The science behind vaccine use

The science behind vaccine use

Both components of the immune system, namely innate immunity and adaptive immunity, are required to generate a strong, effective, protective response to vaccines. Thus the rapidly responding innate immune responses also promote the initial stages of adaptive immunity. We take advantage of this by adding adjuvants to vaccines to initiate these innate responses and so enhance their effectiveness. Innate immune responses are also essential in providing the rapid protective immunity that develops when we use vaccines containing modified live viruses.

There are two major arms of the adaptive immune system. Antibody- mediated responses are optimized for the elimination of extracellular invaders such as bacteria. They are preferentially induced by vaccines containing nonliving antigens. Vaccines can also activate the cell-mediated responses required to eliminate intracellular invaders such as viruses and some specialized bacteria. These cell-mediated responses are preferentially induced by vaccines containing live organisms, especially viruses. The persistence of vaccine-mediated protection is determined by the survival of memory cells that can respond very rapidly to subsequent microbial invasion.

Innate immunity

All animals need to detect and eliminate microbial invaders as fast and as effectively as possible. This immediate defensive response is the task of the innate immune system. Many different innate defense mechanisms have evolved over time and they all respond rapidly to destroy invaders while minimizing collateral damage. Innate immune responses are activated when cells use their pattern recognition receptors to detect either microbial invasion or tissue damage. For example, cells can sense the presence of invading microbes by detecting their characteristic conserved molecules. These are called pathogen-associated molecular patterns (PAMPs). These cells can also sense tissue damage by detecting the characteristic intracellular molecules released from broken cells. These molecules are called damage associated molecular patterns (DAMPs).

The body employs sentinel cells whose job it is to detect PAMPs and DAMPs, and once activated, emit signals to attract white blood cells. The white cells converge on the invaders and destroy them in the process we call inflammation. In addition, animals make many different antimicrobial proteins, such as complement, defensins, and cytokines, that can either kill invaders directly or promote their destruction by defensive cells. Some of these antimicrobial molecules are present in normal tissues whereas others are produced in response to the presence of PAMPs or DAMPs—for example, the damage caused by an injected vaccine.

The innate immune system lacks specific memory and, as a result, each episode of infection tends to be treated identically. The intensity and duration of innate responses such as inflammation therefore remain unchanged no matter how often a specific invader is encountered. These responses also come at a price; the pain of inflammation or the mild toxic effects of vaccines largely result from the activation of innate immune processes. More importantly however, the innate immune responses serve as one of the triggers that stimulate antigen-presenting cells to initiate the adaptive immune responses and eventually result in strong long-term protection (Fig. 2.1).

Adaptive immunity

Adaptive immunity develops when foreign antigens bind to B cell or T cell antigen receptors and trigger strong defensive responses. Adaptive immune responses are the basis of successful vaccination. There is a growing tendency among immunologists to classify these adaptive immune responses into two types. Type 1, or cell-mediated immunity, is mediated by type 1 helper (Th1) cells. Type 1 responses are responsible for immunity to bacteria, viruses, protozoa, and fungi. They generate some antibodies, strong cytotoxic T cell responses, and also activate macrophages. Type 2 immunity in contrast is mediated by type 2 helper (Th2) cells. These cells promote antibody formation. Antibodies generated by type 2 responses are responsible for the destruction of extracellular bacteria and viruses, parasitic helminths and arthropods, and also for allergic reactions.

Note that antibodies and T cells have quite different functions. Thus antibodies are optimized to deal with the organism itself, such as free virus particles and bacteria. T cells, on the other hand, attack and destroy abnormal cells such as those that develop in virus infections or mutated cancer cells. T cells do not recognize free viruses or bacteria. Thus antibodies produced by B cells constitute the primary immune mechanism against extracellular organisms. On the other hand, viruses, bacteria, and protozoa that can live inside cells can only be controlled by T cells. The T cells can either kill the infected cell or release cytokines that inhibit microbial growth. Vaccine usage must take this major divide between antibodies and T cells into account. It is often necessary to design a vaccine that specifically stimulates a type 1 or type 2 response, depending on the nature of the infectious agent.

