Host-pathogen interactions involve complex interplay between host defenses and pathogen virulence factors. The host innate immune system, including physical barriers and pattern recognition receptors like Toll-like receptors, provide first-line defenses against microbes. Toll-like receptors recognize pathogen-associated molecular patterns and initiate signaling pathways that launch immune responses. The adaptive immune system also provides host defenses through acquired immunity and memory responses. Pathogens have evolved virulence strategies like toxins and capsules to overcome these host defenses. Understanding the molecular interactions between hosts and pathogens is crucial for developing new therapeutic approaches.

1. HOST PATHOGEN
INTERACTION:
CURRENT VIEWS
Department of Veterinary Pathology,
College of Veterinary Science & A.H., DUVASU (India)
Neeraj Kumar Gangwar
PhD Scholar

2. Microbes are found everywhere. Most have no ill effects due to protective mechanisms
in our body.
• Robert Koch established that infectious diseases were caused by microbes.
Kinds, Goldsby and Osborne, 2007

3. •Organisms begin to cause any obvious damage to a host by invading tissues and/or
producing toxins, they then become pathogens.
Host Defenses
Skin and mucosal secretions
• Non-specific local responses (e.g., pH)
•Non-specific inflammatory responses
•Specific immune responses (e.g.,
lymphocytes)
Pathogen Defenses- virulence factors
Ability of a pathogen to adhere to a host
•Ability of a pathogen to colonize
(overcome) a host
•Ability of a pathogen to evade host
defenses

5. Defense mechanisms present in host
Resistance: ability of the host to prevent infection from occurring & infectious
disease from developing
Resistance is normally aided by-
Barriers to infection: intact, functional epithelial surfaces (respiratory tract,
gastric acid, antibacterial action of bladder secretions and saliva of oral
cavity)
Immune system
•Nonspecific-
Examples: skin, mucosal surfaces, tears, saliva, gastric juices, & the
immune system.
Disease-specific-
Immunity (resistance) against a particular agent.
•The gut is the largest defense barrier of our body. More than 60% of
immune cells are in the gut mucosa- inhibit uncontrolled inflammatory
reactions.
(Hooper and Mac Pherson, 2010; Sekirov and Finlay, 2009).

6. Interfere with complement
activation
Survive in the phagocyte
Exotoxins
Endotoxins
Exoenzymes
Intracellular pathogens
Capsules prevents phagocytosis
Leukocidins
sneezing, coughing, diarrhea, coitus, pus,
blood, or insect bites

10. THE IMMUNE SYSTEM
Principal function- Protection of host against pathogenic microbes
Two Types
1. Innate Immunity
2. Acquired (Specific) Immunity

11.  Present at birth
 Self-discrimination
 Limited diversity
 Nonspecific defense
 No memory
 Examples: skin, cough,
gastric pH, fever,
Inflammation
(phagocytic cells).
 Appears after contact
with antigen
 Self-discrimination
 Vast diversity
 Specific defense
 Memory responses
 Examples: antibody,
 cytotoxic lymphocytes
Immune System in Vertebrates
Innate Acquired

13. Pathogenic microbes are endowed with “offensive” and “defensive”
functions that separate them from nonpathogenic microbes and
determine the type and outcome of the host-pathogen interaction
(Smith, 1913)
Host defense against invading microbial pathogens is elicited by the
immune system: innate immunity and acquired immunity.
Both recognize invading microorganisms as non-self, which triggers
immune responses to eliminate them (Takeda and Shizuo, 2005)
Both components have been characterized independently, and the
main research interest in the immunology field has been confined to
acquired immunity (Takeda and Shizuo, 2005)

14. Innate immune system is immediately available to combat threats.
There is no complicated method of selecting cells that react to
foreign substances from those that react to self
Innate immune system responds to common structures shared by a
vast majority of threats
These common structures are called pathogen associated molecular
patterns (PAMPs) and are recognized by the cellular receptors (PRR)
(Satoshi and Shizuo, 2008)
Cellular TLRs (a PRR) an important part of the innate immune
system is the humoral complement system that opsonizes and kills
pathogens through the PAMP recognition mechanism. (Satoshi and Shizuo, 2008)

