David Blanco, PhD
CHS A2-087G
MIMG C106
The Immune Response to Bacterial Infections
Bacterial pathogens have, and continue to, evolve to live on or within host
tissues. The mammalian immune system has, and continues to, evolve to
prevent bacterial pathogens from doing so. This long co-evolution is apparent
when one considers the molecular mechanisms involved in bacterial-host
interactions. Many virulence factors expressed by bacterial pathogens have
been shown to function specifically to overcome specific host immune effector
functions. For example, pili expressed by uropathogenic Escherichia coli
mediate specific and tight adherence to urinary epithelia preventing these
bacteria from being washed out by the urinary flow. Yersinia species secrete
anti-phagocytic factors directly into host cell macrophages preventing these
bacteria from being ingested and killed by these phagocytic cells. Similiary, the
mammalian immune system has evolved to recognize general features of
bacterial cells, such as lipopolysacharride (LPS), lipoteichoic acids (LTA),
peptidoglycan (PG), lipoproteins (LP), flagellar proteins, and certain unmethylated DNA sequences (CpG) (all of these are collectively referred to as
Pathogen Associated Molecular Patterns, PAMPs), so that a non-specific
immune response (innate immunity) can be mounted quickly, as well as the
ability to stimulate immune cells that recognize specific, discrete, features of
individual pathogens so that a specific, effective, long-lasting and protective
immune response develops (adaptive immunity). Because of the
interdependence between pathogen and host, mechanisms of bacterial
pathogenesis cannot be considered outside the context of host defense
mechanisms they have evolved to overcome.
The following brief overview of the mammalian immune response to bacterial
infection is intended to supply a basic framework for the lectures that follow.
Specific features of host responses and immunity will be examined in more detail
as we discuss the pathogenic strategies of individual bacterial species.
Immune Response to Bacteria (A Brief Overview)
I. Introduction
A. The immune response to initial infection occurs in three distinct phases:
1) innate immunity – Host defense mechanisms that function within the first
4 hours of encountering a pathogen are those that constitute the innate or
constitutive immune response.
2) early induced (non-adaptive) responses – If innate immune mechanisms
are insufficient to eliminate the pathogen, early induced, but non-adaptive,
mechanisms are recruited. Inflammatory cells may be able to eliminate the
pathogen, or may keep the infection under control until adaptive immune
responses develop. This stage also has important responses leading to the
adaptive immune response. (this stage is also considered to be part of the
innate immune response).
3) specific adaptive responses – Adaptive immune responses are mediated
by T cell lymphocytes (in some cases B-cells mediate this response as
well). They develop and respond over a period of about ten days to specific
features (immunogens) expressed by the invading pathogen and function
almost exclusively against the pathogen against which they were generated.
B.
The immune response to re-infection is rapid (anamnestic response of
adaptive immunity):
1) protective immunity – If sufficient preformed specific antibody or effector
T cells are present, re-infection by a particular pathogen may be
imperceptible - the pathogen may be eliminated before it is able to establish
infection.
2) immunological memory – If preformed specific antibody or effector T cell
levels are insufficient to prevent re-infection by a particular pathogen,
memory immune cells will be activated and the adaptive immune response
will be induced in a much shorter time frame and a much stronger response
as compared to the adpative response to the primary infection. This is
referred to as an “anamnestic” response. The pathogen may be eliminated
before any signs or symptoms of infection develop.
II. Innate immune mechanisms
A. Exterior defenses - Our body surfaces are defended first by epithelia (skin
surface) that provide mechanical, chemical, and microbiological barriers
against potential pathogens. The importance of mechanical barriers, such
as epithelial cells joined by “tight junctions” and the flow of air or fluid over
epithelial surfaces, is obvious in cases where these barriers are
compromised such as in wounds or burns or in people whose mucociliary
escalator is compromised. Surface epithelia also produce chemicals that
inhibit many bacteria. For example, lyzozyme is present in tears and saliva
and is effective at killing many bacterial species, as is the low pH of the
stomach and the defensins and cryptidins (small bactericidal peptides)
present in the respiratory and intestinal mucosa, respectively. In addition,
some mucosal surfaces are colonized by normal flora bacteria, which can
prevent colonization by pathogenic bacteria. An example of this competitive
inhibition is illustrated by the organism Clostridium difficile, a colon gut flora
organism found normally in that setting in very low numbers. This organism,
like other Clostridiae, is nearly ubiquitous and we encounter it all the time,
but usually it is out completed by our normal gut flora. However, when the
number and type of bacterial in the gut are altered by long term antibiotic
therapy, this organism is capable of establishing infection and causing
disease, ranging from diarrhea to life-threatening colitis.
