PLOS Neglected Tropical Diseases: Impairment of T Cell Function in Parasitic Infections

In mammals subverted as hosts by protozoan parasites, the latter and/or
the agonists they release are detected and processed by sensors
displayed by many distinct immune cell lineages, in a
tissue(s)-dependent context. Focusing on the T lymphocyte lineage, we
review our present understanding on its transient or durable functional
impairment over the course of the developmental program of the
intracellular parasites Leishmania spp., Plasmodium spp., Toxoplasma gondii, and Trypanosoma cruzi
in their mammalian hosts. Strategies employed by protozoa to
down-regulate T lymphocyte function may act at the initial moment of
naïve T cell priming, rendering T cells anergic or unresponsive
throughout infection, or later, exhausting T cells due to antigen
persistence. Furthermore, by exploiting host feedback mechanisms aimed
at maintaining immune homeostasis, parasites can enhance T cell
apoptosis. We will discuss how infections with prominent intracellular
protozoan parasites lead to a general down-regulation of T cell function
through T cell anergy and exhaustion, accompanied by apoptosis, and
ultimately allowing pathogen persistence.