Cell Locomotion & Phagocytosis


   In the presence of infection, chemotactic molecules are released from the complement system mast cells and leukocytes. These in turn stimulate macrophages to bind to endothelial surfaces in blood vessels near the site of the infection facilitating their passage through the endothelial wall.

   Once the macrophage is within the tissue fluid, it migrates towards the site of infection across the surface of the extra-cellular matrix.

This migratory process is termed chemotaxis. Its direction is controlled by the concentration gradient of free chemotactic molecules such as C5a.

   The macrophage achieves this migration by means of surface integrin molecules, which are able to bind to proteins within the extra-cellular layer as long as the cell moves up a chemotactic gradient.

Each molecule of integrin attaches to the extra-cellular surface, then is moved by the cyto-skeleton towards the rear of the macrophage. On reaching the trailing end of the macrophage it detaches and is re absorbed within the cell. Other integrin molecules emerge from the leading surface to continue the process.

  Once the chemokine concentration ceases to increase, receptors are no longer stimulated and the integrin stops binding to the extra-cellular surface. The macrophage therefore comes to rest as it is now at the intended destination.

   The macrophage is equipped with CR3 receptors for the C3b residues deposited on the surface of the target bacterium by the complement system. When this binding takes place, the anti-microbial mechanism is triggered. The bacterium is held and enveloped by the macrophage, and is endocytosed, a process called phagocytosis.

   The macrophage attacks the enclosed bacterium in a variety of ways. Enzymes assemble in the phagosome membrane which can reduce oxygen by the addition of an electron to form super-oxide anions. These anions further react to form a variety of reactive oxygen intermediates which can directly damage the bacterial membrane.

Within the body of the macrophage, there are small vesicles called lysosomes, containing enzymes. When a bacterium is endocytosed, the lysosomes may fuse with the vacuole and release an enzyme called myeloperoxidase. This can act further on the peroxides to form even more chemically reactive species. These processes constitute the oxygen dependent mechanism of bacterial killing.

    In the first fifteen minutes after the fusion of a lysosome with the vacuole, the pH rises. During this period cationic peptides called defensins are able to penetrate bacterial membranes and create channels for ions to enter the bacterium, disrupting its metabolism. Additionally, an iron-chelating agent, lactoferrin, is released. This binds free iron, denying it to the bacteria, which need it to reproduce.

    Subsequently, pH falls as hydrogen ions are pumped into the phagosome. This tends to inactivate the defensins, but creates a suitable environment for other lysosomal enzymes which attack bacterial membranes. The lactoferrin is also active under acidic conditions to inhibit bacterial division.

    When destruction of the engulfed organism is complete, debris is ejected.

   The preceding process will occur even if the immune system does not specifically recognise the bacterium.

If, however, the TH1 cells exist with receptors for the bacterial proteins, they recognise bacterial products presented by the macrophage and release lymphokines, notably interferon gamma.

    These lymphokines stimulate the macrophage's killing mechanism, increasing its ability to destroy bacteria with oxygen radicals. Also, another pathway which is not oxygen dependent is activated. This pathway generates reactive nitrogen intermediates, such as nitric oxide, which attacks bacterial membranes.

    The other mechanisms seen in non-activated macrophages are still found but occur more intensely. Lysosomes release cationic proteins which may penetrate bacterial coatings during the high pH alkaline phase...

    ...followed by lytic enzymes which are active in the acid environment which follows.

    The activated macrophage destroys engulfed bacteria rapidly, and through the use of reactive oxygen and nitrogen intermediates will deal with some varieties of bacteria not susceptible to attack by non-activated cells.