B. The complement cascade – Once an organism has crossed the epithelial
barrier, or even before in the case of mucosal pathogens, they will
encounter the complement components. Complement comprises a series
of eleven serum proteins that act together to attack extracellular pathogens
(pathogens which gain an intracellular residence are protected from
complement). Initiation of the complement cascade occurs when the
pathogen is recognized by “early” complement components. Some
complement proteins can directly recognize specific features of bacterial cell
surfaces. Initiation of the cascade by this mechanism constitutes the
antibody-independent or what is referred to as the “alternative pathway” of
complement. Other complement proteins recognize a complex that forms
when mannan-binding lectin, a protein normally circulating in the
bloodstream binds mannose residues on bacterial surfaces. This is referred
to as the “lectin pathway” of complement activation. Complement can also
be activated by antigen-antibody complexes that form when specific
antibody is generated and bind to their respective antigens on the bacterial
cell surface (the result of specific adaptive immunity). This mechanism is
referred to as the antibody-dependent or “classical pathway” of complement
activation. The complement cascade therefore functions as part of the
innate as well as the adaptive immune response to bacterial pathogens.
C. Professional phagocytes – Professional phagocytes are important mediators
of innate and adaptive immunity. These cells are referred to as professional
phagocytes because they possess cell surface receptors for antibody (Fc
receptors) and components of the complement system (CR receptors). The
following are the principle professional phagocytes:
Macrophages mature continuously from circulating monocytes, leave the
blood and migrate into tissues throughout the body. They are found in
especially large numbers in connective tissue, in association with the
gastrointestinal tract, and along certain blood vessels in the liver, lung and
spleen.
Polymorphonuclear neutrophils, or PMNs, also play a particularly important
role in the earliest response to infection. They are recruited rapidly to the
site of infection where they engulf and kill bacterial pathogens. PMNs
contain primary and secondary granules, a distinguishing feature of these
cells, which contain antibacterial enzymes such as acid hydrolases,
lysozyme and bactericidal peptides (defensins). These antibacterial
components constitute what is called the oxygen-independent killing
mechanisms. PMNs also contain oxygen-dependent killing mechanisms as
the result of the granule components NADPH oxidase and myleoperoxidase
which generates products such as superoxides, hydrogen peroxide,
hypochlorus acid, and hydroxy radicals.
Both macrophages and PMNs are capable of engulfing many bacterial
pathogens directly (phagocytosis) and are much more efficient in
phagocytosis when antibody and/or complement is bound to the bacterial
surface (opsonization). They are therefore important both in the initial
control of bacterial infections as well as being major effector cells in the
adaptive immune response (particularly activated macrophages). As
discussed below, these cells also play an important role in the development
of adaptive immunity and determining the type of adaptive immune
response that develops.
III.
Induced but non-adaptive host response to infection
Bacteria that either overwhelm or evade constitutive host defense
mechanisms may be contained by a second wave of non-adaptive but
induced immune defense mechanisms. These responses, characterized
primarily by the recruitment of inflammatory cells and soluble bactericidal
products to the site of infection, involve recognition mechanisms that are
based on relatively invariant molecules (such as LPS, peptidoglycan,
lipoproteins and lipopeptides, lipoteichoic acids, flagella, and CpG
oligonucleotide sequences collectively referred to as PAMPs; pathogen
associated molecular patterns). This inflammatory response is short lived
and does not lead to protective immunity that is long lasting or has memory.
This response is, however, important for keeping the pathogen in check until
an adaptive response is mounted and for influencing the type of adaptive
immune response that predominates during development.
When macrophages and PMNs encounter, ingest, and degrade pathogens
they become activated to secrete a number of cytokines, and the repertoire
of cytokines they secret depends on the nature and pathogenic strategy of
the pathogen. For example, when macrophages engulf and degrade certain
Gram-negative bacteria, they secrete interliukin-1 (IL-1), IL-8, tumor
necrosis factor-α (TNFα), IL-6, and IL-12. IL-8 recruits more phagocytic
cells to the site of infection and together with TNFα activates PMNs. In
terms of specific adaptive immunity, IL-12 induces the differentiation of
CD4+ T cell into Th1. As we will discuss below, Th1 cells are important for
the development of cell-mediated immune responses that are particularly
effective against intracellular pathogens. Other pathogens do not induce IL12 production in macrophages but instead cause the induction of IL-10, that
leads to the development of Th2 cells which are important for the
development of humoral immunity (antibody response), the immune
response that effectively deals with extracellular pathogens.
