Microbiology Quiz: Viral Immune Evasion
20 questions · exam conditions
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Viral Immune EvasionQuestion 1 of 20

The HIV-1 envelope glycoprotein (gp120) is shielded by a dense layer of host-derived N-linked glycans. What is the primary mechanism by which this 'glycan shield' facilitates immune evasion?

It mimics the structure of host glycoproteins, inducing central and peripheral tolerance to the Env protein in developing lymphocytes.
It promotes the binding of mannose-binding lectin, leading to complement activation that paradoxically enhances viral uptake by macrophages.
It sterically occludes conserved, functionally critical protein epitopes on gp120 from recognition and binding by neutralizing antibodies.
It alters the folding of the gp120 protein, locking it in a conformation that has low affinity for the CD4 receptor until it nears the target cell.
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Microbiology Quiz

Microbiology Quiz: Viral Immune Evasion

Practice Viral Immune Evasion in Microbiology with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Viral Immune Evasion, giving you a quick way to practice the rules, question types, and explanations that matter most for Microbiology.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

The HIV-1 envelope glycoprotein (gp120) is shielded by a dense layer of host-derived N-linked glycans. What is the primary mechanism by which this 'glycan shield' facilitates immune evasion?

  1. It mimics the structure of host glycoproteins, inducing central and peripheral tolerance to the Env protein in developing lymphocytes.
  2. It promotes the binding of mannose-binding lectin, leading to complement activation that paradoxically enhances viral uptake by macrophages.
  3. It sterically occludes conserved, functionally critical protein epitopes on gp120 from recognition and binding by neutralizing antibodies. (correct answer)
  4. It alters the folding of the gp120 protein, locking it in a conformation that has low affinity for the CD4 receptor until it nears the target cell.
Explanation: The primary advantage of the glycan shield is physical obstruction. The dense cloud of carbohydrate molecules, which are seen as 'self' by the immune system, covers the underlying protein surface of gp120. This makes it difficult for neutralizing antibodies to access and bind to the conserved epitopes that are critical for the virus's function, such as the CD4 binding site. While the virus needs these sites to be accessible for infection, they are often conformationally masked or transiently exposed, and the glycan shield provides a constant, formidable barrier to antibody-mediated neutralization. The other options describe incorrect or secondary, less significant mechanisms.

Question 2

The NS1 protein of influenza A virus is a multifunctional immune antagonist. One of its key roles is to interfere with the host antiviral response by binding to and inhibiting the cellular protein CPSF30. What is the direct consequence of this NS1-CPSF30 interaction?

  1. It prevents the polyadenylation and nuclear export of host pre-mRNAs, including those for interferon-β and other antiviral proteins. (correct answer)
  2. It sequesters viral dsRNA replication intermediates, preventing their recognition by the cytoplasmic sensor RIG-I.
  3. It blocks the activation of protein kinase R (PKR), thus preventing the shutdown of global protein synthesis in the infected cell.
  4. It inhibits the ubiquitination of RIG-I, a step required for its activation and downstream signaling through MAVS.
Explanation: CPSF30 is a subunit of the Cleavage and Polyadenylation Specificity Factor, a key component of the machinery that processes the 3' end of host pre-mRNAs. By binding to and inhibiting CPSF30, the influenza NS1 protein globally disrupts the processing of host mRNAs. This 'host shutoff' mechanism prevents the production of antiviral proteins, most critically the interferons and interferon-stimulated genes, allowing the virus to replicate without opposition. While NS1 does perform the functions in B and C via different domains or mechanisms, its specific interaction with CPSF30 is responsible for disrupting host mRNA processing (A).

Question 3

Human cytomegalovirus (CMV) expresses the US2 protein, which dislocates MHC class I molecules from the ER to the proteasome. It also expresses the UL18 protein, an MHC class I homolog that binds the inhibitory receptor LIR-1 on NK cells. Which statement best contrasts the strategic purpose of these two evasion mechanisms?

