Roles of Phagocytosis

Earlier we studied the basics of phagocytosis using the figure shown to the right, which shows the formation of the phagosome and the subsequent fusion of lysosomes to form the phagolysosome. Now we want to look at roles that phagocytosis plays at various times during an infection. You will find that it is important in both innate (non-specific) immunity and specific immunity.

Initial Encounters with Pathogen

There are two types of phagocytic cells normally present in interstitial spaces throughout the body, and especially under epithelia. These are the dendritic cells and the macrophages, both of which are the first immunological cells to interact with a pathogen. Both are also similar in that they do not need an specific immune response in order to phagocytize a pathogen. But they have quite different roles.

With dendritic cells, phagocytosis is not for the purpose of destroying the pathogen, but rather for collecting antigen and carrying it to lymphocytes in lymph nodes, the white pulp of the spleen or the MALT. There the dendritic cells present the antigen to the lymphocytes, a step important in starting a specific immune response. Thus, while the phagocytosis and activation of dendritic cells is innate (non-specific), the point of the phagocytosis to help promote a specific immune response. But this process requires five days or so in order to have an effect on the pathogens. With this in mind, we are going to leave dendritic cells until we take up specific immune response and concentrate now on macrophages.

Macrophages start processes that can begin within minutes. At this point, no antibodies or T cell receptors, of course, are available to help in identifying the pathogen. Thus, these initial processes are part of an innate immune response. In addition, macrophages are important in actually destroying pathogens, especially for infections that are more prolonged and that turn into chronic problems.

Recognition of Pathogen

A macrophage must first recognize that it has encountered a microorganism and not a normal cell of the body. Basically, a macrophage has certain proteins in its membrane that either directly or indirectly recognize various molecules that could not be found on human cells. Reflect that microbes have structures such as cell walls that are never a part of a human cell. Thus certain molecules are present on microbes that are quite different than those on a human cell. But the recognition molecules on macrophages are not nearly as precise as antibodies and T cell receptors. The innate recognition molecules only recognize general classes of microbes rather than specific microbes.

The first of these recognition molecules are the toll-like receptors. There are 11 different molecules in this category, and they recognize a wide variety of molecules found only on microbes. The binding of a ligand to a toll-like receptor sets in motion of sequence of events inside the cell that activates various genes important for orchestrating a innate immune response.

Another example of a recognition molecule is the mannose receptor. Some microbes have molecules on their surfaces that contain mannose spaced at specific intervals. (Mannose is a sugar.) Human cells would never have molecules of this type.

In addition, there are three or four more types of known recognition molecules on macrophages. But the total number is not large and they are all coded by typical genes. These are normally expressed; the macrophage does not need to be exposed to the microbe to begin making these innate recognition molecules (unlike for antibodies or T cell receptors).

A few of the recognition molecules are not on the surface of macrophages, but rather found in the blood plasma. One especially important molecule of this sort is C-reactive protein. This is made by the liver and only binds to microbes. Another protein of this type is called mannose binding lectin. Macrophages in turn have molecules on their surface that bind to C-reactive protein or mannose binding lectin. This binding promotes phagocytosis.

A molecule that promotes phagocytosis by binding to a microbe, such as C-reactive protein and mannose binding lectin, is called an opsonin. Another example of an opsonin is C3b, which is discussed on the next webpage. (The best opsonins, however, are antibodies, which are made in a specific immune response.)

Release of Cytokines

In addition to phagocytizing microbes, macrophages (and dendritic cells) release regulatory molecules upon encountering the microbes. These serve mainly as paracrines that set in motion many of the further immunological processes during an infection. The term cytokine refers to any small protein that is released to help coordinate an immune response. Most are released by white blood cells.

Two important cytokines released by macrophages when they encounter a pathogen are TNF-alpha and interleukin-1 (IL-1). The effects of both are similar, except that when TNF-alpha is released in pathological excess, it can trigger dramatic, life-threatening responses. In addition, macrophages, as well as other nearby cells, release cytokines called chemokines.

Roles of Cell Adhesion Molecules

The initial phagocytosis of microbes by resident macrophages is not going to end the infection unless there are just a few, stray microbes present. Any significant infection is going to require reinforcements, and recruiting these is an important role of the cytokines released by the resident macrophages. The first cells recruited are neutrophils, which are present in quantity in the blood and rapidly enter the infected tissue.

The first step in the migration of neutrophils into the tissues is necessarily the binding of the neutrophils to the endothelium of the blood vessels. Without this step, of course, the neutrophils would simply continue to tumble through and out the vessels serving the region. The binding begins when TNF-alpha and IL-1 from the macrophages cause endothelial cells to begin expressing two types of cell adhesion molecules that promote the binding of neutrophils to endothelial cells. These molecules allow the binding to occur in two steps. In the first, adhesion molecules called selectins lightly tether the neutrophil to the endothelium, so that it begins rolling along the surface. In a second step, a much tighter binding occurs through another class of newly expressed adhesion molecules on endothelial cells called ICAMs. These cause a tight adhesion of the neutrophils by binding molecules on the neutrophils called integrins.

Diapedesis and Chemotaxis

Once bound to the endothelium, neutrophils squeeze through gaps between adjacent endothelial cells into the interstitial fluid, a process called diapedesis. Aiding this, an endothelium activated by the cytokines also becomes more permeable than normal.

Once outside the blood vessel, a neutrophil is guided towards an infection by various diffusing chemotactic factors. The chemokines released by macrophages and other cells are an important. Another example is the complement peptide C5a, which is released when the complement system, as discussed on the next page. Certain bacterial molecules are also directly chemotactic.

With time, monocytes in the blood also begin adhering to the endothelium and moving into the infected tissue. Once in the tissues these are called macrophages. While the short-lived neutrophils predominate in an acute response to an infection, the long-lived macrophages are important for chronic infections.