Viruses Evade the Immune Response - Online Tutor, Practice Problems & Exam Prep

Viruses evade the immune response through various mechanisms. They can prevent the effects of antiviral proteins (AVPs) by coating their RNA, thus avoiding detection by pattern recognition receptors (PRRs). Additionally, some viruses interfere with antigen presentation by manipulating MHC class 1 molecules, using fake versions to evade natural killer cells. Lastly, they can mutate surface proteins to escape antibody recognition or exploit antibodies for entry into host cells, enhancing their replication. Understanding these strategies is crucial for developing effective antiviral therapies.

Intro to How Viruses Evade the Immune Response

In this video, we're going to begin our lesson on how some viruses are able to evade the immune response. And so once again, in order to successfully replicate inside of a host, some viruses must evade host defense mechanisms. Now moving forward in our course, we're going to talk about 3 different ways by which some viruses can evade host defenses. And so, what you'll notice is that down below, we're going to mention each of these 3 different ways, but moving forward in their own separate videos, we'll get to talk more details about each of these 3 different ways.

And so the first way that some viruses can evade host defenses is by preventing the effects of antiviral proteins or AVPs. And so, that could involve the use of viral proteins to protect viral mRNA from RIG-like, PRRs, pattern recognition receptors. And again, we'll talk more details about this process moving forward.

Another way by which some viruses can evade host defenses is by viruses interfering with the antigen presentation process. And so what you'll notice is that here is a virus and the virus is interfering with the antigen presentation process so that it will not be recognized by other immune cells. And again, we'll talk more details about this process moving forward.

And then last but not least, the third way by which some viruses can evade host defenses is by some viruses avoiding the effects of bound antibodies. And so, once again, we'll get to talk more about that later and talk about how some viruses are able to mutate to change their surface proteins to avoid being detected by antibodies.

And so this here concludes our brief introduction to how some viruses can evade the immune response, and we'll get to talk more about each of these 3 different ways as we move forward in our course. So I'll see you all in our next video.

1) Viruses Prevent Antiviral Effects of AVPs

In this video, we're going to talk more details about how some viruses are able to avoid the immune response by preventing antiviral effects of antiviral proteins or AVPs. First, we need to recall from some of our previous lesson videos what interferons are and what they do. Recall from those older videos that interferons are cytokines that are produced by virally infected cells. When those virally infected cells produce the interferons, neighboring cells produce antiviral proteins or AVPs. Now, the infected cell is initially going to be able to identify the viral RNA via pattern recognition receptors or PRRs inside of the cytoplasm. For example, a RIG-like receptor or a RIG-like PRR is an example of a pattern recognition receptor that can detect viral RNA. Some viruses are able to coat their RNA with a viral protein that basically hides or protects the viral RNA from the cell's cytoplasmic PRRs.

If we take a look at our image down below, we can get a better understanding of this idea. What you'll notice is that we're showing you how some viruses can protect their RNA and block apoptosis in cells. Notice here what we're showing you is a virus that is coming into a host cell. This big blue thing represents our host cell. We have a virus that is infecting the host cell and it uncoats and releases its nucleic acid. It can be transcribed to form viral mRNA. What you'll notice is that in some cases, viral proteins are able to bind and protect the viral RNA. Those viral proteins can protect the viral RNA and prevent RIG-like receptors or RLRs, which are really just pattern recognition receptors that belong to the host, from binding to the viral RNA. Notice that this big red X here represents the fact that the RIG-like receptor is not able to bind to the viral RNA, which means that the viral RNA is able to be translated to create viral proteins.

In addition, once these viral proteins have been made, many of these viral proteins may inhibit or block the process of apoptosis. What you'll notice is the viral proteins are in orange and the host's apoptosis proteins are in green. The viral proteins can bind to the host, apoptosis proteins and prevent apoptosis from occurring. What we need to recall from some of our previous lesson videos, once again, is that antiviral proteins can cause a virally infected cell to immediately go into apoptosis, and apoptosis prevents viral replication. It's in the virus's best interest to prevent apoptosis. Some viruses are able to inhibit the expression of certain genes in the host cell that will prevent apoptosis so that the virus is able to replicate. Once again, that's what we're seeing down below over here on this side. We're showing you how viral proteins can inhibit host apoptosis proteins from triggering apoptosis. This prevents apoptosis from occurring, allowing the virus to be able to replicate.

This concludes our brief lesson on how some viruses are able to avoid the immune system by preventing antiviral effects by using proteins to prevent recognition through PRRs and also using viral proteins to prevent and block apoptosis. We'll be able to get some practice applying these concepts and learn more as we move forward. I'll see you all in our next video.

