Summary:

The development of naturally acquired immunity (NAI) against Plasmodium falciparum malaria is poorly understood. Previous studies of continuous and intermittent chemoprophylaxis in infants have provided evidence that the age of first exposure to P. falciparum during infancy may be important in determining the development of NAI, as measured by incidence of clinical malaria during the second year of life. These studies suggest that exposure to P. falciparum prior to five months of age does not result in the development of NAI, while exposure to P. falciparum after five months of age leads to development of NAI. The overall objective is to evaluate the effect of exposure to P. falciparum erythrocytic stage antigens during different periods of infancy on the development of NAI. In order to explore the effect of age in the build-up of NAI, a 3-arm randomised double-blind placebo-controlled trial has been designed in an endemic area of southern Mozambique in which exposure to P. falciparum is selectively controlled at different periods during infancy (2.5 to 5.5 months, 5.5 to 10.5 months or none) with monthly chemoprophylaxis with Sulphadoxine-Pyrimethamine+Artesunate. Infants will be enrolled at birth or when aged less than two months from HIV-negative women and allocated to one of three cohorts of 98 children each. Participants will be followed up by active and passive case detection until 11 months of age and by passive case detection from 11 to 24 months. Five cross-sectional surveys will be conducted to obtain blood samples. The risk of clinical malaria and anaemia during the second year of life will be compared between cohorts, as well as its correlation with the type and quality of immune responses (antibodies to several P. falciparum antigens, cytokines), oxidative stress markers and host genetic factors. These results should shed light on the determinants of the development of anti-P. falciparum responses early in life and the potential constraints to early life immunisation.

Background:

The development of a malaria vaccine has proven to be a very difficult task over the years. Understanding the mechanisms of NAI to malaria, that protects adults from death and severe disease, would help greatly in the rational design and deployment of vaccines aimed at duplicating such protection in infants. A major obstacle to this is the non-existence of a general agreement about the rate of onset of acquired immunity (or what constitutes the key determinants of protection) and a consensus regarding the mechanism(s) of protection. Among the possible key factors determining the rate of acquisition of protective immunity to malaria, there has been long-term speculation as to the role of AGE. The evidence to date is limited and difficult to interpret, but previous studies by Partner 1 indicate that gaining an insight into this issue is paramount to understanding the early development of clinical immunity. The studies proposed here are designed to provide critical information regarding at what point during infancy is the optimal time to vaccinate with P. falciparum antigens to result in adequate induction of protective immunity. With many malaria vaccines in the pipeline for future clinical trials, this is a preparatory phase of characterisation and consolidation of knowledge, crucial for moving ahead to vaccine trials. Thus, the relevance of this work is with regard to the administration of vaccines and other strategies to prevent malaria, and the design of future clinical trials of promising candidate interventions specifically targeted for use in developing countries.

Aim:

The goal of this project is to understand the mechanisms of naturally acquired immunity to malaria in infants. This will help in the rational design and deployment of candidate malaria vaccines or other malaria control tools, including the optimal timing for administration during infancy.

The overall objective is to investigate the role of the age of first exposure to Plasmodium falciparum during infancy upon the development of naturally acquired immunity (NAI) to malaria.

In order to achieve this goal and objective by month 48, the following specific objectives will be addressed:

To determine whether exposure to P falciparum erythrocytic stage antigens between 0 to 5.5 months of age, or between 5.5 to 10.5 months of age, affects NAI, measured as risk of clinical malaria between 12 and 24 months of age, as compared to infants with continuous exposure.

• To determine whether exposure to P. falciparum erythrocytic stage antigens between 0 to 5.5 months of age, or between 5.5 to 10.5 months of age, affects the type and quality of immune responses.
• To evaluate the role of oxidative stress measured as the antioxidant / pro-oxidant balance in the development of NAI in children exposed to P. falciparum erythrocytic stage antigens between 0 to 5.5 months of age, or between 5.5 to 10.5 months of age, compared to infants with continuous exposure.
• To study the host genetic factors, which influence the development of NAI to malaria in early life.
• The means to achieve the above objectives will be the carrying out of a randomised double-blind controlled trial in infants in Mozambique who will be followed up through active and passive case detection and cross-sectional studies. The endpoints will be:
• risk of clinical malaria and anaemia in the second year of life in relation to age of first exposure to infection
• quantification of antibody and cytokine responses to P. falciparum protein antigens and toxins over the first two years of life in relation to age of first exposure to infection
• quantification of the antioxidant/pro-oxidant status over the first two years of life in relation to age of first exposure to infection.

