Immunology Fall 2026: Lecture 6 Cellular Innate Immunity Part 2

Overview

Brianne Barker explains how innate immune receptors tailor cellular responses to bacteria versus viruses: NF-κB drives inflammatory cytokines such as TNF, IL-1β, and IL-6, while IRFs induce type I interferons that establish an antiviral state. She traces the interferon pathway from viral sensing through interferon-stimulated genes (ISGs), explains why prolonged inflammation or interferon harms tissues, and introduces how receptor identity and location—including Toll-like receptors (TLRs)—help determine the response.

Key takeaways

PRR signaling helps distinguish bacterial from viral threats: NF-κB commonly drives TNF, IL-1β, and IL-6, whereas IRFs drive type I interferons.
Type I interferon released by an infected cell binds IFNAR on neighboring cells and induces ISGs that make those cells less able to support viral replication.
Antiviral defenses such as PKR-mediated translation inhibition and OAS-associated mRNA degradation also disrupt host-cell functions, so the antiviral state is protective but temporary.
Inflammation is beneficial when localized and short-lived, but systemic activation can cause septic shock and chronic activation can contribute to tissue damage and metabolic dysfunction.
PRR location provides information about the threat: surface TLRs tend to detect microbial lipids and proteins, endosomal TLRs detect nucleic acids, and cytoplasmic RLRs detect viral material.
Innate cytokine signals do more than control pathogens: they also help activate adaptive immunity, making both insufficient and excessive interferon responses consequential.

Chapters

0:00 Course Updates and the 2011 Nobel Prize Assignment

Brianne Barker says class recordings are available on YouTube and the textbook e-book link now works through Moodle.
The first assignment is due Friday and asks students to compare two papers about innate immune receptors and argue which contribution merited the contested 2011 Nobel Prize spot.
Barker says either paper can support a legitimate argument; the class will discuss the issue after assignments are submitted.

4:47 Inflammation Can Protect, Repair, or Cause Disease

Inflammation is an umbrella term for responses involved in host defense, tissue repair, and adaptation to infection, injury, or stress.
Autoimmunity means an immune response against self, allergy targets a harmless foreign antigen such as pollen, and inflammatory disease involves excessive or uncontrolled inflammation.
Inflammation is not inherently harmful: short-lived responses help clear microbes, while excessive or persistent responses can damage tissues.

11:03 Septic Shock and the Long-Term Costs of Chronic Inflammation

When microbial signals such as PAMPs circulate in blood, widespread vessel dilation and leakage can lower blood pressure and volume, promote clotting, and impair organ perfusion—features of septic shock.
Chronic low-level inflammation can damage blood vessels and organs, promote scarring and abnormal tissue growth, and impair muscle insulin signaling.
Possible drivers include unresolved infection, inappropriate responses to gut microbes, genetic factors, and triggers such as lipids associated with obesity.
Barker frames the central rule as a Goldilocks problem: inflammation is useful when appropriately timed and limited, but harmful when excessive or prolonged.

19:13 Why Viral Infection Requires More Than Phagocytosis

An extracellular virus particle is primarily a delivery package: it binds a cell, enters, and uses cellular machinery to produce many new viral particles.
Because viral entry is essential to replication, phagocytosis can inadvertently help deliver viruses into cells rather than solve the infection.
Once viral components and nucleic acid are replicating inside a cell, the cell becomes a virus factory; eliminating that factory may require killing the infected cell.

26:33 Isaacs and Lindemann’s Discovery of Interferon

In 1957, Alick Isaacs and Jean Lindemann found that products from influenza-infected cells could interfere with infection in neighboring cells and named the factor interferon.
Type I interferons—principally interferon-α and interferon-β in this lecture—are rapid innate responses to viral infection.
Type II interferon is interferon-γ and is associated with adaptive responses; type III interferon, including interferon-λ, is especially associated with barrier tissues.

