What is the inflammatory response?

Inflammation is an organism’s protective response to pathogens, infection, or tissue damage and involves the coordinated communication of a variety of immune cells and blood vessels (Figure 1). The host response produces an intricate cascade of molecular signals that activate or change the actions of immune and nonimmune cells. [1].

Physiological and pathological inflammation is an adaptive response that plays a critical role in host defense against infection and disease occurring in response to tissue damage from microbial pathogen, infection, chemical irritation, and other trauma. Inflammation may be acute or chronic in duration. Acute inflammation occurs as the body works to heal itself by resolving of infection or tissue injury. Depending on the amount of damage, the acute phase may be enough to resolve the problem. Conversely, chronic inflammation can be harmful and is linked to serious diseases.

This page discusses the general phenomenon of inflammation and triggers to the process.
 

Figure 1. The inflammatory response. Multiple cell types including granulocytes, NK cells, monocytes, macrophages, T cells and B cells are required for removal of damaged cells or pathogens. Inflammatory inducers, including infection or tissue damage, set off a cascade of cytokines, chemokines, and other proteins to help signal immune cells.
 

What activates inflammation?

Cell signaling pathways are triggered by nucleic acids, proteins, and sugars from pathogens or damaged cells (Figure 2). Some of these triggers are recognized as common molecules and conserved structural patterns and bind to a specific set of pattern recognition receptors (PRRs) found on immune cells [2].

When a pathogen breaches the skin or mucus membrane, cells of the innate immune system mount an immediate response.  To initiate this response, the immune system recognizes pathogen associated molecular patterns (PAMPs) from bacteria and viruses [3].  PAMPs include bacterial and viral nucleic acids and other conserved molecules (Table 1). PAMPs bind several families of surface and intracellular PRRs on immune cells. PRR signaling uses several receptors including Toll like receptors and NLRs (nucleotide-binding, oligomerization domain (NOD)-like receptors), and c-type lectin (receptor for advanced glycation end-products).

The immune system has a set of sensors that detect molecules related to tissue damage known as damage associated molecular patterns (DAMPs).  The immune system triggers DAMPs as a response to cell death during infection from a microbe or toxin. DAMPs can be activated under sterile conditions to have immune cells eliminate dying cells. Once activated, PRRs initiate a signaling cascade that results in an inflammatory response.  Common DAMPs include heat shock proteins (HSP), HMGB-1, uric acid, and extracellular ATP (Table 1) [4, 5].  DAMPs can bind receptors including advanced glycation end products (RAGE), triggering receptors expressed on myeloid cells (TREMs), and ion channels (Figure 2).
 

Table 1. Nonexclusive list of PAMPs and DAMPs that trigger inflammation.

PAMP examplesDAMP examples
Microbial nucleic acidsMitochondrial DNA (mtDNA)
Unmethylated CpG motifsUric acid
Double stranded RNAS100 proteins
Single stranded RNAHeat shock proteins
PeptidoglycansFibronectin
Lipoteichoic acidβ amyloid (Aβ)
Lipopolysaccharide (LPS)Advanced glycation end products (AGEs)
GlycosylphosphatidylinositolHistones

 

Figure 2. Activators to the early stages of inflammation.
Pattern recognition receptors (PRRs) include pathogen and damaged associated molecular patterns (PAMPs and DAMPs). Receptors on innate immune cells recognize inflammation inducers and will activate the immune response.

How to activate inflammation in model systems

The presence of inflammation is indicated by populations of activated immune cells and cytokines. There are multiple inducers for the inflammatory process that can be used on immune cells from blood or tissue samples and directly in animal models.

Endotoxins including LPS are potent, short-lived inducers for inflammation. LPS is used as an agent both in vivo and in vitro to understand molecular pathways and model the immune response [3, 4, 5]. Applying LPS to PBMCs can activate and induce production of cytokines secreted by a wide range of immune cells (Figure 3) [4]. LPS can be applied in a dose and time dependent manner to modulate the inflammatory response. 

Animal LPS challenge models are used in drug discovery to characterize anti-inflammatory therapeutics. LPS administered to animals via intravenously or intradermally allows for systemic or local inflammatory responses [5]. As in cell culture models, the inflammatory response can be modulated based on LPS dose and time. Blood or serum can be analyzed for secreted cytokines.
 

