An Expanding Toolkit for Preclinical Pre-Erythrocytic Malaria Vaccine Development: Bridging Traditional Mouse Malaria Models and Human Trials

Abstract

Malaria remains a significant public health burden with 214 million new infections and over 400,000 deaths in 2015. Elucidating relevant Plasmodium parasite biology can lead to the identification of novel ways to control and ultimately eliminate the parasite within geographic areas. Particularly, the development of an effective vaccine that targets the clinically silent pre-erythrocytic stages of infection would significantly augment existing malaria elimination tools by preventing both the onset of blood-stage infection/disease as well as spread of the parasite through mosquito transmission. In this Perspective, we discuss the role of small animal models in pre-erythrocytic stage vaccine development, highlighting how human liver-chimeric and human immune system mice are emerging as valuable components of these efforts. Malaria continues to cause significant morbidity and mortality despite renewed and concerted efforts to eliminate the causative agents, parasites of the genus Plasmodium. These efforts have largely focused on control of the Anopheles mosquito vectors, and antimalarial drug treatment of symptomatic infected individuals. The 214 million new malaria infections and 438,000 deaths due to malaria in 2015 were disproportionately borne by low income countries where malaria is endemic, and declines in malaria infections since 2000 have been slowest in countries with low resource availability and high malaria burden [1]. In the fight against malaria, effective vaccines preventing both disease and transmission will be important tools to achieve WHO goals of a 90% reduction in disease burden by 2030 [1,2]. In humans, malaria is caused predominantly by the parasite species Plasmodium falciparum and Plasmodium vivax, with the remainder of infections caused by three additional parasite species, Plasmodium malariae, Plasmodium ovale and Plasmodium knowlesi [1].Malaria parasite transmission occurs when a Plasmodium-infected Anopheles mosquito bites and by probing the skin for a blood meal, deposits motile sporozoite stages into the host. Sporozoites pass through the dermis using active motility, enter a capillary and then rapidly home to the liver. Once in the liver sporozoites enter the parenchyma and infect hepatocytes, wherein they then transmogrify into liver stages, which grow, replicate and ultimately differentiate into tens of thousands of first-generation merozoites. Merozoites of human malaria parasites emerge from the liver into the blood stream 7-10 days after transmission, where they undergo cyclic erythrocytic infection and intraerythrocytic replication. This blood-stage infection is responsible for all mortality and clinical symptoms of malaria, as well as infection of a new mosquito vector by sexual stage parasites for onward transmission [3,4]. Stages prior to blood-stage infection (i.e., the sporozoite and liver stages) are collectively known as preerythrocytic (PE) stages of infection and vaccine candidates targeting these stages are collectively termed PE vaccines. By preventing progression to ...

Humoral protection against mosquito bite-transmitted Plasmodium falciparum infection in humanized mice

“… 9 , 10 , 33 , 35 , 47 All current in vitro assays employ infection of hepatocytes in monoculture which is not representative of the sporozoites’ complex journey to the liver or the architecture of the target organ. 26 …”

“… 19 Without robust preclinical evidence for protective efficacy of subunit vaccine candidates against Pf challenge, CHMI trials have resulted in numerous vaccine candidate failures in the past. 20 – 25 Although in vitro assays to evaluate antibody-mediated protection against Pf sporozoite infection have been established using monocultures of hepatoma cells or primary human hepatocytes 26 they do not recapitulate the sporozoites infection route from the skin to the liver and therefore cannot capture antibody activities that block the parasite prior to hepatocyte infection. 13 , 27 , 28 …”

An Opsonic Phagocytosis Assay for Plasmodium falciparum Sporozoites

“…These antibodies can act at multiple points during the journey of sporozoites from the dermis to the liver, and a number of in vitro assays are available to measure the inhibition of distinct processes at multiple steps in this journey. These include assays for the inhibition of motility in the skin as sporozoites search for a capillary, traversal of multiple cell types as sporozoites gain access to the circulation and subsequently to the liver parenchyma, as well as, ultimately, the invasion of hepatocytes by sporozoites (16). While these assays are certainly important tools in the analysis of humoral responses against subunit and whole attenuated sporozoite vaccination, they measure only the neutralization of sporozoite protein functions and do not consider the interaction of these parasite-bound antibodies with other immune components.…”

“…These assays include the sporozoite gliding motility and cell traversal assays that model parasite activities in the skin and liver (7,11), sporozoite invasion assays for hepatocyte infection (12)(13)(14), and assays measuring liver-stage development within primary hepatocytes (15). Antibodies are tested for inhibitory activity in these assays, and the collective results are then used to prioritize candidate antigens for further development (16). However, none of the aforementioned assays currently analyze the downstream effects of anti-sporozoite antibodies in conjunction with the effects of cellular components of the immune system.…”

Humanized Mouse Models for the Study of Human Malaria Parasite Biology, Pathogenesis, and Immunity

“…Perhaps the greatest contribution of the rodent malaria models is in the examination of immunity and protection against an infectious sporozoite challenge after vaccination with whole, attenuated sporozoites. Attenuation was originally generated by gamma irradiation ( 29 ) but can now be achieved by genetic engineering with the precise removal of genes from the parasite genome ( 30 , 31 ) or by treatment of an infectious sporozoite immunization with drugs that prevent BS infection ( 32 , 33 ). Over the last decade, studies utilizing whole sporozoite infection of mice have identified roles for humoral immunity ( 3 , 34 – 37 ) and both peripheral and tissue-resident memory CD8 T cells in the protective response to immunization ( 13 , 38 – 41 ).…”