  1. US2 prevents CD4+ T cell activation, while UL18 prevents B cell antibody production.
  2. US2 helps evade the CTL response, while UL18 acts as a decoy ligand to proactively inhibit the counteracting NK cell response. (correct answer)
  3. Both proteins work to inhibit the CTL response, with US2 blocking peptide loading and UL18 blocking T-cell receptor binding.
  4. US2 blocks the innate immune response by preventing PAMP recognition, while UL18 blocks the adaptive immune response by inducing T-cell anergy.
Explanation: This is a classic example of a virus's two-pronged strategy against cellular immunity. The US2 protein destroys MHC class I molecules, making the infected cell invisible to cytotoxic T lymphocytes (CTLs). However, this action would normally trigger NK cells via the 'missing self' mechanism. To counteract this, CMV expresses UL18. UL18 is a viral mimic of MHC class I that engages an inhibitory receptor (LIR-1) on NK cells, providing the 'don't kill me' signal that was lost by destroying the authentic MHC class I. Therefore, US2 evades the adaptive CTL response, and UL18 evades the innate NK cell response that would otherwise be activated.

Question 4

Many enveloped viruses, including vaccinia virus, incorporate host-derived complement regulatory proteins such as CD55 (Decay-Accelerating Factor) into their envelopes during budding. How does this specific strategy protect the virion from complement-mediated destruction?

  1. It acts as a cofactor for the serum protease Factor I, which proteolytically cleaves and inactivates C3b and C4b.
  2. It binds to the C5b-8 complex on the viral surface, preventing the polymerization of C9 and the formation of the lytic pore.
  3. It directly binds to C1q, preventing the recognition of viral surface antigens by antibodies and blocking classical pathway initiation.
  4. It actively promotes the dissociation of the C3 convertases (C4b2a and C3bBb) that form on the viral surface, halting the amplification cascade. (correct answer)
Explanation: The specific function of CD55, or Decay-Accelerating Factor (DAF), is to accelerate the decay (dissociation) of the C3 and C5 convertase enzymes of the complement system. By incorporating this host protein, the virus co-opts a self-protection mechanism. When C3b or C4b are deposited on the virion surface, the incorporated CD55 prevents the stable assembly of the convertases, thus halting the amplification loop and preventing the downstream formation of the membrane attack complex. Choice A describes the function of CD46 (MCP). Choice B describes the function of CD59 (Protectin). Choice C describes a different mechanism of evasion not related to CD55.

Question 5

A newly characterized poxvirus is found to produce a soluble protein with high structural homology to the extracellular domain of the human IL-10 receptor. This protein is secreted from infected cells. What is the most likely immunological consequence of this viral protein's activity?

  1. It potentiates the anti-inflammatory effects of host IL-10 by increasing its serum half-life, leading to systemic immune suppression.
  2. It acts as a decoy receptor, sequestering host IL-10 and preventing it from suppressing the activation of macrophages and dendritic cells. (correct answer)
  3. It binds to the surface of T helper cells, mimicking IL-10 and directly inducing a shift from a Th1 to a Th2 phenotype.
  4. It binds to the IL-10 receptor complex on host cells, acting as a competitive antagonist and blocking the downstream JAK-STAT signaling pathway.
Explanation: A soluble viral protein that mimics a host cytokine receptor is known as a viroceptor. Its function is to act as a decoy, binding the host cytokine in the extracellular space. In this case, the viral protein would bind to and sequester host IL-10. Since IL-10 is an anti-inflammatory cytokine that suppresses antigen-presenting cells (APCs) like macrophages and dendritic cells, sequestering it would prevent this suppression, leading to a more robust pro-inflammatory response. This is advantageous to some viruses if a strong inflammatory response causes immunopathology that aids dissemination. The other choices describe incorrect mechanisms: potentiating the signal (A), acting as a cytokine mimic (C), or acting as a surface antagonist (D).

Question 6

A virologist is comparing the immune evasion strategies of two viruses. Virus A expresses a protein that binds to the invariant chain (Ii), preventing its cleavage and the subsequent loading of peptides onto MHC class II molecules. Virus B expresses a protein that binds to β2-microglobulin, preventing its association with the MHC class I heavy chain. Which statement correctly predicts the primary immunological impact of these two distinct mechanisms?

