Whole parasite vaccination against malaria - status today and challenges for the future

Bakgrund: Malaria är en av de allvarligaste infektionssjukdomarna i världen. De allvarligaste malariafallen orsakas främst av Plasmodium falciparum som sprids av Anopheles-myggor. Ett vaccin med långvarigt och potent skydd skulle kunna minska dödligheten, men också minska behovet av kontrollåtgärder och problemet med läkemedelsresistens. Subenhetsvaccin är den vaccintyp som kommit längst i kliniska studier men dessa uppvisar begränsad effekt. Helparasitvaccin ger en bredare immunitet vilket kan ge ett mer fullständigt skydd. Syfte: Syftet med denna litteraturstudie var att jämföra effekt och varaktighet i skydd mot P. falciparum hos de två P. falciparum sporozoit (PfSPZ)-helparasitvaccinkandidaterna: RAS (Radiation-attenuated sporozoites) och CPS (Chemoprophylaxis and sporozoites), samt att undersöka betydelsen av vaccindos och administreringssätt. Metod: Arbetet är en litteraturstudie baserat på 14 vetenskapliga studier vilka har erhållits via sökning i PubMed. De aspekter som avhandlas är: vaccineffekt och dess varaktighet, immunsvar och dess korrelation till vaccineffekt, betydelsen av dos och administreringssätt samt vaccinens säkerhetsprofil. Resultat: Litteraturstudien visade att RAS-vaccin och CPS-vaccin kan ge ett potent samt säkert kort- och långvarigt skydd mot homolog kontrollerad human malaria-infektion (CHMI) vid immunisering via myggor eller venös inokulation. Dosen har stor betydelse för vaccineffekten och CPS-vaccin kan uppnå potent skydd vid mycket lägre doser än RAS-vaccin. En del immunmekanismer har visat sig korrelera med skydd men CD8+ T-celler i levern verkar ha störst betydelse för långvarigt sterilt skydd. Det långvariga skyddet mot heterolog kontra homolog CHMI är bristfälligt för både RAS-vaccin och CPS-vaccin. Slutsats: En potent vaccineffekt uppnås med PfSPZ-vaccin mot homolog CHMI vid tillräckligt hög dos, men inte ett långvarigt skydd mot heterolog CHMI vilket begränsar användningen i endemiska områden.Background: Malaria is still one of the most common infectious diseases in the world and there is an overwhelming threat to the development of resistance to different control methods such as drugs and insecticides. A durable vaccine with sterile protection would reduce and maybe eradicate the disease. The most serious cases of malaria are caused by Plasmodium falciparum that is transmitted through the bites of infected Anopheles mosquitoes. The life cycle of malaria is extremely complex and different vaccine candidates have effects at different stages. Naturally acquired immunity develops gradually after many years of clinical episodes but never becomes sterile. RTS,S is the only vaccine candidate who has been in phase III clinical trials. Unfortunately this vaccine has limited efficacy, like many other subunit vaccines, due to rapidly diminishing antibody titers. Whole parasite vaccines have the ability to generate a greater quantity and breadth of antigenic exposure within both the humoral and cellular immunity. This results in stronger immune response and can provide sterile protection. The development of whole parasite vaccines has mainly focused on the pre-erythrocytic stage and the most tested vaccine candidates that are in early clinical trial are radiation-attenuated sporozoites (RAS), chemoprophylaxis and sporozoites (CPS) and genetically attenuated parasites (GAP). Aim: The purpose of this literature study is to examine and compare the vaccine efficacy and durability towards P. falciparum of the two whole parasite vaccine candidates: RAS and CPS and to examine the importance of dose and different routes of administration. Methods: Fourteen different clinical studies were selected from PubMed to be included in this literature study. Different variables were selected for study: the vaccine efficacy and it´s durability after controlled human malaria infection (CHMI) using P. falciparum parasites homologous or heterologous to the vaccine strain, the correlation between the immunogenicity and protection, the importance of the dose and different kinds of administration and vaccine safety. Results: According to the findings in the literature study, direct venous inoculation of RAS-vaccine and CPS-vaccine have the ability to give short and longlasting protection against CHMI using P. falciparum parasites homologous to the vaccine strain. The dose is of great importance to the vaccine efficacy and CPS-vaccine has the ability to give potent protection with much lower doses than RAS-vaccine. Some immune mechanisms in the blood correlate with protection but it seems to be the number of CD8+ T-cells in the liver that are of greatest importance for longlasting and steril protection. Whole parasite vaccines are safe but transient parasitemia is common when using CPS-vaccine. Unfortunately, vaccines with longlasting protection against CHMI using P. falciparum parasites heterologous to the vaccine strain has limited efficacy. Conclusion: RAS-vaccine and CPS-vaccine have the ability to give a potent vaccine efficacy against CHMI using P. falciparum parasites homologous to the vaccine strain when used in sufficiently high doses. Longterm protection against CHMI using P. falciparum parasites heterologous to the vaccine strain is limited and this in turn affects the use in endemic areas. In the future, the vaccine effect can be improved by higher doses, more infectious vaccine strains or vaccine cocktails. An alternative to RAS-vaccine and CPS-vaccine could be direct venous inoculation of late arresting GAP