Role of the Co-Inhibitory Molecule PD-1 (CD279) and its Ligand PD-L1 (CD274) in a Mouse (Mus musculus; Linnaeus, 1758) Model of Malaria

Several mechanisms are involved in the regulation of the immune system allowing the necessary balance between an effective defense against pathogens and tolerance. The control over this immune response is necessary to prevent self damage. The newly described co-inhibitory molecules of the B7/CD28 family, PD-1 and its ligand PD-L1 are involved in the control of the immune response in target organs. PD-1 expression has been found on activated T cells, whereas the endothelial expression of its ligand PD-L1 assures its presence in a high variety of tissues, including the liver. The importance of the PD-1/PD-L1 pathway in controlling autoimmunity during immune responses has been demonstrated using several infection models. Regulatory T cells (Tregs) are also involved in this control by their capacity of inhibiting T cell responses through the production of inhibitory cytokines and by a cell-cell contact mechanism. The implicated molecules are currently under investigation. In this study the PD-1 and PD-L1 expression was analyzed on several cell types under different stimulation conditions. Hence, PD-1 was found on activated T cells whereas PD-L1 was constitutively expressed on bone marrow-derived dendritic cells and induced after stimulation. Tregs showed constitutive expression of PD-1 and PD-L1. Both molecules were induced after stimulation. In vitro experiments using anti-PD-1 and a new generated anti-PD-L1 antibody showed a co-inhibitory function for the PD-1/PD-L1 pathway regulating T cell responses. Nevertheless the suppressive capacity of Tregs was not affected by a PD-1 blockade. The role of the interaction between PD-1 and PD-L1 during infection was analyzed using a mouse model of malaria. The disease transmitted by Plasmodium is characterized by two different stages known as liver and blood stage. During the liver stage, which is asymptomatic, the sporozoites replicate inside the hepatocytes inducing only a low specific CD8+ T cell response against the pathogen. Despite the continuous contact with the parasite, people living in endemic areas are not protected against re-infections. The reasons for that phenomenon are unclear. The parasite’s life cycle follows with the invasion of the erythrocytes, known as the blood stage and provokes their posterior destruction. This results in bouts of fever and anaemia. Only the immunization with gamma-irradiated sporozoitic forms, which are unable to proceed with the blood stage, induces effective and protective CD8+ T cells suggesting that the blood stage negatively regulates liver stage-specific responses. The results obtained in this study showed that the PD-L1 expression is induced in the liver during the blood stage in the mouse model of malaria and that it regulates the liver stage-specific CD8+ T cells which express PD-1. The immunization with an experimental vaccine, that induces liver stage-specific CD8+ T cells without confering protection, together with the blockade of the PD-1/PD-L1 pathway using an antibody against PD-1 during the sporozoite infection, results in an increased number of liver-stage specific IFN-gamma producing CD8+ T cells. Additionally, this enhanced T cell response coincided with a lower parasitemia in antibody-treated mice. In conclusion, the control of the immune response during infection is of major importance in order to avoid cell mediated tissue damage. Along the evolution, Plasmodium as well as other pathogens appears to have been taken advantage of the PD-1/PD-L1 pathway as an escape mechanism to evade the host’s immune system.