F Ndungu, Pwani University, Determining Cellular Correlates of Immunity to Malaria in an Experimental Human Challenge Model of Exposed Adults

Malaria, caused by Plasmodium falciparum parasites, remains a major public health problem in sub-Saharan Africa, affecting over 50% of the population. However, the current control efforts are hampered by a multiplicity of challenges including persistent threats of emerging drug and insecticide resistant parasites and mosquitoes, respectively, and logistical challenges owing to insufficient funding and poor health infrastructure. The main hope for the complete control that will bring down malaria transmission dramatically to the point where elimination is a reality is effective vaccination. However, despite decades of work, the development of an effective antimalarial vaccine has been problematic. Ironically, the substantial investment in developing anti-malaria vaccines has not been matched with similar levels of investment in understanding either the mechanisms of immunity to malaria, or the reasons the limited success with test vaccines. Although, children living with endemic malaria acquire immunity that initially protects them from severe malaria and death, and then recurring episodes of mild disease, the mechanisms that control the severity of symptoms, production of protective antibody, and parasite growth in the immune individual remain major knowledge gaps. Much of what we know today about immunity to malaria comes from animal and human field studies, which are hard to translate directly to the mechanisms that control infection inside the human body. In contrast, deliberate experimental infections of humans with malaria would give more relevant and direct information on the human immune response to infection. Our preliminary data demonstrated variable blood stage parasite growth rates following experimental infections of Kenya adults, and these rates were inversely correlated with the baseline levels of the individual's immunity, indicating that parasite growth rates can be accurate measures of the level of naturally acquired immunity. We are now conducting a prospective screen to determine correlates of antibody-based immunity following experimental human infections of Kenyan adults with different levels of baseline immunity. Because the cellular responses that confers immunity and participates in the mechanisms that produce protective antibodies are not known, I am proposing to take advantage of this unique opportunity to test which cellular responses correlate with the ability of an individual to control infection (inflammation and parasite growth) following experimental malaria infections in Kenyan adults. My study will identify patterns of immune responses that can be used as predictive markers of an individual's ability to control inflammation, produce protective levels of high quality antibodies, and to control parasite growth.

Our preliminary data demonstrated variable parasite growth rates of blood stage Plasmodium falciparum following controlled human malaria infections (CHMI) in Kenyan adults. These growth rates were inversely correlated with levels of natural immunity, indicating that parasite growth rates can be accurate measures of naturally acquired immunity, which remains poorly understood. We are conducting a prospective screen for antibody correlates of immunity, using a high throughput protein micro-array system following CHMI. Cellular immune responses also play critical roles in the control of inflammation, antibody responses, and parasite growth rates. However, it has been difficult to determine cellular correlates of immunity in field studies owing to a lack of information on when natural infections happen and environmental confounding. In contrast, CHMI provide the opportunity to sample before, during and after genetically controlled infections. I will take advantage of this CHMI antibody study and test whether cellular immune responses before and immediately after exposure to blood stage parasites during CHMI of Kenyan adults with different levels of exposure correlate with levels of the host inflammatory response, levels of P. falciparum specific antibody responses after re-exposure, and blood stage parasite growth rates. I will, 1) characterize in detail the cellular immunity prior to malaria challenge, and immediately after first exposure to malaria parasites (days 7, 9 and 14 of CHMI), 2) measure markers of the subsequent inflammatory response during parasite growth, 3) measure the antibody response after CHMI, and 4) correlate the cellular immunity observed in 1) with 2), 3) and with parasite growth rates. My study will identify patterns of cellular responses that predict an individual's ability to a) control inflammation (disease severity), b) produce protective levels of high quality antibodies, and c) control parasite growth.

A) Who will benefit from our research
1) Scientific community and vaccinologists working on malaria: The data set obtained will be of great value to other malaria immunologists, epidemiologists, and vaccine developers, who will be able to interrogate it for their particular areas of immunological or other research interest.
Importantly, the data set can also be used by any scientist to validate the results from this study in children within malaria immunology cohorts.
The results will also help focus mechanistic studies using animal models and human peripheral blood mononuclear cells in vitro towards the specific mechanisms that control inflammation, B cell memory development, and parasite multiplication rates in the immune individual.
The study will identify potential immune regulatory pathways that can be interrogated by interested immunologists from anywhere in the world.
Some of the sera and PBMC remaining from this study will be available to colleagues from other groups for further study, which would help expand the scope of our findings. Moreover, there are other research centres in Africa initiating CHMI platforms and they will be able to link their resultant data sets to our results, and this will help comparing results from different epidemiologic settings.
The identification of immune responses that can be used as predictive markers of the individual's ability to control inflammation, B cell memory (antibody) responses and parasite multiplication rates will be of great value in informing rational vaccine design, monitoring vaccine efficacy and determining indicators for individuals at risk of developing malaria upon infection. Our findings will aid in designing vaccines to target the most protective responses.
2) International organisations and institutions working in global health: Our findings will aid in designing novel control strategies that target the most protective immune responses, and can be used by those organisations such as WHO, MVI, Gates Foundation, MRC, Wellcome Trust and pharmaceutical companies involved in driving the agenda for furthering development of vaccines and therapeutic interventions.
3) Wider Research Community in UK, Kenya and Globally
The experimental approach we are undertaking would also be applicable for other infections for which curative drugs are available, but require detailed immunological analysis in humans. Just like in malaria, it is difficult to find appropriate models for many human diseases and so a lot of immunology and vaccine related work may benefit from experimental medicine. Thus the success of this programme will inform researchers on other diseases for which experimental human infections would help focus research on more relevant mechanisms.
4) Communities living with malaria: The ultimate goal for studies like this one, and the desire of scientists working in malaria immunology is to see populations living with malaria protected from disease and death with effective vaccines. Such a highly cost effective control method would dramatically bring down transmission, making elimination a reality and would benefit countries in sub Saharan Africa greatly as vaccines against malaria would not only save lives and results in healthier nations, but also increase the their economic productivity. Even though this might take years to achieve, the more we understand immunity to malaria the more we increase the possibility of making the right vaccine.
4) Pwani University and Kenya: It cannot be overstated that African universities haven't invested much in research and are lagging behind countries in the more developed parts of the world, and the involvement and part-ownership of this research by Pwani University will no doubt help inspire the realization of the ARL, the university and Kenya as in the long-term; of increasing local scientific capacity. This would be for a more informed society, knowledge generation, innovation and economic development.