HPV – How a cancer-causing virus escapes the immune system

Human Papilloma Virus (HPV) is a member of the Papilloma Viruses which have in fact been evolving and spreading among mammals and birds for millions of years. More than 300 Papilloma viruses have now been identified, and more than 200 are HPV. More than 40 HPV viruses cause infection in humans. HPV is the most common sexually transmitted agent in the world. It can be grouped by genus, species and type. Alpha-genus HPV is classified as high-risk or low-risk according to oncogenic potential. Low-risk types cause anogenital and skin warts, while high-risk HPV types cause cervical, vaginal, penile, anal and head-and-neck cancers, also known as oropharyngeal cancers. HPV reproduces more slowly than other viruses and has therefore developed mechanisms to evade the immune (defense) system. In this article, we will summarize the mechanisms HPV uses to escape the immune system.

Many people have HPV infection, but in most infected individuals, defense mechanisms are highly effective in eliminating the initial HPV infection.

How does our body eliminate HPV infection naturally?

Physical barriers

The cells forming the outer layer of our skin (basal keratinocytes) provide an important barrier to viral entry. Infection occurs when HPV settles in the skin or mucous membranes, and tissue damage increases susceptibility to HPV infection. The mucous membrane provides an important barrier against viral infection by secreting sticky protective fluids and antimicrobial peptides. When HPV enters the extracellular (outside-the-cell) fluid, extracellular proteases modify its capsid (the virus's outer layer), and it is taken into the cell by macropinocytosis. It then reaches the nucleus (cell nucleus) with the help of the endocytic pathway and Golgi organelle. Up to this stage, many viral particles are destroyed by phagocytosis. The nuclear envelope is another barrier preventing HPV DNA from entering the nucleus. Besides these barriers, cells contain powerful antagonists that prevent HPV infection.

Note: antagonist means substances that have an opposing effect.

Innate Immunity

When HPV enters a cell, it is detected by TLR (Toll-like receptor) and other receptors. Certain intracellular pathways are then activated and try to prevent the virus from multiplying. Natural killer cells, Dendritic cells and Langerhans cells are innate immune cells involved in the early response to HPV infection. In fact, the early inflammatory responses to HPV infection are very important for initiating a strong immune defense.

Adaptive (acquired) immunity

To eliminate HPV-infected cells, the adaptive system has among its members T-cells which play very important roles. For T cells to be activated, Dendritic cells first process the antigen and increase expression of the Major Histocompatibility Complex (MHC) to present this antigen to T cells. Binding between a Dendritic cell and a T-cell stimulates T-cell proliferation and increases expression of IL-2 and anti-apoptotic genes. Expression of these genes activates the adaptive immune system. The infected cells are eliminated.

What mechanisms does HPV use to evade the immune system?

Gene expression

One of HPV's mechanisms for evading the immune system is keeping early viral gene expression at a low level. In other words, HPV initially hides its identity. This mechanism is controlled by the E2 gene, and the number of viral antigens is very small. HPV proteins are also not secreted, so infected cells cannot be detected by Antigen-Presenting Cells (APC), and the adaptive immune response is prevented.

Highly immunogenic virions and high expression of viral proteins occur only in the upper layers of the epithelium. Virions are released through cell shedding from the epithelial surface, but immune cells have limited access to the layers close to the epithelial surface, so HPV can evade the immune system.

HPV can also use the host cell's DNA methylation mechanisms to alter gene expression and hide from the immune system. DNA methylation is actually a cellular defense mechanism against foreign DNA. Methylation of the CpG residue of foreign DNA eliminates its pathogenic activity, but some pathogens, such as HPV, can take over these methylation mechanisms and use them for their own benefit. E7, one of HPV's viral oncoproteins, stimulates the DNA Methyltransferase 1 (DNMT1) enzyme, increasing methyltransferase activity. In addition, the E7 protein causes hypermethylation of the CXCL14 promoter region, silencing that gene. CXCL14 is an important chemokine responsible for chemotaxis of many lymphocytes, including dendritic cells, natural killer cells and T cells; hypermethylation reduces this chemokine molecule's activity. The E7 oncoprotein causes methylation in the promoter gene region of the E-kaderin molecule required for Langerhans-cell adhesion. E-kaderin activity decreases, so Langerhans cells cannot adhere sufficiently to infected cells. E6, another HPV oncoprotein, causes methylation near the transcriptional region of IFN-k and suppresses IFN-k activity.

