Download The immunological principles underlying vaccine

Occupational Medicine 2007;57:548–551
doi:10.1093/occmed/kqm109
IN-DEPTH REVIEW
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The immunological principles underlying
vaccine-induced protection in the occupational
health setting
David Baxter
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Abstract
Protection against the large numbers of pathogenic microbes to which humans are constantly exposed is effected through external barriers (skin and mucus membranes), innate barriers (cellular
components and soluble chemical mediators) and adaptive barriers (B and T lymphocytes). This
article reviews the normal mechanisms employed to protect against these pathogenic microbes.
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Key words
Extracellular pathogens; host defences; innate adaptive systems; intracellular pathogens; occupation.
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Host defences
Despite the constant exposure to microbes, serious infectious diseases in humans are generally not common
because barriers to infection are usually highly effective—
these involve external barriers (made up of skin and the
mucous membranes of the respiratory, gastrointestinal
and urogenital tracts), innate barriers (comprising both
cellular and soluble chemical mediators) and adaptive
barriers (based on B and T lymphocytes). Only when
the external barriers are breached will the innate and
adaptive mechanisms be activated.
External barriers
Skin and mucous membranes protect against microbial
invasion in various ways. There is a barrier effect due to
the mechanical rigidity of the stratum corneum; this is
supported by a number of additional processes that impair microbial growth including low moisture content,
acid pH, raised core/surface temperature, secretion of
stratum corneum lipids and the secretion of lysozyme,
defensins and proteinase inhibitors by epithelial cells.
The flow of urine, tears and peristalsis reduces contact
time between the pathogen and cells, limiting the attachment of microbial surface molecules and thus providing
a further protective mechanism [1–3]. The barrier effect
of mucosal membranes is more limited than for skin because these may only be one cell thick—in such situations,
additional protective mechanisms have developed; inEpidemiology and Health Sciences, Stopford Building, Manchester University
Medical School, Oxford Road, Manchester, UK.
Correspondence to: David Baxter, Epidemiology and Health Sciences, Stopford
Building, Manchester University Medical School, Oxford Road, Manchester,
UK. e-mail: [email protected]
cluding, for example, the secretion in the pulmonary
alveoli of surfactant proteins which can act as a bacterial
opsonin [4].
The activities of commensal organisms provide additional protection both through occupation of an ecological niche and competition for food substrate.
Additionally, on the skin and in the vagina, they contribute to the low pH (’5.5) by secreting antimicrobial substances (including bacteriocins) and free fatty acids
generated by the action of microbial lipases on secreted
triglycerides (skin) or peroxide (vagina); in the gastrointestinal tract, they also release antimicrobial metabolites
[2]. Lactic acid secreted by sweat glands helps maintain
the low skin pH.
Fever associated with infection is yet another barrier
which works by decreasing microbe multiplication for
pathogens with impaired replication above 37°C.
Finally, there are secreted antimicrobial molecules, including antibacterial peptides (defensins) and the enzyme
lysozyme (present in saliva), which are produced as a result of innate mechanisms and operate at the external
barrier level.
Innate system
Microbial penetration of the external barrier results in the
activation of the innate system—a system designed to recognize infectious non-host and then eliminate it by activation of the acute inflammatory response [5,6]. The
innate system, alternatively called the constitutive system,
is an evolutionary ancient system with many of the mechanisms used being shared by both plants and animals; this
is in contrast with the adaptive system which appeared
much later in evolution being first seen only in the teleost
vertebrate fish. The recognition function is based on
Ó The Author 2007. Published by Oxford University Press on behalf of the Society of Occupational Medicine.
All rights reserved. For Permissions, please email: [email protected]
D. BAXTER: IMMUNOLOGICAL PRINCIPLES UNDERLYING VACCINE-INDUCED PROTECTION 549
identifying specific microbial molecules and the elimination function on phagocytosis and the activation of various plasma proteins. The response time of the innate
system is immediate and all cells of a particular class,
for example polymorphonuclear leucocytes, are identical.
The innate recognition function requires sentry cells—
mainly immature dendritic cells with assistance from
tissue-based macrophages—to detect microbial invasion
by identifying highly conserved components of pathogenic microbes (termed pathogen-associated molecular
patterns—PAMPs) using germ line-encoded host molecules (designated as pattern-recognition receptors—
PRRs).