  1. Virus A impairs the activation of CD8+ T cells, while Virus B impairs the activation of CD4+ T cells.
  2. Virus A prevents the presentation of exogenous antigens, while Virus B prevents the presentation of both endogenous and exogenous antigens via cross-presentation.
  3. Both viruses will render infected cells more susceptible to killing by Natural Killer (NK) cells due to the 'missing self' phenomenon.
  4. Virus A will primarily affect antigen presentation in professional APCs, while Virus B will affect antigen presentation in all nucleated cells. (correct answer)
Explanation: When analyzing viral immune evasion strategies, focus on which cells express MHC class I versus class II molecules and what happens when these pathways are disrupted. Virus A targets the invariant chain (Ii) that's essential for MHC class II assembly and peptide loading. Since MHC class II molecules are primarily expressed by professional antigen-presenting cells (APCs) like dendritic cells, macrophages, and B cells, this virus specifically impairs antigen presentation in these specialized immune cells. The invariant chain guides proper folding and peptide loading in the endoplasmic reticulum and endosomes of these APCs. Virus B targets β2-microglobulin, which is required for stable MHC class I molecule assembly. Since virtually all nucleated cells express MHC class I molecules to present intracellular peptides to CD8+ T cells, this virus affects antigen presentation across all cell types throughout the body. Option A incorrectly reverses the T cell subsets affected—Virus A (MHC II disruption) impairs CD4+ T cell activation, while Virus B (MHC I disruption) impairs CD8+ T cell activation. Option B mischaracterizes the antigen presentation pathways—both mechanisms affect their respective presentation routes regardless of antigen origin. Option C is wrong because only Virus B would trigger the "missing self" response from NK cells, since NK cells monitor MHC class I expression, not class II. Remember this pattern: MHC class I is ubiquitous (all nucleated cells), while MHC class II is restricted to professional APCs. Viral proteins targeting these pathways will have correspondingly broad or narrow cellular impacts.

Question 7

Epstein-Barr virus (EBV) establishes a persistent infection by targeting B lymphocytes. A hallmark of acute EBV infection (infectious mononucleosis) is a massive, polyclonal B-cell proliferation. This clinical manifestation is a direct consequence of which viral strategy?

  1. The EBV protein LMP1 acts as a constitutively active homolog of the host CD40 receptor, providing potent survival and proliferation signals to infected B cells. (correct answer)
  2. The virus expresses a potent superantigen that cross-links the B-cell receptor of many B-cell clones, leading to antigen-independent activation.
  3. The virus produces a viral cytokine (vIL-10) that protects infected B cells from apoptosis while suppressing the antiviral T-cell response.
  4. The viral genome integrates into the B-cell genome near proto-oncogenes, causing insertional mutagenesis and uncontrolled cell division.
Explanation: EBV's strategy for persistence involves establishing a latent reservoir in memory B cells. To do this, it drives infected B cells to proliferate. A key protein in this process is Latent Membrane Protein 1 (LMP1). LMP1 mimics a constitutively active CD40 receptor, which is a critical co-stimulatory molecule for B-cell activation. This mimicry provides strong, continuous signals for the B cell to proliferate and differentiate, expanding the pool of latently infected cells. While vIL-10 (C) is an important immune evasion molecule, it is the LMP1-driven proliferation that directly causes the polyclonal activation seen clinically. EBV does not typically integrate (D) and does not express a B-cell superantigen (B).

Question 8

A novel RNA virus is found to be highly resistant to the effects of Type I interferons. In infected cells, cytosolic pattern recognition receptors are activated, leading to the phosphorylation and dimerization of the transcription factor IRF3. However, subsequent analysis by immunofluorescence microscopy shows that the activated IRF3 remains in the cytoplasm. Which viral strategy is most consistent with this observation?

