Acute Chronic Inflammation Comparison Essay
IRF5 Ablation Limits Neutrophil Influx in the Inflamed Joint.
The murine model of antigen-induced arthritis relies on s.c. immunization with methylated BSA (mBSA) plus complete Freund’s adjuvant (CFA), followed by intraarticular injection of mBSA into the knee joint (17). The model is characterized by infiltration of polymorphonuclear and mononuclear cells, pannus formation, and erosion of bone and cartilage (17), and is Th17 dependent (18). We have previously reported that synovial macrophages from the AIA-affected joints are characterized by high levels of IRF5 (16). Here, we aimed to determine the role that IRF5 plays in synovial physiology and function in both naïve joints and during inflammatory arthritis. IRF5 deficient mice (IRF5−/−) and their littermate wild-type controls (WT) were immunized with mBSA in CFA and 7 d later, knees were either challenged with mBSA or PBS control (Fig. S1A). Knee swelling of WT mice was induced rapidly and peaked at day 2, after which time swelling decreased. IRF5−/− mice showed significantly less swelling at day 2 in comparison with WT controls (Fig. 1A). Reduced swelling corresponded to a decreased number of cells recovered from excised knee joints at that point (Fig. 1B). Knee pathology at day 2 was further assessed histologically for the degree of inflammatory infiltrate into the synovium and joint cavity, synovial capsule and synovial membrane thickness and bone erosion, respectively (Fig. S1B). IRF5 knockout mice displayed a significantly lower synovial membrane thickening score in the mBSA-challenged knees than WT mice (Fig. 1C). Consistent with resolved knee swelling at the later stages, we observed reduced synovial membrane thickening at day 7 (Fig. S1B).
IRF5 ablation limits neutrophil influx in the inflamed knee. (A) Comparison of knee swelling between inflamed knees of the IRF5−/− and WT animals, expressed as a percentage of swelling of the mBSA-challenged knee compared with the PBS knee. (B) Total number of cells recovered from excised knee joints of the IRF5−/− and WT animals at day 2 of the AIA. (C) Synovial thickness of inflamed knees at day 2 post intraarticular injection of mBSA based on examination of histology slides (Fig. S1B). (D) Ly6G+ cells detected within the synovial capsule of inflamed knees in the IRF5−/− and WT animals, analyzed by immunohistochemical staining and confocal microscopy. Cell nuclei are marked by DAPI staining in blue and Ly6G+ cells are shown in red. Compare 52 ± 17 and 14 ± 3 Ly6G+ cells per field for WT and IRF5−/− (P = 0.0045). (E) Number of Th1, Th17, and γδ T IL17+ cells in the joints of mice at day 7 of AIA, expressed as a percentage of live CD45+ cells. Data show the mean and SEM derived from 10 to 20 mice from three to four independent AIA experiments. Each dot represents an individual mouse. Statistical analysis was performed by one-tailed Mann–Whitney u test. *P < 0.05; **P < 0.01; ***P < 0.001.
AIA model in IRF5−/− mice, innate immune responses. (A) Schematic diagram showing treatment and harvest time points in the antigen-induced arthritis model. (B) Representative images of the knee showing areas of bone erosion (arrowhead), synovial thickening (asterisk), and leukocyte infiltration into joint space (arrows). (C) Total number of macrophages, monocytes, DCs, and pro-IL-1β+ neutrophils recovered from excised knee joints of IRF5−/− and WT animals at day 2 of AIA, expressed as a percentage of live CD45+ cells. Data shown are the mean and SEM derived from 10–20 mice from three to four independent AIA experiments. Each dot represents an individual mouse. Statistical analysis was performed by one-tailed Mann–Whitney u test. *P < 0.05; **P < 0.01.