PAMPs are invariant molecular constituents of infectious microorganisms that are shared by a variety of pathogens, but not usually found in the host. Bacterial PAMPs
include peptidoglycan and lipopolysaccharide (LPS)
located on the surface of gram-positive and -negative cells,
respectively; double-stranded RNA is a key viral PAMP.
PRRs are located on or within cells and in plasma/
tissue fluids where they function as recognition molecules
for PAMPs. Toll-like receptor 2, for example, is the cellbased PRR that recognizes peptidoglycan whereas Tolllike receptor 4 is the cell-based PRR that recognizes LPS.
The complement protein C3 is a recognition molecule for
foreign antigen including bacterial LPS and viruses.
There are an estimated 1000 PRRs in the innate system.
The process of binding between the PRR and its specific PAMP initiates a series of actions leading to microbe
death—these depend on whether the PRR is cell or
plasma based and include opsonization, phagocytosis,
activation of pro-inflammatory signalling pathways,
induction of apoptosis and complement activation.
Complement consists of at least 20 plasma proteins
that have both antigen recognition and effector functions.
Complement attacks microorganisms in one of three
ways. First, particular activated complement proteins
are able to bind to the surface of the microbe facilitating
phagocytosis. Second, activated complement molecules
augment the inflammatory response. And third, the
membrane attack complex (MAC) is directly cytopathic.
Opsonization describes the mechanism whereby invading pathogens are coated with either complement
molecules (for example activated C3) or antibody (for
example IgG). Phagocytic cells (neutrophils, macrophages and dendritic cells) express receptors for IgG
and C3, thus enabling them to lock onto the microbe;
such binding then initiates the change in cytoskeleton
proteins that leads to the formation of the phagosome
in which digestion of the microbe takes place by exposure
to an array of toxic oxidants [7].
Host cells are protected from the destructive effects of
the innate system by the expression of gene encoded selfmolecules on the surface of uninfected host cells—such
molecules are able to prevent activation of innate elimination mechanisms [8].
There are two additional mechanisms that protect
against many viral infections.
(i) The first involves secretion of type I interferons by
virally infected cells. Interferons are generally classified into two types—I and II. All cells are able to
secrete type I interferons, for example in response to
infection, which work either by up-regulating the expression of major histocompatibility complex (MHC)
class I molecules leading to the enhanced recognition
of viral antigens by cytotoxic T cells (Tc) or by directly inhibiting viral replication through synthesizing
a number of enzymes that interfere with the transcription of viral RNA/DNA [9]. Type II interferons (synonym interferon g) are secreted by natural killer (NK)
cells and T cells and activate NK cells, T cells, endothelial cells and macrophages.
(ii) NK cell function is the second important protective
mechanism against viruses. Virus-infected cells may
express viral proteins on the cell surface—these can
be recognized by multiple activatory receptors on NK
cells which then kill the infected cell using the same
final pathway mechanisms as Tc cells—namely, secretion of perforin and granzymes [10]. NK cells also
kill infected cells using antibody-dependent cell cytoxicity (ADCC): this is a mechanism whereby NK
cells recognize those virus-infected cells which have
IgG bound to virus proteins. Because of the potential
damage induced by NK cell activation, NK cells express a number of inhibitory receptors that bind to
the MHC I molecules expressed by normal host cells
and prevent NK cell activation.
Adaptive barrier
The adaptive barrier, like the innate system, has both
a recognition and elimination function. It is an evolutionary response to those pathogens which have developed
the ability to evade host external and innate barriers
and invade tissues; such pathogen evasion strategies can
involve immune system disruption, depletion of immune
cells, tolerance induction and mutation. In contrast to the
innate system, the effectiveness of the adaptive response
is enhanced by previous exposure to the microbe because
of its memory function; in addition, cells of the adaptive
system express surface recognition molecules for antigenic fragments of pathogens that are unique to the particular cell.
The recognition function is based on receptors—either
cell surface (on T and B cells) or circulating in plasma
(antibodies)—that identify specific molecular sequences
or conformational patterns on microbes. The elimination
function is based on T cells (designated cell mediated)
and/or B cells (antibody mediated).
