Role of NK cells and gamma interferon in transplacental passage of Toxoplasma gondii in a mouse model of primary infection

Abstract

Protective immunity in mice infected with Toxoplasma gondii is mainly mediated by NK cells, CD4 and CD8 T cells, and type 1 cytokines, such as gamma interferon (IFN-gamma). To clarify the roles of NK cells and IFN-gamma in protection against primary congenital toxoplasmosis, we used recombination activating gene 2 knockout (RAG-2(-/-)) mice, which lack T and B lymphocytes, in comparison with the wild-type BALB/c model. RAG-2(-/-) mice had a significantly lower risk of fetal toxoplasmosis than BALB/c mice (25 versus 63.9%; P = 0.003). This protection was associated with an increased number of maternal NK cells, IFN-gamma secretion by spleen cells, and decreased parasitemia. In the RAG-2(-/-) mice, NK cell depletion increased both the rate of fetal infection, to 56.5% (P = 0.02), and the blood parasite burden. Conversely, in the BALB/c mice, this treatment did not modify maternofetal transmission or the blood parasite burden. Neutralization of IFN-gamma in both infected RAG-2(-/-) and BALB/c mice decreased congenital Toxoplasma transmission, contrasting with an exacerbation of maternal infection. These data suggest that a partially protective immunity against congenital toxoplasmosis is achieved due to the increased number of NK cells in RAG-2(-/-) mice. However, it seems that IFN-gamma enhances, directly or indirectly, the transplacental transmission.

FIG. 1.

Numbers of NK cells in spleens of T. gondii-infected RAG-2^−/− and BALB/c mice on day 18 of gestation (day 7 postinfection). Spleen cell suspensions were harvested, stained for pan-NK cell DX5⁺ expression, and analyzed by FACScan. The values represent the means plus standard deviations of three mice in each group. Similar results were obtained in two independent experiments.

FIG. 2.

Effect of depletion of NK cells (anti-asialo GM1 Ab) or IFN-γ (XGM1.2 Ab) on maternofetal transmission of T. gondii in RAG-2^−/− and BALB/c mice. The presence or absence of fetal infection was assessed on day 18 of gestation (day 7 postinfection). The values represent the mean percentages of infected fetuses and are representative of three independent experiments.

FIG. 3.

IFN-γ production by spleen cells on day 18 of gestation (day 7 postinfection). The supernatants of spleen cells cultured for 48 h in RPMI were harvested and analyzed for the presence of IFN-γ by enzyme-linked immunosorbent assay. Each bar represents the mean plus standard deviation of triplicate measures of supernatants from three mice and is representative of three independent experiments.

FIG. 4.

Maternal blood parasite burden measured by quantitative real-time PCR on day 18 of gestation (day 7 postinfection). The data represent the means plus standard deviations of three mice. Similar results were obtained in two independent experiments.