Pattern Recognition Receptors (PRRs) - Online Tutor, Practice Problems & Exam Prep

Pattern recognition receptors (PRRs) are crucial for immune cells to detect microbial invasion and host cell damage. Key types include Toll-like receptors (TLRs), C-type lectin receptors (CLRs), NOD-like receptors (NLRs), and RIG-like receptors (RLRs). TLRs and CLRs identify microbe-associated molecular patterns (MAMPs) on cell surfaces, while NLRs and RLRs detect intracellular MAMPs and damage-associated molecular patterns (DAMPs). This detection initiates immune responses, such as cytokine release, essential for inflammation and pathogen elimination.

Pattern Recognition Receptors

In this video, we're going to begin our lesson on pattern recognition receptors or PRRs. Pattern recognition receptors are abbreviated as PRRs. These PRRs are really just cell surface receptors of immune cells that are important for sensing and detecting signs of microbial invasion. As we'll learn moving forward in our course, there are many different types of PRRs, and these different types of PRRs all detect signs of microbial invasion. However, different types of PRRs will detect signs of microbial invasion in different areas of the cell. We'll be able to talk about this as we move forward in our course.

Notice down below we're showing you our map of the lesson on innate immunity on the left-hand side. In our previous lesson videos, we've talked about the first line defenses, the second line defenses, and cells of innate immunity. We've already started talking about the scanning systems that are important for detecting signs of microbes, serving somewhat as security cameras. We've already discussed cell communication, and now we're focusing on these pattern recognition receptors.

Moving forward in our course, we're going to talk about several different types of pattern recognition receptors, including Toll-like receptors or TLRs, C-type lectin receptors or CLRs, NOD-like receptors or NLRs, and RIG-like receptors or RLRs. We'll get to talk about all of these different types of pattern recognition receptors moving forward in our course.

All of these pattern recognition receptors are important for detecting signs of microbial invasion, but they will detect signs of microbial invasion in different areas. Some of the PRRs are found on the cell surface, important for detecting microbial components on the outside of the cell. These PRRs, the pattern recognition surface receptors on the cell surface, are important for detecting signs of microbial components and cell damage on the outside of the cell.

Other PRRs are found embedded in the membranes of phagosomes and endosomes, which are membranes that are brought into the cell either through the process of endocytosis or phagocytosis. Here we have an endosome or a phagosome, which is a membrane that results from phagocytosis, and notice that there are PRRs embedded in the membrane of the phagosome. These are important for detecting microbial components that have been ingested by the cell.

Some PRRs detect components on the outside of the cell; some detect microbial components that have been ingested by the cell; and last but not least, some PRRs are found in the cell's cytoplasm. These PRRs are important for detecting microbial components and damage inside of the cell.

We'll be able to talk about these different types of PRRs, exactly where they are found, and what they help detect as we move forward in our course. But for now, this here concludes our brief introduction to pattern recognition receptors or PRRs. As we move forward in our course, we'll get to learn a lot more. So, I'll see you all in our next video.

MAMPs and DMPs

In this video, we're going to introduce the two terms MAMPs and DAMPs. First, we need to recall from some of our previous lesson videos that immune cells use pattern recognition receptors, or PRRs, to detect different types of signs that will indicate either microbial invasion or host cell damage. The term MAMPs is an abbreviation for Microbe Associated Molecular Patterns. Microbe Associated Molecular Patterns, or MAMPs, represent signs of microbes. Signs of pathogens are referred to as PAMPs, and so, PAMPs are pathogen associated molecular patterns. The terms MAMPs and PAMPs can be used interchangeably to refer to a broad group of different types of molecules that are associated directly with microbes or pathogens. Some examples of MAMPs or PAMPs include lipopolysaccharide or LPS, a component found in the outer surface membranes of gram-negative bacteria; peptidoglycan, a molecule specific to bacterial cell walls; viral nucleic acids; fungal cell walls; and flagellin, a protein component important for bacterial flagella. These are all direct signs of microbes or pathogens, and so these are all examples of what could be considered MAMPs or PAMPs.

DAMPs, on the other hand, are damage associated molecular patterns. These are not direct signs of microbes but are more like indirect signs of microbes and pathogens; however, they are direct signs that indicate host cell damage. These are molecules that actually originate from the host but indicate host cell damage. For example, it could be a phospholipid, a component of a host cell membrane, or something of that nature. Something that belongs to the host that indicates that the host cells have been damaged.