Tissue invading pathogens are either intracellular or
extracellular replicating microbes. The adaptive response
550 OCCUPATIONAL MEDICINE
to intracellular pathogens is cell mediated whereas extracellular microbes are eliminated through antibodies. The
systems are, however, highly integrated with a number of
viruses (obligate intracellular pathogens) being eliminated
during their viraemic phase by neutralizing antibodies.
The following description will focus on the adaptive
response to bacteria and viruses, since these are pathogens
against which a number of commercially available and
effective vaccines have been developed.
Protection against extracellular pathogens
Extracellular pathogens (which include most bacteria)
are attacked primarily by antibodies—these are produced
by B cells and are located on the cell surface or secreted
in serum, other extracellular fluids and on mucous
membranes.
Antibodies recognize virtually every kind of molecule
including macromolecules, dietary products (polysaccharides, lipids and proteins) and nucleic acids. Each B
cell has a unique receptor that recognizes the specific
molecular sequence or conformational pattern on a particular microbe. Genes that code for the B-cell receptor
(BCR) undergo random mutation and related processes
that enable as many as 109 BCRs with separate specificities to be generated. Each B cell will express many thousands of copies of this unique BCR on its cell surface.
Antigen-responsive naive B cells develop in the bone
marrow from where they enter the peripheral lymphoid
tissues. Antigen recognition takes place here—the spleen
in the case of blood borne pathogens, lymph nodes in the
case of microbes that enter through the skin and mucous
membranes and mucosal-associated lymphoid tissue in
the case of ingested or inhaled antigens. Antigen binding
to the BCR will subsequently result in B-cell activation,
proliferation and differentiation.
Binding of the naive BCR with its specific antigen does
not immediately result in activation of the cell, rather
it requires additional signals that are provided by a
TH2 cell—such antigens, which include proteins, are
termed T-dependent antigens. The antibody produced
during initial activation of the adaptive response is IgM;
subsequently, the B cell produces increasing amounts
of IgG—a process termed isotype switch. In contrast,
polyvalent carbohydrate molecules can directly activate
the B cell by cross linking a number of BCRs—such carbohydrate antigens are termed T-independent antigens.
Activation of a B cell by T-independent vaccines leads to
IgM production with only limited isotype switch to IgG
and very little memory cell production. T-independent
antigens (for example, polysaccharide molecules which
make up the capsule of gram-negative bacteria or subunit
polysaccharide vaccines like ‘Pneumovax’) directly activate the B cell with the production of IgM—this is clearly
a useful protective mechanism since IgM is an efficient
complement-fixing antibody generating cytopathic
MACs. However, because this response does not involve
TH2 cells, there are no co-stimulatory signals or cytokines, and thus only very small amounts of IgG are produced and no memory cells; furthermore, this process
only occurs in older children and adults and not infants
or young children—hence, the recommendation that
such vaccines should not be used in the very young.
Following invasion by a microbe to which there has
been no prior exposure, the organism will be phagocytosed by immature dendritic cells; the process is initiated
by PAMP:PRR binding leading to changes in the cytoskeleton proteins that cause the development of the digestive phagosome [11]. Subsequently, the microbe is
degraded and antigenic fragments are bound to MHC
II molecules which then migrate to the cell surface where
they display the MHC II:antigen complex. While this
process is happening internally, the now activated mature
dendritic cells migrate along lymph channels to the draining lymph node where they encounter naive TH2 cells, each
with their own unique T-cell receptor (TCR)—multiple
copies of which are found on the cell surface. Identifying
and then binding of the MHC II:antigen complex to the
specific TH2 receptor then activates the naive T cell,
causing it to proliferate.
Simultaneously, fragments or whole microbial organisms killed at the invasion site as a result of the innate
response pass along lymph channels to the same draining
lymph nodes where they come into contact with B cells,
each with their own unique BCR: many thousands of
copies of the identical BCR are found on the cell surface.
Binding to the B cell through the specific immunoglobulin receptor that recognizes the antigenic fragment results
in its endocytosis, processing through the phagosome and
presentation as an MHC II:antigen complex (as with the
dendritic cell).
