Nonreplicating, Cyst-Defective Type II Toxoplasma gondii Vaccine Strains Stimulate Protective Immunity against Acute and Chronic Infection

Live attenuated vaccine strains, such as type I nonreplicating uracil auxotroph mutants, are highly effective in eliciting lifelong immunity to virulent acute infection by Toxoplasma gondii. However, it is currently unknown whether vaccine-elicited immunity can provide protection against acute infection and also prevent chronic infection. To address this problem, we developed nonreverting, nonreplicating, live attenuated uracil auxotroph vaccine strains in the type II Delta ku80 genetic background by targeting the deletion of the orotidine 5'-monophosphate decarboxylase (OMPDC) and uridine phosphorylase (UP) genes. Deletion of OMPDC induced a severe uracil auxotrophy with loss of replication, loss of virulence in mice, and loss of the ability to develop cysts and chronic infection. Vaccination of mice using type II Delta ku80 Delta ompdc mutants stimulated a fully protective CD8(+) T cell-dependent immunity that prevented acute infection by type I and type II strains of Toxoplasma gondii, and this vaccination also severely reduced or prevented cyst formation after type II challenge infection. Nonreverting, nonreplicating, and non-cyst-forming Delta ompdc mutants provide new tools to examine protective immune responses elicited by vaccination with a live attenuated type II vaccine.