The presence of bacteria within the body provokes a protective inflammatory response (Figure 1) characterized by the clinical findings of redness, swelling, warmth, and pain at the site of infection. These signs are due to increased blood flow, increased capillary permeability, and the escape of fluid and cells into the tissue spaces. The increased permeability is due to several chemical mediators, of which histamine, prostaglandins, bradykinin, and leukotrienes are the most important. Complement components, C3a and C5a, also contribute to increased vascular permeability. Bradykinin is also an important mediator of pain.
Fig1. Inflammation. The inflammatory response can be caused by two different mechanisms. Left: Pyogenic bacteria (e.g., Staphylococcus aureus) cause inflammation via antibody- and complement-mediated mechanisms. Right: Intracellular bacteria (e.g., Mycobacterium tuberculosis) cause inflammation via cell-mediated mechanisms. IL-2 = interleukin-2.
Neutrophils and macrophages are phagocytes that play an important role in the inflammatory response. Neutrophils pre dominate in acute pyogenic infections, whereas macrophages are more prevalent in chronic or granulomatous infections. Gamma interferon, which activates macrophages and enhances their microbicidal action, is produced by activated helper T cells. Macrophages produce important “proinflammatory” cytokines: tumor necrosis factor (TNF), interleukin-1 (IL-1), and interleukin-6 (IL-6), which is the main inducer of the acute-phase response. The response is a coordinated series of nonspecific events that occur early in infection and involve several proteins produced in the liver, most notably, C-reactive protein which opsonizes bacteria, mannose-binding protein, which can activate complement, and lipopolysaccharide (endotoxin)-binding protein, which is produced in response to gram-negative bacteria.
Neutrophils and macrophages are attracted to the site of infection by chemokines that are produced by tissue cells in the infected area, local endothelial cells, and resident poly morphonuclear neutrophils (PMNs) and macrophages. Important pro-inflammatory chemokines include interleukin-8 and complement component C5a, which attract primarily PMNs, and monocyte chemotactic protein 1 (MCP-1) and macrophage inflammatory protein (MIP). PMNs make up approximately 60% of the leukocytes in the blood, and their numbers increase significantly during infection (called leukocytosis) due to production of granulocyte-stimulating factors (granulocyte colony-stimulating factor [G-CSF] and granulocyte-macrophage colony-stimulating factor [GM-CSF] by macrophages.
Circulating PMNs are signaled to adhere to the endothelium through interactions with surface-localized proteins expressed on the endothelium (intracellular adhesion molecule [ICAM]) and PMN surface proteins (selectins and integrins). ICAM proteins on the endothelium are induced by inflammatory mediators, such as IL-1 and TNF, produced by macrophages in response to bacterial infection. The increase in surface-exposed ICAM proteins ensures that PMNs selectively adhere to the site of infection. Increased permeability of capillaries as a result of histamine, kinins, and prostaglandins allows PMNs to migrate through the capillary wall to reach the focus of infection. This migration is called diapedesis and takes several minutes to occur.
The bacteria are phagocytosed by PMNs into a membrane bound vacuole (phagosome). This process is enhanced when immunoglobulin G (IgG) antibodies or the C3b component of complement (both opsonins) is bound to the surface of the bacteria in a process called opsonization. Binding of these molecules to the bacterial surface facilitates phagocytosis because the outer cell membranes of PMNs (and macrophages) have receptors both for the Fc portion of IgG and for C3b. Upon engulfment, the respiratory burst pathway is triggered inside the phagosome where NADPH oxidase produces superoxide radicals and superoxide dismutase produces hydrogen per oxide. These highly reactive compounds (often called reactive oxygen intermediates) are microbicidal.
Nitric oxide (NO) is another important microbicidal agent. It is a reactive nitrogen intermediate that is synthesized by an inducible enzyme called nitric oxide synthase in response to stimulators such as endotoxin. NO participates in oxidative killing of ingested microbes phagocytosed by neutrophils and macrophages. Overproduction of NO contributes to the hypo tension seen in septic shock because it causes vasodilation of peripheral blood vessels.
The killing of the organism within the neutrophil phagosome is a two-step process that consists of degranulation followed by production of hypochlorite. In degranulation, membrane-bound lysosomal granules fuse with the phagosome, emptying cytolytic enzymes, such as nucleases and proteases, into the vesicle. This converts the phagosome into a structure called the phagolysosome, where the actual killing of microorganisms occurs. The most important bactericidal mechanism is the production of hypochlorite ion by myeloperoxidase, which damages bacterial cell membranes.
Additional bactericidal mechanisms involve lactoferrin which chelates iron from the bacteria, lysozyme which degrades peptidoglycan in the bacterial cell wall, and cationic proteins which damage bacterial membranes.
Macrophages also migrate, engulf, and kill bacteria by using essentially the same processes as PMNs do, but there are a few differences:
(1) Macrophages do not make hypochlorite ion; however, they do produce hydrogen peroxide and superoxide by respiratory burst.
(2) Certain organisms such as the agents of tuberculosis, brucellosis, and toxoplasmosis are preferentially ingested by macrophages rather than PMNs and may remain viable and multiply within these cells; granulomas formed during these infections contain many of these macrophages.
