The Sinnis Laboratory is part of the Johns Hopkins Malaria Research Institute, committed to the pursuit of basic science research that translates into solutions targeting one of the most important infectious diseases in the world. Malaria, caused by protozoan parasites of the genus Plasmodium, is transmitted to humans by infected Anopheles mosquitoes. The evolution of drug resistance by the parasite, and insecticide resistance by the mosquito, have created an urgent need for new strategies to control and ultimately eliminate malaria.

We focus on the transmission of sporozoites from mosquito to mammalian host and how these parasites overcome physical and immunological obstacles to establishing infection. Sporozoites, the infective stage of the malaria parasite, make an impressive journey from the midgut wall of the mosquito where they emerge from oocysts, to their final destination in the mammalian liver (see picture below). Using biochemical, cell biological and genetic approaches, as well as intravital imaging, we aim to understand the molecular interactions between sporozoites and their mosquito and mammalian hosts that enable the parasite to initiate infection.

The Skin Phase of Infection

Mosquitoes inoculate sporozoites into the dermis of the mammalian host as they probe for blood. Once inoculated, sporozoites actively move through the dermis to fine a blood vessel, which they penetrate to enter the circulation and go to the liver. Using a rodent malaria model we have determined the sporozoite inoculum (Medica & Sinnis 2005), the kinetics with which sporozoites exit the dermis (Yamauchi et al. 2007), and the probability of infection after a single infectious bite (Aleshnick et al., unpublished data). Our work suggests that this is a significant bottleneck for the parasite. Intra-vital imaging of sporozoites in the skin, has led to the demonstration that sporozoites recognize blood vessels and alter their speed and trajectories when in the vicinity of a vessel (Hopp et al. 2015). Current work focuses on the host innate and adaptive immune response to sporozoites at the inoculation site and understanding the mechanism(s) by which sporozoites recognize and penetrate blood vessels.

Structure-function studies of the circumsporozoite protein (CSP)

CSP is the major surface protein of the sporozoite, forming a dense coat on the surface of the parasite. CSP is also the basis of RTS,S, the only malaria vaccine candidate to show efficacy in Phase III clinical trials. Despite its importance, our understanding of CSP structure and function is limited and we believe that elucidating how this protein functions will inform future malaria vaccine design.

Our previous studies demonstrate that CSP has two conformational states, an adhesive conformation in which the TSR domain is exposed and a nonadhesive conformation in which the N-terminus masks this domain. As sporozoites travel from mosquito midgut to mammalian liver, the cell-adhesion domain is masked, maintaining sporozoites in a migratory state (Coppi et al., 2011). Upon arrival in the liver, the N-terminal domain is proteolytically cleaved and the cell-adhesion domain is exposed (Coppi et al., 2005; Coppi et al., 2011). Processing is triggered by the highly sulfated heparan sulfate proteoglycans of the liver, which serves as a signal for the sporozoite to switch from a migratory to an invasive state (Coppi et al 2007). We are currently focused on determining the structure of the N-terminus of CSP, the function of the central repeat region and the identity of the parasite protease that cleaves CSP.

The role of parasite proteases in the establishment of malaria infection.

Our work on CSP and another sporozoite adhesin, TRAP (Ejigiri et al 2012), demonstrates the vital role that parasite proteases play in sporozoite motility and infectivity. We are actively working to identify the CSP protease and have expanded our work to dissect the function of other Plasmodium serine and cysteine proteases expressed by pre-erythrocytic stages. Ultimately the goal of this work is to screen inhibitors of critical proteases and validate them as drug targets.