Adaptive immune responses can be considered to proceed in four major steps (Fig. 2.2). These are: Step 1: Antigen capture and processing; Step 2: Helper T cell activation; Step 3: B cell and/or cytotoxic T cell mediated responses that eliminate the invaders; and Step 4: The generation and survival of large populations of memory cells. It is these memory cells that provide a vaccinated animal with the ability to respond rapidly and effectively to subsequent microbial infection. When designing an effective vaccine, consideration must be given to optimizing each of these four steps. This can be accomplished by careful selection of appropriate antigens and by the addition of adjuvants that enhance all stages of the process.

Step 1: Antigen capture and processing

The induction of adaptive immune responses requires the activation of antigen-presenting cells, primarily dendritic cells. This activation is mediated by cytokines generated during the initial innate response. These activated dendritic cells are needed to capture and process exogenous antigens.

Exogenous antigens

One type of foreign antigen is typified by the invading bacteria that grow in the tissues and extracellular fluid. These live outside cells and so are called exogenous antigens. Exogenous antigens must first be captured, processed, and presented in the correct fashion to helper T cells if they are to be recognized as foreign. This is the responsibility of specialized antigen-processing cells.

The most important of these antigen-processing cells are dendritic cells (DCs). DCs have a small cell body with many long cytoplasmic processes known as dendrites. These dendrites increase the efficiency of antigen trapping and maximize the area of contact between DCs and other cell types (Fig. 2.3). Dendritic cells are found throughout the body and form networks in virtually every tissue. They are especially prominent in lymph nodes, skin, and mucosal surfaces, sites where invading microbes are most likely to be encountered.

The number of dendritic cells varies considerably among tissues. Thus DCs are present in high numbers within the dermis. As a result, intradermally administered vaccines are readily recognized. Likewise circulating DCs are common in well-vascularized muscles, the preferred site of injection for many vaccines. There are fewer DCs in the subcutis and adipose tissue, thus explaining why these are usually a less effective routes for vaccine administration.

When DCs encounter foreign antigens and are stimulated by “danger signals,” such as DAMPs from tissue damage or PAMPs from infection or from a vaccine, they mature rapidly. This maturation and activation causes DCs to migrate toward the source of the antigen, either at the injection site or in the draining lymph node. (In the absence of these danger signals, DCs will not be activated. Most inactivated vaccines thus require adjuvants to generate the required danger signals.) The activated, mature DCs capture antigens by phagocytosis. If they ingest bacteria, they can usually kill them. The pH within the phagosomes of DCs is less acidic than in other phagocytic cells, and as a result the ingested microbial antigens are not totally degraded but fragments are preserved. These antigen fragments are bound to specialized receptors called major histocompatibility complex (MHC) class II molecules and expressed on the DC surface. The DCs carry these MHC-bound antigen fragments to lymph nodes where they are presented to helper T cells. The DCs embrace the T cells while the T cells palpate the DCs for the presence of MHC-bound antigen fragments. If the T cell antigen receptors can bind any of these presented fragments, the T cells will respond.

MHC class II molecules are dendritic cell surface receptors that bind processed peptide fragments from exogenous antigens. Exogenous antigen processing involves multiple steps (Fig. 2.4). First, the antigen must be endocytosed and taken into the cell. The ingested proteins are broken up by proteases into peptide fragments of varying length. The endosomes containing these peptide fragments then fuse with other endosomes carrying newly synthesized MHC class II molecules. Once an antigen peptide binds to an MHC molecule, the complex moves to the cell surface. When it reaches the cell surface, the MHC-peptide complex is exposed and made available for inspection by any passing T cell. It has been calculated that an antigen-processing DC contains about 2 × 105 MHC class II molecules that can present peptide fragments to T cells. If costimulation is provided, a single T cell can be activated by binding 200 to 300 of these peptide-MHC complexes. It is therefore possible for a single antigen-processing dendritic cell to present many different antigens to multiple T cells simultaneously. Only a few DCs are needed to trigger a strong T cell response, and one dendritic cell may activate as many as 3000 T cells.

Because helper T cells must recognize MHC-antigen complexes to respond to an antigen, MHC class II molecules effectively determine whether an animal can mount an immune response. Class II molecules can bind some, but not all, peptides created during antigen processing so they select those antigen fragments that are to be presented to the T cells. The response to vaccines is thus controlled by an animal’s set of MHC genes—its MHC haplotype.