IV.
Adaptive immunity to infection
Adaptive immunity is the antigen-specific, T cell dependent, immune
response that develops when innate immune defense mechanisms are
unable to control an infection. This response develops over several days,
the time it takes antigen-dependent T and B cell lymphocytes to proliferate
and differentiate to effector cells.
Adaptive immune responses can be generally divided into two types: 1)
Humoral immunity (antibody response) and 2) Cell mediated immunity
(CMI).
1) Humoral or antibody mediated immunity – In general, humoral immune
responses are most effective against extracellular pathogens. Specific
antibody serve three main functions in host defense. i) Neutralization:
Binding of antibody to bacterial toxins prevents them from interacting with
the host cell targets. The neutralized toxins, bound to specific antibody, can
then be ingested and degraded by macrophages. Neutralization also
involves antibody binding to specific bacterial adhesins that mediate the
attachment of the bacterial pathogen to its target host cells or tissues.
Antibody binding to these adhesins inhibits bacterial colonization ii)
Opsonization: Bacteria in extracellular spaces can become opsonized
(coated with specific antibody). These opsonized bacteria are recognized
by macrophages that bind the bacterial to the macrophage via the Fc
portion of the antibody molecule and the Fc receptor on the macrophage
surface. The bound bacteria are then injested and degraded. iii)
Complement activation: Bacteria bound with specific antibody can activate
the “classical pathway” of complement. Specific complement components
that recognize these antigen-antibody complexes induce the complement
cascade that can result in direct killing of the bacteria, via the final
components of the complement cascade (Membrane Attack Complex,
MAC) or ingestion and degradation by macrophages (via complement
receptors on the macrophage surface).
2) Cell-mediated immunity (CMI) – In general, CMI responses are most
effective against intracellular pathogens. Intracellular pathogens can reside
either within phagocytic vesicles or free in the host cell cytoplasm. Two
types of CMI reponses have evolved to deal with these two pathogenic
strategies. i) Pathogens that survive within phagocytic vesicles of
macrophages, such as Mycobacterium tuberculosis, are controlled by a T
cell dependent mechanism of macrophage activation. M. tuberculosis
prevents fusion of the phagocytic vesicle with lysosomal vesicles of the
macrophage. A specific class of T lymphocyte, known as the T h1 cell,
activates the macrophage inducing fusion of lysosomes with the phagosome
(phagolysosomal fusion), hence killing of the Mycobacteria. The Th1 cell
also secretes cytokines to recruit additional macrophages to the site of
infection. ii) Infections by pathogens that reside free in the cytoplasm, such
as viruses and certain bacteria (e.g. Shigella and Listeria), are controlled by
CD8+ cytotoxic T cell lymphocytes (CTL). CTLs actually kill the infected
cell, releasing the intracellular pathogens to the extracellular environment
where they can be ingested and degraded by activated macrophages.
How is the adaptive immune response induced and how is it determined
whether the response will be primarily humoral (antibody) or one that is cellmediated (CMI).
When professional phagocytes, such as macrophages, engulf and degrade
pathogens, they process molecules (degrade proteins, complex
carbohydrates, glycoproteins, glycolipids) and present them on their surface
together with molecules of the immune system known as the major
histocompatibility complex (MHC). The processed molecules can be
presented with different MHC molecules (MHC class I and MHC class II)
depending on whether the pathogen resided within a phagocytic vesicle
(MHC class II) or free in the cytoplasm (MHC class I). The pathogenic
strategy of the pathogen can also influence the repertoire of cytokines
expressed by the macrophage. The macrophage, presenting molecules
and secreting cytokines, migrates to draining lymph nodes where T and B
cells are activated. Although the process is not yet fully understood and is
much more complicated than presented here, a popular (simplified) model is
that naïve T-helper cells (Th0 CD4+ T cells) will develop into the Th1 type T
cells in the presence of interleukin-12 (IL-12) and interferon- (IFN) and into
Th2 cells in the presence of IL-4 and IL-10. Th1 are crucial for activating
macrophages while Th2 cells activate B cells that ultimately mature into
antibody-producing plasma cells. These responses generally occur when
the processed bacterial molecules are presented in combination with MHC
class II molecules. When the bacterial processed molecules are presented
in combination with MHC class I molecules, Th0 CD8+ T cells develop, with
the help of the Th1 response, into CTLs (cytotoxic lymphocytes). CTLs
(MHC I) and activated macrophages (MHC II, Th1) are the effector cells
against primarily intracellular pathogens. Antibody responses (MHC II, T h2)
are the effectors of primarily extracellular pathogens.