  1. The virus produces a decoy dsRNA-binding protein that sequesters viral RNA, preventing its initial detection by RIG-I.
  2. The virus encodes a protease that cleaves the adaptor protein MAVS from the mitochondrial membrane.
  3. The virus produces a protein that specifically blocks the importin-α/β pathway required for the nuclear translocation of IRF3. (correct answer)
  4. The virus secretes a soluble decoy receptor for IFN-α/β, preventing the activation of the JAK-STAT pathway in neighboring cells.
Explanation: The question describes a specific point of inhibition in the interferon production pathway. The steps leading up to IRF3 activation (phosphorylation and dimerization) are intact, but the final step, nuclear translocation, is blocked. This pinpoints the mechanism of evasion to be interference with nuclear import. Several viruses are known to do this, for example, by targeting the nuclear pore complex or components of the nuclear import machinery. Choices A and B describe mechanisms that would block the pathway before IRF3 phosphorylation. Choice D describes a mechanism to block the response to interferon in other cells, not the initial production of interferon within the infected cell.

Question 9

A latent DNA virus is found to express a viral microRNA (v-miRNA) with a seed sequence that is highly complementary to the 3' untranslated region (UTR) of the host mRNA for TAP1. What is the most direct immunological consequence of this v-miRNA's expression in the infected cell?

  1. Increased surface expression of MHC class I molecules, making the cell a better target for cytotoxic T lymphocytes.
  2. Degradation of the viral genome by the host RNA-induced silencing complex (RISC), leading to clearance of the latent infection.
  3. Enhanced stability of the TAP1 protein, resulting in more efficient loading of self-peptides and stronger inhibitory signaling to NK cells.
  4. Inhibition of TAP1 translation, leading to reduced peptide transport into the ER and decreased presentation of viral antigens on MHC class I. (correct answer)
Explanation: When you encounter questions about viral microRNAs and immune evasion, focus on how viruses manipulate host cellular machinery to avoid detection. This question tests your understanding of the MHC class I antigen presentation pathway and how viral interference affects immune recognition. The correct answer is D because viral miRNAs function like host miRNAs - they bind to complementary sequences in target mRNAs and inhibit translation or promote degradation. Since this v-miRNA targets TAP1 mRNA, it will reduce TAP1 protein production. TAP1 (Transporter associated with Antigen Processing) is essential for transporting cytoplasmic peptides into the endoplasmic reticulum, where they load onto MHC class I molecules. Without adequate TAP1, fewer viral peptides reach the ER, resulting in reduced viral antigen presentation and helping the virus evade CD8+ T cell recognition during latency. Answer A is backwards - reduced TAP1 would decrease, not increase, MHC class I surface expression. Answer B misunderstands the mechanism; RISC doesn't target viral DNA genomes, and the miRNA is viral-encoded, not targeting viral RNA. Answer C incorrectly suggests the miRNA would enhance TAP1 stability, when complementary binding to the 3' UTR typically inhibits translation, plus it mentions "self-peptides" and "inhibitory signaling to NK cells," which confuses the MHC class I pathway with NK cell inhibition. Remember that many DNA viruses encode miRNAs specifically to downregulate host immune components. Always trace through the antigen presentation pathway: peptide generation → TAP transport → ER loading → surface presentation → T cell recognition.

Question 10

Mouse Mammary Tumor Virus (MMTV) encodes a superantigen (SAg) that is critical for its life cycle. The SAg non-specifically activates T cells expressing particular Vβ segments in their T-cell receptors. What is the key long-term advantage this provides for viral persistence?

  1. The massive initial cytokine release damages lymphoid architecture, creating a privileged site for viral replication.
  2. The SAg preferentially activates the target cells for MMTV infection (B cells and dendritic cells), expanding the pool of infectible cells.
  3. The initial massive activation of T cells is followed by activation-induced cell death, deleting a large fraction of the T-cell repertoire and impairing the specific antiviral response. (correct answer)
  4. The SAg specifically stimulates regulatory T cells, leading to global immunosuppression that allows the virus to replicate unchecked.
Explanation: The defining characteristic of a superantigen response is an initial, massive, non-specific T-cell activation, followed by a crash. This crash involves widespread apoptosis (activation-induced cell death) and anergy of the responding T-cell clones. By deleting entire families of T cells based on their Vβ chain usage, the virus creates significant holes in the host's T-cell repertoire. This immunosuppressive effect impairs the host's ability to mount a coordinated and specific T-cell response against actual viral antigens, thus facilitating long-term viral persistence. MMTV's primary targets are B cells and dendritic cells, but it is the manipulation of the T-cell response that ensures its survival.

