7.1 Mucosal Immunity

MODULE 7

Up to this point we have focused on descriptions of immune responses that would occur following immune activation in inflamed tissues and lymph nodes. However, since most human pathogens actually enter the body through the mucosal tissues, it is important to now focus on mucosal immunity and highlight how immune responses operate in these tissues. As described in module 1, mucosal tissues include the gastrointestinal tract, the respiratory tract, and the urogenital tract. See figure 10.1 and figure 10.2.

Mucosal tracts are lined with specialized ciliated epithelial cells that actively secrete mucus. The cilia function to impede the colonization of pathogens and the sticky mucus is loaded with proteins and enzymes which further protect the underlying tissues from invasion by pathogens. Included in the mucus are IgA and IgM antibodies which can block pathogens and neutralize toxins.

A particular characteristic of the gastrointestinal tract is the existence of an incredible diversity of commensal microorganisms, mostly bacterial species, which reside within our gut but do not cause disease. In fact, our bodies are loaded in number with more bacterial cells than human cells! For these particular bacterial species, our bodies provide a rich source of food and a constant warm environment for them to grow; but we also benefit from the presence of these bacteria. Not only do they aid in our digestive process, but they compete with pathogens that may invade our digestive tracts by food or water contamination. See figure 1.2.

Moreover, the proper development of the immune system is highly dependent upon the presence of these commensal populations, as exemplified by experiments involving "germ free" animals. These animals which are raised in completely sterile environments do not establish commensal populations (microbial flora) and experience impaired development of their immune systems.

Although commensals are not harmful in the lumen (opening) of the tracts, they can cause disease if they are able to cross the epitheial cell layer and propagate in the underlying tissue called the lamina propria. Therefore a key function of MALT is to keep the commensals "in their place."

For more detailed information about the role of the bacterial microflora see:

Adaptive immune responses to non-mucosal tissues depend upon the migration of antigens and antigen presenting cells to secondary lymphoid organs (lymph nodes and spleen) that may be at a significant distance from the site of infection. By contrast, mucosal tracts are associated with MALT (mucosal associated lymphoid tissue) which is composed of structural subcomponents of the tissue itself. Therefore, adaptive immune responses can be initiated locally, and at a distance by draining of fluid from the lamina propria. See figure 10.4.

A critical component of MALT is the presence of specialized epithelial cells called microfold cells (M cells) which actively sample antigens from the lumen. M cells take up the antigens by phagocytosis and endocytosis. Rather than destroying these antigens, the M cells pass them to the underlying lymphoid tissue where APCs, T-cells and B-cells congregate. See figure 10.6. Additionally, dendritic cells residing in the lamina propria can capture luminal antigens by reaching between the epithelial layer as shown in figure 10.7. The constant sampling of luminal antigens results in a population of activated effector cells in the lamina propria of a healthy gut. See figure 10.8.

In contrast to what occurs in damaged tissues, the activation state of lamina propria residing cells is not associated with an inflammatory response. This is because the resident APCs, dendritic cells and macrophages, do not express TLRs and other inflammatory PRRs.

Once a B or T lymphocyte has been activated in the mucosal tissues, it will become committed to serving these tissues. Upon activation, these cells begin to express adhesion molecules that optimize their interaction with the mucosal tracts, and the cells will tend to "home" to the mucosal tissues. See figure 10.10 and figure 10.11.

A clear summary of key features of adaptive immunity in mucosal tissues is presented in figure 10.17.

7.2 Immunological Memory

The concept of immunological memory as a defining component of adaptive immunity has been referenced frequently in this course. In this section we take a more detailed view of the distinctive characteristics between naïve, effector and memory lymphocytes.

Figure 10.18 illustrates the time course of events that occur in a primary response and a secondary response to a particular pathogen. Note that since antibodies can persist for several months in the circulation, a person can be protected from re-infection by the original pathogen without necessarily calling upon a secondary response. However, since the antibodies do eventually break down, and the effector cells are not long-lived, protection against subsequent exposure that occurs after a much later time period will require activation of memory cells.

Recall that both memory B-cells and memory T-cells are produced during a primary immune response. See figure 10.19.

Memory B-cells are distinct from naïve B-cells in several ways:

Isotypes: While naïve B-cells express IgM, memory B-cells have typically undergone isotype switching. Therefore, IgM will not generally be produced during a secondary response. See figure 10.21.
Affinity Maturation: During the proliferative phase of activation, B-cells undergo hyper mutation. Therefore, memory B-cells will likely carry antibody receptors that display a higher affinity for antigen. See figure 10.21 and figure 10.22.
Activation: Memory B-cells are more readily activated than naïve B-cells, and therefore a secondary response will lead to plasma cell secretion of antibodies in a significantly shorter time span as compared to a primary response. However, memory B-cells do still require T-cell help.

