Page 919 - Veterinary Immunology, 10th Edition

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FIG. 28.3 The points in the life cycle of Toxoplasma gondii at
which the immune system can exert a controlling influence.

A toxoplasma protein called profilin is a ligand for TLR11 in
mice. Ligation of TLR11 on dendritic cells signals through the
MyD88 pathway and stimulates IL-12 and IFN-γ production by

natural killer (NK) and T cells. The IL-12 and IFN-γ in turn trigger a
strong Th1 response. Activated Th1 cells secrete IFN-γ in response
to Toxoplasma ribonucleoproteins. This IFN-γ activates
macrophages, permitting lysosome-vacuole fusion, enhancing

autophagy, and killing the intracellular organisms. Cytotoxic T cells
can also destroy Toxoplasma tachyzoites and Toxoplasma-infected
cells on contact. Toxoplasma-infected dendritic cells are attacked and
killed by NK cells. The T. gondii tachyzoites, however, may convert

into cysts containing bradyzoites. These cysts are weakly
immunogenic and do not stimulate inflammation. It is possible that
the cyst stage is not recognized as foreign so that Toxoplasma can
persist indefinitely within tissues.

Th1-mediated macrophage activation is important in many

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