Question 11

A researcher infects cultured human cells with a mutant adenovirus that lacks a functional E1B 55K protein. They observe that upon infection, the host cell protein p53 becomes stabilized and initiates apoptosis at a high frequency. This finding suggests the primary immune evasion function of the wild-type E1B 55K protein is to:

  1. prevent activation of the extrinsic apoptotic pathway by blocking Fas ligand signaling from cytotoxic lymphocytes.
  2. function as a viral Bcl-2 homolog, binding to pro-apoptotic proteins Bak and Bax to prevent mitochondrial outer membrane permeabilization.
  3. bind to the p53 tumor suppressor protein and, in conjunction with other viral proteins, target it for ubiquitin-mediated proteasomal degradation. (correct answer)
  4. act as a direct inhibitor of executioner caspases, such as caspase-3 and caspase-7, thereby blocking the final stage of apoptosis.
Explanation: The experimental evidence directly links the absence of E1B 55K with the stabilization of p53 and subsequent apoptosis. This strongly implies that the function of E1B 55K is to counteract p53. The known mechanism is that the E1B 55K protein, along with the E4 orf6 protein, forms an E3 ubiquitin ligase complex that targets p53 for degradation. This prevents p53 from sensing viral replication as DNA damage and triggering apoptosis. The other options describe valid viral anti-apoptotic strategies, but they are not consistent with the specific experimental observation involving p53. For instance, the adenovirus E1B 19K protein, not E1B 55K, functions as a Bcl-2 homolog (B).

Question 12

A patient is infected with a virus that expresses a protein specifically blocking the TAP transporter. Despite this, the patient's immune system mounts a robust cytotoxic response and effectively clears the infection, primarily mediated by Natural Killer (NK) cells. Which of the following statements best explains this outcome?

  1. The TAP blockade also prevents the loading of inhibitory viral peptides onto non-classical MHC molecules like HLA-E, leading to NK cell activation.
  2. The absence of MHC class I molecules on the infected cell surface fails to engage inhibitory KIR and NKG2A receptors on NK cells, triggering a 'missing self' response. (correct answer)
  3. The viral infection causes upregulation of stress ligands like MICA/MICB, which are recognized by the activating receptor NKG2D on NK cells, bypassing the need for MHC-I recognition.
  4. Viral dsRNA released from lysed cells activates Toll-like receptors on NK cells, leading to a potent, IFN-γ-dependent cytotoxic response that is not specific to infected cells.
Explanation: The virus's strategy of blocking the TAP transporter prevents peptide loading onto MHC class I molecules, effectively evading recognition by cytotoxic T lymphocytes (CTLs). However, this leads to decreased MHC class I expression on the cell surface. NK cells survey for the presence of 'self' MHC class I using inhibitory receptors (like KIRs and the CD94/NKG2A complex). When these receptors are not engaged due to the absence of MHC class I, the inhibitory signal is lost, and NK cells are activated to kill the 'missing self' target cell. While stress ligand upregulation (C) can contribute, the primary and most direct consequence of MHC-I downregulation is the loss of inhibition described in B.

