Innate Immune Signaling: PRRs, cGAS-STING, MyD88, NF-κB, and Neutrophil Activation
Pattern Recognition Receptors (PRRs) and the Neutrophil Response
PRRs recognize microbial activity in the tissue via patterns on microbes (PAMPs) or tissue damage (DAMPs). The transcript frames this as “pattern recognition receptors” that detect extracellular signals and trigger a cascade to fight infection.
The transcript lists a class of receptors by name-like wording: “nucleotide binding, the oligomerization, domain receptors” which corresponds to NOD-like receptors (NLRs) or related PRRs in innate immunity.
Activation of PRRs leads to activation of gene expression to generate proteins that will coordinate a defense and recruit other cells – a chain-like, stepwise process (a chain-link effect).
The example event: neutrophil/macrophage encounters a microbe (e.g., Gram-negative bacteria detected via LPS). This recognition starts the cascade that ends in production of inflammatory mediators and recruitment of more immune cells.
The cGAS-STING dinucleotide second messenger pathway (dinucleotide activation described in the transcript)
Upon activation, a signaling intermediate called a dinucleotide is produced (described here as cyclic GMP-AMP, cGAMP).
The transcript explains cGAMP as:
Note: the speaker emphasizes that cGAMP is a dinucleotide, containing two nucleotides (a guanine and an adenosine with phosphates).
cGAMP binds to a sensor named STING (Stimulator of Interferon Genes) and activates it.
The transcript then states that STING activation triggers the adaptor MYD88 and downstream signaling to transcription factors.
Important caveat to contextualize: canonical biology typically places STING signaling through TBK1 and IRF3 to drive type I interferon production, while MYD88 is a canonical adaptor for many TLR/IL-1R pathways. The transcript’s model links STING to MYD88, which is an atypical or simplified depiction; be aware of the difference in standard pathways.
The dinucleotide/STING step is part of a broader innate sensing network that ultimately leads to gene expression for inflammatory mediators.
Numerical/formula reference here:
Gene expression: transcription, translation, and transcription factors
Gene expression proceeds in two main steps: transcription and translation:
End product of transcription is a messenger RNA (mRNA) strand, which serves as the template to make protein.
Transcription factors regulate the expression of genes; transcription factors mentioned include Interferon Regulatory Factor 3 (IRF3) and Nuclear Factor kappa B (NF-κB).
Interferon Regulatory Factor 3 (IRF3):
Described as a transcription factor that stimulates the expression of interferons when activated.
Promoters:
The transcript describes a promoter as a chemical that acts like a starter box for a gene and “promotes” expression. In the model, promoters help initiate transcription for genes encoding defense mediators.
The two-step model is presented as a simplified view of a much more complex reality with many small factors, post-transcriptional modifications, and regulatory checks.
MyD88 and the NF-κB cascade: upregulation of inflammatory mediators
Activation of PRRs (exemplified by STING in the transcript) leads to activation of signaling molecules that converge on NF-κB.
Nuclear Factor kappa B (NF-κB):
Once produced/activated, NF-κB translocates to the nucleus and induces the transcription of a broad set of genes involved in the immune response.
Outcome of NF-κB-driven transcription:
Upregulation of pro-inflammatory cytokines (e.g., interleukins, TNF-α) and co-stimulatory molecules (CD markers).
Interleukins and tumor necrosis factor (TNF-α) are highlighted as typical pro-inflammatory mediators produced in this cascade.
Co-stimulatory molecules (CD markers):
These molecules are upregulated to provide the necessary “costimulation” for activating other immune cells and promoting a robust immune response.
The transcript emphasizes that the neutrophil/macrophage’s recognition of a microbe triggers a cascade that results in the production of these inflammatory mediators and co-stimulatory signals to recruit and activate additional immune cells.
The trigger: PAMPs and LPS from Gram-negative bacteria
The model: a neutrophil/macrophage encounters a microbe and uses pattern recognition receptors (PRRs) to detect microbial patterns.
