Phases Of Inflammation

Understanding The 3 Phases Of Inflammation

Understanding The 3 Phases Of Inflammation

The body’s response to injury is called inflammation. Physical traumas such as sprain, strain or bruise are most common, whereas injuries can also occur from bacterial or viral infections, heat or any sort of chemical injury. Trauma causes direct damage to cells in the immediate area of injury, causing bleeding. A stream of events is initiated due to bleeding that results in the inflammatory process, which promotes healing of the injured tissue.

Acute and chronic are terms commonly used to refer to the duration or the length of the problem, giving inaccurate information about the actual stage of inflammation. Progression from acute to chronic inflammation can result from persistent injury or individual factors such as, diabetes, corticosteroid use, blood disorders, etc.

• Inflammatory Response: Acute swelling stage (Phase 1)
  

This is a fundamental type of response by the body to disease and injury. It is characterized by the classical signs of dolor, calor, rubor and tumor – pain, heat, redness and swelling. Inflammation is a key part of the body’s defense system, an indispensable protective response by the body’s system of self-defense. Innumerable causes (mosquito bite, a splinter, a virus infection, a bruise, a broken bone) can trigger an inflammatory response and dispatch cells and chemicals to the site to repair the damage.

Acute swelling is short lived, lasting for only a few days. If it lasts longer, it is referred to as chronic inflammation. Chronic inflammation may last for weeks, months or beyond.

• Subacute: Repair and Regeneration (Phase 2)
  

In this phase 2, special cells go into the area of the damaged tissue and start building new tissues. The subacute stage is the time of healing and repair. New collagen fibers are laid down in a disorganized manner in the form of a scar and there are weak links between each fiber. This new tissue is fragile and must be handled gently as it can be easily injured.

• Chronic: Remodelling and Maturation (Phase 3)
  

As the healing process continues, the tissue starts to remodel, strengthen and improve its cellular organization. Signs of inflammation are absent and scar tissue starts maturing. Maturation refers to the growth of fibroblasts to fibrocytes and remodelling refers to the organization of and shrinking of collagen fibres along lines of stress. There is less new collagen formation, but increased organization of collagen fibers, and stronger bonds between them. 

In order to determine if the condition of the injury is in the acute, subacute or chronic inflammatory stage, an adequate case history is needed along with assessment. The assessment should include a visual scan, active muscle testing, passive range of motion testing and resisted isometric muscle tests and palpation of the structure involved.

1.3.1 Inflammation

The inflammatory phase is the immediate response to the trauma and sets about preparing the groundwork for the remaining two phases. The wound swells and there is the inevitable bleeding which is a primary mechanism through which debris and toxins can be removed. Coagulation is needed for wound protection and hemostasis. Soon after an injury, inflammatory cells are sent to the site of the wound and a fibrin plug is formed. This consists of platelets embedded into polymerized fibrinogen, fibronectin, vitronectin, and thrombospondin. It immediately fends of any bacteria as well as providing temporary coverage. While converging within the plug, platelets aggregate and release growth factors such as platelet-derived growth factor (PDGF) and transforming growth factor (TGF). Inflammatory cells such as neutrophils and macrophages aid in wound debridement whereby dead cells are removed. They both produce key growth factors as well as mediators that help fuel the repair process. Over the following 2–3 days, derma and inflammatory cells at the wound site produce a powerful arrangement of growth factors and cytokines. From previous granulation, the presence of macrophages, fibroblasts, and endothelial cells initiates the process of wound contraction .