The observed difference in the number of leukocytes recovered from mBSA-challenged knees at day 2 compared with the PBS controls (Fig. 1B) was mirrored by the increase in myeloid cell numbers, i.e., CD11b+F4/80hiCD64hi macrophages, CD11b+F4/80dim monocytes, CD11b+F4/80+CD11c+ dendritic cells (DCs), and neutrophils (Fig. S1C). There was no change in the number of CD11b−F4/80−CD11c+ dendritic cells (Fig. S1C). Due to the likelihood of substantial influx into the neutrophil pool from the bone marrow (BM) when harvesting knees, we used pro-IL1β as a marker for activated neutrophils (19) and focused on synovial-activated neutrophils CD11b+F4/80−Ly6G+pro-IL1β+. The number of synovial-activated neutrophils was significantly reduced in inflamed knees of IRF5−/− mice (Fig. S1C), whereas the numbers of synovial monocytes, macrophages, and DCs were unaffected (Fig. S1C). This was confirmed by immunohistochemical staining of slides for Ly6G+ cells and cell counting, which demonstrated a clear reduction in synovial neutrophil infiltrate in the IRF5−/− animals (Fig. 1D). Neutrophils are thus the only myeloid cells, whose influx or propagation depends on IRF5 during early stages of AIA.
IL-17 produced by CD4+ (Th17) cells or γδ T cells plays an important role in the pathogenesis of AIA (18). Thus, we analyzed the number of IL-17 producing Th17 and γδ T cells in the joints of mice at days 2 and 7 of the disease. Minimal CD4+ and no γδ T-cell populations were detected at day 2, whereas by day 7, the T-cell response in WT mBSA knees was increased compared with day 2 (Fig. S2A). However, in IRF5−/− animals the T-cell response at day 7 was significantly reduced, affecting both Th17 and γδ T IL17+ cells, as well as IFNγ-producing Th1 cells (Fig. 1E). We also observed a reduction in the levels of IFNγ and IL-17A mRNA and of cytokines facilitating generation of Th1/Th17 cells, e.g., IL-1β, IL-6, IL-12p40, and IL-23p19 (Fig. S2B).
AIA model in IRF5−/− mice, adaptive immune responses. (A) IRF5 WT and KO were immunized with mBSA and challenged with mBSA or PBS control 7 d later. Knees were excised on day 2 or 7 of disease and analyzed using flow cytometry. Cell suspensions obtained from knees were stained for CD45+ live and CD4 expression to identify CD4+ T cells. Statistical analysis performed by two-way ANOVA and Bonferroni‘s multiple comparison. (B) IL17, IFN-γ, IL-1β, IL-6, IL-12p40, and IL-23p19 mRNA in whole joint extracts from arthritic mice at day 2 of AIA. Data shown are the mean and SEM derived from 10–20 mice from three to four independent AIA experiments. (C) Proliferation of lymphocytes after 48-h stimulation with αCD3 or mBSA. The data are mean and SEM derived from six independent lymphocyte cultures from lymph nodes (LNs) of IRF5−/− and WT arthritic mice. Statistical analysis was performed by one-tailed paired t test. (D) Total number of B cells in the lymph nodes and spleen. The data are mean and SEM derived from 10 to 15 independent cell preparations from LNs and spleens of the IRF5−/− and WT immunized mice. Each dot represents an individual mouse. (E) Levels of total IgG1 and IgG2a in the serum. The data are mean and SEM using 3–5 mice treated from a representative AIA experiment. Statistical analysis was performed by two-tailed unpaired t test. *P < 0.05; **P < 0.01; ***P < 0.001; ****P ≤ 0.0001.
To assess a potential effect of IRF5 deficiency on the immunization process preceding the intraarticular challenge, inguinal lymph nodes were harvested 7 d after immunization. Lymph node cell suspensions were stimulated with α-CD3 or mBSA for 48 h and T-cell proliferation was measured. Both stimulations resulted in a significant increase in lymphocyte proliferation, including proliferation of CD4+ and CD8+ T cells, but proliferative responses were unaffected by IRF5 deficiency (Fig. S2C). As B cells appear to play a critical role in arthritis pathogenesis (20), and express variable levels of IRF5, we examined B-cell numbers and function during the immunization of the IRF5−/− animals. No CD19+ B cells were detected in the joint, whereas the number of CD19+ B cells in the blood, spleen, and lymph node remained unaffected (Fig. S2D). Total B-cell (IgG1 and IgG2a) responses in the serum were assessed at day 2 of AIA and revealed no significant change for IgG1 and a reduction in the levels of IgG2a (Fig. S2E). This is in agreement with the previously documented role for IRF5 in the control of the IgG2a locus (21).
