Pathogenesis of human cytomegalovirus in the immunocompromised host

Abstract

Human cytomegalovirus (HCMV) is a herpesvirus that infects ~60% of adults in developed countries and more than 90% in developing countries. Usually, it is controlled by a vigorous immune response so that infections are asymptomatic or symptoms are mild. However, if the immune system is compromised, HCMV can replicate to high levels and cause serious end organ disease. Substantial progress is being made in understanding the natural history and pathogenesis of HCMV infection and disease in the immunocompromised host. Serial measures of viral load defined the dynamics of HCMV replication and are now used routinely to allow intervention with antiviral drugs in individual patients. They are also used as pharmacodynamic read-outs to evaluate prototype vaccines that may protect against HCMV replication and to define immune correlates of this protection. This novel information is informing the design of randomized controlled trials of new antiviral drugs and vaccines currently under evaluation. In this Review, we discuss immune responses to HCMV and countermeasures deployed by the virus, the establishment of latency and reactivation from it, exogenous reinfection with additional strains, pathogenesis, development of end organ disease, indirect effects of infection, immune correlates of control of replication, current treatment strategies and the evaluation of novel vaccine candidates.

Fig. 1. Overview of human cytomegalovirus entry into target cells and the establishment of latency in non-permissive myeloid cells.

a | Human cytomegalovirus (HCMV) glycoproteins on the surface of the virion engage with receptors on the surface of cells and can drive entry by multiple processes in a cell type-dependent manner. In fibroblasts, glycoprotein H (gH), gL and gO form a trimer that binds to platelet-derived growth factor-α (PDGFRα) and co-receptors. This binding triggers gB to fuse directly with the plasma membrane at neutral pH. In permissive epithelial and endothelial cells, gH and gL form a pentameric complex with three other proteins encoded within the ULb′ region (UL128–UL130–UL131). This pentameric complex binds to neuropilin 2 and triggers pH-dependent endocytosis. The fusion activity of gB becomes relevant for escape from the endosome. For both cell types, once the capsid and associated tegument proteins are released into the cytoplasm they move independently to the nucleus, where virion DNA interacts with the nuclear pore complex to transition into the nucleus. Infection of myeloid cells (including potential sites of latency) involves macropinocytosis. In myeloid cells where HCMV establishes latency (that is, CD34⁺ cells), activation of epidermal growth factor receptor (EGFR) and integrin-mediated src family kinase (SFK) signalling via gB and pentamer, respectively, is required for the trafficking of HCMV DNA contained within the capsid to the nucleus via recycling endosomes. An overview of cellular receptors is provided in ref.. b | Establishment of latency is dependent on effective silencing of major immediate early (MIE) gene expression. In CD34⁺ cells, this is likely a combination of host and viral-encoded events including a failure of virion transactivators (for example, pp71) to enter the nucleus coupled with a host environment of high levels of transcriptional repressors of the MIE promoter (MIEP). The result is establishment of a repressive chromatin phenotype driving MIEP silencing that is maintained by viral UL138 gene expression. c | Cellular differentiation to a dendritic cell promotes induction of transcription from the MIE locus through the activity of host chromatin remodelling enzymes. This process is responsive to inflammatory cytokine signalling (for example, tumour necrosis factor (TNF) and interleukin 6 (IL-6)) through extracellular signal-regulated kinase (ERK) and SFK signalling pathways. Binding sites for multiple transcription factors (for example, nuclear factor-κB (NF-κB), cAMP response element binding protein (CREB), activator protein 1 (AP1)) in the MIEP have been hypothesized to be important for the control of MIE gene expression upon reactivation. Part a is adapted from Murray et al., 2018 CC BY 4.0 (https://www.mdpi.com/2076-0817/7/1/30).

Fig. 2. Viral and host functions in human cytomegalovirus latency and reactivation.

a | In healthy individuals, a robust innate and adaptive immune response restricts human cytomegalovirus (HCMV) reactivation and replication. HCMV counters this with an armoury of measures to disable all arms of the immune response. Recognition by CD8⁺ T cells is limited by major histocompatibility complex (MHC) class I downregulation and prevention of antigen loading and presentation at the cell surface. Similarly, MHC class II presentation to CD4⁺ T cells is prevented by similar strategies including the expression of a viral interleukin-10 (IL-10) homologue that promotes MHC class II downregulation. Loss of MHC class I can potentially activate natural killer cell recognition and killing according to the ‘missing self hypothesis’, thus HCMV promotes the expression of an HLA-E inhibitory receptor as well as numerous gene products that disable natural killer activating receptors and upregulate natural killer inhibitory receptors. The interferon response is disabled at multiple points of the viral life cycle. Specifically, HCMV gene products interfere with DNA sensing pathways to prevent activation including inhibitors of IFI16 (for example, pp65 and US28) and cGAS–STING (UL31 and pp71). Interferon signalling is also disabled via an interaction of IE72 with the signal transducer and activator of transcription (STAT) transcription factor. HCMV also modulates the bio-activity of cytokines through expression of β-chemokine receptors that bind and sequester host cytokines. Additionally, HCMV encodes numerous α-chemokines that mimic CXCL1 and CXCL2 activity to modulate the recruitment to, and activity of, immune cells at the site of infection. b | Potential roles for immunosuppression in HCMV infection and reactivation. HCMV establishes latency in CD34⁺ progenitor cells. Myeloid or dendritic cell progenitor (step Ba) differentiation into macrophages or dendritic cells promotes cellular reactivation (step Bb), production of infectious virus, and subsequent infection and replication in multiple permissive tissue cells (step Bc). HCMV-specific T cells can recognize cellular reactivation (step Bd) or disseminated infection (step Be). Additionally, B cells produce virus-neutralizing antibodies (step Bf) or non-neutralizing antibodies that likely recognize viral cell surface antigens on reactivating cells (step Bb) or newly infected cells (step Bc). This will promote the recruitment of antibody-dependent effector functions (steps Bg and Bh) to target the infected cells. A second site of viral persistence is hypothesized to be tissue-resident endothelial cells (step Bi), although whether they are seeded via differentiation from a latently infected CD34⁺ progenitor or by direct infection in tissue is unknown. Hypothetically, these latently infected endothelial cells are activated by an undefined stimulus and thus, following viral replication, can be recognized by immune responses mediated by T cells (step Bj) and B cells (steps Bk and Bl) as described for macrophages and dendritic cells. In the context of immunosuppression, pre-existing T cell responses will be reduced; in individuals who are seropositive, this leads to reduced control of both cellular and clinical reactivation. In individuals who are seronegative experiencing primary infection (who have no latent HCMV reservoir in CD34⁺ cells), the major impact of immunosuppression is a reduction in the generation of new T cell and B cell responses, reducing control of replication in permissive cells (steps Be, Bf and Bh). These processes are likely exacerbated through inflammation (allogeneic T cells or co-infection), enhancing cellular reactivation and viral replication in individuals who are seropositive (steps Bm and Bn) and viral replication in infected individuals who are seronegative (step Bn). T_H1 cell, T helper 1 cell.