15. Pathogen Associated Molecular Patterns (PAMPs)
– Gene products unique to microbes - Molecular Signatures of
microbial invaders
– Conserved and Invariant among a given class microbes
– Essential for microbial survival
LPS (Gram negative bacteria)
Peptidoglycan (Gram positive bacteria) (Kiyoshi and Shizuo, 2005)
Pattern Recognition Receptors (PRRs) -- Recognize PAMPs
– PRRs are expressed constitutively in the host and detect the pathogens
regardless of their life-cycle stage.
– PRRs are germline encoded, nonclonal, expressed on all cells of a given
type and independent of immunologic memory.
– Signals to induce inflammatory cytokines and activate host defense
Mechanisms (Kiyoshi and Shizuo, 2005)
Innate Immunity: Recognition of Microbial Nonself ?

16. The identification of surface or intracellular receptors (e.g. Toll-like, Nod, etc.)
equipped to respond to a variety of bacterial products has sparked a renewed interest
in the study of the interaction of microbial pathogens with the innate immune system.
Stimulation of Toll-like receptors (TLRs) by bacterial agonists results in host
inflammatory responses necessary to control infection.
Professional phagocytes play pivotal roles in sensing bacteria through pathogen-
associated molecular patterns (PAMPs), Danger-associated molecular patterns
(DAMPs) by various pathogen recognition receptors (PRRs), including Toll-like
receptors (TLRs), NOD-like receptors (NLRs), RIG-I–like receptors, C-type lectin
receptors and absence in melanoma 2(AIM2)–like receptors
(Chen and Nuñez, 2010; Davis et al., 2011)
The phagocytes also emit various alarms to further amplify innate immune responses
(Mosser and Edwards, 2008).

17. Receptors that detect infection. TLRs recognize microbes by binding to pathogen-
associated molecular patterns (PAMPs).
Abbr: lipopolysaccharide (LPS), lipoteichoic acid (LTA), lipoproteins (LP),
glycophosphatidylinositol (GPI).
Other pattern-recognition receptors for pathogens have been identified, such as:
transmembrane C-type lectin receptors (CLRs) which detect fungi; secreted receptors
(collectins, ficolins, and pentaxins) which activate innate defenses involving
complement and phagocytosis; cytosolic RIG-1-like receptors (RLRs) which detect
viruses; and cytosolic nucleotide-binding oligomerisation domain and leucine-rich
repeat-containing receptors (NLRs) which detect pathogens and stress signals.

18. TLRs are transmembrane proteins expressed by cells of the innate immune system,
which recognize invading microbes and activate signaling pathways that launch
immune and inflammatory responses to destroy the invaders.
TLR First identified in fruit fly Drosophila. (1985 by C. N. Volhard)
In mammals, the TLR family - TLR1−TLR11.
Two new members, TLR12 and TLR13 have been discovered in murine cells
Mammalian TLRs consist of an extracellular portion containing leucine-rich
repeats, a transmembrane region and a cytoplasmic tail, called the TIR (Toll-IL-1R
(Interleukin-1-Receptor) (SABiosciences.com Pathways 2008)
TLRs are mostly (but not exclusively) present on the membrane of immune and
epithelial cells (T. Kawai and S. Akira, 2006) and NODs are present in the cytoplasm
of enteric cells (E.C.Lavelle, 2010).
TLRs and NODs are capable of recognizing conserved molecular motives, generally
divided in microbe-associated molecular patterns (MAMPS, expressed by resident
microbiota) and pathogens-associated molecular patterns (PAMPS, produced by
microbial invaders).

19.  TLRs are predominantly expressed in tissues –
 involved in immune function (spleen & peripheral
blood leukocytes)
 as well as those exposed to the external
environment (lung & GIT).
 TLRs are located on the plasma membrane with the
exception of TLR3, TLR7, TLR8, TLR9 which are
localized in the endosomal compartment .
(Nishiya T & DeFranco AL. 2004. Ligand-regulated chimeric receptor approach reveals
distinctive subcellular localization and signaling proper ties of the Toll-like receptors. J
Biol Chem. 279(18):19008-17.)