Manipulating inflammation to improve healing

5.2.1.1 Cells regulating inflammation.

The inflammatory phase is characterized by the tissue infiltration of a series of leukocytes, including polymorphonuclear (PMN) leukocytes, monocytes/macrophages, and T cellscells. These leukocytes mediate essential processes for normal wound healing by fighting pathogenic organisms, removing damaged tissue and apoptotic/necrotic cells, producing growth factors, and promoting extracellular matrix (ECM) remodeling. Very early leukocyte infiltration can be mediated by tissue resident mast cells, which release vasodilative histamine and proteases as well as proinflammatory cytokines. The importance of resident mast cells in wound healing has been examined by the use of mast cell-deficient Kit mutant mice; Kit is an essential tyrosine kinase receptor driving mast cell development in mice. For example, in Kit mutant mice, neutrophil recruitment into the site of injury is reduced, supporting the idea that mast cells promote neutrophil recruitment . However, Kit mutant mice exhibit phenotypes that extend beyond mast cell deficiency, and new mouse models more specifically targeting mast cells have been generated . These latter mice express Cre recombinase under the control of mast cell protease genes to obtain Kit-independent mast cell-deficient mice. Using these mice, it has been demonstrated that mast cells may be dispensable for normal wound healing [11,12]. Finally, pharmacological mast cell inhibition by disodium cromoglycate reduces inflammation and scar formation in mice, leaving open the question of whether the alteration of specific functions of mast cells, rather than the number of mast cells, might improve wound healing.

Regardless of whether mast cells are involved, PMN leukocytes are generally considered to be the first responders following tissue damage. These cells clear debris and provide protection against infection if the body’s barrier function is compromised. Neutrophils also release enzymes such as elastase and proteases as well as reactive oxygen species that can cause bystander damage to otherwise healthy tissue. In the mouse cutaneous excisional wounding model, antibody-induced neutrophil depletion accelerated wound closure in adult wild-type and diabetic mice . In contrast, delayed wound closure in aged wild-type mice was further delayed by neutrophil depletion. Together, these studies indicate that the influence of neutrophils on wound healing may depend on the host environment. Further investigation of diverse functions of neutrophils and of neutrophil subsets in wound healing is needed, including the complex communication between neutrophils and other immune cells that may lead to either enhanced or impaired healing.

Monocytes/macrophages follow neutrophils into the site of injury either by egressing from the blood or migrating and proliferating from their local pool. They remove damaged tissue and necrotic or apoptotic cells through phagocytosis, produce cytokines/growth factors, and present antigen to adaptive immune cells such as T cells. Macrophages play roles in both tissue damage and repair based on studies involving the selective depletion of wound macrophages in different phases of healing in different injury models. These studies indicated that wound macrophages are involved in healing responses, including angiogenesis and collagen deposition.

Under noninflammatory conditions, peripheral tissues contain primarily tissue resident M2-like macrophages that contribute to tissue homeostasis. Upon tissue injury or infection, M1-like activation is induced by the engagement of pattern recognition receptors with damage-associated molecular patterns (DAMPs) or pathogen-associated molecular patterns (PAMPs), respectively. Studies have focused on the macrophage phenotypes expressed during wound healing following tissue injury. Indeed, M1-like macrophages appear at the early stage of inflammation, and they are replaced by M2-like macrophages .

Different ontogenies of macrophages may also influence macrophage phenotypes during wound healing. In myocardial infarction, Ly6Chi monocytes/M1-like macrophages and CD11c+ dendritic monocytes/macrophages accumulate during the initial proinflammatory phase, followed by Ly6Clo/M2-like macrophages associated with antiinflammatory or healing response. Ly6Chi monocytes/M1-like macrophages and CD11c+ dendritic monocytes/macrophages were reported to be bone marrow-derived under inflammatory conditions. However, it remains to be established whether Ly6Clo/M2-like macrophages that accumulate in healing tissue are derived from tissue resident progenitors or from Ly6Chi monocytes/M1-like macrophages during a phenotype conversion that may be regulated by the tissue environment . Moreover, since Ly6Clo monocytes can contribute to proinflammatory responses in some inflammatory injury models, the precise role(s) of monocyte subsets may be specific to each situation.