Taken together, the deficiency limits the neutrophil influx into the inflamed joint in the early stages of arthritis and leads to a reduction in the Th1/Th17 and γδT IL-17+ cells in the joint at the later stages.
Synovial Macrophages in the Naïve and Inflamed Knee.
The phenotype and origin of macrophages in the knee joint in the steady state or during inflammatory arthritis are not known. Recent work by Misharin et al. demonstrated that the ankle synovial lining of naïve mice consists of a heterogeneous population of macrophages, with the majority being true tissue-resident cells (CD11b+CD11cintF4/80+CD64+MHCII−) and about a fifth originating from bone marrow (CD11b+CD11cintF4/80+CD64+MHCII+) (14). Applying a similar gating strategy (SI Appendix, FACS Gating Strategy and Representative Plots) we demonstrate that the total number of CD11b+F4/80+CD64+ macrophages in both ankle and knee is reduced in IRF5−/− mice (Fig. S3A). The reduction seemed to be mainly due to the effect on the number of MHCII+ macrophages (Fig. 2A) that display the highest level of IRF5 expression within macrophage populations (Fig. 2B). Of interest, we find a higher representation of MHCII+ macrophages in the knee than in the ankle (Fig. 2A), whereas MHCII− macrophages in the ankle express higher levels of IRF5 than their counterparts in the knee (Fig. 2B). These data highlight differences between the two joints in naïve animals.
Characterization of synovial macrophages in naïve knees and during AIA. (A) Number of tissue resident MHCII+ macrophages in both ankle and knee joints of naïve IRF5−/− mice and WT controls. (B) IRF5 expression within tissue resident macrophage populations defined by mean fluorescence intensity (MFI) and normalized to MFI of IRF5-deficient animals. Data shown are the mean and SEM derived from 6 mice from a representative experiment. Statistical analysis was performed by two-way ANOVA with Bonferroni’s correction for multiple comparisons. *P < 0.05. (C) Number of CD11b+F4/80dim monocytes and CD11b+F4/80hi macrophages in the mBSA-challenged knees or PBS controls of CCR2−/− and WT mice, expressed as a percentage of live CD45+ cells. (D) Number of MHCII+ or CD206+ macrophages in the mBSA-challenged knees or PBS controls of IRF5−/− and WT animals, expressed as a percentage of CD11b+F4/80hi cells. Data shown are mean and SEM derived from 4–7 mice from a representative AIA experiment (A and C) or 10–20 mice from three to four independent AIA experiments (C and D). Each dot represents an individual mouse. Statistical analysis was performed by one-tailed Mann–Whitney u test. *P < 0.05; **P < 0.01; ***P < 0.001.
Characterization of synovial macrophages. (A) Number of tissue resident CD64hi macrophages in both the ankles and knees of naïve IRF5−/− mice and WT controls. Data shown are mean and SEM derived from 4 to 7 mice from a representative experiment. (B) Number of Ly6Chi monocytes and macrophages, expressed as a percentage of CD11b+F4/80dim and CD11b+F4/80hi cells, respectively. (C) Number of CD64hi macrophages, expressed as a percentage of CD11b+F4/80dim and CD11b+F4/80hi cells, respectively. (D) Number of CD64hi macrophages in the mBSA-challenged knees or PBS controls of IRF5−/− and WT animals, expressed as a percentage of CD11b+F4/80hi cells. Data shown are mean and SEM derived from 10 to 11 mice from two to three independent AIA experiments. Each dot represents an individual mouse. Statistical analysis was performed by one-tailed Mann–Whitney u test. *P < 0.05; **P < 0.01; ***P < 0.001.
