Mechanisms Underlying Mumps Virus-induced Innate Immune Responses In The Mouse Testis

Background and objectives:Mumps virus (MuV) infection is highly tropism to the testis and frequently causes orchitis, which may lead to male infertility. Although the introduction of vaccines effectively control the pandemic of MuV, mumps orchitis has been recently recurred world-wide. Understanding the mechanisms underlying the innate immune responses to MuV infection and antiviral defense against MuV in the testis can aid in the development of preventive and therapeutic approaches for mumps orchitis. This study aimed to investigate the MuV-induced innate immune responses in the mouse testis and MuV replication in testicular cells.Materials and methods:Primary testicular cells were isolated from C57BL/6 or Toll-like receptor (TLR) knockout (TLR2-/-, TLR3-/-, TLR4-/-) mice. Virus titers were measured using crystal violet staining on Vero cells. Gene expression was examined using real-time quantitative RT-PCR. Cytokines level were measured using ELISA. Proteins were determined by Western blot, immunohistochemistry and immunofluorescence. Specific siRNA for target genes were used to knockdown gene expression.Results:MuV induces innate immune responses in mouse Sertoli and Leydig cells through TLR2 and retinoic acid-inducible gene I (RIG-I) signaling, which result in the production of proinflammatory cytokines and chemokines, including TNF-α, IL-6, MCP-1, CXCL10, and type 1 interferons (IFN-α and IFN-β). By contrast, MuV does not induce the cytokine production in male germ cells. In response to MuV infection, Sertoli cells produce higher levels of proinflammatory cytokines and chemokines but lower levels of IFN-a and IFN-β than Leydig cells. The local injection of MuV into the testis triggers the testicular innate immune responses in vivo, and suppresses testosterone synthesis by Leydig cells.MuV can infect major testicular cells, including Sertoli, Leydig, macrophages, and germ cells. MuV differentially replicates in the testicular cells, which is associated with cell type-specific antiviral defense responses. MuV rapidly replicates in Sertoli cells. MuV also significantly replicates in Leydig cells and macrophages. By contrast, MuV does not significantly replicate in male germ cells. Accordingly, Leydig cells and macrophages produce relatively high levels of type 1 IFNs and antiviral proteins compared with Sertoli and germ cells in response to MuV infection. Notably, autophagy restricts MuV replication in male germ cells and macorphages.Conclusions:MuV can induce innate immune responses in mouse Sertoli and Leydig cells through TLR2 and RIG-I signaling. Testicular cells limit MuV replication via IFN response and/or autophagy. This is the first study examining the innate immune responses to MuV infection and viral replication in testicular cells. The results provide novel insights into the mechanisms underlying the testicular defense against MuV infection.