Host cells that are infected with a virus should undergo apoptosis. However, some viruses can prevent host cells from undergoing apoptosis and thus allowing the virus to continue to replicate within the infected cell. All of the following are ways viruses prevent host cell apoptosis except which of these answers?

• A. Covering viral RNA with viral proteins so host cell RIG-Like PRRs do not bind to the viral RNA.
• B. Inhibiting the expression of host cell genes which trigger apoptosis during an infection.
• C. Evolving antigenic variation to avoid detection by host antibodies and immune cells.
• D. Preventing the expression of host antiviral proteins which trigger apoptosis.

2) Viruses Interfere with Antigen Presentation in Infected Cells

In this video, we're going to talk about yet another way that some viruses can evade the immune system, and that is by interfering with antigen presentation in infected cells. First, we need to recall from way back in some of our previous lesson videos that infected host cells present intracellular antigens on their surface via MHC class I molecules. This process allows those infected host cells to alert cytotoxic T cells, and those cytotoxic T cells can induce apoptosis in those infected cells. Now, some viruses have evolved mechanisms to prevent the movement of MHC class I molecules to the surface of the cell, and that is a mechanism for how some viruses are able to evade the immune system. However, also recall that natural killer cells that are part of our immune system are programmed to identify host cells that lack MHC class I molecules, and then they can immediately destroy those cells that lack MHC class I. And so that's a mechanism that counteracts those viruses that prevent the movement of MHC class I to the surface.

Now to counteract this, some viruses have evolved a special mechanism of producing fake MHC class I molecules that are going to be presented on the cell surface in order to trick the natural killer cells. Host cells that are presenting these fake MHC class I molecules are not going to be destroyed by natural killer cells, and that will allow the virus to replicate and evade the immune system. If we take a look at our image down below, we'll be able to get a better understanding of these concepts and how viruses can interfere with the antigen presentation process.

Notice over here on the left-hand side of our image, we're showing you a host cell that has normal MHC class I molecules on its surface. Also notice that we're showing you a virus here that is getting ready to infect this host cell. This virus has evolved mechanisms to cause the cell to present fake MHC class I molecules. Now notice that this host cell is presenting fake MHC class I molecules on its surface after being infected by the virus. These fake MHC class I molecules can trick the cells of the immune system. Natural killer cells and cytotoxic T cells cannot recognize the infected cell that has those fake MHC class I molecules. Therefore, these immune system cells, the natural killer cells and the cytotoxic T cells do not respond to this infection. This is going to mean that the infected cell is not going to be killed, and the virus that's inside of the infected cell is going to be able to replicate. This means that the virus has been able to avoid the immune system.

This here concludes our brief lesson on how some viruses are able to interfere with antigen presentation and create fake MHC class I molecules in infected cells in order to evade the immune system. We'll be able to practice these concepts moving forward. So, I'll see you all in our next video.

A virus that causes the infected host cell to express and present 'fake' MHC I molecules on its surface is evading detection from which host immune cells?

• A. T_H cells & NK cells.
• B. T_C cells & NK cells.
• C. T_REG cells & T_C cells.
• D. NK cells & macrophages.

3) Viruses Avoid the Effects of Antibodies

This video, we're going to talk about another way that some viruses can evade the immune system, and that is by avoiding the effects of antibodies. Similarly to bacteria, viruses can also rapidly change their surface structures due to mutations that accumulate in their genome during replication. These mutations that accumulate in the viral genome can change its surface structures, and those changes in the surface structures allow the virus to once again evade the adaptive immune response, specifically evading the antibodies. Some viruses use antibodies to enhance their infection by the process of antibody-dependent enhancement, also known as ADE for short. Viral particles that are bound by antibodies in this process of ADE are engulfed by macrophages where the virus is actually able to actively replicate.

If we take a look at our image down below over here on the left-hand side, we're showing you how some viruses are able to mutate to avoid antibodies. Notice over here on the left-hand side we're showing you our normal virus and this virus is going to have viral surface proteins. What you'll notice is that the viral surface proteins over here on the left-hand side are pink and circular. Through mutations that can accumulate as this virus replicates, these mutations can lead to changes in the surface molecules of the virus. Now, this virus has new viral surface proteins, and the surface proteins have gone from pink circles to being these blue squares. Thus, the antibodies that used to be effective towards the circular pink viral proteins are no longer effective towards the bluish square surface proteins. By mutating, the virus is able to evade the effects of the antibodies.