To elucidate the source of macrophages infiltrating the knee in response to mBSA challenge, AIA was performed using C-C chemokine receptor 2 deficient mice (CCR2−/−), which lack CCR2 expression on classical Ly6C+ monocytes essential for their trafficking (22). The influx of CD11b+F4/80dim monocytes and CD11b+F4/80hi macrophages was almost completely abolished in the mBSA-challenged joints of CCR2−/− mice (Fig. 2C). Ly6C was expressed on 80–90% of monocytes and macrophages in the knee joints of immunized mice, with 50–60% of those being Ly6Chi (Fig. S3B). This finding represents a drastic increase in the amount of Ly6Chi cells compared with steady-state joints without intraarticular antigen challenge. A total of 60% of macrophages in PBS-treated knees and 90% of macrophages in the mBSA-challenged knees were CD64hi (Fig. S3C). IRF5 deficiency did not affect monocyte, macrophage, or DC infiltrate into the mBSA-challenged knee (Fig. S1B), but expression of CD64 (Fc gamma receptor 1) on macrophages was reduced (Fig. S3D). Consistent with our previously reported observations (15), we observed a shift toward an alternatively activated macrophage phenotype in the IRF5−/− animals, with a lower number of macrophages expressing MHC II and higher numbers expressing CD206 (Fig. 2E).
Taken together, these data suggest that in the antigen-driven arthritis model, Ly6C+ circulating monocytes are preferentially recruited into the inflamed joint and give rise to Ly6C+ inflammatory macrophages. The presence of IRF5 appears to be important for differentiation of monocytes into CD64+ macrophages and for the establishment of the MHCII+ inflammatory macrophage phenotype in the arthritic joint.
IRF5 Controls Neutrophil Influx via Regulation of Chemokine Production Such as CXCL1.
Because the recruitment of neutrophils into the knees of IRF5−/− animals was significantly reduced, we set out to examine whether neutrophils themselves were affected by the loss of IRF5. Firstly, we measured the levels of IRF5 expression in neutrophils in WT mice and found them to be significantly lower compared with monocytes and macrophages in the knee (Fig. 3A). Secondly, we examined the migratory capacity of neutrophils toward the neutrophil chemoattractant CXCL2 using the EZ-Taxiscan system, which allows visualization of cell migration in shallow, linear gradients and provides real-time analysis of neutrophil movement (23). Neutrophils were isolated from the air pouch, created on the dorsal surface of the mice and injected with zymosan (24) (Fig. S4A). Approximately 80% of the recovered cells after injection of 100 μg of zymosan were CD11b+Ly6G+ neutrophils. The Euclidean distance that neutrophils covered when migrating toward the attractant was comparable in WT and IRF5−/− cells (Fig. 3B and Fig. S4B). Notably, basal neutrophil movement, in the absence of any chemoattractant, was not affected either. Hence, we conclude that lack of IRF5 does not affect the intrinsic capacities of neutrophils to migrate toward a chemokine. To assess whether secretion of neutrophil attracting chemokines was altered in IRF5−/− mice, we analyzed levels of known neutrophil chemoattractants, such as CXCL1, CXCL2, CXCL10, chemokine (C-C motif) ligand (CCL) 3, CCL4, CCL5, in the supernatants of synovial leukocytes isolated from mBSA-treated knees, by Luminex analysis. We found that the levels of CXCL1 were significantly affected by the loss of IRF5 (Fig. 3C).
Neutrophil recruitment in IRF5−/− mice is limited due to the decrease in CXCL1 secretion. (A) IRF5 expression within cell populations of the inflamed knee defined by MFI. Data shown are the mean and SEM derived from 10–20 mice from three to four independent AIA experiments. (B) The tracks of air pouch neutrophilic infiltrate in an EZ-Taxiscan migrating toward CXCL2 (Fig. S4B) were analyzed to show the Euclidean distance each cell traveled in a 45-min time period. Data shown are the average and SEM of three independent experiments each including 9–20 movies per treatment. (C) Levels of CXCL1 in the supernatants of the synovial leukocytes isolated from the mBSA-treated knees. Data are shown as mean with 95% confidence interval from 6 mice from a representative AIA experiment. (D) Number of infiltrating cells and neutrophils in bronchoalveolar lavage fluid (BAL) of the IRF5−/− and WT mice challenged with LPS intranasally. Each dot represents an individual mouse. (E) Levels of CXCL1 in BAL of LPS-challenged animals. Data are shown as mean with 95% confidence interval from 9–10 mice from two independent experiments. Statistical analysis was performed by one-tailed Mann–Whitney u test. *P < 0.05; **P < 0.01.
