Interestingly, the treatment with mDC-derived L particles also resulted in a significant reduction (P< 0.01) of CD83 surface expression (gray bar), supporting the hypothesis that L particles are indeed responsible for CD83 downmodulation in uninfected bystander DCs. Taken together, the above reported data clearly demonstrate that CD83 is usually downmodulated in uninfected bystander DCs and that this reduction occurs in a time-delayed fashion compared to infected mDCs. parallel with full virions, called H particles. L particles contain viral proteins but lack the viral capsid and DNA. Therefore, these particles are not infectious but are able to transfer several viral proteins. Incubation of mDCs with L particles indeed reduced CD83 expression on uninfected bystander DCs, providing for the first time evidence that functional viral proteins are transmitted via L particles from infected mDCs to uninfected bystander cells, thereby inducing CD83 downmodulation. IMPORTANCEHSV-1 has developed a number of strategies to evade the host's immune system. Among others, HSV-1 contamination of mDCs results in an inhibited Daunorubicin T cell activation caused by degradation of CD83. Interestingly, CD83 is usually lost not only from HSV-1-infected mDCs but also from uninfected bystander cells. The release of so-called L particles, which contain several viral proteins but lack capsid and DNA, during contamination is usually a common phenomenon observed among several viruses, such as human cytomegalovirus (HCMV), Epstein-Barr computer virus, and HSV-1. However, the detailed function of these particles Daunorubicin is usually poorly comprehended. Here, we provide for the first time evidence that functional viral proteins can be transferred to uninfected bystander mDCs via L particles, revealing important biological functions of these particles during lytic replication. Therefore, the transfer of viral proteins by L particles to modulate uninfected bystander cells may represent an additional strategy for viral immune escape. == INTRODUCTION Rabbit Polyclonal to PPP2R3C == Dendritic cells (DCs) are known as the most potent antigen-presenting cells (APCs) due to their unique ability to primary naive T cells. Thus, they are vital to induce effective antiviral immune responses. In their immature state, DCs reside as sentinels of the immune system in almost all peripheral tissues until they encounter and take up antigens, resulting in maturation of DCs. As a consequence, expression of major histocompatibility complex (MHC) classes I and II as well as of costimulatory molecules, Daunorubicin such as CD40, CD80, CD86, and also CD83, is strongly induced (13). As CD83 is not expressed on immature, tolerogenic DCs but is usually highly upregulated during DC maturation, this protein has become one of the best surface markers for mature DCs (35). Nevertheless, CD83 is also expressed on subsets of activated T cells, B cells, granulocyte precursor cells, myelocytes, neutrophils, and thymus epithelial cells as well as on regulatory T cells (510). In addition to this membrane-bound CD83 molecule (mCD83), a soluble form of CD83 (sCD83), consisting of the extracellular Ig domain name of mCD83, also has been explained previously (11,12). This soluble form, which was shown to possess potent immunosuppressive properties, is usually released from activated DCs as well as from B cells and can be detected at low levels in sera of healthy individuals (11) and at highly elevated concentrations in patients suffering from malignant disorders (12). Using animal models, it could be demonstrated that a recombinant expressed sCD83 molecule inhibits disease-associated symptoms in experimental autoimmune encephalomyelitis as well as in an inflammatory bowel disease model. In addition, sCD83 was shown to prevent graft rejection in different transplantation models (1317). In contrast, mCD83 expressed on human mature DCs (mDCs) has been suggested to have costimulatory properties, sincein vitroknockdown of mCD83, using small interfering RNA (siRNA) technology, resulted in a significantly reduced stimulatory capacity of these DCs (18,19). Furthermore, several viruses, especially herpesviruses, including human cytomegalovirus (HCMV), varicella-zoster computer virus, and also herpes simplex virus 1 (HSV-1), modulate the surface expression of mCD83, resulting in inhibition of T cell proliferation and thus in reduced antiviral immune responses (2022). HSV-1 is the prototype of the alphaherpesvirus subgroup and is characterized by low species specificity and, in addition, by an extremely fast lytic replication cycle. During replication, not only are total virions, called H particles (heavy particles), created, but also so-called L particles (light particles) are released. L particles resemble H particles, but they lack the capsid and viral DNA. Interestingly, it has been shown previously that.