An example is lipopolysaccharide (LPS) from Gram-negative bacteria, which serves as a recognizable marker for PRRs.
Once PRRs bind their ligand (e.g., LPS), this triggers the signaling cascade culminating in NF-κB activation and cytokine production.
The transcript notes that the signaling can be initiated by any of several PRRs (the two pink folded structures in the diagram symbolize PRRs).
Inflammation and the functional consequences
The signaling cascade results in the production of pro-inflammatory mediators which attract and activate additional immune cells to the site of infection.
The inflammatory response is classically characterized by the cardinal signs: heat (calor), redness (rubor), swelling (tumor), pain (dolor), and loss of function (functio laesa).
The production of interleukins and TNF-α drives vasodilation and vascular permeability, facilitating leukocyte recruitment.
Co-stimulatory molecules enable activation of immune cells (e.g., T cells and neutrophils) at the site of infection or at nearby lymphoid tissues.
The model emphasizes that mediators are produced on demand (not pre-stored): cells synthesize these signals when needed to mount an infection-specific defense.
On-demand synthesis and the “on-the-ground” view
The transcript underlines that cytokines and other mediators are not stored; they are synthesized when needed by the responding cells.
This on-demand production allows the immune system to tailor its response to the particular microbe present in the local environment.
Simplified mental model and practical takeaways
A neutrophil encounters a microbe and recognizes it via PRRs (e.g., recognizing LPS on Gram-negative bacteria).
PRR activation initiates a signaling cascade that involves cGAS-STING-like sensing and, in the speaker’s model, an adaptor (MYD88) pathway leading to transcription factor activation.
NF-κB and IRF3 drive gene expression of inflammatory cytokines and interferons, as well as co-stimulatory molecules.
Pro-inflammatory cytokines (e.g., interleukins, TNF-α) recruit and activate more immune cells; co-stimulatory molecules facilitate further activation.
The inflammatory response produces classic signs: heat, redness, swelling, pain, and sometimes loss of function.
While multiple pathways exist in real biology, this lecture presents a cohesive, simplified map to help students understand the sequence from pathogen detection to an inflammatory response.
Connections, context, and considerations
This content ties into foundational principles of innate immunity: pattern recognition, signal transduction, transcriptional regulation, and inflammatory mediator production.
It connects to the broader concept that gene expression is regulated by transcription factors (e.g., IRF3, NF-κB) and promoter architecture, leading to targeted protein production.
Real-world relevance includes understanding how initial innate responses shape subsequent adaptive immunity and how dysregulation can contribute to excessive inflammation or sepsis.
Ethical/practical note: The transcript contains some simplifications and a few inaccuracies (e.g., STING signaling via MYD88 is not the canonical pathway). When studying from multiple sources, cross-check canonical signaling steps (e.g., STING -> TBK1 -> IRF3 -> type I interferons) and treat the presented model as a simplified teaching diagram with a note on its limitations.
Key terms to remember
PRR: Pattern Recognition Receptors
PAMP: Pathogen-Associated Molecular Pattern
DAMP: Damage-Associated Molecular Pattern
cGAMP: cyclic GMP-AMP dinucleotide
STING: Stimulator of Interferon Genes
MYD88: Myd88 adaptor protein (TLR/IL-1R signaling; transcript links it to STING in a simplified model)
NF-κB: Nuclear Factor kappa B
IRF3: Interferon Regulatory Factor 3
ILs: Interleukins
TNF-α: Tumor Necrosis Factor alpha
CD markers: Co-stimulatory molecules on immune cells
LPS: Lipopolysaccharide
Transcription and Translation: the two-step process of gene expression
Promoter: DNA region that initiates transcription
In short: PRR detects a signal → signaling cascade → transcription factors (NF-κB, IRF3) activate gene expression → cytokines and co-stimulatory molecules produced on demand → recruitment and activation of more immune cells → inflammation signs appear.