Question 13

A virologist performs a pulse-chase experiment on cells infected with a novel DNA virus. They find that newly synthesized MHC class I heavy chains associate correctly with β2-microglobulin and TAP, but the fully assembled peptide-MHC complexes fail to exit the endoplasmic reticulum for transport to the cell surface. This suggests the virus employs a protein that:

  1. targets folded MHC class I complexes for dislocation back into the cytosol and subsequent proteasomal degradation.
  2. acts as a high-affinity 'tether,' retaining properly loaded MHC class I molecules within the ER lumen. (correct answer)
  3. specifically blocks the function of the TAP transporter, preventing peptide entry into the ER.
  4. encodes a microRNA that targets the β2-microglobulin transcript, preventing its translation and MHC I assembly.
Explanation: The experimental data show that MHC class I assembly and peptide loading are proceeding normally, but the final complex is trapped in the ER. This points to a mechanism that interferes with ER-to-Golgi transport. Several viruses, such as adenovirus (E3/19K protein) and CMV (US3 protein), use proteins that bind to assembled MHC class I molecules and retain them in the ER, effectively preventing their transit to the cell surface where they would be surveyed by CTLs. Choice A (dislocation) would lead to degradation of the heavy chains. Choice C (TAP block) would prevent peptide loading in the first place. Choice D (miRNA) would prevent the initial assembly by depleting β2-microglobulin.

Question 14

A researcher infects cultured human cells with a mutant adenovirus that lacks a functional E1B 55K protein. They observe that upon infection, the host cell protein p53 becomes stabilized and initiates apoptosis at a high frequency. This finding suggests the primary immune evasion function of the wild-type E1B 55K protein is to:

  1. prevent activation of the extrinsic apoptotic pathway by blocking Fas ligand signaling from cytotoxic lymphocytes.
  2. function as a viral Bcl-2 homolog, binding to pro-apoptotic proteins Bak and Bax to prevent mitochondrial outer membrane permeabilization.
  3. bind to the p53 tumor suppressor protein and, in conjunction with other viral proteins, target it for ubiquitin-mediated proteasomal degradation. (correct answer)
  4. act as a direct inhibitor of executioner caspases, such as caspase-3 and caspase-7, thereby blocking the final stage of apoptosis.
Explanation: The experimental evidence directly links the absence of E1B 55K with the stabilization of p53 and subsequent apoptosis. This strongly implies that the function of E1B 55K is to counteract p53. The known mechanism is that the E1B 55K protein, along with the E4 orf6 protein, forms an E3 ubiquitin ligase complex that targets p53 for degradation. This prevents p53 from sensing viral replication as DNA damage and triggering apoptosis. The other options describe valid viral anti-apoptotic strategies, but they are not consistent with the specific experimental observation involving p53. For instance, the adenovirus E1B 19K protein, not E1B 55K, functions as a Bcl-2 homolog (B).

Question 15

A population experiences a typical seasonal influenza outbreak in Year 1. In Year 2, a severe global pandemic occurs, and the causative influenza A virus possesses a hemagglutinin (HA) subtype not previously observed in humans. Which statement best explains the drastically increased severity in Year 2?

  1. Antigenic drift in Year 1 caused minor HA mutations that were partially recognized by memory cells, while antigenic shift in Year 2 introduced a novel HA segment, resulting in a naive population with no pre-existing immunity. (correct answer)
  2. Antigenic shift in Year 1 created a localized epidemic, while rapid antigenic drift of the viral polymerase gene in Year 2 created a far more virulent strain that overwhelmed the population's immunity.
  3. The Year 1 virus underwent antigenic drift that decreased its affinity for human sialic acid receptors, while the Year 2 virus experienced a shift that enhanced its binding affinity, leading to more efficient infection.
  4. The Year 1 outbreak was caused by influenza B, which is limited to antigenic drift, while the Year 2 pandemic was due to an influenza A strain that acquired a new neuraminidase (NA) segment via reassortment.
Explanation: This scenario contrasts seasonal epidemics with global pandemics. Seasonal outbreaks are driven by antigenic drift, which involves the accumulation of point mutations in the HA and NA genes. This allows the virus to partially evade existing immunity, but cross-reactive memory responses still provide some protection. Pandemics are caused by antigenic shift, a process where influenza A viruses exchange entire gene segments (reassortment), often between human, avian, and swine viruses. This can introduce a completely novel HA or NA subtype into the human population, for which there is no pre-existing immunity, leading to severe, widespread disease. Choice A correctly identifies drift for the mild outbreak and shift for the severe pandemic. B reverses the terms. C incorrectly links drift to decreased affinity. D incorrectly focuses on NA for a pandemic (while possible, HA shifts are the primary drivers) and makes an unsupported assumption about influenza B.

