Malaria, a disease caused by the parasite Plasmodium, is one of the world's major infectious diseases. With approximately 627.000 deaths a year, there is desperate need for an effective vaccine. Though a number of vaccine-candidates have been developed, they have yet to achieve the level of efficacy necessary to eliminate malaria. It has been shown previously that healthy human volunteers bitten by malaria-infected mosquitoes while taking chloroquine, medicine that prevents malaria, are fully protected against a subsequent malaria challenge. This is called CPS-immunization. The unprecedented effectiveness of CPS-immunization makes it a good model to identify what immune responses protect against malaria, to further guide vaccine development. In this study we will use CPS-immunization to induce protection against malaria in healthy subjects and then analyse their immune response to a malaria challenge infection.

Rationale:

Malaria, a disease caused by the parasite Plasmodium, is one of the world's major infectious diseases. With approximately 627.000 deaths a year, it is both a chief cause of morbidity and mortality as well as a significant contribution to ongoing poverty in endemic countries. Ultimately, the key to malaria control and hopefully eradication, would be an effective vaccine. Though a number of vaccine-candidates have entered the pipeline of pre-clinical and clinical development, they have yet to achieve the level of efficacy necessary for effective malaria prevention. It has been shown previously that healthy human volunteers can be fully protected against malaria infection with a homologous parasite by immunization with Plasmodium parasites while taking chloroquine chemoprophylaxis (ChemoProphylaxis and Sporozoites, CPS-immunization). The unprecedented efficacy of CPS-immunization makes it a unique model to identify pre-erythrocytic target antigens for the development of a subunit vaccine. Identification of antigens that play a significant role in the development of sterile protection against malaria will provide a basis for the development and evaluation of more effective sub-unit candidate vaccines.

Primary objective:

• To delineate the antibody repertoire directed against the pre-erythrocytic stages of Plasmodium falciparum induced by CPS-immunization.

Secondary objectives:

• To assess the functionality of CPS-immunization induced antibodies.
• To determine T-cell antigen specificities in CPS-immunized, protected volunteers.

Exploratory objectives:

• To assess the functionality of CPS-immunization induced T-cells.
• To explore the adaptive and innate immune responses during CPS-immunization and early malaria infection.

Study design:

This is a single-centre, randomized open-label study. A total of 15 volunteers will be divided into two groups, one scheduled to receive CPS immunization (Group 1, n=10) and one to receive only chloroquine prior to malaria challenge (Group 2, n=5).

Study population:

The study population will be comprised of male and female healthy subjects. A total of 15 subjects will be enrolled to participate in the study and randomized.

Intervention:

In the immunization group a total of four CPS immunizations will be performed, with 15 bites from Plasmodium infected mosquitoes per immunization, over a period of four months, during which volunteers will take chloroquine prophylaxis. The control group will take only chloroquine prophylaxis during this period. All volunteers will undergo Controlled Human Malaria Infection (CHMI) by exposure to 5 bites from Plasmodium falciparum sporozoite infected mosquitoes.

Inclusion Criteria:

1. Subject is aged ≥ 18 and ≤ 35 years and in good health.
2. Subject has adequate understanding of the procedures of the study and agrees to abide thereby.
3. Subject is able to communicate well with the investigator, is available to attend all study visits, lives in proximity to the trial centre or is willing to stay in a hotel close to the trial centre during part of the study (day 5 post-infection until three days post-treatment). The subject will remain within the Netherlands during the challenge period, not travel to a malaria-endemic area during the study period, and is reachable (24/7) by mobile telephone throughout the entire study period.
4. Subject agrees to inform his/her general practitioner (GP) about participation in the study and to sign a request to release by the GP any relevant medical information concerning possible contra-indications for participation.
5. Subject agrees to refrain from blood donation to Sanquin or for other purposes throughout the study period and for a defined period thereafter according to Sanquin guidelines.
6. For female subjects: agrees to use adequate contraception and not to breastfeed for the duration of study.
7. Subject has signed informed consent.

Exclusion Criteria:

1. Any history, or evidence at screening, of clinically significant symptoms, physical signs or abnormal laboratory values suggestive of systemic conditions, which could compromise the health of the volunteer during the study or interfere with the interpretation of the study results. These include, but are not limited to, any of the following:

   1.1 Body weight <50 kg or Body Mass Index (BMI) <18.0 or >30.0 kg/m2 at screening.

   1.2 A heightened risk of cardiovascular disease, as determined by: an estimated ten year risk of fatal cardiovascular disease of ≥5% at screening, as determined by the Systematic Coronary Risk Evaluation; history, or evidence at screening, of clinically significant arrhythmia's, prolonged QT-interval or other clinically relevant ECG abnormalities; or a positive family history of cardiac events in 1st or 2nd degree relatives <50 years old.

   1.3 Functional asplenia, sickle cell trait/disease, thalassaemia trait/disease or G6PD deficiency.

   1.4 History of epilepsy in the period of five years prior to study onset. 1.5 Positive Human Immunodeficiency Virus (HIV), Hepatitis B Virus (HBV) or Hepatitis C Virus (HCV) screening tests.

   1.6 Chronic use of i) immunosuppressive drugs, ii) antibiotics, iii) or immune modifying drugs within three months prior to study onset (inhaled and topical corticosteroids and oral anti-histamines exempted) or expected use of such during the study period.

   1.7 History of malignancy of any organ system (other than localized basal cell carcinoma of the skin), treated or untreated, in the past 5 years.

   1.8 Any history of treatment for severe psychiatric disease in the past year. 1.9 History of drug or alcohol abuse one year prior to study onset, or positive urine toxicology test for cocaine or amphetamines at screening or prior to infection.

2. Females: positive urine pregnancy test at screening or prior to infection.

3. Any history of malaria, positive serology for P. falciparum, or previous participation in any malaria study.

4. Known hypersensitivity to or contra-indications to any antimalarials, or history of severe reactions to mosquito bites.

5. Receipt of any vaccinations in the 3 months prior to the start of the study or plans to receive any other vaccinations during the study period or up to 8 weeks thereafter.

6. Participation in any other clinical study in the 30 days prior to the start of the study or during the study period.

7. Being an employee or student of the department of Medical Microbiology of the Radboudumc or the department of Internal Medicine.

8. Any other condition or situation that would, in the opinion of the investigator, place the subject at an unacceptable risk of injury or render the subject unable to meet the requirements of the protocol.