T lymphocytes infiltrate damaged tissue in the late inflammatory phase and remain in the tissue during the remodeling phase for weeks or longer. In classical studies, congenitally athymic nude mice that lack a normal T cell system exhibited an increased fibrotic response, suggesting that T cells may limit fibrosis . A series of studies using monoclonal antibody-induced T cell depletion suggested that a subpopulation of T cells stimulates wound healing . T cells residing in nonwounded skin of mice have been identified as γδ T cells, which may help to maintain homeostasis as well as direct wound healing . Similar skin resident T cells have been observed in wounded skin . Dermal T cells may execute a variety of functions during wound healing through secreting soluble factors such as keratinocyte growth factors  and insulin-like growth factor (IGF) . Interestingly, but not surprisingly, epidermal T cells are sensitive to metabolic conditions such as hyperglycemia  and obesity in mice  and in humans . In mouse myocardial infarction, Foxp3+CD4+ regulatory T cells appear to improve healing by modulating monocyte/macrophage differentiation . While the significance of T lymphocytes in human wound healing remains obscure , CD4+ T helper 2 (TH2) cells may promote healing through the production of the key Th2 cytokines such as IL-4, IL-5, IL-10, and IL-13 .

Mesenchymal stem/stromal cells (MSCs) can migrate to sites of injury from their place of residence in perivascular spaces of various tissues and/or through the circulation . MSCs can be isolated from various sources such as adipose tissue, bone marrow, and peripheral blood  and expanded for therapeutic use, which has been shown to accelerate wound healing. Moreover, endogenous MSC mobilization can be enhanced by systemic pharmacological interventions. Cytokines and growth factors such as granulocyte colony stimulating factor, granulocyte macrophage colony stimulating factor, hepatocyte growth factor, vascular endothelial growth factor (VEGF), and a combination of these have been used to promote wound healing through endogenous MSC mobilization , while these therapies also mobilize hematopoietic stem and progenitor cells (HSPCs) and endothelial progenitor cells (EPCs) . The chemokine receptor CXCR4 is expressed on MSCs, HSPCs, and EPCs and plays important roles both in cell retention in the bone marrow (BM) and in recruitment to the wound site. The CXCR4 antagonist AMD3100, especially when combined with other mobilizing factors, has been shown to release stem and progenitor cells from their anchor bone marrow, thus driving their mobilization. Combined with low-dose tacrolimus or IGF-1, AMD3100 improves skin wound healing and bone fracture healing through increased MSC and/or EPC accumulation at the wound site. MSCs may promote wound healing by transdifferentiation into multiple cell types . Perhaps more significantly, MSCs have notable immunomodulatory effects on the surrounding environment following implantation and can secrete a variety of factors as well as induce factor secretion from other neighboring cells , especially those with antiinflammatory effects .

The supply of leukocytes from hematopoietic sources also participates in the regulation of the inflammatory response as cells infiltrating injured tissues from the blood need to be replenished. Studies on the healing of myocardial infarction as well as stroke revealed that the turnover of monocytes/macrophages in the acute inflammatory phase is rapid with an average tissue residence time of 20 h and that proinflammatory monocytes, perhaps as well as neutrophils, given their shorter lifespans, are constantly replenished by blood-borne cells at the site of damage. The regulation of cell supply from the blood is dependent on multiple biological events: production and mobilization from the hematopoietic organs such as bone marrow and spleen, transendothelial migration to tissue interstitial space (cell recruitment), cell survival and proliferation, and emigration from the tissue. Regulators for these processes could occur at multiple levels from the control of gene expression to environmental cues, including adhesion molecules and receptor and cytokines/chemokines expression, as reviewed elsewhere. Notably, the chemokine CCL2/MCP-1 plays essential roles in both myeloid mobilization from the bone marrow and its infiltration into the inflamed tissue. At least in mice, spleen and lymphoid organs serve as monocyte reservoirs that may be deleterious for wound healing when the supply is greater than needed for efficient repair, such as in proinflammatory conditions including hypercholesterolemia and atherosclerosis. 

A study demonstrated that brain injury activates hematopoietic stem cells to produce myeloid cells in mouse bone marrow . Our research has shown that hindlimb ischemia injury in mice causes expansion and mobilization of hematopoietic progenitor cells in the bone marrow. These studies indicate that multiple sources of inflammatory cells and hematopoietic stem and progenitor cells participate in the regulation of inflammation in peripheral tissues during wound healing, although the associated mechanisms are largely unknown.