In addition to DNA methylation, HPV also manipulates histone modification to evade the immune system. Histone modification is another epigenetic mechanism regulating gene expression. HPV38 E7 affects the polycomb protein of zeste homolog 2 (EZH2), and Histone 3 Lysine 27 trimethylation (H3K27me3) occurs in the TLR9 promoter region, reducing TLR9 transcriptional activity. The HPV16 E7 protein also affects Histone deacetylase 1 (HDAC1) and Histone demethylation JARID 1B, reducing Histone 3 Lysine 4 trimethylation (H3K4me3) in the TLR9 transcription start region and thereby reducing TLR9 expression. In addition, the HPV E7 protein inhibits IRF1 transcriptional activity, and substances released in response to IRF1 stimulation, such as IFN-b, decrease significantly.

Nuclear Factor-kB (NF-kB) is one of the immune system's most important transcription factors. It regulates inflammation through substances such as cytokines and growth factors. Oncoproteins of high-risk HPV types inhibit K310 methylation in the p65 gene region, reducing NF-kB activation and the release of proinflammatory cytokines. HPV16 E6 and E7 proteins also bind to the p300/CBP-associated factor (PCAF), again reducing NF-kB activation and consequently IL-8.

Disruption of intracellular pathways and proteins

HPV oncogenes can use protein-protein interactions to disrupt the host cell's protein and signaling functions. The HPV18 E16 protein acts directly on Non-receptor Tyrosine Kinase 2 (TYK2) to reduce phosphorylation of TYK2 and the transducer and activator of Transcription 2 (STAT2). This also disrupts the Interferon (IFN) response. Another mechanism discovered in recent years is the c-GAS-STING pathway's inhibition by HPV. Normally, c-GAS is a molecule that senses foreign cytosolic DNA, while STING is a protein that initiates innate immune signals against viruses. The HPV18 E7 oncoprotein targets the c-GAS-STING pathway, suppressing type 1 IFN production and reducing the cell's antiviral immune response.

Another evasion mechanism used effectively by HPV oncogenes is posttranslational modification and protein degradation. For example, in keratinocytes, high-risk HPV types affect the ubiquitin carboxyl-terminal hydrolase 1 (UCHL1) protein by increasing its activity. UCHL 1 inhibits ubiquitination of TNF receptor-associated Factor-3 (TRAF3), reducing IRF 3 phosphorylation. The UCHL1 protein also suppresses the NF-kB signaling pathway. Interaction between the HPV E6 protein and the E6AP molecule disrupts pro-IL-1B production; in other words, high-risk HPV types actually try to suppress proinflammatory processes.

HPV does not kill the epithelial cells it infects and does not cause detectable viremia in the body. As a result, the antigen recognition and presentation process cannot function properly. HPV hides from the immune system by affecting various molecules involved in antigen presentation. HPV-16 suppresses proteasomes (PSM8 and PSM9), which initiate antigen presentation by degrading systolic proteins. It also reduces synthesis of transporter molecules (TAP1 and TAP2). It inhibits Antigen-Presenting Cells (APC). It also affects MHC, the most important molecule in antigen presentation. The task of MHC protein is to present antigens to T-lymphocytes, members of the adaptive immune system. HPV16 E7 and E5 oncoproteins block MHC-1 by causing the MHC-1 molecule to remain in the Golgi organelle, and also suppress MHC-2 by disrupting chaperone-protein maturation. As a result, T cells become less able to detect foreign antigens, allowing HPV to hide from the adaptive system as well.

The L2 protein in HPV's capsid structure disrupts the normal functioning of Dendritic cells and Langerhans cells. By blocking these cells in a way, it also disrupts antigen presentation.

It changes T-cell activity

HPV tries to suppress T cells through various mechanisms. It inhibits the cytotoxic effect by shifting the Th1 cellular response toward a Th2 humoral response. It also reduces expression of the CD1d molecule, which activates the adaptive immune system and plays an important role in Th1/Th2 conversion. HPV also activates the CTLA-4 molecule, which drives T lymphocytes and B lymphocytes into apoptosis, and the programmed cell-death mechanism.