Human Cytomegalovirus-Induced Autophagy Prevents Necroptosis of Infected Monocytes.

Key to the viral dissemination strategy of human cytomegalovirus (HCMV) is the induction of monocyte survival, where monocytes are normally short-lived cells. Autophagy is a cellular process that preserves cellular homeostasis and promotes cellular survival during times of stress. We found that HCMV rapidly induced autophagy within infected monocytes. The early induction of autophagy during HCMV infection was distinctly required for the survival of HCMV-infected monocytes, as repression of autophagosome formation led to cellular death of infected cells but had no effect on the viability of uninfected monocytes. The inhibition of caspases was insufficient to rescue cell viability of autophagy-repressed infected monocytes, suggesting that autophagy was not protecting cells from apoptosis. Accordingly, we found that HCMV blocked the activation of caspase 8, which was maintained in the presence of autophagy inhibitors. Necroptosis is an alternative form of cell death triggered when apoptosis is impeded and is dependent on RIPK3 phosphorylation of MLKL. Although we found that HCMV activated RIP3K upon infection, MLKL was not activated. However, inhibition of autophagy removed the block in RIPK3 phosphorylation of MLKL, suggesting that autophagy was protecting infected monocytes from undergoing necroptosis. Indeed, survival of autophagy-inhibited HCMV-infected monocytes was rescued when MLKL and RIPK3 were suppressed. Taken together, these data indicate that HCMV induces autophagy to prevent necroptotic cell death in order to ensure the survival of infected monocytes and thus facilitate viral dissemination within the host.IMPORTANCE Human cytomegalovirus (HCMV) infection is endemic throughout the world, with a seroprevalence of 40 to 100% depending on geographic location. HCMV infection is generally asymptomatic, but can cause severe inflammatory organ diseases in immunocompromised individuals. The broad array of organ diseases caused by HCMV is directly linked to the systematic spread of the virus mediated by monocytes. Monocytes are naturally programmed to undergo apoptosis, which is rapidly blocked by HCMV to ensure the survival and dissemination of infected monocytes to different organ sites. In this work, we demonstrate infected monocytes also initiate necroptosis as a "trap door" death pathway in response to HCMV subversion of apoptosis. HCMV then activates cellular autophagy as a countermeasure to prevent the execution of necroptosis, thereby promoting the continued survival of infected monocytes. Elucidating the mechanisms by which HCMV stimulates monocyte survival is an important step to the development of novel anti-HCMV drugs that prevent the spread of infected monocytes.

Human Cytomegalovirus Mediates Unique Monocyte-to-Macrophage Differentiation through the PI3K/SHIP1/Akt Signaling Network.

Blood monocytes mediate the hematogenous dissemination of human cytomegalovirus (HCMV) in the host. However, monocytes have a short 48-hour (h) lifespan and are not permissive for viral replication. We previously established that HCMV infection drives differentiation of monocytes into long-lived macrophages to mediate viral dissemination, though the mechanism was unclear. Here, we found that HCMV infection promoted monocyte polarization into distinct macrophages by inducing select M1 and M2 differentiation markers and that Akt played a central role in driving differentiation. Akt's upstream positive regulators, PI3K and SHIP1, facilitated the expression of the M1/M2 differentiation markers with p110δ being the predominant PI3K isoform inducing differentiation. Downstream of Akt, M1/M2 differentiation was mediated by caspase 3, whose activity was tightly regulated by Akt in a temporal manner. Overall, this study highlights that HCMV employs the PI3K/SHIP1/Akt pathway to regulate caspase 3 activity and drive monocyte differentiation into unique macrophages, which is critical for viral dissemination.

Human Cytomegalovirus Glycoprotein-Initiated Signaling Mediates the Aberrant Activation of Akt.