Now over here on the right-hand side, we're also showing you how viruses can actually use antibodies themselves to enter cells in the process of antibody-dependent enhancement or ADE. In this image, you'll notice that there is a virus, and the virus is being bound by these antibodies. But these antibodies are actually going to assist with the virus's replication. These antibodies will actually enhance the ability for this virus to replicate. Notice that the virus is saying "Engulf me, I dare you" to the macrophage. As these antibodies are bound, it leads to phagocytosis. However, this virus is able to evade destruction and it is actually able to replicate inside of the macrophage via this process. The virus replicates within the macrophage, evading the antibodies and evading the immune system. This here concludes our brief lesson on how some viruses are able to avoid the effects of antibodies in order to evade the immune response. We'll be able to get some practice applying this as we move forward. So I'll see you all in our next video.

Some viruses want to be engulfed by phagocytic immune cells. These viruses use some of the same survival mechanisms that bacterial pathogens use to survive and replicate within a phagocyte. Those mechanisms include all of the following except which of these answers?

• A. Escaping the phagosome after being engulfed by the phagocyte.
• B. Having defenses against the digestive and hydrolytic enzymes of the phagolysosome.
• C. Preventing the phagosome and lysosome from fusing to create the phagolysosome.
• D. Preventing antibodies from binding the virus and triggering the process of phagocytosis.

Which of the following statements about viruses is false?

• A. They can enter the host cell via endocytosis.
• B. They are known to colonize the surface of the skin.
• C. They have the ability of preventing apoptosis of infected host cells.
• D. They may enter host cells by fusing their viral envelopes with the host cell membrane.
• E. They may suppress the expression of MHC I molecules on host cells.

Here’s what students ask on this topic:

How do viruses evade the immune response by preventing the effects of antiviral proteins (AVPs)?

Viruses can evade the immune response by preventing the effects of antiviral proteins (AVPs) through several mechanisms. One key strategy involves coating their RNA with viral proteins, which hides the viral RNA from pattern recognition receptors (PRRs) like RIG-like receptors (RLRs) in the host cell's cytoplasm. This prevents the host cell from recognizing the viral RNA and initiating an antiviral response. Additionally, some viral proteins can inhibit apoptosis, the programmed cell death that would normally occur in infected cells. By blocking apoptosis, the virus ensures that the host cell remains alive long enough for the virus to replicate. These strategies allow the virus to avoid detection and destruction by the host's immune system.

How do viruses interfere with antigen presentation to evade the immune system?

Viruses can interfere with antigen presentation by manipulating MHC class I molecules. Normally, infected cells present viral antigens on their surface via MHC class I molecules, alerting cytotoxic T cells to induce apoptosis. Some viruses prevent MHC class I molecules from reaching the cell surface, thus evading detection. To counteract this, natural killer (NK) cells destroy cells lacking MHC class I. However, some viruses produce fake MHC class I molecules, tricking NK cells into not destroying the infected cell. This allows the virus to replicate without being targeted by the immune system.

How do viruses avoid the effects of antibodies?

Viruses can avoid the effects of antibodies through rapid mutation of their surface proteins. These mutations alter the viral surface structures, making previously effective antibodies unable to recognize and neutralize the virus. Additionally, some viruses exploit a process called antibody-dependent enhancement (ADE). In ADE, antibodies bound to the virus facilitate its entry into host cells like macrophages, where the virus can replicate. This not only helps the virus evade the immune response but also enhances its replication within the host.

What role do pattern recognition receptors (PRRs) play in the immune response to viral infections?

Pattern recognition receptors (PRRs) are crucial in the immune response to viral infections. They detect viral components, such as RNA, within the host cell. RIG-like receptors (RLRs) are a type of PRR that recognize viral RNA in the cytoplasm. Upon detection, PRRs trigger signaling pathways that lead to the production of interferons and other cytokines, which activate antiviral defenses. These defenses include the production of antiviral proteins (AVPs) that inhibit viral replication and promote apoptosis of infected cells. By recognizing and responding to viral components, PRRs play a key role in initiating the immune response against viral infections.

What is antibody-dependent enhancement (ADE) and how does it benefit viruses?

Antibody-dependent enhancement (ADE) is a process where antibodies facilitate viral entry into host cells, enhancing viral replication. In ADE, antibodies bind to viral particles but instead of neutralizing them, they promote their uptake by immune cells like macrophages through Fc receptors. Once inside the macrophage, the virus can evade destruction and replicate. This process benefits the virus by increasing its ability to infect host cells and evade the immune response. ADE is a significant concern in the development of vaccines and therapies, as it can potentially worsen the infection rather than prevent it.