30:15 How Type I Interferon Warns Neighboring Cells

A virus-infected cell detects viral material, often its genome, through a pattern-recognition receptor (PRR) and changes gene transcription.
The infected cell secretes type I interferon, which binds IFNAR—the type I interferon receptor—on neighboring cells.
Interferon signaling puts uninfected neighbors into an antiviral state, making them less able to support viral replication if virus reaches them.

36:44 Interferon-Stimulated Genes Build the Antiviral State

IFNAR signaling activates thousands of interferon-stimulated genes (ISGs), which share responsive regulatory elements such as the interferon-stimulated response element (ISRE).
Different viruses may be opposed by different combinations of ISGs; the exact virus-specific combinations remain an active area of research.
ISGs have varied mechanisms, so the antiviral state is a coordinated program rather than the effect of one antiviral gene.

39:22 PKR and OAS Block Replication by Disrupting Cell Functions

Protein kinase R (PKR) can inhibit translation, limiting the protein production a virus needs to replicate.
The 2′,5′-oligoadenylate synthetase (OAS) pathway activates an enzyme that degrades mRNA.
These mechanisms are not selective for viral material: they also suppress the host cell’s translation or degrade its mRNA, making the antiviral state damaging if sustained.
Barker compares antiviral-state cells to a firebreak: temporarily sacrificing local cell function can help prevent infection from spreading through tissue.

45:35 Interferon’s Goldilocks Effect on Health and Immunity

Type I interferon can cause systemic symptoms including fever, chills, and nausea because many cells have interferon receptors and enter an altered state.
Many viral infections produce flu-like symptoms, but those symptoms alone do not establish infection with influenza virus.
Too little interferon can leave antiviral defense and adaptive immunity impaired; too much or poorly timed interferon can harm the body.
Strong antiviral signaling can suppress antibacterial responses, helping explain why severe influenza may be followed by dangerous bacterial infection.

51:00 Bacterial and Viral Responses Also Prepare Adaptive Immunity

Barker contrasts bacterial-associated TNF, IL-1β, and IL-6 with viral-associated type I interferons, IFN-α and IFN-β.
Which PRR is activated helps determine the cytokines produced and the downstream response suited to the pathogen.
Innate signals also help activate adaptive immune cells; the danger hypothesis describes how innate detection of danger helps signal that an adaptive response is warranted.

54:04 NF-κB and IRF Route PRR Signals to Different Cytokines

PRR signaling activates transcription factors that drive cytokine gene expression.
NF-κB promotes inflammatory cytokines such as TNF, IL-1β, and IL-6, which Barker groups as the classic bacterial response.
Interferon regulatory factors (IRFs) promote interferon production and the antiviral response.
The NF-κB-versus-IRF distinction is a simplified framework for understanding how receptor signaling helps tailor innate immunity.

58:33 PRR Families Use Cellular Location to Identify Threats

PRR families include TLRs, C-type lectin receptors (CLRs), RIG-I-like receptors (RLRs), NOD-like receptors (NLRs), and absent in melanoma 2-like receptors (ALRs).
Some PRRs are on the cell surface, others are in intracellular compartments, and others are in the cytoplasm.
Location supplies context: surface receptors can detect extracellular microbes, endosomal receptors can detect material from engulfed microbes, and cytoplasmic receptors often detect viral components.

1:00:45 TLR Discovery Links Fly Development to Mammalian Immunity

Christiane Nüsslein-Volhard identified the Drosophila gene toll while studying embryo development; removing it disrupted dorsal–ventral patterning.
Later experiments showed that adult flies without Toll were highly susceptible to fungal infection, revealing an immune function as well as a developmental one.
Mammals have about 13 TLRs, broadly divided into cell-surface receptors that detect lipids and proteins and endosomal receptors that detect nucleic acids.
Surface TLRs generally activate NF-κB and inflammatory cytokines, while endosomal TLRs often activate IRFs and interferon; CLRs are surface receptors, RLRs detect viral material in the cytoplasm, and Barker postpones detailed NLR coverage.

Summary, takeaways, and chapters were generated by AI from the video's transcript and may contain errors. The video belongs to its creator, Brianne Barker.