The role of the human cytomegalovirus UL111A gene in down-regulating CD4+ T-cell recognition of latently infected cells: implications for virus elimination during latency

Abstract

The capacity of human cytomegalovirus (HCMV) to establish and maintain a latent infection from which it can later reactivate ensures its widespread distribution in the population, but the mechanisms enabling maintenance of latency in the face of a robust immune system are poorly understood. We examined the role of the HCMV UL111A gene, which encodes homologs of the immunosuppressive cytokine interleukin-10 in the context of latent infection of myeloid progenitor cells. A UL111A deletion virus was able to establish, maintain, and reactivate from experimental latency in a manner comparable with parental virus, but major histocompatibility complex class II levels increased significantly on the surfaces of cells infected with the deletion virus. Importantly, there was an increase in both allogeneic and autologous peripheral blood mononuclear cells and CD4 ؉ Tcell responses to UL111A deletion virusinfected myeloid progenitors, indicating that loss of the capacity to express IntroductionHuman cytomegalovirus (HCMV) is a species-specific ␤-herpesvirus that infects a majority of the world's population. 1 During primary productive infection, both innate and adaptive immune responses are activated, and replicating virus is eventually cleared from the host. However, the virus is able to evade complete immune clearance by establishing and maintaining a life-long latent infection in hematopoietic cells, specifically those of the myeloid lineage. [2][3][4][5][6][7] During latency, detectable infectious virus production ceases, the viral genome is maintained as an extrachromosomal plasmid 8 at a low viral genome copy number, 7 and only a subset of viral genes remain transcriptionally active. [9][10][11][12][13][14] Reactivation from latency results in reinitiation of the full replicative cycle, with production of new infectious virus.Reactivation of HCMV from latency during states of reduced immune surveillance has profound implications for the management of patients with hematologic and immune deficiency disorders. In these clinical contexts, HCMV reactivation can lead to tissue infection causing major morbidity and mortality. As a result, the presence of HCMV seropositivity and thus of latent virus in donors or recipients of unrelated stem cell transplantations is an adverse prognostic feature despite major advances in HCMV monitoring and management. 15,16 Reactivation of HCMV is also of importance in seropositive patients with nontransplantation cellular immunodeficiencies, such as those resulting from human immunodeficiency virus infection, 17 or in seropositive patients with iatrogenically induced immunodeficiency, such as that resulting from treatment with the anti-CD52 antibody Campath. 18,19 There is no known therapy for the elimination of virus that persists during latency. Instead, where necessary, medical treatment has concentrated on the preemptive treatment or the prophylaxis of viral reactivation. Even here, no single strategy prevents reactivation while universally avoiding unnecessary therapy. There is an urge...

Viral Interleukin-10 Expressed by Human Cytomegalovirus during the Latent Phase of Infection Modulates Latently Infected Myeloid Cell Differentiation

“…Specifically, in four independent experiments, both parental virus and viral IL-10 deletion virus reactivated at the same time (mean time of 12 days after reactivation stimulus), and the frequencies of reactivation were very similar for the parental virus (1.2 ϫ 10 Ϫ4 ) and viral IL-10 deletion virus (1.3 ϫ 10 Ϫ4 ). These results were comparable to the rates of reactivation that we have previously reported for these viruses (9).…”

“…Primary human CD34 ϩ myeloid progenitors were latently infected as previously described (8,22), and cells and culture supernatants were harvested at day 8 postinfection (p.i.). We have previously demonstrated that this viral IL-10 deletion virus infects and maintains a latent infection in CD34 ϩ myeloid progenitors as efficiently as the parental virus does (9). Our routine analyses of viral genome load when new virus stocks were generated or when new infection experiments were undertaken confirmed equal rates of latent infection with these two viruses (see Fig.…”

“…As DCs are the most potent antigen-presenting cell type, suppression of differentiation of latently infected myeloid progenitors toward a DC is likely to enhance the ability of latent virus to limit presentation of latency-associated viral peptides to HCMVspecific T cells. In this respect, we reported that latently infected cells were unable to evade detection by CD4 ϩ T cells in the absence of the ability to express viral IL-10 and that this was concomitant with increased MHC class II by these latently infected cells (9). Thus, modulation of DC differentiation by viral IL-10 may act to render latently infected cells less immunogenic and so limit/evade immune detection to enhance the ability of HCMV to persist in a latent state in the human host.…”

“…We did not identify a significant difference in the timing or frequency of reactivation of viral IL-10 deletion virus in comparison to parental virus using our previously described reactivation assay, whereby latently infected cells were cocultured with monolayers of primary human foreskin fibroblasts to stimulate reactivation, and monolayers were monitored daily for the appearance of cytopathic effect (CPE) as an indicator of virus reactivation (8,9,22). Specifically, in four independent experiments, both parental virus and viral IL-10 deletion virus reactivated at the same time (mean time of 12 days after reactivation stimulus), and the frequencies of reactivation were very similar for the parental virus (1.2 ϫ 10 Ϫ4 ) and viral IL-10 deletion virus (1.3 ϫ 10 Ϫ4 ).…”

“…The vast majority of characterization of functions has come from studies using recombinant viral IL-10 proteins, although some have also assessed function during productive infection with viruses with deficient viral IL-10 proteins. Much less is known about the function of viral IL-10 in the context of latent infection, which, to date, has been limited to a single report that viral IL-10 expressed during latent infection of primary human myeloid progenitor cells modulates cell surface MHC class II levels and renders these cells refractory to recognition by both allogeneic and autologous CD4 ϩ T cells (9). In this study, we examined (i) the production of proinflammatory cytokines linked to the control of cellular differentiation and (ii) the cellular differentiation pattern of primary human myeloid progenitor cells latently infected with parental virus or virus in which the gene encoding viral IL-10 has been deleted (viral IL-10 deletion viruses), providing evidence for a role of viral IL-10 in modulating the differentiation ability of latently infected cells.…”