Breaking barriers: a leap ahead in Plasmodium biology Experimentally induced blood stage malaria infection as a tool for clinical research Christian R. Engwerda 1, 2 , Gabriela Minigo 3 , Fiona H. Amante 1 , and James S. McCarthy 1, 2 1 The Queensland Institute of Medical Research, Brisbane, QLD, Australia 2 The School of Medicine, University of Queensland, Brisbane, QLD, Australia 3 Global and Tropical Health Division, Menzies School of Health Research and Charles Darwin University, Darwin, NT, Australia A system for experimentally induced blood stage malar- ia infection (IBSM) with Plasmodium falciparum by di- rect intravenous inoculation of infected erythrocytes was developed at the Queensland Institute of Medical Research (QIMR) more than 15 years ago. Since that time, this system has been used in several studies to investigate the protective effect of vaccines, the clear- ance kinetics of parasites following drug treatment, and to improve understanding of the early events in blood stage infection. In this article, we will review the devel- opment of IBSM and the applications for which it is being employed. We will discuss the advantages and disadvantages of IBSM, and finish by describing some exciting new areas of research that have been made possible by this system. Controlled human malaria infections Controlled human malaria infection (CHMI) [1] is now used to test vaccines and drugs, as well as to examine physiological and immunological responses to malaria parasites. CHMI can be broadly divided into sporozoite- induced malaria infection (SIM) and induced blood stage malaria infection (IBSM). SIM allows both liver stage and blood stage infection to develop, whereas IBSM results in blood stage infection only. The two different CHMI systems each have advantages and disadvantages that are dis- cussed below and summarized in Box 1. However, the main focus of this review will be on the development and utility of IBSM for malaria research. The development of IBSM for clinical studies Deliberate malaria infection by parenteral injection of infected erythrocytes was first described by Julius Wagner-Jauregg [Nobelprize.org Julius Wagner-Jauregg – Biography ( http://www.nobelprize.org/nobel_prizes/ medicine/laureates/1927/wagner-jauregg-bio.html)] who won the Nobel Prize in 1927 for the description of malar- iotherapy for treatment of syphilis, then a lethal disease. Malariotherapy was widely practiced in both Europe and the United States for the treatment of syphilis until the 1950s when the curative effect of penicillin was recognized. The mechanism of action of malariotherapy was through induc- tion of fever in patients which then killed the temperature- sensitive Treponema pallidum spirochaetes responsible for syphilis. IBSM using each of the major malaria-causing species was described, as well as the use of infected mosqui- toes to induce infection [2]. After the abandonment of malar- iotherapy for syphilis, experimental studies using IBSM continued, particularly for testing the efficacy of experimen- tal schizonticidal antimalarials [3–9]. Owing to ethical and safety concerns, direct intravenous inoculation of malaria-infected donor blood was abandoned in the 1980s. However, experimental SIM infection using mosquitoes continued, and to this day is being widely used to study protective immunity [10]. This approach is cur- rently being further facilitated by the development by Hoffman and colleagues of good manufacturing practice (GMP) production of Plasmodium falciparum sporozoites that are cryopreserved in liquid nitrogen [11]. These can be subsequently thawed and inoculated (ClinicalTrials.gov, registration: NCT01624961). This process offers the possi- bility of obviating the need for maintenance of an insectary on-site, with the attendant expense, technical challenges, and regulatory requirements. The reconstitution of IBSM, led by Allan Saul in the early 1990s, required overcoming an array of ethical, safety, and technical challenges. Overcoming the ethical and safety concerns was facilitated by recognition of the extensive and long-term experience with blood and blood product transfusion, and the increases in the use of allogenic trans- plantation, including cryopreserved bone marrow. The ethi- cal status of studies using deliberate infection of human volunteers with a range of pathogens, including viruses [12] and enteric pathogens [13], has been subject to detailed independent review by regulatory authorities in multiple jurisdictions, and has been deemed to be an ethically acceptable activity [14]. Technical advances that have facil- itated the development of the IBSM system include the ability to maintain P. falciparum in continuous culture, the ability to cryopreserve intraerythrocytic parasites and thaw them for later infusion, and the development of Review Corresponding authors: Engwerda, C.R. (chrisE@qimr.edu.au); McCarthy, J.S. (James.McCarthy@qimr.edu.au). Keywords: malaria; vaccines; drugs; immunity; pathophysiology; IBSM. 1471-4922/$ – see front matter ß 2012 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.pt.2012.09.001 Trends in Parasitology, November 2012, Vol. 28, No. 11 515