Testing candidate effectors contributing to resistance to pneumoviruses

Pneumoviruses are members of the Paramyxoviridae family, negative sense single stranded RNA viruses. The 3 major members of the pneumovirus genus are human respiratory syncytial virus (huRSV), bovine respiratory syncytial virus (boRSV) and their murine counterpart, pneumonia virus of mice (PVM). In humans, huRSV mainly infects young children and can lead, in the worst cases, to infantile bronchiolitis, which is the first cause of children hospitalization and concerns 2.3% of children under the age of one. Many laboratories are studying this disease in order to better understand the role played by the immune system on its pathogenesis and to create an efficient and safe vaccine that does not exist for now. In bovine, boRSV generates stable epidemics in calves under 6 months of age and causes important economic losses in terms of veterinary costs and loss of productivity. These viruses infect the respiratory tract and generate a disease that importantly depends on the genetic background of the host. The quality of the immune response plays a considerable role in the expression as well as in the outcome of the disease, especially during its first instants. The study of pneumoviruses diseases has been widely developed using murine models of infection by heterologous huRSV. It is now assumed that this model, although providing interesting information, only weakly reproduces human disease on the contrary to infection of mice with their homologous pneumovirus: PVM. This model has been broadly developed in our laboratory, by highlighting the similarities between murine, human and bovine diseases, i.e. an important morbidity, intra-pulmonary granulocytic influx, viral amplification and evolution toward acute respiratory distress syndrome. The present work consisted in studying potential effectors of the resistance of mice against PVM infection: two proteic and two cellular effectors. A first working hypothesis was to evaluate the role of Carcinoembryonic antigen-related cell adhesion molecule 1 (Ceacam1) as a potential receptor for PVM. This membrane protein present at the surface of respiratory epithelial cells is known to be mutated in SJL/J mice, the most resistant strain to PVM infection. We have thus infected highly PVM-susceptible 129/Sv mice and compared their susceptibility to Ceacam1a knock-out 129/Sv mice. Our results have permitted to discard Ceacam1a as a receptor to PVM. The second tested protein is bovine Mx1 (boMx1) GTPase. We wished to assess if its expression could lead, as it is the case for numerous negative sense single stranded RNA viruses, to a resistance to PVM. Transgenic mice producing boMx1, engineerd in our laboratory in an FVB/J background, have been infected and their susceptibility has been compared to wild-type FVB/J mice. We could evidently demonstrated that boMx1 induces resistance to PVM. We have thus, for the first time, demonstrated that an Mx protein induces resistance to a pneumovirus. We have then compared 2 inbred mice strains presenting opposite phenotypes concerning their susceptibility to PVM: SJL/J (resistant) and 129/Sv (susceptible). Our goal was to evaluate if the difference in resistance is present during the first moments of infection. Therefore, we have focussed on both cell types that first interact with the virus: respiratory epithelial cells and alveolar macrophages. Virus tropism and respiratory epithelial permissivity to viral amplification were compared between both strains. In the same way, alveolar macrophages’ phenotypes (phagocytosis, susceptibility to viral infection, cytokine production) were compared between SJL/J and 129/Sv. Our results could not demonstrate any difference concerning respiratory epithelium. On the other hand, a pivotal role could be attributed to alveolar macrophages in the first instants of the infection. SJL/J alveolar macrophages presented greater phagocytic capacity, increased resistance to virus replication and earlier and higher cytokine production. Our results concerning the role of alveolar macrophages as a key effector of the immune response during the infection of mice by PVM raises new interrogations. Differences observed between SJL/J and 129/Sv alveolar macrophages regarding their resistance to viral infection, their greater phagocytic capacity and their cytokine secretion will have to be further investigated in order to understand the relative importance of these three characteristics in the resistance process of SJL/J to PVM infection. A particular attention will have to be given to the candidate cytokines in order to determine if one of them plays a particular role in the disease outcome. Moreover, the anti-PVM property of boMx1 raises the question of why boMx1 possesses an anti-PVM property but no anti-boRSV effect. Comparison of the interaction of boMx1 with PVM or boRSV could help understanding this anti-PVM effect.TESTING CANDIDATE EFFECTORS CONTRIBUTING TO RESISTANCE TO PNEUMOVIRUSE