20.  In addition to the TLRs, two NOD (nucleotide
oligomerization domain) proteins in the cytoplasm, have
recently been found to play an important role in the
innate defense against E. coli & S. aureus.
 The NOD proteins contain leucine-rich repeats very
similar to those in TLRs that recognize specific
components of these bacteria (diaminopimelic acid) and
form a cytoplasmic signaling platform with other proteins
known as the inflammasome.
 This signaling leads to IL-1 & IL-8 production.
 While it is well established that a strong innate
defense response to bacteria is essential for
survival, the most important role of this TLR
activation in the long term, may be in the induction
of the adaptive immune response.

21. •TLRs are located on the plasma membrane with the exception of TLR3, TLR7, TLR9
which are localized in the endosomal compartment (Nishiya & DeFranco , 2004).
•TLR2 is essential for the recognition of a variety of PAMPs from Gram-positive
bacteria, including bacterial lipoproteins, lipomannans and lipoteichoic acids.
•TLR3 is implicated in virus-derived double-stranded RNA.
•TLR4 is predominantly activated by lipopolysaccharide. G negative bacteria.
•TLR5 detects bacterial flagellin and TLR9 is required for response to unmethylated
CpG DNA.
•TLR7 andTLR8 recognize small synthetic antiviral molecules [Jurk et al., 2002], and
single-stranded RNA was reported to be their natural ligand [Heil et al., 2004].
•TLR11(Keating et al., 2007) has been reported to recognize
uropathogenic E.coli (Lauw et al., 2005 (Zhang et al., 2004) and a profilin-like protein
from Toxoplasma gondii .
•PAMPs include various bacterial cell wall components such as lipopolysaccharide
(LPS),peptidoglycan (PGN) and lipopeptides, as well as flagellin, bacterial
DNA and viral double-stranded RNA.
•DAMPs include intracellular proteins such as heat shock proteins as well as protein
fragments from the extracellular matrix.

22. International Immunol., 2005

25. TLR Signaling Pathways
MyD88-dependent pathway
MyD88-dependent pathway leads to the production of inflammatory cytokines
[Adachi et al., 1998]. It is common to all TLRs, except TLR3.
TLR4 and TLR2 signaling requires the adaptor TIRAP/Mal, which is involved in
the MyD88-dependent pathway [Horng et al., 2002].

26. MyD88-independent pathway leads to stimulation of IFN-β and the
maturation of dendritic cells.
TLR3 triggers the production of IFN-β in response to double-stranded
RNA, in a MyD88-independent manner, through the adaptor
TRIF/TICAM-1 [Yamamoto et al., 2002]
TRAM/TICAM-2 is another adaptor molecule involved in the MyD88-
independent pathway [Heil et al., 2004] which function is restricted to
the TLR4 pathway [Yamamoto et al., 2003]
MyD88-independent pathway

28. TLR3, TLR7, TLR8 and TLR9 recognize viral nucleic acids and induce type I
IFNs. The signaling mechanisms leading to the induction of type I IFNs
differ depending on the TLR activated.
They involve the interferon regulatory factors, IRFs, a family of transcription
factors known to play a critical role in antiviral defense, cell growth and
immune regulation.
TLR3 and TLR4 activate IRF3 and IRF7 [Doyle S. et al., 2002], while TLR7
and TLR8 activate IRF5 and IRF7 [Schoenemeyer A. et al., 2005].
Furthermore, type I IFN production stimulated by TLR9 ligand CpG-A has
been shown to be mediated by PI(3)K and mTOR [Costa-Mattioli M. &
Sonenberg N. 2008].