Human cytomegalovirus (HCMV) is a major cause of morbidity and mortality among immunocompromised and immunonaive individuals. HCMV-induced signaling initiated during viral entry stimulates a rapid noncanonical activation of Akt to drive the differentiation of short-lived monocytes into long-lived macrophages, which is essential for viral dissemination and persistence. We found that HCMV glycoproteins gB and gH directly bind and activate cellular epidermal growth factor receptor (EGFR) and integrin ß1, respectively, to reshape canonical Akt signaling within monocytes. The remodeling of the Akt signaling network was due to the recruitment of nontraditional Akt activators to either the gB- or gH-generated receptor signaling complexes. Phosphoinositide 3-kinase (PI3K) comprised of the p110ß catalytic subunit was recruited to the gB/EGFR complex despite p110δ being the primary PI3K isoform found within monocytes. Concomitantly, SH2 domain-containing inositol 5-phosphatase 1 (SHIP1) was recruited to the gH/integrin ß1 complex, which is critical to aberrant Akt activation, as SHIP1 diverts PI3K signaling toward a noncanonical pathway. Although integrin ß1 was required for SHIP1 recruitment, gB-activated EGFR mediated SHIP1 activation, underscoring the importance of the interplay between gB- and gH-mediated signaling to the unique activation of Akt during HCMV infection. Indeed, SHIP1 activation mediated the increased expression of Mcl-1 and HSP27, two Akt-dependent antiapoptotic proteins specifically upregulated during HCMV infection but not during growth factor treatment. Overall, our data indicate that HCMV glycoproteins gB and gH work in concert to initiate an HCMV-specific signalosome responsible for the atypical activation of Akt required for infected monocyte survival and ultimately viral persistence.IMPORTANCE Human cytomegalovirus (HCMV) infection is endemic throughout the world regardless of socioeconomic conditions and geographic locations with a seroprevalence reaching up to 100% in some developing countries. Although asymptomatic in healthy individuals, HCMV can cause severe multiorgan disease in immunocompromised or immunonaive patients. HCMV disease is a direct consequence of monocyte-mediated systematic spread of the virus following infection. Because monocytes are short-lived cells, HCMV must subvert the natural short life-span of these blood cells by inducing a distinct activation of Akt, a serine/theonine protein kinase. In this work, we demonstrate that HCMV glycoproteins gB and gH work in tandem to reroute classical host cellular receptor signaling to aberrantly activate Akt and drive survival of infected monocytes. Deciphering how HCMV modulates the cellular pathway to induce monocyte survival is important to develop a new class of anti-HCMV drugs that could target and prevent spread of the virus by eliminating infected monocytes.

HCMV modulation of cellular PI3K/AKT/mTOR signaling: New opportunities for therapeutic intervention?

Human cytomegalovirus (HCMV) remains a major public health burden domestically and abroad. Current approved therapies, including ganciclovir, are only moderately efficacious, with many transplant patients suffering from a variety of side effects. A major impediment to the efficacy of current anti-HCMV drugs is their antiviral effects are restricted to the lytic stage of viral replication. Consequently, the non-lytic stages of the viral lifecycle remain major sources of HCMV infection associated with transplant recipients and ultimately the cause of morbidity and mortality. While work continues on new antivirals that block lytic replication, the dormant stages of HCMV's unique lifecycle need to be concurrently assessed for new therapeutic interventions. In this review, we will examine the role that the PI3K/Akt/mTOR signaling axis plays during the different stages of HCMV's lifecycle, and describe the advantages of targeting this cellular pathway as an antiviral strategy. In particular, we focus on the potential of exploiting the unique modifications HCMV imparts on the PI3K/Akt/mTOR pathway during quiescent infection of monocytes, which serve an essential role in the dissemination strategy of the virus.

Aberrant regulation of the Akt signaling network by human cytomegalovirus allows for targeting of infected monocytes.

Primary peripheral blood monocytes are responsible for the hematogenous dissemination of human cytomegalovirus (HCMV) following a primary infection. In order to facilitate viral spread, HCMV extends the naturally short 48-h lifespan of monocytes by stimulating a non-canonical activation of Akt during viral entry, which leads to the increased expression of a specific subset of antiapoptotic proteins. In this study, global analysis of the Akt signaling network showed HCMV induced a more robust activation of the entire network when compared to normal myeloid growth factors. Furthermore, we found a unique interplay between HCMV-activated Akt and the stress response transcription heat shock factor 1 (HSF1) that allowed for the synthesis of both cap- and internal ribosome entry site (IRES)-containing antiapoptotic mRNAs such as myeloid cell leukemia-1 (Mcl-1) and X-linked inhibitor of apoptosis (XIAP), respectively. As generally a switch from cap-dependent to IRES-mediated translation occurs during cellular stress, the ability of HCMV to concurrently drive both types of translation produces a distinct milieu of prosurvival proteins needed for the viability of infected monocytes. Indeed, we found inhibition of XIAP led to death of ∼99% of HCMV-infected monocytes while having minimal effect on the viability of uninfected cells. Taken together, these data indicate that the aberrant activation of the Akt network by HCMV induces the upregulation of a unique subset of antiapoptotic proteins specifically required for the survival of infected monocytes. Consequently, our study highlights the possibility of exploiting these virus-induced changes to prevent viral spread in immunocompromised patients at high-risk for HCMV exposure.