Human cytomegalovirus reprogrammes haematopoietic progenitor cells into immunosuppressive monocytes to achieve latency

Abstract

The precise cell type hosting latent human cytomegalovirus (HCMV) remains elusive. Here, we report that HCMV reprogrammes human haematopoietic progenitor cells (HPCs) into a unique monocyte subset to achieve latency. Unlike conventional monocytes, this monocyte subset possesses higher levels of B7-H4, IL-10 and inducible nitric oxide synthase (iNOS), a longer lifespan and strong immunosuppressive capacity. Cell sorting of peripheral blood from latently infected human donors confirms that only this monocyte subset, representing less than 0.1% of peripheral mononuclear cells, is HCMV genome-positive but immediate-early-negative. Mechanistic studies demonstrate that HCMV promotes the differentiation of HPCs into this monocyte subset by activating cellular signal transducer and activator of transcription 3 (STAT3). In turn, this monocyte subset generates a high level of nitric oxide (NO) to silence HCMV immediate-early transcription and promote viral latency. By contrast, the US28-knockout HCMV mutant, which is incapable of activating STAT3, fails to reprogramme the HPCs and achieve latency. Our findings reveal that via activating the STAT3-iNOS-NO axis, HCMV differentiates human HPCs into a longevous, immunosuppressive monocyte subset for viral latency.

Interferon-responsive genes are targeted during the establishment of human cytomegalovirus latency

“…US28-mediated signalling is critical to latency: US28-deleted viruses, or the loss of G-protein coupling by the US28 mutant R129A, result in lytic infection of undifferentiated myeloid cells 19,21,23,25 . Similarly, infection of US28-WT-expressing THP-1 cells with US28-deletion viruses leads to complementation and the establishment of latency; both the US28-R129A and empty vector cell lines fail to establish latent infection under these conditions 21 .…”

“…The viral GPCR US28 is expressed during both lytic and latent infection of HCMV. While US28 is dispensable for lytic replication in vitro 76,77 , it is essential for the establishment and maintenance of HCMV latency in early myeloid lineage cells 19,21,23,25 . This is attributable, in part, to the ability of US28 to suppress the major immediate early promoter; a US28 function specific for undifferentiated myeloid cells 18,21,23,25 .…”

“…We hypothesised that this ability of US28 to so profoundly regulate viral IE gene expression in undifferentiated myeloid cells was likely via US28-mediated modulation of host protein abundance and, therefore, we wanted to determine whether such US28-driven changes could be important for the establishment or maintenance of HCMV latency. While previous work has used targeted arrays to assess US28-mediated effects on myeloid cells 21,23,25 ; here we have used an unbiased proteomic screen to understand how US28 reprograms host cells in order to support latent infection. Our screen compared host protein abundance in control THP-1 cells or THP-1 cells which express either WT-US28 or the US28 signalling mutant, US28-R129A.…”

“…One viral factor that suppresses MIEP activity is the G-protein coupled receptor (GPCR) US28, a virally encoded chemokine receptor homologue, which is expressed de novo during latency as well as being delivered to cells with the incoming virion 19–24 and this incoming viral US28 is functional 25 . US28 modulates the signalling pathways of early myeloid cells; it attenuates MAP kinases, NF-κB, and c-fos, whilst activating STAT3 and iNOS 21,23,25 . All of these contribute to the repression of MIEP activity.…”

“…All of these contribute to the repression of MIEP activity. This US28-mediated signalling is so critical to latency that US28-deleted viruses, or the loss of G-protein coupling by the US28 mutant R129A, result in lytic infection of undifferentiated myeloid cells 19,21,23,25 . Furthermore, when examined, these US28-mediated effects on cell signalling did not occur during lytic infection or in permissive cells 21 , implying that US28 represses the MIEP during latency but does not impair reactivation following cellular differentiation.…”