Human cytomegalovirus triggered necroptosis is suppressed by sequestration of MLKL in the nucleus of infected monocytes

Abstract

The systemic spread of human cytomegalovirus (HCMV) is associated with severe morbidity and mortality in immunocompromised and immunonaive patients. Hematogenous dissemination of HCMV to different organ sites is facilitated by peripheral blood monocytes. Circulating monocytes have a short lifespan due, in part, to their intrinsic biological programming to initiate caspase 8-mediated apoptosis upon entry into the circulation from the bone marrow. We previously reported that HCMV extends the lifespan of infected monocytes by blocking procaspase 8 cleavage, yet the precise viral mechanism responsible for suppressing caspase 8 activity remains unknown. Here, we demonstrate that HCMV entry into monocytes rapidly increases the abundance of the antiapoptotic cellular FLICE-like inhibitory protein long (cFLIPL), which prevents procaspase 8 cleavage into active caspase 8. However, others have demonstrated that inhibition of caspase 8 opens a "trapdoor" cell death response termed necroptosis. Accordingly, we found the increased levels of cFLIPL, along with a co-stimulatory signal from toll like receptor 3 (TLR3), activates the receptor-interacting protein kinase 3 (RIPK3) responsible for initiating necroptosis. Despite triggering of the necroptotic cascade within infected monocytes, the final execution of this death pathway is thwarted by nuclear sequestering of mixed lineage kinase domain like pseudokinase (MLKL), the executioner of necroptosis. Together, our data reveal a multitude of countermeasures employed by HCMV to obstruct cellular antiviral death responses within infected monocytes. ImportanceHCMV is highly prevalent in the adult population with a seroprevalence of 50-80% in the United States. Although immunocompetent individuals are generally asymptomatic, HCMV infection can cause multiorgan disease in immunocompromised and immunonaive patients. Peripheral blood monocytes are responsible for the systemic dissemination of HCMV. However, the inherently short lifespan of monocytes combined with the induction of antiviral cellular death responses requires HCMV to circumvents cell death pathways to allow for viral spread. In this work, we show that HCMV induces cFLIPL levels to inhibit caspase 8-mediated apoptosis. However, the inhibition of apoptosis, combined with TLR3 activation, triggers a secondary cell death pathway termed necroptosis. As a countermeasure to block necroptosis, HCMV sequesters MLKL within the nucleus of infected monocytes. Defining the precise mechanisms through which HCMV stimulates survival will provide insight into novel therapeutics able to target infected monocytes.