Parasite signalling and host responses in experimental and human African trypanosomiasis

Abstract

African trypanosomiasis or sleeping sickness is a disease of tropical Africa caused by the unicellular-flagellated protozoan parasite, Trypanosoma brucei (T b.), which is transmitted by its insect vector, the tsetse fly. In humans, there are two forms of the disease: a chronic form in West and Central Africa, caused by T. b. gambiense, and an acute form that occurs in East and Southern Africa and is caused by T. b. rhodesiense. The parasite exhibits a great capability of evading immune control, mainly by genetic switching of its variant surface glycoprotein (VSG). Furthermore, trypanosomes secrete a trypanokine, trypanosome-derived lymphocyte-triggering factor (TLTF) that activates CD8+ T-cells to produce interferon-gamma (IFN-gamma) which in turn stimulates parasite growth.

In view of the role of IFN-gamma in inducing parasite growth, aspects of the signalling system for this cytokine during TLTF secretion were studied. Secretion of TLTF was induced by IFN-gamma in a dose dependent manner. This constitutive and active secretion was dependent on tyrosine protein kinase activity. Specific blockage of this pathway resulted in lower yields of TLTF. An ELISA system was used to measure levels of TLTF and anti-TLTF antibodies during early and late stages of the disease in mice. IFN-gamma and IFN-gamma receptor gene knockout mice and their wild type littermates were used to illustrate the role of IFN-gamma in the generation of antibodies to TLTF. Wild type mice showed high parasitemia, accompanied by high TLTF levels and low anti-TLTF antibodies at day 3 post-infection (p.i.); 21 days p.i low TLTF levels together with increased anti-TLTF antibodies was observed. In contrast, IFN- gamma/- mice exhibited very low parasitemia, low TLTF and anti-TLTF antibody levels. Furthermore, IFN-gamma R- /- mice revealed very high parasitemia, increased TLTF levels, but decreased antiTLTF antibodies. This data demonstrates a role for IFN-gamma in the generation of neutralizing antibodies to TLTF.

Levels of anti-TLTF antibodies in the sera and cerebrospinal fluid (CSF) of patients at different stages of the Gambian form of African trypanosomiasis were assessed in an attempt to establish a reliable diagnostic assay for CNS involvement in human African trypanosomiasis (HAT). While the sera illustrated no significant difference between disease stages, CSF samples demonstrated a clear difference in anti-TLTF antibodies levels between the early and late stage that may be useful in determining the early stage of the CNS involvement. Interactions between TLTF and human first trimester brain cells were examined. TLTF induced proliferation of human first trimester forebrain cells and IFN-gamma production at the mRNA and protein levels, mainly by astrocytes. The data illustrates for the first time a direct effect of a parasite factor on human brain cells. Targeting TLTF during the course of the disease should be considered for preventing the lethal neurological complications of human African trypanosomiasis.