Dendritic Cells and Hepatocytes Use Distinct Pathways to Process Protective Antigen from Plasmodium in vivo

Abstract

Malaria-protective CD8+ T cells specific for the circumsporozoite (CS) protein are primed by dendritic cells (DCs) after sporozoite injection by infected mosquitoes. The primed cells then eliminate parasite liver stages after recognizing the CS epitopes presented by hepatocytes. To define the in vivo processing of CS by DCs and hepatocytes, we generated parasites carrying a mutant CS protein containing the H-2K ^b epitope SIINFEKL, and evaluated the T cell response using transgenic and mutant mice. We determined that in both DCs and hepatocytes CS epitopes must reach the cytosol and use the TAP transporters to access the ER. Furthermore, we used endosomal mutant (3d) and cytochrome c treated mice to address the role of cross-presentation in the priming and effector phases of the T cell response. We determined that in DCs, CS is cross-presented via endosomes while, conversely, in hepatocytes protein must be secreted directly into the cytosol. This suggests that the main targets of protective CD8+ T cells are parasite proteins exported to the hepatocyte cytosol. Surprisingly, however, secretion of the CS protein into hepatocytes was not dependent upon parasite-export (Pexel/VTS) motifs in this protein. Together, these results indicate that the presentation of epitopes to CD8+ T cells follows distinct pathways in DCs when the immune response is induced and in hepatocytes during the effector phase.

Author Summary

Malaria causes the deaths of 0.5–2 million people each year, mainly in Africa. A safe and effective vaccine is likely needed for the control or eradication of this disease. Immunization by irradiated malaria-infected mosquitoes has been shown to protect people against malaria. Irradiated parasites do not divide and cause infection but are capable of activating specialized killer cells called CD8+ T cells, which can protect against live parasites. Because vaccinating people with irradiated mosquitoes is not practical, we wanted to understand which parasite molecules are targeted by CD8+ T cells. These molecules may then be formulated into a safe and effective vaccine. CD8+ T cells do not automatically recognize every parasite molecule, but instead fragments of parasite proteins must be displayed on the surface of infected cells to be seen by CD8+ T cells. Our data show that CD8+ T cells recognize parasite proteins secreted by the parasite into the infected cell. This suggests that such proteins could be important components of malaria vaccines.