Expected results:

Determining the optimal timing for administering candidate malaria vaccines or other malaria control tools during infancy, one of the main objectives of this project, will have important implications for other international research projects and initiatives that aim at developing such vaccines or control tools.

The recent availability of increased funding and initiatives dedicated to health care problems in developing countries, including the development of candidate malaria vaccines, enlarges the likelihood of licensing a safe and effective vaccine in the next few years. However, it is very important to clarify the potential constraints to early life immunisation and determine whether immunising or administering other malaria-control tools according to the EPI (expanded programme of immunisation) schedule adequately induce a protective response or whether implementation later in infancy is more appropriate. If this hypothesis was shown to be true, and exposure to P. falciparum erythrocytic stage antigens in the first months of life is not capable of appropriately priming the neonatal immune system to develop an effective protective immunity later on, whilst exposure after 5 months of age proves to be better at inducing adequate immune responses and effective protection, then the standards for the design of trials of candidate malaria vaccines in infants would have to be re-evaluated. Eventually, this could mean having to alter the immunisation schedule for malaria vaccines accordingly, and evaluating the implications that not following the EPI scheme could have on the effectiveness of such a prevention strategy. In addition, the target population for vaccination will continue to be infants younger than six months of age, and if an immature immune system disables them from responding to vaccination, more research would have to be devoted to immune enhancement strategies (new adjuvants, delivery systems, etc.) to develop vaccine formulations that could potentially overcome the neonatal unresponsiveness.

Coordinator:

Partners:

Nº	Principal Scientific Participants	Official Address	Other Information
2	Jahit Sacarlal	Centro de Investigaçao em Saúde de Manhiça (Manhiça Health Research Centre) CP 1929 Maputo Mozambique	Tel: +258 1 810002 Fax: +258 1 810002 E-mail: jahit.sacarlal@manhica.net
3	David Roberts	University of Oxford – Blood Research Laboratory John Radcliffe Hospital Headington UK-OX3 9BQ Oxford United Kingdom	Tel: +44 1865 447971 Fax: +44 1865 447957 E-mail: david.roberts@ndcls.ox.ac.uk
4	Evelin Schwarzer	Università di Torino – Department of Genetics, Biology and Biochemistry Via Santena 5 bis IT-10126 Torino Italy	Tel: +39 011 670 5846 Fax: +39 011 670 5845 E-mail: evelin.schwarzer@unito.it
5	Louis Schofield	Walter and Eliza Hall Institute of Medical Research 1G, Royal Parade, Parkville Victoria 3050 Australia	Tel: +61 3 9345 2474 Fax: +61 3 9347 0852 E-mail: schofield@wehi.EDU.AU
6	Peter Le Souëf	The University of Western Australia School of Paediatrics and Child Health c/o Princess Margaret Hospital for Children, GPO Box D184 Perth Western Australia 6840 Australia	Tel: +61 8 9340 8173 Fax: +61 8 9388 2097 E-mail: peterles@paed.uwa.edu.au
7	Chetan E. Chitnis	International Centre for Genetic Engineering PO Box 10504 Aruna Asaf Ali Marg 110067 New Delhi India	Tel: + 91 11 26177357 / 26189358/60/61 Fax: +91 11 26162316 E-mail: cchitnis@icgeb.res.in
8	Denise L. Doolan	Malaria Program - Naval Medical Research Center 503 Robert Grant Avenue, Rm. 3W41/3W16 Silver Spring, MD 20910-7500 United States of America	Tel: +1 301 319 7575 Fax: +1 301 319 7545 E-mail: dooland@nmrc.navy.mil Website: http://www.nmrc.navy.mil