Figure 3. Inflammation induced by LPS measured by Human 65plex ProCartaPlex Assay. Human PBMCs were stimulated with 10 μg/mL LPS for 24 (d1), 48, and 72 h (d3).  Cell culture supernatant was probed for the following 65 markers with Invitrogen ProcartaPlex Human 65-plex panel (Thermo Fisher Scientific Cat. No. EPX650-10065-901) kit: APRIL, BAFF, BLC, CD30, CD40L, ENA-78, Eotaxin, Eotaxin-2, Eotaxin-3, FGF-2, Fractalkine, G-CSF, GM-CSF, GROα, HGF, IFN-α, IFN-ɣ, IL-1α, IL-1β, IL-2, IL-2R, IL-3, IL-4, IL-5, IL-6, IL-7, IL-8, IL-9, IL-10, IL-12p70, IL-13, IL-15, IL-16, IL-17A, IL-18, IL-20, IL-21, IL-22, IL-23, IL-27, IL-31, IP-10, I-TAC, LIF, MCP-1, MCP-2, MCP-3, M-CSF, MDC, MIF, MIG, MIP-1α, MIP-1β, MIP-3α, MMP-1, NGF-β, SCF, SDF-1α, TNF-α, TNF-β, TNF-R2, TRAIL, TSLP, TWEAK, and VEGF-A.

Activating inflammation in the CNS and brain is challenging as it is protected by the blood-brain barrier. Mouse models are often used to overexpress inflammatory agents including β amyloid [9]. LPS can be used as a challenge model to induce neuronal damage and activate inflammation [10]. Microglial cells (Table 2), a type of tissue resident macrophage (Figure 4) are stimulated and recruited to the area of injury. Serum from harvested brain samples can be measured for neural specific injury markers including GFAP, S100B, NF-H, and UCH-L1.

Table 2. Nonexclusive list for tissue resident microglial cells.

SpeciesMarker typeMarkerClone (flow cytometry)Location
MouseGeneral phenotypicsCD11bM1/70Surface
CD4530-F11Surface
F4/80BM8Surface
FunctionalLy-6CHK1.4Surface
Siglec-HeBio440cIntracellular
CX3CR12A9-1Surface
CD115AFS98Surface
Tmem119V3RT1GOszSurface
Sall1NRNSTNXIntracellular

 

Brain tissue stained with TMEM119 antibody to identify microglial cells. Positive cells are outlined in brown and have cell nucleus stained blue.

Figure 4. Microglial cells in brain tissue. 
Immunohistochemistry of paraffin-embedded human brain tissue slide using 27585-1-AP (TMEM119 antibody) at dilution of 1:1000 (under 40x lens) heat mediated antigen retrieved with Tris-EDTA buffer (pH 9).

References
  1. Hawiger J, Zienkiewicz J (2019) Decoding inflammation, its causes, genomic responses, and emerging countermeasures. Scand J Immunol 90:e12812.
  2. Newton K, Dixit VM. Signaling in innate immunity and inflammation. Cold Spring Harb Perspect Biol. 2012 Mar 1;4(3):a006049
  3. Tang D, Kang R, Coyne CB, Zeh HJ, Lotze MT. PAMPs and DAMPs: signal 0s that spur autophagy and immunity. Immunol Rev. 2012 Sep;249(1):158-75. 
  4. Rock KL, Kono H. The inflammatory response to cell death. Annu Rev Pathol. 2008;3:99-126. 
  5. Roh JS, Sohn DH. Damage-Associated Molecular Patterns in Inflammatory Diseases. Immune Netw. 2018;18(4):e27. Published 2018 Aug 13. 
  6. Meneses G, Rosetti M, Espinosa A, Florentino A, Bautista M, Díaz G, Olvera G, Bárcena B, Fleury A, Adalid-Peralta L, Lamoyi E, Fragoso G, Sciutto E. Recovery from an acute systemic and central LPS-inflammation challenge is affected by mouse sex and genetic background. PLoS One. 2018 Aug 22;13(8):e0201375. 
  7. Cook DB, McLucas BC, Montoya LA, Brotski CM, Das S, Miholits M, Sebata TH. Multiplexing protein and gene level measurements on a single Luminex platform. Methods. 2019 Apr 1;158:27-32. 
  8. Seemann, S., Zohles, F. & Lupp, A. Comprehensive comparison of three different animal models for systemic inflammation. J Biomed Sci 24, 60 (2017). 
  9. Fang F, Yu Q, Arancio O, Chen D, Gore SS, Yan SS, Yan SF. RAGE mediates Aβ accumulation in a mouse model of Alzheimer's disease via modulation of β- and γ-secretase activity. Hum Mol Genet. 2018 Mar 15;27(6):1002-1014.
  10. Sheng JG, Bora SH, Xu G, Borchelt DR, Price DL, Koliatsos VE. Lipopolysaccharide-induced-neuroinflammation increases intracellular accumulation of amyloid precursor protein and amyloid beta peptide in APPswe transgenic mice. Neurobiol Dis. 2003 Oct;14(1):133-45. 

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