These two processes occur in the same part of the
lymph node with the result that the B cell with the
MHC II:antigen complex on its surface now comes into
contact with the activated TH2 cell whose receptors are
specific for this antigen. Binding of these, termed linked
recognition, results in the TH2 cell activating the B cell to
become a plasma cell with the production and secretion
initially of IgM, and then IgG; in addition, a subset of
B cells differentiate to become long-lived memory cells.
The whole process takes a minimum of 10–14 days. Subsequent exposure to the microbe results in activation of
the memory B cells leading to more rapid production
of antibody—often within 1–2 days—and this is the basis
of immunological B cell memory.
There are five major antibody isotypes designated
IgM, IgG, IgA, IgD and IgE: IgA is subdivided into
IgA1 and IgA2 subtypes and IgG is subdivided into
IgG1, IgG2, IgG3 and IgG4 subtypes. The isotypes differ
in their functionality; thus IgM primarily activates complement whereas IgG1 is a highly effective opsonizing
antibody. Prevention of infection can be the result of
D. BAXTER: IMMUNOLOGICAL PRINCIPLES UNDERLYING VACCINE-INDUCED PROTECTION 551
antibodies binding to attachment organelles so preventing infection—neutralizing antibodies; such antibody
types may also block an exotoxin secreted by the microbe.
Alternatively, antibody attachment to the surface of a microbe can activate complement-facilitating opsonization
and generate the MAC. IgG attached to the surface of
a microbe will facilitate phagocytosis because phagocytes
have a FCgIII receptor, which is able to ligate such bound
antibody. ADCC is the final mechanism by which antibodies are able to cause microbial death.
Protection against intracellular pathogens
Intracellular pathogens are subdivided into those pathogens that replicate within the cytosol and those that replicate within the phagosome (for example Mycobacteria).
Pathogens that replicate in the cytosol are eliminated by
Tc cells and pathogens that replicate in intracellular
vesicles are eliminated by TH1 cells.
Cytosolic pathogens include most viruses and some bacteria (for example Listeria). Following cell attachment and
penetration and using a virus as an example, the following
steps occur. Viral genetic material is released and transcription occurs leading to the production of viral proteins,
which are then assembled into new virions. As the virus
replicates, intracellular defence systems lead to disruption
of some virions: these viral fragments are then loaded onto
MHC I molecules and subsequently transported to the cell
surface. Tc cells, each with a unique TCR, constantly circulate throughout the body sampling the cell surfaces looking for signs of infection; binding of the TCR with the
MHC I (containing the viral fragment) activates the Tc cell
which releases (by exocytosis) cytokines (including perforin
and granzymes) that cause the infected cell [12] to undergo
programmed cell death (termed apoptosis). Although all
cell types express MHC I molecules on their cell surface,
only antigen-presenting cells (macrophages, monocytes
and B cells) are able to activate TH1 cells.
TH1 cells recognize antigen through MHC II:microbial
antigen complexes expressed by infected macrophages.
Activation of TH1 cells results in a cell-mediated immune
response directed primarily at intracellular bacterial infections (e.g. Mycobacterium tuberculosis and Listeria monocytogenes); as a result of the TH1 cell secreting the IFN-g and
TNF-a cytokines, macrophages in particular are activated
and kill the infected cell. A common feature of both these
infections is the secretion of either IL-12 during the innate
immune response or IFN-g by NK cells (causing macrophages to secrete IL-12). IFN-g also enhances the production of opsonizing and complement-fixing IgG
antibodies. It should be noted that a subset of TH1 cells
can also activate B cells leading to the development of
antibody-secreting and memory B cells [13].
Tc and TH1 cell activation and proliferation typically
takes 7 days with an in vivo expansion of between 100
and 5000 antigen-specific cells being reported [5]. As
the microorganism is eliminated and antigen correspondingly declines, the majority (.95%) of effector cells are
removed by apoptosis.
Conclusion
Humans have evolved protective mechanisms against
pathogen attack. While generally effective, limited effectiveness against particular pathogens together with evolving pathogen evasion strategies results in ongoing
susceptibility to certain infections. Immunization, by inducing specific adaptive immune responses, can be used
to provide protection against some of these pathogens.
The process of immunization will be considered in the
next article.
Conflicts of interest
None declared.
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