If we take a look at our image down below, which is not provided here, you'll notice that we are discussing a macrophage, an immune system cell that has many different types of PRRs, many different types of pattern recognition receptors. These different types of PRRs can detect either MAMPs or DAMPs in different regions. Some PRRs are found on the surface, and these are capable of detecting microbe or pathogen associated molecular patterns, or MAMPs, on the outside of the cells. Notice that we are showing you these PRRs that are positioned on the cytoplasmic membrane that are detecting signs of microbes outside the cell, including lipopolysaccharide, peptidoglycan, and flagellin. Other types of PRRs are going to be embedded in the membranes of endosomes or phagosomes, and they can detect MAMPs like viral DNA or RNA or bacterial DNA that have been ingested by the cell. We also have cytoplasmic PRRs that can detect either MAMPs or DAMPs, where DAMPs are damage associated molecular patterns. Those are going to detect damage to cellular components.

This concludes our brief introduction to the terms MAMPs and DAMPs: molecular associated and microbe associated molecular patterns, and damage associated molecular patterns. We'll be able to apply some of these concepts as we move forward in our course. I'll see you all in our next video.

Which of the following are PRRs least likely to detect?

• A. Glycolysis enzymes.
• B. Peptidoglycan.
• C. Flagellin.
• D. Lipopolysaccharide.
• E. Foreign nucleotide sequences.

Toll-Like Receptors (TLRs)

In this video, we're going to introduce Toll like receptors or TLRs for short. The most well-characterized type of PRR, or pattern recognition receptor, is the Toll like receptors. Now, Toll like receptors are commonly abbreviated as TLRs, and these Toll like receptors or TLRs are membrane-embedded PRRs or membrane-embedded pattern recognition receptors that are used to detect MAMPs outside of the cell or MAMPs that have been ingested by the cell through the process of endocytosis or phagocytosis.

The TLRs or Toll like receptors can either be found in the cell membrane, in the cell cytoplasmic membrane, or they could be found embedded in the membranes of endosomes and phagosomes, which are these membrane vesicles that result from endocytosis or phagocytosis. The TLRs that are found embedded in the cell's cytoplasmic membrane will face outward on the cell surface in order to detect MAMPs on the outside of the cell. But the TLRs that are found in the endosomes or phagosomes are going to face inward in order to monitor what is engulfed via endocytosis or phagocytosis.

Again, these TLRs are going to recognize MAMPs, such as, for example, microbial nucleic acids. Like, for example, double stranded RNA, which is found in some types of viruses but is not common in the host cell at all. Once a microbial invasion has been detected, once a MAMP has been detected by a TLR, it can then initiate some kind of immune response, such as the release of a cytokine, for example.

If we take a look at our image down below over here on the left-hand side, notice that we're showing you our map of the lesson on the scanning systems of innate immunity, which are going to serve to detect signs of microbes and serve somewhat as security cameras. These pattern recognition receptors or PRRs are going to help detect signs of MAMPs and DAMPs, microbe-associated molecular patterns and damage-associated molecular patterns.

There are many different types of pattern recognition receptors, but here in this video, we're focusing on the Toll like receptors or the TLRs. Again, these Toll like receptors or TLRs are membrane-embedded pattern recognition receptors. They're going to be found embedded in the membrane of the cytoplasmic membrane, and they will be facing towards the outside of the cell, where they can detect signs, like, for example, lipopolysaccharide, peptidoglycan, or flagellin, or they can be found embedded in the membranes of endosomes or phagosomes, and they're going to be facing inwards so that they can detect what has been ingested within the endosome or phagosome.

This here concludes our brief lesson on these Toll like receptors or TLRs, and we'll be able to learn about the other types of pattern recognition receptors as we move forward in our course. So I'll see you all in our next video.

Toll-like receptors (TLRs) bind molecules on pathogens. Why is this helpful to the immune response?

• A. It provides a highly specific response to very small and highly unique areas on an individual pathogenic microbe, providing the most specific and selective response possible.
• B. It provides a general response to broad categories of molecules/cells that should NOT be in our system, as we don't have these molecules on our own cells.
• C. These secreted molecules help bind pathogens and then direct them to receptors on the immune system cells that are best capable of eliminating them from our systems. TLRs are delivery mechanisms for the immune responses.
• D. TLRs are capable of directly lysing (destroying) the microbes, helping our immune responses by eliminating pathogens.