Fig. 3. Prevalence of human cytomegalovirus antibodies and incidence of infection in immunocompromised individuals.

a | Prevalence of prior human cytomegalovirus (HCMV) infection is high (~90%) in individuals with HIV infection and intermediate (~60%) in those awaiting haematopoietic stem cell transplantation (SCT). The solid organ transplantation (SOT) patient group can be divided further according to prevalence of antibodies in the donor as well as the recipient. b | Once individuals become immunocompromised either through immunosuppression after transplantation or because the CD4⁺ T cell count in individuals who are HIV-positive falls below 100 cells per microlitre of blood, they are at risk of HCMV viraemia, which can be detected by PCR. In the SOT group, the risk is highest in those with primary infection (donor positive–recipient negative (D+R–)), intermediate in those at risk of reactivation or reinfection (D+R+) and lowest in those at risk of reactivation only (D–R+). This illustrates that pre-existing natural immunity against HCMV provides substantial protection against exogenous (reinfection) and endogenous (reactivation) sources of virus. Note that the incidence of end organ disease (EOD) declines in parallel with reduced detection of viraemia. In the SCT group, the risk of viraemia and EOD is as high as in D+R– individuals in the SOT group, despite the SCT patients being R+. Comparison of the SCT group with D–R+ individuals in the SOT group shows that reactivation dominates R+ SCT recipients and that ablation of their bone marrow greatly reduces immunity acquired in the past. The incidence of both viraemia and EOD is intermediate in individuals who are HIV-positive. Figure drawn from information provided in refs^,,,,–.

Fig. 4. Distribution of peak viral loads in immunocompromised individuals.

a | Peak viral loads approximate to a normal distribution in the donor positive–recipient negative (D+R–) solid organ transplantation (SOT) patient subgroup. b | By contrast, the distribution is shifted strongly to the left in the D+R+ subgroup where the recipient has natural immunity pre transplantation (green). c | Natural immunity does not prevent low viral loads resulting from reactivation (or either reactivation or reinfection (part b)). d | Following haematopoietic stem cell transplantation (SCT), the peak viral loads are relatively low yet patients in this subgroup have a high risk of end organ disease (EOD) (see Fig. 3). This shows that SCT patients are susceptible to a low viral load that would be unlikely to cause EOD in SOT. e | Individuals with HIV have a high viral load distribution, similar to that seen after D+R– SOT. Figure drawn using data provided in refs^,,,–.

Fig. 5. Two distinct strategies used to reduce human cytomegalovirus disease in allograft recipients.

In the case of prophylaxis (upper panel), an antiviral drug is given from the time of transplantation (as soon as the patient can tolerate oral medication) for a fixed period of time, with clinical trials for solid organ transplantation (SOT) supporting durations of either 100 or 200 days^,. This strategy is effective in preventing end organ disease (EOD), but patients are at risk once again after prophylaxis is stopped, including with strains of human cytomegalovirus (HCMV) resistant to the drug used for prophylaxis (late-onset disease). In the case of pre-emptive therapy (PET) (lower panel), patients are monitored frequently to determine whether HCMV DNA is detectable by PCR. Individuals with low viral loads continue to be monitored, but those with a viral load above a defined threshold are given antiviral therapy until two consecutive blood tests can no longer detect HCMV DNA. Patients continue to be monitored and may require a subsequent episode of PET. Humoral and cell-mediated responses are superior in SOT managed using PET and late-onset disease is uncommon^,. For haematopoietic stem cell transplantation (SCT), valganciclovir prophylaxis cannot be used because of bone marrow toxicity of the drug. Letermovir is safe enough to be used for prophylaxis and is combined with PET. If the individual fails to respond to treatment (that is, the viral load does not show at least a one-log reduction over 2 weeks), refractory HCMV infection is diagnosed. This may be due to poor host responses and/or the selection of strains resistant to the antiviral drug being administered. At present, foscarnet is commonly used to treat ganciclovir-resistant strains of HCMV, but has severe side effects. Phase II results for maribavir are encouraging with phase III randomized clinical trial (RCT) results expected in 2021.