A Herpesvirus Virulence Factor Inhibits Dendritic Cell Maturation through Protein Phosphatase 1 and IκB Kinase

Abstract

Dendritic cells are sentinels in innate and adaptive immunity. Upon virus infection, a complex program is in operation, which activates IB kinase (IKK), a key regulator of inflammatory cytokines and costimulatory molecules. Here we show that the ␥ 1 34.5 protein, a virulence factor of herpes simplex viruses, blocks Toll-like receptor-mediated dendritic cell maturation. While the wild-type virus inhibits the induction of major histocompatibility complex (MHC) class II, CD86, interleukin-6 (IL-6), and IL-12, the ␥ 1 34.5-null mutant does not. Notably, ␥ 1 34.5 works in the absence of any other viral proteins. When expressed in mammalian cells, including dendritic cells, ␥ 1 34.5 associates with IKK␣/␤ and inhibits NF-B activation. This is mirrored by the inhibition of IKK␣/␤ phosphorylation, p65/RelA phosphorylation, and nuclear translocation in response to lipopolysaccharide or poly(I:C) stimulation. Importantly, ␥ 1 34.5 recruits both IKK␣/␤ and protein phosphatase 1, forming a complex that dephosphorylates two serine residues within the catalytic domains of IB kinase. The amino-terminal domain of ␥ 1 34.5 interacts with IKK␣/␤, whereas the carboxyl-terminal domain binds to protein phosphatase 1. Deletions or mutations in either domain abolish the activity of ␥ 1 34.5. These results suggest that the control of IB kinase dephosphorylation by ␥ 1 34.5 represents a critical viral mechanism to disrupt dendritic cell functions.Herpes simplex virus 1 (HSV-1), a member of the Herpesviridae, establishes both latent and lytic infection (45). Upon infection of the mucosal tissues, HSV encounters a variety of cells, including dendritic cells (DCs), that bridge innate and adaptive immunity (7). Immature DCs are able to capture and process viral antigens. When activated, DCs express a high level of costimulatory molecules. Moreover, DCs release inflammatory cytokines to promote DC maturation. A prominent feature of DCs is to activate naive T cells, where myeloid mucosal and lymph node-resident DCs are responsible for HSV-specific T cell activation (1,40,47).A complex program is in operation upon DC maturation, which is coupled to Toll-like receptor (TLR)-related pathways (34). For example, when exposed to lipopolysaccharide (LPS) or viral proteins (3, 24, 41), TLR4 activates the two arms of downstream signaling. In this process, TLR4 recruits TRIF via an adaptor, TRAM, and migrates to the endosomal membrane, where it activates TANK binding kinase 1 (TBK1) and interferon (IFN) regulatory factor 3 (IRF3). In parallel, TLR4 engages with MyD88 through the adaptor TIRAP (also called Mal) at the plasma membrane, which facilitates the formation of a complex consisting of TRAF6, TAK1, TAB2, and IRAK. This relays signals to IB kinase (IKK), containing the IKK␣, ⌱⌲⌲␤, and IKK␥ subunits. Activated IB kinase phosphorylates IB proteins to trigger their ubiquitination and proteosome-mediated degradation, leading to the nuclear translocation of NF-B, which usually exists as a p65 (RelA)/p50 heterodimer (12). Accordingly, TLR activation upr...

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