Cytomegalovirus Immune Evasion Mechanism

Background of Cytomegalovirus

Human cytomegalovirus (HCMV) is a double-stranded DNA virus that belongs to the beta subfamily of herpesviruses and is the DNA virus with the largest genome among human herpesviruses. The full length of the HCMV genome is approximately 240 kb, consisting of unique long (UL) and unique short (US) sequences, with inverted repeats at both ends. HCMV has a complex proteome, and it is estimated that HCMV has more than 200 open reading frames encoding proteins. HCMV replication is tightly regulated by multiple steps, and gene expression can be divided into immediate early, early and late stages. The glycoprotein (GP) rich in the HCMV envelope is the main determinant of its tissue affinity and plays an important role in the virus entering cells and spreading between cells.

Figure 1. Human cytomegalovirus entry into target cells and the establishment of latency in non-permissive myeloid cells.(Griffiths P, et al.; 2021)

After the body is first infected with HCMV, the virus cannot be completely cleared by the body's immune response and enters a latent infection period, where the virus is excreted for a long time or intermittently. HCMV has various latent sites, and the viral DNA has been detected in monocytes, dendritic cells, megakaryocytes, myeloid progenitor cells and endothelial cells in the bone marrow. In immunocompromised people (including organ transplant recipients, hematopoietic stem cell transplant recipients, HIV-infected patients, and patients receiving immunomodulatory drugs), latent HCMV infection is more likely to be reactivated and invade the lungs, gastrointestinal tract, and central nervous system, and retina and other organs and tissues, causing serious complications and even death. Cytomegalovirus (CMV) infection can be transmitted through multiple ways such as close contact, blood or tissue exposure, perinatal exposure, and sexual exposure. The innate immune barrier is the body's first line of defense against CMV invasion. When the barrier is damaged or if the development is not perfect (such as blood-fetal barrier), the body will be damaged by CMV. During the infection process, CMV is first recognized by non-specific immune cells such as natural killer (NK) cells, monocyte-macrophages, and dendritic cells (DC), initiating the innate immune response. Innate immunity can recognize pathogens, trigger cell signaling cascades, regulate the body's production of interferon (IFN) and pro-inflammatory factors, thereby triggering the body's antiviral response and activating the adaptive immune response. The adaptive immune response can generate protective immunity against CMV through the direct antiviral effect of specific lymphocytes and the secretion of various cytokines, and control virus replication.

Figure 2. Viral and host functions in human cytomegalovirus latency and reactivation. (Griffiths P, et al.; 2021)

CMV has formed a complex anti-immune response mechanism during its long-term coexistence with the host. It can not only use its huge genome to induce the synthesis of multiple components to participate in the anti-infection response of the host, but also regulate host cells to achieve immune evasion. The immune escape mechanism is an important research topic in the field of CMV. Analyzing its mechanism is of great scientific significance for the design of safe and efficient new vaccines and the development of antiviral drugs. Now we will introduce the research on CMV immune evasion mechanism.

T Cell Exhaustion

Due to long-term exposure to persistent antigen stimulation and inflammation in chronic infections, T cells gradually lose their effector functions and memory T cell characteristics also begin to be lost, which is called T cell exhaustion. It is mainly manifested by the gradual loss of effector functions (such as defective cytokine production), increased and sustained expression of multiple inhibitory receptors, metabolic disorders, and changes in cellular transcription and epigenetics. The increase in soluble immunosuppressive factors and defective T cell function are the main causes of T cell exhaustion.

1. Cytokines: Cytokines and the inflammatory environment play an important role in T cell activation and differentiation. IL-10 is a pleiotropic cytokine that has a bidirectional immunoregulatory effect in CMV infection, mainly in negative regulation, helping the virus achieve immune escape.
2. Inhibitory receptors: Under normal conditions, PD-1 is expressed briefly during the activation of effector T cells and then returns to baseline levels; while in chronic infection, PD-1 expression on Tex cells remains at a high level. Studies have shown that in patients with severe sepsis, compared with the non-activated group, patients in the CMV reactivation group showed high levels of PD-1, accompanied by CD⁸⁺ T cell exhaustion and a reduction in the proportion of multifunctional CD⁸⁺ T cells. Other studies have found that during CMV infection, the expression of PD-1 is up-regulated in functionally exhausted virus-specific CD⁸⁺ T cells. Blocking PD-1 can restore the vitality of exhausted T cells and reduce viral load.
3. Immune regulatory or suppressor cells: CD⁴⁺ T cells antagonize CD⁸⁺ T cell exhaustion to a certain extent. If the number of CD⁴⁺ T cell lymphocytes decreases, the probability of CMV reactivation increases.

Interfering with NK Cells

CMV can evade the killing effect of NK cells by regulating the inhibitory or activating signals of NK cells. Inhibitory receptors include CD94/NKG2C, immunoglobulin-like transcript receptors, etc. among killer cell lectin-like receptors, while NKG2D and NKp30 and others are activating receptors. Multiple gene loci in the CMV genome, such as UL135, UL142 and UL148, have been proven to inhibit NK cell recognition and killing through different mechanisms. UL135 inhibits the formation of immune synapses by remodeling the actin cytoskeleton; glycoproteins encoded by UL142 and UL148A inhibit NK cell activation by downregulating the expression of the NKG2D ligand MICA on the cell surface; UL148 affects the CD2 costimulatory molecule CD58 (LFA-3) expression. In addition, the m02 and m145 family members in CMV can also express MHC-class I molecular analogues, which bind to NK cell inhibitory receptors, inhibit NK cell activation, and protect infected cells from being killed.

Interference with Humoral Immunity

CMV can directly interfere with viral antibody production and effector functions. Studies have found that the glycoproteins gp34 and gp68 encoded by the CMV RL11 gene and UL118-UL119 gene can compete with FcγR on immune cells to bind to the IgGC end, blocking the viral protein-IgG complex from binding to receptors on the surface of phagocytes and NK cells. These processes block antibody-dependent cell-mediated cytotoxicity and the classic complement activation pathway. Interestingly, studies have found that deletion of these two genes significantly enhances the ability of anti-CMV polyclonal IgG antibody-mediated FcγR activation, the ability of immune cells to clear viruses is enhanced.