Memory T-cells are distinct from naïve T-cells also in a number of ways:

Cell Markers: Effector and memory T-cells can be distinguished from naïve T-cells on the basis of differential expression of a variety of cell surface and intracellular proteins. See figure 10.27. The expression of these proteins is related to functional characteristics of the effector and memory T-cells. For example, effector and memory Tc cells express molecules required for target cell killing (Granzyme and FasL), while these proteins are absent from naïve T-cells.
Activation: Like memory B-cells, memory T-cells are also more easily activated than naïve cells. Many circulate through peripheral tissues rather than strictly through lymphoid organs, and they can be stimulated directly by tissue APCs. Furthermore, they do not so stringently rely on co-stimulation by CD28/B7 interaction.
Sub-populations: There are two sub-populations of memory T-cells, effector memory cells, and central memory cells. See figure 10.29.
Effector memory cells: These memory cells tend to home directly to peripheral inflamed tissues and will quickly differentiate into effector cells upon activation by APCs.
Central memory cells: These memory cells tend to home to secondary lymphoid tissues where they can contribute to activation of B-cells.

7.3 Pathogen Evasion of Immune Responses

As discussed in module 1, most microorganisms do not cause noticeable disease because the innate and adaptive immune responses efficiently neutralize or eliminate them before they can cause harm. Some pathogens cause significant disease by producing toxins. See figure 9.27. More commonly pathogens can colonize and persist within the human body because they have evolved mechanisms that subvert immune system effector functions.

Examples of such mechanisms include: antigenic variation, latency, inhibition of humoral immunity, inhibition of inflammation, blocking of antigen presentation and processing pathways, and general immunosuppression.

Antigenic variation involves a pathogen's ability to bring about genetic changes in antigen structures so that the specificity of previously formed pathogen specific antibodies, memory B-cells and T-cells will no longer bind to the pathogen.

The influenza virus is notorious for its propensity to mutate every flu season. This means that immune responses to previously encountered flu virus may not be effective in the following season. This kind of gradual change is called antigenic drift. See figure 11.2.

For an animated view of this figure see:

Since influenza type A has a segmented genome and call also infect pigs and birds, a more serious type of antigenic change can occur if there is co-infection of human, pig and / or bird flu virus in the same cell. Such co-infection can result in the formation of a radically different sub-type of virus for which our immune systems can be completely naïve. Such changes are called antigenic shifts and can result in rapidly spreading pandemics (world wide infections). See figure 11.3.

For an animated view of this figure see:

Non-viral pathogens are also capable of antigenic variation. The protozoan parasite Trypanosoma brucei, has an elaborate genetic mechanism that allows it to circumvent humoral responses to a major surface protein called VSG (variable surface glycoprotein). See figure 11.4. Neisseria gonorrhoeae, the bacterial pathogen that causes the sexually transmitted disease gonorrhoeae is another example of a pathogen that can vary a key exterior structure to evade humoral responses.

We have considered previously, intracellular bacterial pathogens which have evolved mechanisms to colonize within our own host cells. These pathogens typically have developed mechanisms that enable them to escape destruction within phagocytic cells.

For more information regarding immune evasion mechanisms of pathogenic bacteria see:

Viruses are notorious for the incredible diversity of mechanisms which they have evolved for subverting immune responses. See figure 11.5, figure 11.6, and figure 11.7.

For an animated example of a viral mechanism to block antigen processing and presentation see:

7.4 Vaccination

The study of immunology as a scientific and medical discipline has its roots in the procedure of vaccination. In module 1 we considered the work of Edward Jenner and his successful procedure for vaccinating against small pox, which ultimately led to the complete eradication of this dreaded disease.

Indeed, world wide vaccination efforts have had a tremendous impact on reducing the incidence of once greatly feared diseases, many of which caused significant complications and yearly mortalities. See figure 1.27.

For an excellent resource for information on vaccines and the infectious diseases which are targeted see:

The ideal vaccine is designed to mimic the targeted pathogen so as to generate a population of memory B and T-cells which can effectively neutralize and eliminate the pathogen should it be encountered later. The vaccine should ideally be safe and effective in the majority of vaccinated people in a given population. Since no vaccine will be 100% effective, we rely on the phenomenon of "herd immunity" to keep a pathogen from being transmitted within a community. If the majority of vaccinated individuals have established a stable pool of relevant memory cells this will minimize the odds of the pathogen's ability to be propagated and spread to others who did not respond well or who have not been vaccinated.

Most vaccines are first administered in children so as to prevent development of many infectious diseases that are particularly threatening to young children. The current recommended schedule of U.S. childhood vaccinations is shown in figure 14.3.

Vaccines currently in use fall into one of three categories: whole organism live attenuated vaccines, whole organism killed vaccines, sub-unit vaccines. See figure 14.7.

