Protracted Sterile Protection withPlasmodium yoeliiPre‐erythrocytic Genetically Attenuated Parasite Malaria Vaccines Is Independent of Significant Liver‐Stage Persistence and Is Mediated by CD8⁺T Cells

Abstract

Irradiation-attenuated sporozoite vaccinations confer sterile protection against malaria infection in animal models and humans. Persistent, nonreplicating parasite forms in the liver are presumably necessary for the maintenance of sterile immunity. A novel vaccine approach uses genetically attenuated parasites (GAPs) that undergo arrested development during liver infection. The fate of GAPs after immunization, their persistence in vaccinated animals, and the immune mechanisms that mediate protection are unknown. To examine the developmental defects of genetically attenuated liver stages in vivo, we created deletions of the UIS3 and UIS4 loci in the Plasmodium yoelii rodent malaria model (Pyuis3[-] and Pyuis4[-]). The low 50% infectious dose of P. yoelii in BALB/c mice provides the most sensitive infectivity model. We show that P. yoelii GAPs reach the liver, invade hepatocytes, and develop a parasitophorous vacuole but do not significantly persist 40 h after infection. A single dose of Pyuis4(-) sporozoites conferred complete protection, but full protection by Pyuis3(-) sporozoites required at least 2 immunizations. CD8(+) T cells were essential for protection, but CD4(+) T cells were not. Our results show that genetically distinct GAPs confer different degrees of protective efficacy and that live vaccine persistence in the liver is not necessary to sustain long-lasting protection. These findings have important implications for the development of a P. falciparum GAP malaria vaccine.

Preerythrocytic, live-attenuated Plasmodium falciparum vaccine candidates by design

“…Immunization of mice with uis3 Ϫ , uis4 Ϫ , or p52 Ϫ parasites induced complete, long-lasting protection against infectious sporozoite challenge (7-9, 11), demonstrating that rodent malaria GAPs are highly effective vaccines. The GAP-induced protection was mediated mainly by CD8 ϩ T cells (9,13,14), but antibodies also contributed to protection (9).To assess the potential to create a GAP vaccine for human malaria, we deleted the P52 and P36 loci in P. falciparum. The deletions did not affect the parasites throughout most of the life cycle, including sporozoite production of the attenuated lines, but resulted in significant growth defects in a hepatocytic cell line.…”

“…Immunization of mice with uis3 Ϫ , uis4 Ϫ , or p52 Ϫ parasites induced complete, long-lasting protection against infectious sporozoite challenge (7-9, 11), demonstrating that rodent malaria GAPs are highly effective vaccines. The GAP-induced protection was mediated mainly by CD8 ϩ T cells (9,13,14), but antibodies also contributed to protection (9).…”

“…Recently, preerythrocytic stages of the rodent malaria parasites Plasmodium berghei and Plasmodium yoelii were attenuated by deletion of preerythrocytic-stage-expressed genes named Up-regulated in Infectious Sporozoites (UISs). UIS3 and UIS4 (7)(8)(9) are proteins of the liver-stage parasitophorous vacuole membrane, the principal host-parasite interface during liver infection (7,10). Deletion of UIS3 and UIS4 led to complete arrest of early liver-stage development after hepatocyte infection (7)(8)(9).…”

“…UIS3 and UIS4 (7)(8)(9) are proteins of the liver-stage parasitophorous vacuole membrane, the principal host-parasite interface during liver infection (7,10). Deletion of UIS3 and UIS4 led to complete arrest of early liver-stage development after hepatocyte infection (7)(8)(9). Deletion of another UIS gene, P52, encoding a putative GPI-anchored protein (11,12), and P36, a gene encoding a putative secreted protein (12), also resulted in developmental arrest at the early stage of hepatocyte infection.…”

TAP‐mediated processing of exoerythrocytic antigens is essential for protection induced with radiation‐attenuated Plasmodium sporozoites

“…In the mouse model, RAS-induced protection is dependent on CD8 + T cells [17] that recognize MHC I:peptide complexes derived from exoerythrocytic (EE) stage Ags expressed on either parenchymal or nonparenchymal liver cells [18]. More recently, we and others have confirmed the importance of CD8 + T cells as the key effectors in protective immunity induced in mice with genetically attenuated Plasmodium berghei [19,20] and Plasmodium yoelii [21,22] sporozoites. In the P. berghei RAS model, protracted protective immunity is linked to the presence of liver CD8 + T effector memory (T EM ) cells, swift producers of IFN-γ in response to infectious sporozoite challenge.…”