Evasion of Innate Cytosolic DNA Sensing by a Gammaherpesvirus Facilitates Establishment of Latent Infection

Abstract

Herpesviruses are DNA viruses harboring the capacity to establish lifelong latent-recurrent infections. There is currently limited knowledge on viruses targeting the innate DNA sensing pathway and also on how the innate system impacts on the latent reservoir of herpesvirus infections. Here we report that murine gammaherpesvirus MHV68, in contrast to alpha- and beta-herpesviruses, induce very limited innate immune responses through DNA-stimulated pathways, which correspondingly played only a minor role in control of MHV68 infections in vivo. Similarly, Kaposi's sarcoma-associated herpesvirus also did not stimulate immune signaling through the DNA sensing pathways. Interestingly, a MHV68 mutant lacking the deubiquitinase (DUB) activity, embedded within the large tegument protein ORF64, gained the capacity to stimulate the DNA-activated STING pathway. We found that ORF64 targeted a step in the DNA-activated pathways upstream of the bifurcation into the STING and AIM2 pathways, and lack of the ORF64 DUB was associated with impaired delivery of viral DNA to the nucleus, which instead localized to the cytoplasm. Correspondingly, the ORF64 DUB active site mutant virus exhibited impaired ability to establish latent infection in wild type but not STING-deficient mice. Thus, gammaherpesviruses evade immune activation by the cytosolic DNA sensing pathway, which in the MHV68 model facilitates establishment of infections.

Activation and Regulation of DNA-Driven Immune Responses

“…Herpesviruses were recently demonstrated to also have evolved a mechanism to counteract the exposure of viral DNA to cytosolic DNA sensors (76). It was first reported that the HSV-1 capsid is ubiquitinated in the cytosol in macrophages and that this stimulates a degradation pathway leading to the release of DNA into the cytosol (64).…”

“…It was first reported that the HSV-1 capsid is ubiquitinated in the cytosol in macrophages and that this stimulates a degradation pathway leading to the release of DNA into the cytosol (64). All herpesviruses encode a deubiquitinase in the large tegument protein, and it has now been reported that this enzymatic activity is essential for the efficient delivery of viral DNA to the nucleus versus the cytoplasm and that alpha-, beta-, and gammaherpesviruses lacking the deubiquitinase activity stimulate elevated expression of type I IFN through the STING pathway (76).…”

“…At present, there are data to support that this occurs through at least 2 pathways. First, in murine fibroblasts and myeloid cells lacking STING, the activation of NF-B and the induction of NF-B-stimulated genes are impaired (75,76). The STING-dependent pathway for the activation of NF-B was proposed to involve TRAF6 acting upstream of TBK1 to trigger the classical IB kinase ␣/␤ (IKK␣/␤) pathway (75) (Fig.…”

Murine Gammaherpesvirus 68 Pathogenesis Is Independent of Caspase-1 and Caspase-11 in Mice and Impairs Interleukin-1β Production upon Extrinsic Stimulation in Culture

“…Similarly, the removal of AIM2, a cytosolic DNA sensor that activates the inflammasome in response to murine cytomegalovirus (MCMV), leads to an impaired NK cell response and an increase in virus titers during infection (17). The loss of the inflammasome components NLRP3 or ASC, but not the AIM2 sensor, results in an increase of MHV68 genomes in the spleen (18). Thus, the inflammasome response is a critical innate defense with the potential to activate immune cells to control herpesvirus replication.…”

“…EBV encodes a miRNA that reduces transcription of the NLRP3 sensor protein (23). The KSHV open reading frame 63 (ORF63) tegument protein impairs both the NLRP1-and the NLRP3-mediated inflammasomes (24), while the MHV68 ORF64 deubiquinase impairs sensing that reduces type I IFN and inflammasome responses upon infection of dendritic cells (18). Gammaherpesviruses activate and inhibit inflammasome responses in culture (19-21, 23, 24), but the influence of caspase-1-dependent inflammatory responses on pathogenesis in vivo is unclear.…”