Endogenous antigens

A second type of invading organism is typified by viruses that invade cells and force them to make new viral proteins. These “endogenous antigens” are processed by the cells in which they are synthesized.

The function of T cell–mediated immune responses is the detection and destruction of cells producing abnormal or foreign proteins. The best examples of such cells are those infected by viruses. Viruses take over the protein-synthesizing machinery of infected cells and use it to make new viral proteins. To control virus infections, cytotoxic T cells must be able to recognize a virus-infected cell by detecting the viral proteins expressed on its surface. T cells can detect these endogenous antigens, but only after they have been processed and bound to MHC class I molecules (Fig. 2.5).

Living cells continually break up and recycle the proteins they produce. As a first step, the protein is tagged with a small protein called ubiquitin. These ubiquinated proteins are recognized by proteasomes, powerful tubular complex proteases. Ubiquinated proteins are inserted into the inner channel of the proteasome, where they are broken into 8 to 15 amino acid peptides like a meat grinder. Some of these peptide fragments are rescued from further destruction by attachment to transport proteins. These fragments are then transported to a newly formed MHC class I molecule. If they fit the MHC antigen-binding site, they will be bound. Once loaded onto the MHC, the MHC-peptide complex is carried to the cell surface and displayed to any passing T cells.

A cell can express about 106 MHC-peptide complexes at any one time. A minimum of about 200 MHC class I molecules loaded with the same viral peptide is required to activate a cytotoxic T cell. Thus the MHC-peptide complexes can provide passing T cells with fairly complete information on virtually all the proteins being made by a cell. Analyses of peptide binding indicate that the binding groove of a class I molecule can bind over a million different peptides with various levels of affinity. The number is not unlimited, however, and binding is not always strong. As a result, not all of the antigens within a vaccine may induce a protective immune response. Thus these MHC molecules also determine whether an animal can respond to a specific antigen.

Step 2: Helper T cell activation

There are several populations of lymphocytes with antigen-binding receptors. These include helper and regulatory T cells that control immune responses, cytotoxic T cells that destroy abnormal cells, and B cells that produce antibodies. Each of these cell types will respond to any vaccine antigens that can bind to their receptors.

Helper T cells

Helper T cells are found in follicles and germinal centers within lymph nodes. Each helper T cell is covered by about 30,000 identical antigen receptors. If these receptors bind sufficient antigen in the correct manner, the T cell will initiate an immune response. It does this by secreting multiple cytokines and expressing new cell surface molecules. The other antigen-responsive cell populations, B cells and cytotoxic T cells, cannot respond properly to antigens unless they too are stimulated by helper T cells. There are four major types of helper T cells. These are called: helper 1 (Th1), helper 2 (Th2), helper 17 (Th17), and regulatory (Treg) cells, and each is distinguished by the mixture of cytokines that they secrete as well as their functions. For vaccination purposes, the most important populations are the Th1 and Th2 cells.

T cell antigen receptors (TCRs) form a large diverse repertoire. Any foreign antigen that enters the body will probably encounter and be able to bind to the receptors on at least one T cell. Each T cell has receptors of a single specificity. T cell antigen receptors only recognize antigens attached to MHC molecules. They cannot recognize or respond to free antigen molecules.

The binding of an antigen-MHC complex to a T cell antigen receptor is usually not sufficient by itself to trigger a helper T cell response. Additional signals are needed for the T cell to respond fully. For example, adhesion molecules must bind the T cells and antigen-presenting cells firmly together and permit prolonged, strong signaling between them. TCR-antigen binding triggers the initial signaling steps. Receptors on antigen-presenting cells bind to their ligands on T cells and amplify these signals. T cells must also be stimulated by cytokines secreted by the antigen-presenting cells. These cytokines determine the way in which a T cell responds to antigen by turning on some pathways and turning off others.

