Dual recognition of herpes simplex viruses by TLR2 and TLR9 in dendritic cells

Abstract

Dendritic cells (DCs) express multiple Toll-like receptors (TLR) in distinct cellular locations. Herpes simplex viruses (HSV) have been reported to engage both the surface TLR2 and intracellular TLR9 in conventional DCs. However, the contributions of these TLRs in recognition of HSV and the induction of proinflammatory cytokines in DCs remain unclear. Here, we demonstrate that a rare population of HSV, both in laboratory strains and in primary clinical isolates from humans, has the capacity to activate TLR2. This virus population is recognized through both TLR2 and TLR9 for the induction of IL-6 and IL-12 secretion from bone marrow-derived DCs. Further, we describe a previously uncharacterized pathway of viral recognition in which TLR2 and TLR9 are engaged in sequence within the same DC. Live viral infection results in two additional agonists of TLR2 and TLR9. These results indicate that in cells that express multiple TLRs, pathogens that contain multiple pathogenassociated molecular patterns can be detected in an orchestrated sequence and suggest that the innate immune system in DCs is optimized to linking uptake and degradation of pathogens to microbial recognition.cytokines ͉ innate immunity ͉ Toll-like receptor ͉ viral infection ͉ plasmacytoid dendritic cell I nnate recognition of viruses by the mammalian immune system is critical in providing both the immediate antiviral effects, mediated in large part by type I IFNs (1), and in inducing appropriate classes of adaptive immune responses required for clearing viral infection (2, 3). Distinct mechanisms are used to recognize viruses and the viral replication intermediates produced during viral infections. Whereas most cells use the retinoic acid inducible gene-I to recognize certain ssRNA virus infections (4) and melanoma differentiation-associated gene 5 to recognize picornaviruses (5), plasmacytoid dendritic cells (pDCs) use exclusively the Toll-like receptors (TLRs) to recognize ssRNA and dsDNA viruses (4, 6). More recently, a TLR-independent cytosolic recognition of DNA has been shown to play a major role in the induction of type I IFNs (7,8).For innate recognition of herpes viruses, at least three pathways have been described. The first pathway involves the detection of viral genomic dsDNA by the TLR9. This pathway is used by the pDCs to recognize Herpes simplex virus (HSV)-1 (9, 10) and HSV-2 (10). TLR9-mediated recognition also represents the predominant pathways in non-pDCs, such as splenic CD11c ϩ DCs upon HSV-1 infection (9). Further, systemic inoculation of UVirradiated HSV-2 results in IFN␣ production in an myeloid differentiation factor (MyD)88 and TLR9-dependent manner (10). Similarly, in vivo recognition of a beta herpes virus, murine cytomegalovirus, has been shown to depend on TLR9 and MyD88 (11,12). The second type of TLR-herpesvirus interaction occurs when virions engage surface TLR2 on DCs and macrophages. Peritoneal macrophages (13) and microglial cells (14) secrete inflammatory cytokines in response to HSV-1 in a TLR2-dependent manner. T...

Herpesviral infection and Toll-like receptor 2

“…A large fraction of this TLR2-dependent recognition of HSV by DCs requires TLR9. Furthermore, subspecies which do not stimulate TLR2 may still stimulate TLR9 (Sato et al, 2006). Together, these observations highlight the redundancy in innate detection and suggest the possibility of greater importance for TLR9 relative to TLR2 in detection of HSV.…”

“…The relative importance of TLR2 signaling in HSV infection has also been called into question by the in vitro observation that a rare population of HSV, both in laboratory strains and in primary clinical isolates from humans of a variety of tissues and mucosal secretions, has the capacity to activate TLR2 (Sato et al, 2006). This study found that clinical and some laboratory HSV isolates generally exist as a collection of subspecies of viral clones, most of which do not activate TLR2.…”

“…This strategy may be beneficial to the virus in order to induce upregulation of viral entry receptors but may also serve the host in favoring a rapid respond to viral invaders (Bieback et al, 2002;Zhu et al, 2007). Thus far, a large body of works demonstrate that TLR2 has been implicated in the recognition of structural components of several viruses (Bieback et al, 2002;Duesberg et al, 2002;Dolganiuc et al, 2004;Cooper et al, 2005), as well as members of the herpesvirus family, including HSV, VZV, murine gammaherpesvirus-68 (MHV-68), CMV, and EBV (Compton et al, 2003;Kurt-Jones et al, 2004;Wang et al, 2005a;Boehme et al, 2006;Sato et al, 2006;Szomolanyi-Tsuda et al, 2006;Gaudreault et al, 2007;Ariza et al, 2009;Michaud et al, 2010).…”

“…A number of recent studies have demonstrated that TLRs expressed by peritoneal macrophages and microglial cells, which are key immune cells that survey the brain parenchyma, are pivotal in identifying and generating the first line of innate immune responses against viral pathogens in the central nervous system , 2005Olson and Miller, 2004;Aravalli et al, 2005;Town et al, 2006). Although specific viral ligands for TLR2 have not been identified (Sato et al, 2006), which may have major roles in virus entry and initiate innate immune recognition at this early stage of the viral life cycle, recent studies suggest that TLR2 signaling may be involved in innate responses to HSV.…”

“…It is anticipated that, as with other viruses, such as LCMV (Zhou et al, 2005), the elimination of these responses will result in a defective acquired immune response. It was recently shown that TLR2 and TLR9 act in a cooperative manner to activate conventional DCs (cDC)s during infection with HSV (Sato et al, 2006), and that TLR2 and TLR9 are sequentially engaged by HSV clones recognized by TLR2. A large fraction of this TLR2-dependent recognition of HSV by DCs requires TLR9.…”