29. Ligands-
Bacterial cell-surface lipopolysaccharides (LPS), lipoproteins,
lipopeptides, and lipoarabinomannan; proteins such as flagellin from
bacterial flagella; double-stranded RNA of viruses; or the
unmethylated CpG islands of bacterial and viral DNA; and also of the CpG
islands found in the promoters of eukaryotic DNA; as well as certain other
RNA and DNA molecules.
Endogenous ligands-
Produced as a result of non-physiological cell death.(Kawai, Taro; Shizuo Akira (20
April 2010))
Fibrinogen (involved in blood clotting), heat shock proteins (HSPs), HMGB1,
extracellular matrix components and self DNA (it is normally degraded by
nucleases, but under inflammatory and autoimmune conditions it can form a
complex with endogenous proteins, become resistant to these nucleases and gain
access to endosomal TLRs as TLR7 or TLR9).

31. Takeda and Akira, 2005

32. TLR signaling pathways originate from the cytoplasmic TIR domain. A TIR
domain-containing adaptor, MyD88, associates with the cytoplasmic TIR domain
of TLRs, and recruits IRAK to the receptor upon ligand binding. IRAK then
activates TRAF6, leading to the activation of the IjB kinase (IKK) complex
consisting of IKKa, IKKb and NEMO/IKKc. The IKK complex phosphorylates
IjB, resulting in nuclear translocation of NF-jB which induces expression of
inflammatory cytokines.
TIRAP, a second TIR domain-containing adaptor, is involved in the MyD88-
dependent signaling pathway via TLR2 and TLR4. In TLR3- and TLR4-mediated
signaling pathways, activation of IRF-3 and induction of IFN-b are observed in a
MyD88-independent manner.
A third TIR domain-containing adaptor, TRIF, is essential for the MyD88-
independent pathway. Non-typical IKKs, IKKi/IKKe and TBK1, mediate activation
of IRF-3 downstream of TRIF.
A fourth TIR domain-containing adaptor, TRAM, is specific to the TLR4-mediated
MyD88-independent/TRIF-dependent pathway.

33. Innate Defense Against Bacteria- TLR-1,2,4,5,6
Two NOD (nucleotide oligomerization domain) proteins in the cytoplasm, have
recently been found to play an important role in the innate defense against E. coli &
S. aureus. The NOD proteins contain leucine-rich repeats very similar to those in
TLRs that recognize specific components of these bacteria. This signaling leads to
IL-1 & IL-8 production
Innate Defense Against Viruses-TLR-3,7 and 8.
Viral nucleic acids contain PAMPs that are recognized by intracellular TLRs. These
TLRs are located on the intracellular endosome membranes. The TLRs found on
endosomes are TLR3, TLR7, TLR8 and TLR9.
TLR3 activates immune cells in response to double-stranded viral RNA.
It should be noted that only plasmacytoid cells use the TLR pathway for viral
defense. Other cells use RIG1 (retinoic acid inducible gene1)-like helicases (RLHs)
to recognize viral PAMPs which results in primarily an IFN response (Barton, G.M.
and R. Medzhitov, 2003, Uematsu, S. et al, 2005).

34. Innate Defense Against Parasites
Toxoplasma gondii, the common parasite causing
toxoplasmosis in humans, binds to the newly
discovered TLR11.
In leishmaniasis, which affects 10 million people,
TLR2 and TLR4 are required for proper parasite
control, due to the activity of inducible nitric
oxide (iNOS).
A factor induced by TLR4 activation, neutrophil
elastase, is also important for the leishmanicidal
activity of macrophages (Kropf et al, 2004).
An intact TLR signaling system has been shown to contribute to the severe cerebral
malarial infection that is often lethal (Coban et al, 2007).

