate goal is that POPULATIONS OF LOW- AND MIDDLE-INCOME COUNTRIES will benefit from improved vaccines (perhaps tailor-made for the environment), resulting in a reduced burden of infectious diseases, enhanced human capital and improved economic development. An area of particular benefit may be the development of effective vaccines for endemic infections such as HIV, tuberculosis and malaria. For these infections, vaccine development so far has been challenging and vaccine responses impaired, or vaccine efficacy modest in tropical LICs. Effective new vaccines are also needed in LICs for emerging and re-emerging viral infections, such as Ebola and Zika, and for infections (such as bacterial causes of gastroenteritis) which are prone to the development of drug resistance.

POPULATIONS OF HIGH INCOME COUNTRIES will also, in the long-term, benefit from improved vaccines, as well as from a reduced risk from endemic, emerging and re-emerging infectious diseases with reservoirs in low- and middle-income countries.