C-Type Lectin Receptors (CLRs)

This video, we're going to introduce the c type lectin receptors or CLRs. Another type of PRR or pattern recognition receptor that's located on the cell surface are the c type lectin receptors. These c type lectin receptors are commonly abbreviated as CLRs. These c type lectin receptors or CLRs are cell surface pattern recognition receptors or PRRs that bind to carbohydrate MAMPs on the microbial surfaces. Some microbes have carbohydrate MAMPs on their surfaces. These c type lectin receptors are found on our own immune cells, and they can bind to the carbohydrate MAMPs that are found on these microbial surfaces. Now lectins themselves, which are found within the name c type lectin receptor, are proteins that bind to specific carbohydrates. That's really what lectins are. If we take a look at our image down below, over here on the left-hand side, once again, we're showing you our map of the lesson on the scanning systems of innate immunity, which serve to detect signs of microbes and serve as, somewhat like security cameras. Here, we're talking about the pattern recognition receptors, specifically the c type lectin receptors. These c type lectin receptors are going to be membrane embedded receptors on the cell surface. What you'll notice is that these c type lectin receptors have a green region right here that we refer to as the lectin protein. The lectin protein is capable of binding to carbohydrates like this one that are found on the microbial surface. Our host cell is able to use the CLRs to bind and recognize carbohydrate MAMPs found on microbe surfaces. Once the c type lectin receptor has bound a carbohydrate MAMP on the microbial cell surface, it can then initiate some kind of immune response. For example, perhaps the release of some kind of cytokine. This concludes our brief lesson on c type lectin receptors or CLRs. We'll be able to apply some of these concepts as we move forward and learn about the other pattern recognition receptors as we move forward. So see you in the next video.

Which of the following is not considered a MAMP (Microbe/Pathogen-Associated Molecular Pattern)?

• A. Carbohydrates on microbial surfaces.
• B. Peptidoglycan from gram + bacteria.
• C. Damaged host cell DNA.
• D. Lipopolysaccharide (LPS).

Nucleotide-Binding & Oligomerization Domain (NOD)-Like Receptors

In this video, we're going to briefly discuss the nucleotide binding and oligomerization domain-like receptors, or in other words, for short, just the Nod-like receptors. And so these Nod-like receptors are commonly abbreviated as NLRs. And unlike the other two types of pattern recognition receptors that we've talked about in our previous lesson videos, including the toll-like receptors and C-type lectin receptors, which are membrane-bound receptors, the Nod-like receptor is not membrane-bound. Instead, the Nod-like receptor, or NLR, is a cytoplasmic receptor, meaning that it is not embedded in the membrane and it is dissolved within the cytoplasm. And so this Nod-like receptor or NLR is a cytoplasmic receptor protein or a cytoplasmic PRR, a cytoplasmic pattern recognition receptor, that is going to detect intracellular MAMPs or DAMPs. So these Nod-like receptors, which are cytoplasmic PRRs, they play an important role in the immune response of macrophages and dendritic cells. When an NLR detects a MAMP, it can combine with other cytoplasmic proteins to form what is known as an inflammasome. This inflammasome can be defined as capable of activating pro-inflammatory cytokine production, leading to inflammation.

If we take a look at our image down below, once again on the left-hand side we have our map of the lesson on the scanning systems, and we're focusing specifically on the Nod-like receptors here. Notice that these Nod-like receptors can be found in macrophages and dendritic cells, and notice that they are cytoplasmic pattern recognition receptors. They're found, dissolved in the cytoplasm. They are not embedded in membranes. When these Nod-like receptors, or NLRs, bind to MAMPs and DAMPs inside the cell, here's an NLR binding to some kind of MAMP or DAMP, it can lead to the formation of what is known as an inflammasome. The inflammasome will form, which will activate cytokine production. Here we have the inflammasome formation, and the formation of the inflammasome will trigger the release of cytokines, and those cytokines can trigger inflammation. Inflammation is an entire response that can help to eliminate the microbes that are causing problems. And so, what you can see here is that these Nod-like receptors are, again, cytoplasmic PRRs that detect signs of intracellular MAMPs and DAMPs, and can lead to some kind of response that can ultimately help to eliminate those microbes or pathogens.