Question 16

Human cytomegalovirus (CMV) expresses the US2 protein, which dislocates MHC class I molecules from the ER to the proteasome. It also expresses the UL18 protein, an MHC class I homolog that binds the inhibitory receptor LIR-1 on NK cells. Which statement best contrasts the strategic purpose of these two evasion mechanisms?

  1. US2 prevents CD4+ T cell activation, while UL18 prevents B cell antibody production.
  2. US2 helps evade the CTL response, while UL18 acts as a decoy ligand to proactively inhibit the counteracting NK cell response. (correct answer)
  3. Both proteins work to inhibit the CTL response, with US2 blocking peptide loading and UL18 blocking T-cell receptor binding.
  4. US2 blocks the innate immune response by preventing PAMP recognition, while UL18 blocks the adaptive immune response by inducing T-cell anergy.
Explanation: This is a classic example of a virus's two-pronged strategy against cellular immunity. The US2 protein destroys MHC class I molecules, making the infected cell invisible to cytotoxic T lymphocytes (CTLs). However, this action would normally trigger NK cells via the 'missing self' mechanism. To counteract this, CMV expresses UL18. UL18 is a viral mimic of MHC class I that engages an inhibitory receptor (LIR-1) on NK cells, providing the 'don't kill me' signal that was lost by destroying the authentic MHC class I. Therefore, US2 evades the adaptive CTL response, and UL18 evades the innate NK cell response that would otherwise be activated.

Question 17

The NS1 protein of influenza A virus is a multifunctional immune antagonist. One of its key roles is to interfere with the host antiviral response by binding to and inhibiting the cellular protein CPSF30. What is the direct consequence of this NS1-CPSF30 interaction?

  1. It prevents the polyadenylation and nuclear export of host pre-mRNAs, including those for interferon-β and other antiviral proteins. (correct answer)
  2. It sequesters viral dsRNA replication intermediates, preventing their recognition by the cytoplasmic sensor RIG-I.
  3. It blocks the activation of protein kinase R (PKR), thus preventing the shutdown of global protein synthesis in the infected cell.
  4. It inhibits the ubiquitination of RIG-I, a step required for its activation and downstream signaling through MAVS.
Explanation: CPSF30 is a subunit of the Cleavage and Polyadenylation Specificity Factor, a key component of the machinery that processes the 3' end of host pre-mRNAs. By binding to and inhibiting CPSF30, the influenza NS1 protein globally disrupts the processing of host mRNAs. This 'host shutoff' mechanism prevents the production of antiviral proteins, most critically the interferons and interferon-stimulated genes, allowing the virus to replicate without opposition. While NS1 does perform the functions in B and C via different domains or mechanisms, its specific interaction with CPSF30 is responsible for disrupting host mRNA processing (A).

Question 18

Mouse Mammary Tumor Virus (MMTV) encodes a superantigen (SAg) that is critical for its life cycle. The SAg non-specifically activates T cells expressing particular Vβ segments in their T-cell receptors. What is the key long-term advantage this provides for viral persistence?

  1. The massive initial cytokine release damages lymphoid architecture, creating a privileged site for viral replication.
  2. The SAg preferentially activates the target cells for MMTV infection (B cells and dendritic cells), expanding the pool of infectible cells.
  3. The initial massive activation of T cells is followed by activation-induced cell death, deleting a large fraction of the T-cell repertoire and impairing the specific antiviral response. (correct answer)
  4. The SAg specifically stimulates regulatory T cells, leading to global immunosuppression that allows the virus to replicate unchecked.
Explanation: The defining characteristic of a superantigen response is an initial, massive, non-specific T-cell activation, followed by a crash. This crash involves widespread apoptosis (activation-induced cell death) and anergy of the responding T-cell clones. By deleting entire families of T cells based on their Vβ chain usage, the virus creates significant holes in the host's T-cell repertoire. This immunosuppressive effect impairs the host's ability to mount a coordinated and specific T-cell response against actual viral antigens, thus facilitating long-term viral persistence. MMTV's primary targets are B cells and dendritic cells, but it is the manipulation of the T-cell response that ensures its survival.