Whole organism live attenuated vaccines: These are composed of weakened pathogen that is still infectious but which does not cause disease. For example, the Sabin polio vaccine, which is the one taken orally in sugar droplets, is composed of three strains of polio virus that were grown in prolonged culture, resulting in viruses that can still infect host cells but which no longer cause polio. The great advantage of this type of vaccine is that it closely mimics a true disease state, and thus exposes the immune system to relevant antigens. Furthermore, in the circumstances of viral pathogens, the ability of an attenuated vaccine to still invade a host cell increases the likely-hood that Tc memory cells can be generated against endogenous viral antigens. On the other hand, this class of vaccines can be more risky, particularly in people who have weakened immune systems, as they may succumb to disease from the vaccine it self. In the case of the Sabin polio vaccine, this did occur in 1 in 1 million cases, and since polio is nearing eradication, it is no longer administered in many countries, being replaced by the killed version, the Salk vaccine.
Whole organism killed vaccines: These are composed of the whole pathogen that has been inactivated or killed. Thus the microorganism is no longer infectious, but it does still retain predominant antigens that can be presented to the immune system. Since the organism can not invade host cells (significant for intracellular pathogens such as viruses) there is not an opportunity for activating memory Tc cells. Th memory and B memory cells can be generated, and for many vaccines this may be sufficient.
Sub-unit vaccines: These are composed of purified antigens and do not contain the whole pathogen. For example, the tetanus and diphtheria vaccines are simply inactivated protein toxins (toxoids); and the hepatitis B vaccine is a recombinant protein derived from the surface of the virus. Sub-unit vaccines are typically mixed with adjuvants, which are non-specific immunostimulatory agents that act on PRRs to activate APCs.

A relatively new type of vaccine is created by genetic modification of a non-pathogenic carrier organism to deliver an antigen of an unrelated pathogen. These are called recombinant live vaccines. See figure 14.12.

7.5 Immunodeficiency

Immunodeficiency describes a status of reduced ability to mount immune responses to infectious microorganisms. Individuals with immunodeficiency succumb to diseases caused by opportunistic pathogens, which under healthy circumstances would not cause detectable symptoms.

Immunodeficiency can be acquired by infection by the human immunodeficiency virus (HIV) as discussed in the next topic, or it can be acquired by environmental factors such as treatment with immunosuppressive drugs for autoimmune diseases or by chemotherapy for cancer. These are called secondary immunodeficiency diseases.

Primary immunodeficiency diseases are caused by inheritance of defective genes that control some component of either innate or adaptive immunity. See figure 11.9

7.6 AIDS

In the early 1980s the Center for Disease Control (CDC) in Atlanta, Georgia, began receiving alarming reports of clusters of young patients who were succumbing to disease from opportunistic infections. See figure 11.30.

It was soon determined that these patients had depleted numbers of CD4 Th cells and the disease was given the name acquired immune deficiency syndrome (AIDS). The causative agent for AIDS was identified in 1983 with the discovery of the human immunodeficiency virus (HIV). HIV is transmitted primarily by sexual contact with someone who is infected with the virus; and amongst IV drug users by sharing needles with someone who is infected. Transmission of HIV through blood transfusions was once a common way of contracting AIDS. Fortunately, HIV contamination of the blood supply is rare now that donated blood is routinely screened for HIV. The virus can also be transmitted to new born infants of infected mothers during pregnancy, birth or by breast-feeding.

With more than 30 million people infected, HIV has had a significant global impact, particularly in sub-Saharan Africa where millions have died from AIDS. See figure 1.28 and figure 11.19.

We now understand an impressive level of detail about HIV, yet despite more than two decades of intensive research there is still no cure and no vaccine for AIDS. Why not?

To understand the complexity of the problem, it is important to understand the structure and life cycle of HIV.

HIV Structure
HIV is a retrovirus with an RNA genome that is transcribed in reverse by the enzyme reverse transcriptase, a characteristic that is typical of retroviruses. The genome is packaged inside a protein nucleocapsid that is further enclosed in a lipid membrane called the envelope. The envelope is studded with a protein called gp41 to which another protein called gp120 is attached. Since gp120 is a predominant structure on the surface of the virus, it is considered to be a major surface antigen that could serve as a primary target of a humoral response by the immune system. See figure 11.20.

HIV Life Cycle
The HIV life cycle follows a well characterized sequence of events. See figure 11.22:

HIV gp120 binds to CD4 and a chemokine co-receptor on Th cells.
The envelope fuses with the Th cell membrane allowing delivery of the genome and capsid contents into the cytoplasm.
Reverse transcriptase creates a DNA copy (cDNA) of the viral genome.
Viral cDNA enters the nucleus and integrates into the Th cell's chromosome creating a provirus.
If the T-cell is activated, this "wakes up" the provirus and the genes are transcribed.
Provirus mRNA is translated into viral proteins in the cytoplasm.
A viral protease processes the proteins appropriately so that they can be reassembled into functional viral particles.
The mature viral particles exit the cell by budding out from the membrane, leading to death of the Th cell.

For animated view of the figure 11.22 HIV life cycle see:

AIDS Disease Progression
Patients initially respond to HIV as they would to any viral infection. Antibodies are formed against major HIV surface antigens, HIV specific Th cells vigorously proliferate and secrete cytokines and HIV specific Tc cells vigorously attack virus infected cells. During the first few weeks of infection, patients experience flu like symptoms which eventually subside as the immune responses "take out" the majority of detectable viruses. At the point that HIV specific antibody can be detected in a patient, they are said to have undergone seroconversion, a characteristic that serves as a key clinical diagnostic indicator of exposure to HIV. See figure 11.23.