This here concludes our brief lesson on the Nod-like receptors or NLRs, and we'll be able to get some practice applying some of these concepts, and then we'll talk about the final type of pattern recognition receptor, the RIG-like receptor. So I'll see you all in our next lesson video.

Why are NOD-like receptors (NLRs) important for macrophage cells?

• A. Macrophages commonly ingest infectious material and use NLRs to detect intracellular MAMPs.
• B. Macrophages are unstable and commonly have intracellular damage and use NLRs to detect DAMPs.
• C. Macrophages use NLRs to detect MAMPs which triggers the formation of RIG-like receptors.
• D. Macrophages are the only cells able to detect viral infection and use NLRs to detect viral RNA.

Retinoic Acid Inducible Gene (RIG)-Like Receptors

This video, we're going to briefly discuss retinoic acid inducible gene like receptors, or for short, RIG like receptors. Now these RIG like receptors are a second type of cytoplasmic PRR in addition to the nod like receptors that we talked about in some of our previous lesson videos. And so once again, these RIG like receptors are cytoplasmic PRRs that are capable of detecting cytoplasmic mAMPS or cytoplasmic microbe-associated molecular patterns. Now, specifically, these RIG like receptors are cytoplasmic proteins or cytoplasmic PRRs that detect viral RNA. And, these RIG like receptors that are capable of detecting viral RNA in the cytoplasm are found in most types of host cells.

Now the RIG like receptors are capable of distinguishing between host cell RNA and viral RNA by two features that we have listed down below. The first feature is that viral RNA can be double-stranded. And double-stranded RNA is quite unique and really only found in some types of viruses. And host RNA is not double-stranded, so that makes it easy to distinguish the host cell's RNA from viral RNA. Now a second important feature that helps distinguish host RNA from viral RNA is that host cell RNA is going to have a 5 prime cap as well as a poly A tail, a 3 prime poly A tail.

However, viral RNA does not have a 5 prime cap or a poly A tail. And so that is another way to distinguish between host cell RNA and viral RNA. And so if we take a look at our image down below over here on the left-hand side, notice we have our map of the lesson on the scanning systems of innate immunity, which serve to detect signs of microbes and host cell damage, and they serve somewhat as these security cameras. And so here in this video, we're focusing specifically on pattern recognition receptors, specifically these RIG like receptors. And so, these RIG like receptors, again are capable of detecting viral RNA.

And so when a virus infects a cell, so here we have an infecting virus, it will uncoat and release its genetic material, and that viral genetic material can be transcribed through the process of transcription to make viral RNA, either viral single-stranded RNA or viral double-stranded RNA. And so regardless if it's viral single-stranded RNA or viral double-stranded RNA, the RIG like receptors, which are down below right here, are capable of distinguishing the host cell's RNA, which notice the host cell's RNA is over here. It is single-stranded, and it has a 5 prime cap and a poly A tail, from the viral RNA. And so, that's because, again, the viral RNA is, not going to have the 5 prime cap or poly A tail, and also viral RNA can be double-stranded. And so these RIG like receptors will detect these viral RNA molecules, and upon detecting the viral RNA molecules, it could initiate some kind of immune response, like, for example, the release of a cytokine, perhaps an interferon, for example, which we'll get to talk about the interferon response in more detail later in our course.

But for now, this here concludes our brief lesson on these RIG like receptors, and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.

What defining features of viral RNA allows the RIG-like receptors to distinguish infecting viral RNA from the host cell’s RNA?

• A. Viral RNA has a 5’ cap while host cell RNA does not.
• B. Viral RNA has a poly-A tail while host cell RNA does not.
• C. Viral RNA can be double stranded while host cell RNA is single stranded.
• D. Viral RNA has a circular formation while host cell RNA is linear.
• E. None of the above.

In addition to peptidoglycan, which molecule(s) unique to bacteria would PRRs recognize?

• A. Lipopolysaccharide (LPS).
• B. Fungal cell walls.
• C. Flagellin proteins.
• D. Tubulin proteins found in cilia.
• E. A and C.
• F. B and D.
• G. All of the answers would be recognized by PRRs.

Toll-like receptors are receptor proteins on ___________.

