Innate Mechanisms for the Recruitment and Activation of Neutrophils

Neutrophils are highly efficient effectors of innate immunity. They are categorized as either resting or activated, according to their secretory and cell membrane activity. Recruitment of resting, circulating neutrophils by the innate immune response occurs rapidly in a tightly controlled process consisting of 2 events:

• neutrophil adhesion to the vascular endothelium through cell-adhesion molecules (CAMs) on leukocytes as well as on endothelial cells primarily in postcapillary venules

• transmigration of the neutrophils through the endothelium and its extracellular matrix, mediated by chemotactic factors

Activation of vascular endothelial cells is triggered by various innate immune stimuli, such as LPS, physical injury, thrombin, histamine, or leukotriene release. Neutrophil rolling—a process by which neutrophils bind loosely and reversibly to nonactivated endothelial cells—involves molecules on both cell types that belong to at least 3 sets of CAM families:

• selectins, especially L-, E-, and P-selectin

• integrins, especially leukocyte function–associated antigen 1 (LFA-1) and macrophage-1 antigen (Mac-1)

• immunoglobulin superfamily molecules, especially intercellular adhesion molecule 1 (ICAM-1) and ICAM-2

The primary events are mediated largely by members of the selectin family and occur within minutes of stimulation (Fig 1-1). Nonactivated neutrophils express L-selectin, which mediates a weak bond to endothelial cells by binding to specific selectin ligands. Upon exposure to the triggering molecules described in the previous section, endothelial cells become activated, expressing in turn at least 2 other selectins (E and P) by which they can bind to the neutrophils and help stabilize the interaction in a process called adhesion. Subsequently, other factors, such as platelet-activating factor (PAF), various cytokines, and bacterial products can induce upregulation of the β-integrin family. As integrins are expressed, the selectins are shed, and neutrophils then bind firmly to endothelial cells through the immunoglobulin superfamily molecules.

Figure 1-1 Four steps of neutrophil migration and activation. 1, In response to innate immune stimuli, such as bacterial invasion of tissue, rolling neutrophils within the blood vessel bind loosely and reversibly to nonactivated endothelial cells by selectins. 2, Exposure to innate activating factors and bacterial products (dotted arrow) activates endothelial cells, which in turn express E- and P-selectins, β-integrins, and immunoglobulin superfamily molecules to enhance and stabilize the interaction by a process called adhesion.3, Chemotactic factors triggered by the infection induce transmigration of neutrophils across the endothelial barrier into the extra-cellular matrix of the tissue. 4, Finally, neutrophils are fully activated into functional effector cells upon stimulation by bacterial toxins and phagocytosis.

(Illustration by Barb Cousins, modified by Joyce Zavarro.)

After adhesion, various chemotactic factors are required to induce transmigration of neutrophils across the endothelial barrier and extracellular matrix into the tissue site. Chemotactic factors are short-range signaling molecules that diffuse in a declining concentration gradient from the source of production within a tissue to the vessel. Neutrophils have receptors for these molecules and are induced to undergo membrane changes that cause migration in the direction of highest concentration. Numerous such factors include:

• complement products, such as the anaphylatoxin C5a

• fibrin split products

• certain neuropeptides, such as substance P

• bacteria-derived formyl tripeptides, such as N-formyl-methionyl-leucyl-phenylalanine (fMLP)

• leukotrienes

• α-chemokines, such as IL-8

Activation of neutrophils into functional effector cells begins during adhesion and transmigration but is fully achieved upon interaction with specific signals within the injured or infected site. The most effective triggers of activation are bacteria and their toxins, especially LPS. Other innate or adaptive mechanisms (especially complement) and chemical mediators (such as leukotrienes and PAF) also contribute to neutrophil activation. Neutrophils, unlike monocytes or lymphocytes, do not leave a tissue to recirculate but remain and die.