bind to the surfaces of invading bacteria and viruses and prevent them from attaching to and entering target host cells (neutralization of the pathogen) (see Table 4-1). Antibodies are able to act as neutralizing agents because they are bulky molecules that can block interaction between microbial surface proteins and the receptors they recognize on host cells. Antibodies can also neutralize toxic proteins produced and secreted by bacteria by binding to the toxins and preventing them from binding to host cell surface receptors, thereby blocking their toxic effects (toxin neutralization). In addition, antibodies can directly neutralize the catalytic activities of secreted bacterial enzymes such as proteases, nucleases, or glycosidases that normally work to degrade host extracellular molecules and allow the pathogen to disseminate throughout the body.
An example of protective antibody neutralization of a bacterial toxin is the antibody response to Corynebacterium diphtheriae, the cause of diphtheria, a serious toxin-mediated disease of children. Corynebacteria often live innocuously in the upper respiratory tract, but when they become infected with a corynebacteriophage encoding the diphtheria toxin gene, they are then able to produce and secrete the diphtheria toxin, which enters cells lining the respiratory tract and bloodstream. Diphtheria toxin is one of the most potent bacterial toxins known and can kill many types of human cells. The action of the toxin in the throat is evident from a grayish or whitish “pseudo-membrane” patch that consists of dead epithelial cells and mucus. In some cases, this membrane can grow to the point at which it causes asphyxiation. If the toxin makes it through the bloodstream to the heart, it can cause death due to heart failure. The most effective protective response to infection by toxin-producing C. diphtheriae, which is also the response elicited by the antidiphtheria vaccine, is the production of antibodies that bind to diphtheria toxin and prevent it from binding to and killing human cells (neutralization of the toxin).
Affinity and Avidity
A characteristic of antibody binding to antigens, which is of critical importance in assessing the effectiveness of the antibody, is the avidity of the antigen-binding site for the epitope it binds. Avidity is a combination of affinity (the strength of the binding interaction between an antigen-binding site and an epitope) and valence (the number of antigen-binding sites available for binding epitopes on an antigen). A single epitope can elicit a mixture of antibodies that vary considerably in affinity. This variation may arise because the body cannot know in advance what epitopes it will encounter, so it produces a variety of antibodies with differing antigen-binding sites, some of which will have a high affinity for a particular antigen. In fact, as the antibody response to an epitope develops, the B cells producing antibodies with the highest affinity will proliferate the most, and eventually those high-affinity antibodies will predominate.
High affinity is important, but it is not sufficient to ensure that an antibody bound to an epitope will retain its hold on the epitope. Because the binding between antigen-binding sites and epitopes is noncovalent, the interaction is reversible. Thus, there is an off-rate, as well as an on-rate, associated with antibody binding to an epitope. The importance of valence is that an antibody with a higher valence will be significantly less likely to detach from the antigen to which it is bound. If two antigen-binding sites of an antibody monomer bind to two adjacent epitopes on an antigen, the probability that both of them will detach at the same time is much lower than the probability that a single antigen-binding site will detach from its epitope. Thus, higher valence can improve the apparent strength of binding of an antibody to an epitope by orders of magnitude.
The differences in affinity and avidity between the first responders IgM/IgA and the adaptive IgG antibodies become important when one considers the roles that these different antibodies play in the immune response. Antibodies, such as IgM and sIgA, that appear early and have lower affinity and less specificity for the invading microbe need to have a higher valence number so that they can increase the avidity for the foreign antigen by binding more epitopes, up to 4 for sIgA or 10 for IgM. High avidity for an antigen is more effective at neutralizing microbes and toxins, blocking their binding and entry into cells. For IgM, high avidity significantly enhances the activation of complement and clustering of receptors to stimulate immune cells. Moreover, high avidity leads to agglutination that enhances opsonization of the microbes by phagocytic cells and clearance of the microbe from the circulation. Rapid clearance is a desirable feature because the longer antibody-antigen complexes remain in circulation, the more likely they are to deposit in the kidneys or other blood-filtering organs, where the complexes can activate complement and cause an inappropriate inflammatory response that damages the organ.
In contrast, IgG is a monomer and can bind only two epitopes, but because it is the product of an adaptive immune response, its low valency and hence low avidity is countered by its much higher affinity and specificity for the foreign antigen. Circulating IgG antibodies can not only bind very tightly to neutralize a specific microbe or toxin, but also mediate opsonization of the specific microbe through binding to Fc receptors on phagocytes. Antigen-bound IgG can also mediate complement activation through binding to a C1 complex.
Cytotoxic T Cells, Also Known as Cytotoxic T Lymphocytes (CTLs)
Cytotoxic T Lymphocytes: Critical Defense against Intracellular Pathogens
Like PMNs and NK cells (in ADCC), cytotoxic T cells, also called CD8+ T cells or cytotoxic T lymphocytes (CTLs), kill infected host cells, but through a different mechanism. By entering cells, intracellular pathogens are protected from antibodies and complement. As such, killing infected host cells is often the only way to attack these invaders. CTLs and NK cells (in ADCC) are important parts of this defense response. The difference between CTLs and NK cells is that CTLs have T cell receptors (TCRs) that are specific to a particular epitope from a microbial antigen. Thus, while NK cells kill host cells infected with a variety of intracellular pathogens (see previous sections and chapter 3), CTLs kill only cells infected with a specific intracellular pathogen (see Figure 3-3).
CTLs have two mechanisms for killing infected cells. In the first, the CTL binds to a microbial antigen displayed on the surface of an infected cell using a TCR on its surface. (How the microbial antigen comes to be displayed on the cell surface will be described later.) This interaction then triggers the CTL to release granules containing two proteins, perforin (a pore-forming cytolysin that pokes holes in the host cell membrane) and proteolytic enzymes (granzymes) that enter the cell through the pore and trigger programmed cell death (apoptosis) in the infected cell (Figure 4-5). This type of attack kills the infected host cell but not the microbes. Instead, the released microbes are taken up by nearby activated macrophages that are better able to kill them. In the second mechanism, the CTL also releases a second pore-forming cytolysin from the granules, called granulysin. Granulysin is somewhat ineffective in lysing host cells, but it is very effective at killing bacteria. Presumably, granulysin kills bacteria the way perforin kills eukaryotic cells: by creating holes in the bacterial membranes and collapsing the proton motive force that bacteria use to gain energy. Additionally, perforin may also help deliver other yet-to-be-discovered antibacterial lysins or toxins into intracellular compartments of host cells.
Figure 4-5. Cytotoxic T cell (CTL) action on infected host cells. Cytotoxic T cells (CTLs) mediate their action on infected cells through the release of perforin, granzyme, and granulysin from granules. (A) The cytotoxic T cell recognizes an infected target cell (e.g., macrophage) containing bacteria through T-cell receptor binding to a bacterial antigen displayed via MHC I. (B) The cytotoxic T cell releases granules containing perforin, granzymes, and granulysin. Perforin creates pores in the infected target cell membrane. Granzyme and granulysin enter the target cell through the pores created by perforin. (C) Granzyme triggers signaling pathways that lead to apoptosis of the infected cell. (D) Granulysin binds to and kills