When something harmful or irritating affects a part of our body, there is a biological response to try to remove it. The signs and symptoms of inflammation can be uncomfortable but are a show that the body is trying to heal itself.
- Inflammation is the body's attempt at self-protection to remove harmful stimuli and begin the healing process.
- Inflammation is part of the body's immune response.
- Infections, wounds, and any damage to tissue would not be able to heal without an inflammatory response.
- Chronic inflammation can eventually cause several diseases and conditions, including some cancers and rheumatoid arthritis.
What is inflammation?
Inflammation is part of the body's immune response.
It can be beneficial when, for example, your knee sustains a blow and tissues need care and protection. However, sometimes, inflammation can persist longer than necessary, causing more harm than benefit.
Our immediate reaction to a swelling is to try and decrease it. However, it is important to remember that inflammation is an essential part of the healing process.
The first stage of inflammation is often called irritation, which then becomes inflammation. Inflammation is followed by the discharging of pus. The granulation stage comes next, and new tissue is formed in the wound.
Without inflammation, infections and wounds would never heal.
When a person is born, certain defenses in the immune system are naturally present in the body. This is known as innate immunity.
It is different from adaptive immunity, which we develop after an infection or vaccination when the body "learns" to fight a specific infectious agent.
Innate immunity is generally nonspecific, while adaptive immunity is specific to a particular pathogen. Inflammation is one example of an innate immune response.
Symptoms of inflammation vary depending on whether the reaction is acute or chronic.
The effects of acute inflammation can be summed up by the acronym PRISH. They include:
- Pain: The inflamed area is likely to be painful, especially during and after touching. Chemicals that stimulate nerve endings are released, making the area more sensitive.
- Redness: This occurs because the capillaries in the area are filled with more blood than usual.
- Immobility: There may be some loss of function in the region of the inflammation.
- Swelling: This is caused by a buildup of fluid.
- Heat: More blood flows to the affected area, and this makes it feel warm to the touch.
These five acute inflammation signs only apply to inflammations of the skin. If inflammation occurs deep inside the body, such as in an internal organ, only some of the signs may be noticeable.
For example, some internal organs may not have sensory nerve endings nearby, so there will be no pain, such as in certain types of lung inflammation.
Symptoms of chronic inflammation present in a different way. These can include:
- mouth sores
- chest pain
- abdominal pain
- joint pain
Inflammation is caused by a number of physical reactions triggered by the immune system in response to a physical injury or an infection.
Inflammation does not necessarily mean that there is an infection, but an infection can cause inflammation.
Three main processes occur before and during acute inflammation:
- The small branches of arteries enlarge when supplying blood to the damaged region, resulting in increased blood flow.
- Capillariesbecome easier for fluids and proteins to infiltrate, meaning that they can move between blood and cells.
- The body releases neutrophils. A neutrophil is a type of white blood cell filled with tiny sacs that contain enzymes and digest microorganisms.
A person will notice inflammation symptoms after these steps take place.
An acute inflammation is one that starts rapidly and becomes severe in a short space of time. Signs and symptoms are normally only present for a few days but may persist for a few weeks in some cases.
Examples of diseases, conditions, and situations that can result in acute inflammation include:
Chronic or acute inflammation
These are the two types of inflammation that differ in how quickly symptoms escalate and how long they last.
The following table shows the key differences between acute and chronic inflammation:
|Caused by||Harmful bacteria or tissue injury||Pathogens that the body cannot break down, including some types of virus, foreign bodies that remain in the system, or overactive immune responses|
|Duration||A few days||From months to years|
|Outcomes||Inflammation improves, turns into an abscess, or becomes chronic||Tissue death and the thickening and scarring of connective tissue|
What is chronic inflammation?
This refers to long-term inflammation and can last for several months and even years. It can result from:
- failure to eliminate whatever was causing an acute inflammation
- an autoimmune disorder that attacks normal healthy tissue, mistaking it for a pathogen that causes disease
- exposure to a low level of a particular irritant, such as an industrial chemical, over a long period
Examples of diseases and conditions that include chronic inflammation:
Rheumatoid arthritis involves chronic inflammation.
Although damaged tissue cannot heal without inflammation, chronic inflammation can eventually cause several diseases and conditions including some cancers, rheumatoid arthritis, atherosclerosis, periodontitis, and hay fever.