Question 19

A virologist is comparing the immune evasion strategies of two viruses. Virus A expresses a protein that binds to the invariant chain (Ii), preventing its cleavage and the subsequent loading of peptides onto MHC class II molecules. Virus B expresses a protein that binds to β2-microglobulin, preventing its association with the MHC class I heavy chain. Which statement correctly predicts the primary immunological impact of these two distinct mechanisms?

  1. Virus A impairs the activation of CD8+ T cells, while Virus B impairs the activation of CD4+ T cells.
  2. Virus A prevents the presentation of exogenous antigens, while Virus B prevents the presentation of both endogenous and exogenous antigens via cross-presentation.
  3. Both viruses will render infected cells more susceptible to killing by Natural Killer (NK) cells due to the 'missing self' phenomenon.
  4. Virus A will primarily affect antigen presentation in professional APCs, while Virus B will affect antigen presentation in all nucleated cells. (correct answer)
Explanation: When analyzing viral immune evasion strategies, focus on which cells express MHC class I versus class II molecules and what happens when these pathways are disrupted. Virus A targets the invariant chain (Ii) that's essential for MHC class II assembly and peptide loading. Since MHC class II molecules are primarily expressed by professional antigen-presenting cells (APCs) like dendritic cells, macrophages, and B cells, this virus specifically impairs antigen presentation in these specialized immune cells. The invariant chain guides proper folding and peptide loading in the endoplasmic reticulum and endosomes of these APCs. Virus B targets β2-microglobulin, which is required for stable MHC class I molecule assembly. Since virtually all nucleated cells express MHC class I molecules to present intracellular peptides to CD8+ T cells, this virus affects antigen presentation across all cell types throughout the body. Option A incorrectly reverses the T cell subsets affected—Virus A (MHC II disruption) impairs CD4+ T cell activation, while Virus B (MHC I disruption) impairs CD8+ T cell activation. Option B mischaracterizes the antigen presentation pathways—both mechanisms affect their respective presentation routes regardless of antigen origin. Option C is wrong because only Virus B would trigger the "missing self" response from NK cells, since NK cells monitor MHC class I expression, not class II. Remember this pattern: MHC class I is ubiquitous (all nucleated cells), while MHC class II is restricted to professional APCs. Viral proteins targeting these pathways will have correspondingly broad or narrow cellular impacts.

Question 20

The NLRP3 inflammasome is assembled when the host cell detects signs of stress or infection, leading to caspase-1 activation and secretion of IL-1β. Some poxviruses encode proteins that contain only a pyrin domain (PYD) and no other functional domains. How does this 'PYD-only' protein most likely function to evade the immune response?

  1. It binds directly to pro-IL-1β, preventing its cleavage by active caspase-1.
  2. It acts as a competitive inhibitor, binding to the PYD of the host adaptor protein ASC and preventing its oligomerization. (correct answer)
  3. It functions as a ubiquitin ligase, targeting NLRP3 for proteasomal degradation before it can be activated.
  4. It inserts into the mitochondrial membrane, stabilizing it and preventing the release of ROS that would normally activate NLRP3.
Explanation: Inflammasome assembly relies on homotypic protein-protein interactions between specific domains, including the pyrin domain (PYD) and the caspase activation and recruitment domain (CARD). The host adaptor protein ASC has a PYD that interacts with the PYD of NLRP3, and a CARD that interacts with the CARD of pro-caspase-1. A viral 'PYD-only' protein is a classic example of molecular mimicry. It can bind to the PYD of ASC, preventing ASC from interacting with NLRP3 or from forming the large oligomeric 'specks' necessary for robust caspase-1 activation. This effectively short-circuits the assembly of the entire inflammasome complex.