• A. Granulocytes that destroy parasitic helminths.
• B. Phagocytes that recognize foreign or microbe-associated molecules.
• C. Viruses entering the body which stimulate the immune system.
• D. Mucus producing cells that trigger mucus production when they bind with an allergen.

Which type of pattern recognition receptor when bound to MAMPs triggers the formation of the inflammasome?

• A. NOD-Like Receptors.
• B. RIG-Like Receptors.
• C. Toll-Like Receptors.
• D. C-Type Lectin Receptors.

Toll-like receptors:

• A. Are cytokines.
• B. Each recognize a specific "danger" molecule.
• C. Are embedded in cellular membranes.
• D. Are part of adaptive immunity.
• E. b and c.

______ on phagocytes bind to PAMPs on bacteria, triggering uptake & destruction of the bacterial pathogens?

• A. PRRs.
• B. PAMPs.
• C. AMPs.
• D. PMNs.

Here’s what students ask on this topic:

What are Pattern Recognition Receptors (PRRs) and their role in the immune system?

Pattern Recognition Receptors (PRRs) are cell surface receptors found on immune cells that play a crucial role in detecting microbial invasion and host cell damage. They recognize specific molecular patterns associated with pathogens (MAMPs) or host cell damage (DAMPs). Upon detection, PRRs initiate immune responses, such as cytokine release, which are essential for inflammation and pathogen elimination. Key types of PRRs include Toll-like receptors (TLRs), C-type lectin receptors (CLRs), NOD-like receptors (NLRs), and RIG-like receptors (RLRs). Each type of PRR is specialized to detect microbial components in different cellular locations, ensuring a comprehensive immune surveillance system.

What are the different types of Pattern Recognition Receptors (PRRs) and their functions?

There are several types of Pattern Recognition Receptors (PRRs), each with specific functions:

• Toll-like receptors (TLRs): Membrane-bound receptors that detect MAMPs on the cell surface or within endosomes/phagosomes.
• C-type lectin receptors (CLRs): Cell surface receptors that bind to carbohydrate MAMPs on microbial surfaces.
• NOD-like receptors (NLRs): Cytoplasmic receptors that detect intracellular MAMPs and DAMPs, leading to inflammasome formation and cytokine release.
• RIG-like receptors (RLRs): Cytoplasmic receptors that detect viral RNA, distinguishing it from host RNA, and initiate antiviral responses.

These PRRs collectively ensure the immune system can detect and respond to a wide range of microbial invasions and cellular damage.

How do Toll-like receptors (TLRs) function in the immune response?

Toll-like receptors (TLRs) are membrane-bound PRRs that detect MAMPs either on the cell surface or within endosomes/phagosomes. TLRs on the cell surface recognize microbial components like lipopolysaccharides and flagellin, while those in endosomes/phagosomes detect ingested microbial nucleic acids. Upon recognizing MAMPs, TLRs initiate immune responses by activating signaling pathways that lead to the production of cytokines and other inflammatory mediators. This helps to recruit and activate other immune cells, enhancing the body's ability to eliminate pathogens and initiate an effective immune response.

What are MAMPs and DAMPs, and how do they relate to PRRs?

MAMPs (Microbe-Associated Molecular Patterns) and DAMPs (Damage-Associated Molecular Patterns) are molecular signals detected by PRRs. MAMPs are molecules associated with microbial pathogens, such as lipopolysaccharides, peptidoglycan, and viral nucleic acids. DAMPs, on the other hand, are molecules released from damaged host cells, indicating cellular injury. PRRs, such as TLRs, CLRs, NLRs, and RLRs, recognize these patterns and trigger immune responses. MAMPs and DAMPs help the immune system distinguish between microbial invaders and host cell damage, ensuring appropriate and targeted immune reactions.

What is the role of NOD-like receptors (NLRs) in the immune system?

NOD-like receptors (NLRs) are cytoplasmic PRRs that detect intracellular MAMPs and DAMPs. Unlike membrane-bound PRRs, NLRs are dissolved in the cytoplasm and play a crucial role in the immune response of macrophages and dendritic cells. Upon detecting MAMPs or DAMPs, NLRs can form inflammasomes, which are protein complexes that activate pro-inflammatory cytokine production. This leads to inflammation, a critical response for eliminating pathogens and repairing tissue damage. NLRs thus help in recognizing intracellular threats and initiating appropriate immune responses.