Leukocyte recruitment

In fast flow, cells ride in the centre of the vessel and a cell-poor layer of plasma runs along the wall. In an inflamed venule the picture changes: fluid has leaked out, red cells are concentrated and stack into rouleaux in the central stream, and flow slows (stasis). Leukocytes are pushed out of the axial stream toward the endothelium. This is margination, the precondition for adhesion.

Even in health, roughly half of the neutrophils in the vascular space are not circulating freely: they move slowly along the walls of small vessels, especially in the lungs, spleen and liver. This marginating pool is not counted by a blood count; only the circulating pool is. Any shift between the two pools changes the measured white count without any change in how many neutrophils the body has.

Lecture image

Rolling. Selectins are lectins: they bind sugar groups, chiefly sialyl-Lewis X on leukocyte glycoproteins. The bonds form and break quickly, so the neutrophil tumbles along the wall, bind–detach, bind–detach. P-selectin is stored in the endothelial Weibel–Palade bodies and reaches the surface within minutes of histamine or thrombin; E-selectin is made over 1–2 hours after IL-1 or TNF; L-selectin sits on the leukocyte itself.

Tight adhesion. Rolling slows the cell enough for chemokines (above all IL-8) displayed on the endothelium to signal through its receptors. Within seconds this switches the leukocyte's β₂ integrins into a high-affinity shape: LFA-1 (CD11a/CD18) and Mac-1 (CD11b/CD18). Meanwhile TNF and IL-1 have induced their endothelial ligands, ICAM-1 (for LFA-1 and Mac-1) and VCAM-1 (for the β₁ integrin VLA-4 on monocytes and lymphocytes). Integrin meets ligand, and the cell stops and flattens.

The arrested neutrophil crawls to a junction between endothelial cells, where PECAM-1 (CD31) on the leukocyte binds PECAM-1 on the endothelial cells, guiding it through. It then secretes collagenases (type IV) that open the basement membrane, and enters the interstitium. Diapedesis happens mainly in postcapillary venules, the same vessels that leak, in gaps that histamine-driven endothelial contraction has already widened.

Each binds a G-protein-coupled receptor. The side of the cell facing the higher concentration polymerizes actin into a leading edge, myosin contracts at the back, and the cell crawls up the gradient. The same signals also activate the neutrophil: integrins switch on, granules are primed, and the oxidative burst becomes possible.

Which cell, when. Neutrophils dominate the first 6–24 hours: there are more of them in blood, they respond faster to chemokines, and they attach firmly to E- and P-selectin. They then die by apoptosis within a day or two, and monocytes, which survive longer and become macrophages, dominate by 24–48 hours. Exceptions: Pseudomonas keeps recruiting neutrophils for days; viral infections bring lymphocytes first; allergic reactions and parasites bring eosinophils.

LAD type I (autosomal recessive) is a defect of CD18, the β₂ chain shared by LFA-1 and Mac-1. Neutrophils can roll, but they cannot stop, so they never leave the blood. Everything follows from that one failed step:

Lecture image

LAD type II (autosomal recessive) is a defect in fucosylation: the transporter that brings GDP-fucose into the Golgi is faulty, so cells cannot build fucose-containing sugars. Leukocytes lack sialyl-Lewis X, the selectin ligand, so rolling fails. Red cells lack the fucose-containing H antigen: the Bombay phenotype. Children also have intellectual disability and short stature; infections are milder than in LAD I.

The adhesion slide warns that catecholamines and corticosteroids inhibit neutrophil adhesion. Their effect on the blood count is a pool shift. Epinephrine (stress, exercise, a seizure) and glucocorticoids loosen marginated neutrophils from the walls, a process called demargination, so they join the circulating pool and the measured count rises within hours. Glucocorticoids also lower adhesion-molecule expression, release neutrophils from the marrow and delay their apoptosis, while lowering eosinophils and lymphocytes.