Inflammation needs to be well managed.
Is inflammation painful?
When people have inflammation, it often hurts.
People will feel pain, stiffness, discomfort, distress, and even agony, depending on the severity of the inflammation. The type of pain varies. It can be described as constant and steady, throbbing and pulsating, stabbing, or pinching.
Inflammation primarily causes pain because the swelling pushes against the sensitive nerve endings. This sends pain signals to the brain.
Other biochemical processes also occur during inflammation. They affect how nerves behave, and this can enhance pain.
As mentioned earlier in this article, inflammation is part of the healing process. Sometimes, reducing inflammation is helpful, though not always necessary.
Non-steroidal anti-inflammatory drugs (NSAIDs) can be taken to alleviate the pain caused by inflammation.
They counteract an enzyme that contributes to inflammation. This either prevents or reduces pain.
Examples of NSAIDs include naproxen, ibuprofen, and aspirin, which are available to purchase online.
Avoid the long-term use of NSAIDs unless advised by a doctor. They increase a person's risk of stomach ulcers, which can result in severe, life-threatening bleeding.
NSAIDs may also worsen asthma symptoms, cause kidney damage, and increase the risk of having a stroke or heart attack.
Acetaminophen, such asparacetamol or Tylenol, can reduce pain without affecting the inflammation. They may be ideal for those wishing to treat just the pain while allowing the healing factor of the inflammation to run its course.
Corticosteroids, such as cortisol, are a class of steroid hormones that prevent a number of mechanisms involved in inflammation.
There are two sets of corticosteroids:
Glucocorticoids: These are prescribed for a range of conditions, including:
- temporal arteritis
- inflammatory bowel disease (IBS)
- systemic lupus
- allergic reactions
Creams and ointments may be prescribed for inflammation of the skin, eyes, lungs, bowels, and nose.
Mineralocorticoids: These are used to treat cerebral salt wasting, and to replace important hormones for patients with adrenal insufficiency.
The side effects of corticosteroids are more likely if taken by mouth. Taking them with inhalers or injections can reduce the risk.
Inhaled medications, such as those used long-term to treat asthma, raise the risk of developing oral thrush. Rinsing the mouth out with water after each use can help prevent oral thrush.
Glucocorticoids can also cause Cushing's syndrome, while mineralocorticoids can cause high blood pressure, low blood potassium levels, connective tissue weakness, and problems with the levels of acids and alkalis in body tissue.
Herbs for inflammation
Discuss any possible use of herbal supplements with a doctor.
Harpagophytum procumbens: Also known as devil's claw, wood spider, or grapple plant, this herb comes from South Africa and is related to sesame plants. Some research has shown it may have anti-inflammatory properties. Various brands are available to purchase online.
Hyssop: Thisis mixed with other herbs, such as licorice, for the treatment of some lung conditions, including inflammation. The essential oils of hyssop can lead to life-threatening convulsions in laboratory animals. Caution is advised.
Ginger: This has been used for hundreds of years to treat dyspepsia, constipation, colic, and other gastrointestinal problems, as well as rheumatoid arthritis pain. Ginger may be purchased online in supplement form.
Turmeric: Current research is looking into the possible beneficial effects of turmeric in treating arthritis, Alzheimer's disease, and some other inflammatory conditions. Curcumin, a substance found in turmeric, is being invested for the treatment of several illnesses and disorders, including inflammation. Supplements with turmeric and curcumin are available.
Cannabis: This contains a cannabinoid called cannabichromene, which has been shown to have anti-inflammatory properties. However, cannabis is not legal in many places.
There are several foods that can have been shown to help reduce the risk of inflammation, including:
- olive oil
- nuts, such as walnuts and almonds
- leafy greens, including spinach and kale
- fatty fish, such as salmon and mackerel
- fruit, including blueberries and oranges
Avoid eating foods that aggravate inflammation, including:
- fried foods, including French fries
- white bread, pastry, and other foods that contain refined carbohydrates
- soda and sugary drinks
- red meat
- margarine and lard
While these dietary solutions do not alone hold the key to controlling inflammation, they can help prime the immune system to react in a measured way.
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