35. TLR activation by pathogens can lead to serious medical consequences, such as sepsis and
autoimmune diseases.
Negative modulators of TLR activation have been identified, and their important role in
reducing the inflammatory response has been demonstrated in animal models
(Brown, 2006, Luke & O’Neill, 2007, Luke & O’Neill, 2008).
The TAM family members are one example. TAM receptor ligands Gas6 and ProS, play an
important role. The inhibitory effect requires the synthesis of SOCS1 (suppressor of
cytokine signaling 1)
Loss of function of the three members of this family (Tyro3/Axl/Mer) in a triple knockout
mouse results in a profound dysregulation of the immune response. This includes massive
splenomegaly and lymphadenopathy, lymphocyte infiltration into all tissues, and high
levels of autoimmunity
(Luke & O’Neill, 2007)
Fine Tuning TLR Activation

36. Negative regulation of TLR signaling
Stimulation of TLRs by microbial components triggers the induction of inflammatory
cytokines such as TNF-a, IL-6 and IL-12.
When all these cytokines are produced in excess, they induce serious systemic disorders
with a high mortality rate in the host.
It is therefore not surprising that organisms have evolved mechanisms for modulating their
TLR- mediated responses.
IRAK-M inhibits dissociation of IRAK-1/IRAK-4 complex from the receptor. MyD88s
blocks association of IRAK-4 with MyD88. SOCS1 is likely to associate with IRAK-1 and
inhibits its activity. TRIAD3A induces ubiquitination-mediated degradation of TLR4 and
TLR9. TIR domain containing receptors SIGIRR and T1/ST2 are also shown to negatively
modulate TLR signaling

37. Negative regulation of TLR signaling via IRAK

38. TLR involve in immune disorders
TLR-mediated pathway is responsible for the development
of atherosclerosis
(Bjorkbacka et al. 2004; Michelsen et al. 2004).
The MyD88-dependent pathway is seemingly involved in
allograft rejection
(Goldstein et al., 2003)

40. Mechanisms of Viral Pathogenesis
 Avoiding immune responses
 Avoiding antiviral effects of interferon
 Interferons alter regulatory responses of cell in event of viral
infection
 Helps limit viral replication
• Some viruses encode specific proteins to interrupt inhibition of viral
replication
 Regulation of host cell death by viruses
 Kill host after production of large numbers of viral copies
 Allows spread to other cells
 Viruses prevent apoptosis
 Inhibits protein that regulates apoptosis
 Block antigen presentation of MHC class I
 No sign of infection
 Cause production of “counterfit” MHC class I molecules
 All appears “well”

42. The binding of NS1 protein of influenza A virus to RIG-I prevents the
downstream activation of IFN response factor-3 (IRF-3), effectively stopping the
transcriptional induction of IFN-ß. NS1 also binds and prevents the cleavage and
polyadenylation specificity factor (CPSF) component of the cellular pre-mRNA
processing machinery, which may enhance the interferon antagonistic effects
(Mibayashi, M, et al., 2007).
CrmA from Cowpox virus and p35 from Autographa californica nuclear
polyhedrosis virus (AcNPV) have antiapoptotic potential (Chang and Yang 2000;
Shi 2002).
CrmA protein is a caspase as well as granzyme-B inhibitor and blocks apoptosis
induced by CTL, TNF or Fas (Ploegh 1998).

43. These anti-death mechanisms include
(i) modulation of the anti-apoptotic members of the Bcl-2 family, resulting in
inhibition of formation of ‘apoptosome’,
(ii) inactivation of the tumour suppressor p53, and
(iii) caspase inhibition
Host Pathogen Interaction During Apoptosis
Viruses have evolved proteins that are able to inhibit or delay the host protective
actions by targeting strategic points in the apoptotic pathways
(Granville et al 1998)
PKR is an important cellular target that viruses inactivate to prevent apoptosis

44. Avoiding Host Defenses by Bacteria
 Avoiding being killed by complement
proteins
› Gram-negative cells susceptible to MAC attack
 MAC has little effect on gram-positive cells
› Certain bacteria can circumvent killing by
complement (MAC)
 Termed serum resistant
 Bacterial cells hijack protective mechanism used by host
cells
 Inhibits formation of MAC

46.  Avoiding destruction by phagocytosis
 Preventing encounters with phagocytes
Some pathogens prevent phagocytosis by
avoiding phagocytic cells
 Some cells destroy complement components that attract
phagocytes through
• C5a peptidase – degrades component C5a
• Producing membrane-damaging toxins – kills
phagocytes by forming pores in membrane

