IL-10 and regulatory T cells limit the pathogenicity of African trypanosome infection

During natural as well as experimental African trypanosomiasis, a range of severity of disease patterns is associated with different host-trypanosome combinations. As compared to Trypanosoma congolense, Trypanosoma brucei causes a pathogenic infection in C57BL/6 mice associated with important tissue damage, increased anemia burden and decreased survival. The increased pathogenicity in T. brucei infected mice correlated with uncontrolled IFN-gamma production, marginal IL-10 production and continuous stimulation of classically activated monocytic cells (M1), producing the pathogenic molecules TNF-alpha and NO. Conversely, the decreased pathogenicity of T. congolense infected mice was associated with induction of IL-10 production coinciding with the down-regulation of the IFN-gamma response and M1 de-activation. The mechanisms implicated in this differential regulation of the immune response in T. congolense and T. brucei infected mice had not been identified. At the initiation of the present work, regulatory T cells (Tregs) has been proposed to suppress the activity of T cells and other immune cells by producing IL-10. Therefore we thought to evaluate whether Tregs could influence the development of the pathogenic immune response in trypanotolerant/resistant and trypanosusceptible/susceptible mouse models. Our results show that naturally occurring CD4+Foxp3+ Tregs expanded in the spleen and the liver of T. congolense infected C57BL/6 mice. These cells produced IL-10, limited the production of IFN-gamma by CD4+ and CD8+ effector T cells and down-regulated the activation M1 cells, resulting in reduced TNF-alpha production. Treg-mediated suppression did not hamper parasite clearance but was beneficial for the host survival by limiting the tissue damage. Based on these results and on the observation that Tregs did not expand in T. brucei infected mice, we have assessed whether experimental expansion of Tregs could decrease pathogenicity of the disease in trypanosusceptible mice. CD28 superagonist antibody treatment mediated expansion of IL-10 producing Tregs, down-regulated IFN-gamma production by T cells was well as TNF-alpha production by M1 cells, triggered the development of alternatively activated monocytic cells (M2), delayed the onset of liver injury, diminisehd the anemia burden and prolonged the survival of T. brucei infected mice. However, the CD28 superagonist treatment revealed ineffective when delivered therapeutically to chronically infected mice. Therefore, we employed another modality to increase IL-10 levels during T. brucei infection, namely by delivering IL-10 using a adeno-associated viral vector (AAV). This treatment suppressed IFN-gamma production, diminished TNF-alpha production by M1 cells, induced M2 activation and reduced the pathogenicity of the disease. Together, these date indicated that expansion of Tregs (through CD28 superagonist treatment) or delivery of IL-10 (through AAV treatment) could restore the balance between pro- and anti-inflammatory signals and hereby limit the pathogenicity of African trypanosomiasis. Having shown that the CD28 superagonist and the IL-10-AAV treatment in T. brucei infected mice limited the production of the pathogenic compounds TNF-alpha and NO by M1 cells and induced M2 activation, we thought interesting to scrutinize the phenotype of monocytic cells in T. brucei infected C57BL/6 mice. We show that CD11b+Ly6c+CD11c-. Monocytes gradually accumulated in the spleen, the liver and the lymph nodes of T. brucei infected mice and differentiated to CD11b+Ly6c+CD11c+TIP-DCs, producing elevated levels of TNF-alpha and iNos. In addition, the CD28 superagon