When DCs present their antigen load to helper T cells, they generate three signals. The first signal is delivered when T cell antigen receptors bind antigen fragments attached to MHC class II molecules. The second signal provides the cells with additional critical costimulation through contact with other cell surface receptors such as adhesion molecules. The third signal determines the direction in which naïve helper T cells will develop and is provided by secreted cytokines. For example, some microbial antigens trigger DCs to secrete interleukin-12 (IL-12). These are called DC1 cells because their IL-12 activates Th1 cells and so triggers type 1 responses. Other microbial antigens can cause DCs to secrete IL-4, and IL-6. These cytokines stimulate Th2 differentiation and are produced by DC2 cells. They stimulate type 2 responses. It may also be that the same dendritic cell can promote a type 1 or a type 2 response, depending on the dose and type of antigen it encounters. The response may also depend on its location. For example, DCs from the intestine or airways seem to preferentially secrete IL-4 and thus promote type 2 antibody responses.

Because MHC molecules can bind many different antigenic peptides, any individual peptide will only be displayed in small amounts. T cells must be able to recognize these few specific peptide-MHC complexes among a vast excess of MHC molecules carrying irrelevant peptides. The number of MHC-peptide complexes signaling to the T cell is also important because the stimulus needed to trigger a T cell response varies. For example, at least 8000 TCRs must bind antigen for a helper T cell to become activated in the absence of a molecule called CD28, but only about 1000 TCRs need be engaged if CD28 is present. The duration of signaling also determines a T cell’s response. In the presence of appropriate antigens, T cells need to bind DCs for less than 15 seconds. T cells can make 30 to 40 DC contacts within a minute. Sustained signaling is however required for maximal T cell activation. Thus during prolonged cell interactions, each MHC-peptide complex may trigger up to 200 TCRs. This serial triggering depends on the kinetics of TCR-ligand interaction. CD28, for example, reduces the time needed to trigger a T cell and lowers the threshold for TCR triggering. Adhesion molecules stabilize the binding of T cells to antigen-processing cells and allow the signal to be sustained for hours.

Naïve T cells have strict requirements for activation. They must receive a sustained signal for at least 10 hours in the presence of costimulation or for up to 30 hours in its absence. This level of costimulation can only be provided by dendritic cells, which supply high levels of costimulatory and adhesion molecules. This is relevant to vaccine usage because once primed, memory T cells become much more responsive to the same antigen.

Step 3: B and T cell responses

B cell responses

As described earlier, the division of the adaptive immune system into two major components is based on the need to recognize two distinctly different forms of foreign invaders: exogenous and endogenous. Antibodies deal with the microbe. They attack bacteria and free virus particles and parasites. Antibodies also provide the first line of defense against organisms that can survive and grow within cells. However, once these organisms succeed in entering cells then cytotoxic T cells are needed to kill the infected cell, release cytokines that inhibit microbial growth, or prevent pathogen survival within cells.

In general, B cell responses are affected by the dose of antigen administered. At priming, higher antigen doses favor the induction of plasma cells over memory cells. Lower doses generally favor memory cell production. At revaccination, higher doses of antigen favor stronger responses. Thus increased doses of antigen, within limits, promote greater B cell responses. On the other hand, very high doses of antigen exert less selective pressure on B cell responses so that cells do not have to compete for limited amounts of antigen and hence lower affinity antibodies can be produced. Adjuvants, by providing the required “danger” signals reduce the dose of antigen needed to induce a protective response. Modified live vaccines that cause local inflammation are “naturally adjuvanted” and are thus more immunogenic than killed/inactivated vaccines. Very few killed vaccines can induce high, sustained antibody responses after a single dose.

B cells are activated within the lymph nodes draining the vaccine injection site. When a B cell encounters an exogenous antigen that binds its receptors, with appropriate costimulation it will respond by secreting these receptors into body fluids, where they are called antibodies. Each B cell is covered with about 200,000 to 500,000 identical antigen receptors (BCRs). Unlike the TCRs, however, BCRs can bind soluble antigens. Antibodies are simply BCRs released into body fluids; they all belong to the family of proteins called immunoglobulins.

Although the binding of antigen to a BCR is an essential first step, this alone is usually insufficient to activate B cells. Complete activation of a B cell also requires multiple costimulatory signals from helper T cells and their cytokines.

When helper T cells “help” B cells they start the process that leads to B cell division and differentiation into antibody-secreting cells. They stimulate B cell proliferation and survival. The “help” also triggers somatic mutation within B cells and thus results in an increase in antibody-binding affinity (Fig. 2.6).

Jan 21, 2021 | Posted by in GENERAL | Comments Off on The science behind vaccine use

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