Funktionelle Bedeutung von VCAM-1 und CD4+ T-Zellen fuer die Induktion einer protektiven Immunantwort bei der murinen Toxoplasmose

In the present study the functional role of the vascular cell adhesion molecule (VCAM)-1 was analysed for the first time in a murine infectious disease, namely toxoplasmosis. To dissect the precise function of VCAM-1, conditional VCAMflox/flox MxCre mice were used in which the VCAM-1 gene was neonatally inactivated by IFN-alpha-induced Cre-loxP-mediated deletion. These VCAM-1 mutant mice did not express VCAM-1 on cerebral blood vessel endothelial cells and remained VCAM-1 negative after Toxoplasma gondii infection. The constitutive expression of VCAM-1 on the epithelium of the choroid plexus as well as on the ependymal cells of the ventricular wall was unaffected and expression further increased after T.gondii infection. In infected VCAMflox/flox MxCre mice resistance against T. gondii was abolished and mice died of a chronic Toxoplasma encephalitis (TE) caused by a failure to control parasites in the CNS. Unexpectedly, the recruitment of inflammatory leukocytes to the CNS was unimpaired. However, VCAMflox/flox MxCre mice showed a significantly reduced level of T.gondii-specific antibodies and a decrease in the frequency and activation state of intracerebral, T.gondii-specific T cells. Furthermore, the microglial activation was markedly reduced. Another aim of present study was to determine the functional role of CD4+ T cells in the induction of a T.gondii-specific immune response in CD4-depleted, T.gondii-resistant mice, which lack CD4+ T cells. Non-depleted wild-type (wt) mice were used as controls. Following oral infection with cysts of a beta-Galactosidase-expressing Toxoplasma strain the majority of CD4-depleted mice succumbed to a necrotizing TE within 28 days p.i., whereas control mice survived. Nevertheless, the frequency and activation state of beta-Galactosidase-specific CD8+ T cells in CD4-depleted, T.gondii-resistant mice was not impaired. Antigen-specific CD8+ T cells from the spleen and brain of CD4-depleted mice were also able to produce IFN-gamma and TNF-alpha and showed cytotoxic activity, although these functions were slightly reduced in the brain of CD4-depleted mice compared to control mice. The activation state of cerebral macrophages and microglia cells in CD4-depleted mice was similar to wt mice as determined by MHC class (cl.) II expression and TNF-alpha production. In contrast, the level of T.gondii-specific antibodies in serum was drastically reduced in CD4-depleted mice than in wt animals. Moreover, the absence of CD4+ T cells resulted in the complete loss of T.gondii-specific IgG and IgM antibodies in the liquor. Adoptive transfer of immune serum obtained form T.gondii-infected wt mice improved the resistance and the survival rate of CD4-depleted mice. The functional role of CD4+ T cells in the induction of a T. gondii-specific antibody response was also analysed in T.gondii-susceptible, CD4-depleted mice and T.gondii-susceptible, MHC cl. II-deficient mice, which lack conventional CD4+ T cells. Similar to T.gondii-resistant mice, the infection of T.gondii-susceptible mice resulted in a strong reduction of T.gondii-specific antibodies. The impaired resistance to T.gondii in MHC cl. II-deficient mice could be temporarily overcome by administration of Toxoplasma-specific immune serum. In contrast to the B cell response the induction of a CD8+ T cell response was independent of CD4+ T cells in T.gondii-susceptible mice. Although further investigation of non-conventional CD4+ T cells from MHC cl. II-deficient mice showed that these cells are capable of producing IFN-gamma in a CD1-dependent mannerƒz non-conventional CD4+ T cells did not contribute to the protection against T. gondii. Moreover, the induction of a T. gondii-specific antibody response and CD8+ T cell response was independent of these cells. Taken together these data reveal, that VCAM-1 and CD4+ T cells are indispensable for the control of the intracellular parasite T. gondii. Whereas VCAM-1 is critical for the induction of a T.gondii-specific antibody response, a T.gondii-specific T cell response and for the appropriate activation of macrophages and microglia cells, it is not necessary for the recruitment of leukocytes to the brain. CD4+ T cells are also crucial for the induction of a T.gondii-specific antibody response. In contrast to VCAM-1, CD4+ T cells play no significant role in the induction of a T.gondii-specific CD8+ T cell response and are not necessary for the activation of macrophages and microglia cells.