Interleukin-5 Mediates Parasite-Induced Protection against Experimental Autoimmune Encephalomyelitis: Association with Induction of Antigen-Specific CD4CD25 T Regulatory Cells.

Objective: To examine if the protective effect of parasite infection on experimental autoimmune encephalomyelitis (EAE) was due to interleukin (IL)-5, a cytokine produced by a type-2 response that induces eosinophilia. We hypothesize that, in parasite infections, IL-5 also promotes expansion of antigen-specific T regulatory cells that control autoimmunity.

Methods: larvae were used to infect Lewis rats prior to induction of EAE by myelin basic protein. Animals were sham treated, or given blocking monoclonal antibodies to interleukin 4 or 5 or to deplete CD25 T cells. Reactivity of CD4CD25 T regulatory cells from these animals was examined.

Results: Parasite-infected hosts had reduced severity and length of EAE. The beneficial effect of parasitic infection was abolished with an anti-IL-5 or an anti-CD25 monoclonal antibody (mAb), but not anti-IL-4 mAb. Parasite-infected animals with EAE developed antigen-specific CD4CD25 T regulatory cells earlier than EAE controls and these expressed more than controls. Treatment with IL-5 also reduced the severity of EAE and induced expressing CD4CD25 T regulatory cells.

Interpretation: The results of this study suggested that IL-5 produced by the type-2 inflammatory response to parasite infection promoted induction of autoantigen-specific CD25 T regulatory cells that reduced the severity of autoimmunity. Such a mechanism may explain the protective effect of parasite infection in patients with multiple sclerosis where eosinophilia is induced by IL-5, produced by the immune response to parasites.