A Review of the Potential Role of Human Cytomegalovirus (HCMV) Infections in Breast Cancer Carcinogenesis and Abnormal Immunity

Abstract

Previously recognized classical human onco-viruses can regulate complex neoplastic events, and are estimated to play a role during carcinogenesis in 15-20% of cancer cases. Although the DNA and gene products of several viruses have been found in breast tumors, none of the classical onco-viruses have definitely been linked to the initiation of breast cancer. However, recent evidence shows that human cytomegalovirus (HCMV) gene products are found in >90% of tumors and metastases of breast cancers, and their increased expression can be correlated to a more aggressive breast cancer phenotype. Supporting the active role of HCMV in breast cancer, a specific HCMV strain, HCMV-DB, was recently shown to exert oncogenic transformational activity in breast epithelial cells in vitro, and to give rise to fast-growing, triple-negative breast tumors when injected into immune deficient mice. The same observation holds true for clinical studies implying increased HCMV protein expression in triple negative breast cancer biopsies. In addition to functionally being able to hijack tumor-promoting cellular events, HCMV is known to exhibit a wide range of immunosuppressive effects, which can have radical impact on the tumor microenvironment. HCMV infected cells can avoid recognition and elimination by the immune system by orchestrating polarization of immunosuppressive type II macrophages, preventing antigen presentation, by expressing T cell inhibitory molecules, and possibly, by the induction of regulatory T (Treg) cell responses. These actions would be especially deleterious for the antigenic activation and proliferation of tumor specific CD8⁺ cytotoxic T lymphocytes (CTLs), whose effector functions have recently been targeted by successful, experimental immunotherapy protocols. The recognition of alternative causes and drivers of breast cancer is a pivotal research topic for the development of diagnostics and novel, effective preventive and therapeutic strategies targeting both tumor cells and their microenvironments.

Keywords: T cells; antiviral treatment; breast cancer; cancer; check-point inhibitors; dendritic cells; human cytomegalovirus; triple negative breast cancer; tumor associated macrophages; tumor immunology.

Figure 1

Schematic picture of the HCMV clinical isolate. The HCMV viral genome consists of linear double stranded DNA, ranging from 220 to 240 kB in length. The genome is comprised of unique long (UL) and a unique short (US) regions, which are flanked by inverted open reading frame (ORF) repeats, named terminal repeat long (TRL), internal repeat long (IRL), internal repeat short (IRS), and terminal repeat short (TRS) regions. The latency regions consisting of 19 ORFs (denoted UL133-151) are highlighted. They are present in the low-passage HCMV clinical strains, but are missing in the HCMV laboratory strain AD169. The HCMV-DB clinical strain is reported to share 99% genomic similarity to Toledo and JP clinical strains, but is mutated in RL13 and UL9 genes [11]. A previous functional annotation of these protein products showed that UL9 encodes for a membrane glycoprotein that is involved in viral growth suppression, whereas RL13 encodes for a 60S ribosomal protein. RL13a has a role in epithelial tropism.

Figure 2

The normal immune response against virus infections. Cytotoxic CD8+ T cells (CTLs) are effector cells, which display a critical role in the immune defense against intracellular pathogens, including HCMV, and in tumor surveillance. They can recognize viral or mutant cellular peptides presented by MHC class I molecules on their target cells, and exclusively eliminate the virally infected or malignant cells. Their function is supported by antigen specific CD4+ T cells, which are activated by an antigenic peptide presented by MHC class II histocompatibility complexes on the antigen-presenting cells (APCs), such as macrophages and dendritic cells. In addition to inducing a B cell dependent antibody response, CD4+ T cells are crucial for the initial antigenic activation of naïve CD8+ T cells, which contribute to the pool of sustaining memory CD8+ T cells. IL-2 production from antigen stimulated CD4+ T cells activates natural killer (NK) cells, which play an important role in the immune surveillance against HCMV infected or malignant cells, including HCMV infected macrophages [18,19]. FasL, Fas ligand; TRAIL-R, tumor necrosis factor (TNF)-related apoptosis-inducing ligand; TCR, T cell receptor; NK, natural killer cell; MHC, major histocompatibility complex.

Figure 3

HCMV infection affects several cell types in the tumor. HCMV infection in epithelial cells is pro-tumorigenic, and contributes to the polarization of M1 to M2 macrophages or TAMs. The key TAM-mediated immunosuppressive effects are related to TAMs’ ability to recruit the CD4+ T regulatory (Treg) cells. In addition, tumor cells secrete immunosuppressive cytokines, such as TGF-β and IL-10, which are conductive to proliferation of the TAMs and induction of the CD8+ Treg cells [43]. TAM/Treg shift results to unfavorable, immune suppressive effects, resulting in tumorigenic tolerization and immune escape, thereby promoting cancer progression [43,53]. The CD8+ Tregs can eliminate the activated tumor specific CD4+ T cells, and inhibit actions of both CD8+ T cells and NK cells. The CD4+ Tregs can counteract both the tumor specific CD8+ CTL, and B cell activation and proliferation. HCMV infection and IL-10 secretion from the tumor cells promote an immature phenotype in dendritic cells, which contributes to the reduced antigen-presenting capacity, and increased elimination of the activated T cells through Fas/FasL and TRAIL pathways [42]. The tumor escape is supported when the aberrant activation of the T-cell receptor alone in mature T cells produces a long-lived state of functional unresponsiveness, anergy. Tumor cells express death ligands (PD-L1), of which receptors are expressed on the surface of the immune cells, contributing to the lower proliferation, cytokine production, and their cytotoxic abilities.