48.  Avoiding destruction by
phagocytosis
 Mechanisms include
 Capsule
 Interfere with alternative pathway
of complement activation
• Bind host regulatory protein to
inactivate C3b
 M protein
 Binds complement regulatory
protein
• Inactivates C3b
 Fc receptors
 Foil opsonization
• Bind Fc region of antibodies
interferes with binding to
bacteria

49.  Surviving within the phagocyte
 Allows bacteria to hide from antibodies and control immune
response
 Mechanisms include
 Escape from phagosome
 Escapes before phagosome-lysosome fusion
• Allows bacteria to multiply in cytoplasm
 Preventing phagosome-lysosome fusion
 Avoids exposure to degradative enzymes of lysosome
 Surviving within phagolysosome
 Delay fusion to allow organism time to equip itself for growth
within phagosome

50.  Avoiding antibodies
 Mechanisms
 IgA protease
 Cleaves IgA antibodies
 Antigenic variation
 Alteration of surface antigens
• Allows bacteria to stay ahead of antibody production
 Mimicking host molecules
 Pathogens can cover themselves with molecules that resemble
normal host “self” molecules

51. The secreted serine/threonine protein kinagse G (PKnG) of Mycobacterium tuberculosis is
mediator of phagososme maturation inhibition. (Walburger, et al., 2004)
Lapaque et al. discussed that Brucella has an unconventional LPS that confers
resistance to antimicrobial attacks and modulates the host immune response.
Rieder et al. show that the VacA secreted protein of Helicobacter pylori, which
targets epithelial cells, can also target cells of the immune system and induce
immunosuppression.
MacMicking describes a recently identified family of GTPases that seem to control
the replication of vacuolar pathogens by regulating the trafficking of bacterial-
containing phagosomes. This is indeed an emerging field and it is expected that
more elements of innate immune control of bacterial pathogens

52. Significant progress has been made over the past years in the understanding of TLR function
[Kawai & Akira, 2011]
TLRs are essential receptors in host defense against pathogens by activating the innate
immune system, a prerequisite to the induction of adaptive immune responses.
TLR-mediated signaling is paramount in eradicating microbial infections and promoting
tissue repair, the regulation must be tight.
TLRs are implicated in a number of inflammatory and immune disorders and play a role in
cancer [Rakoff-Nahoum & Medzhitov , 2009]
Many single nucleotide polymorphisms have been identified in various TLR genes and are
associated with particular diseases. Several therapeutic agents targeting the TLRs are now
under pre-clinical and clinical evaluation [Hennessy et al., 2010]
TLR-Targeted Therapeutics

53. Therapeutic agents targeting the TLRs must be able to antagonize
the harmful effects resulting without affecting host defense
functions.
Innate immune system with drugs targeting TLRs, to prevent or treat
human inflammatory and autoimmune diseases as well as cancer,
appears to be promising
IRAK4-deficient human blood cells still displayed anti-viral
responses via production of IFNa/b and IFNc, and IRAK4-deficient
patients suffer from pyogenic infections, but are resistant to viruses,
fungi, and parasites, as well as many other bacteria
(Picard et al., 2003)
A new TLR7 agonists useful for treating a carcinoma (e.g. Bladder
cancer) or viral infection (Romagne and Tiollier, 2007).

54. We now know that innate immunity plays an important role in the initiation
of an immune response that follows the activation of antigen-specific acquired
immunity.
Although signaling pathways via TLRs are now being unveiled, there still
remain several unanswered questions. (Takeda and Akira, 2005)
A complete understanding of the mechanisms of innate immunity will be
helpful for the future development of innovative therapies for manipulation of
infectious diseases, cancer and allergies.
(Takeda and Akira, 2005)
Vaccine for diseases such as AIDS, Hepatitis C, Malaria and even cancers
might be made more effective by supplementing them with TLR activators
that stimulate dendritic cells, in turn to produce effective adaptive immune
response.
Future Prospects