In the BALB/c mouse model, primary infection with Toxoplasma gondii during the second third of gestation leads to a high percentage of infected foetuses. However, immunity induced by infection contracted before pregnancy prevents parasites from crossing the placenta and completely protects the foetuses, as well as the pregnant women. In order to clarify the roles of CD4+, CD8+ T lymphocytes and IFN-gamma in this protection, pregnant BALB/c mice were treated with depleting monoclonal antibodies against CD4, CD8, IFN-gamma, or control antibody. Only the foetuses of the groups treated with anti-CD8 and anti-IFN-gamma antibodies developed congenital toxoplasmosis. The maternal production of IFN-gamma was depressed in the mice depleted of CD4 and CD8 cells (P < 0.001). Determination of the blood parasite load demonstrated that materno-foetal transmission of T. gondii correlates with maternal parasitaemia. Together, these results show that CD8+ T lymphocytes and IFN-gamma play an important role in protection against congenital toxoplasmosis during reinfection.

The stimulation of DC by CD4(+) T cells is known to condition DC to activate naïve CD8(+) T cells, predominantly via CD40-CD40L interactions. It has been proposed that a critical consequence of DC conditioning is the induction of CD70 expression. Whether and how CD70 induction contributes to CD8(+) T cell responses in the absence of CD40-CD40L interactions are unknown. CD8(+) T cell responses to adenoviral- or DC-based immunization of CD40-deficient mice revealed a CD40-independent, CD4(+) T cell-dependent pathway for CD70 induction on conventional DC. This pathway and subsequent CD8(+) T cell responses were enhanced by, but not dependent on, concomitant activation of TLR and in part, used TRANCE and LIGHT/LTalphabeta stimulation. Blocking TRANCE and LIGHT/LTalphabeta during stimulation reduced the immunogenicity of CD40-deficient DC. These data support the hypothesis that induction of CD70 expression on DC after an encounter with activated CD4(+) T cells is a major component of CD4(+) T cell-mediated licensing of DC. Further, multiple pathways exist for CD4(+) T cells to elicit CD70 expression on DC. These data in part explain the capacity of CD40-deficient mice to mount CD8(+) T cell responses and may provide additional targets for immunotherapy in situations when CD40-mediated licensing is compromised.

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