Genetically attenuated Plasmodium berghei liver stages induce sterile protracted protection that is mediated by major histocompatibility complex Class I-dependent interferon-gamma-producing CD8+ T cells.

At present, radiation-attenuated plasmodia sporozoites ( gamma -spz) is the only vaccine that induces sterile and lasting protection in malaria-naive humans and laboratory rodents. However, gamma -spz are not without risks. For example, the heterogeneity of the gamma -spz could explain occasional breakthrough infections. To avoid this possibility, we constructed a double-knockout P. berghei parasite by removing 2 genes, UIS3 and UIS4, that are up-regulated in infective spz. We evaluated the double-knockout Pbuis3(-)/4(-) parasites for protective efficacy and the contribution of CD8(+) T cells to protection. Pbuis3(-)/4(-) spz induced sterile and protracted protection in C57BL/6 mice. Protection was linked to CD8(+) T cells, given that mice deficient in beta (2)m were not protected. Pbuis3(-)/4(-) spz-immune CD8(+) T cells consisted of effector/memory phenotypes and produced interferon- gamma . On the basis of these observations, we propose that the development of genetically attenuated P. falciparum parasites is warranted for tests in clinical trials as a pre-erythrocytic stage vaccine candidate.