The document discusses insect immunity, highlighting the innate immune system's mechanisms and responses to pathogens, particularly focusing on physical barriers, humoral, and cell-mediated immunity. It elaborates on various components such as antimicrobial peptides, signaling pathways like the Toll and IMD pathways, and cellular responses like phagocytosis and melanization. The conclusion suggests that insects rely exclusively on their innate immune response, which adapts to microbial challenges, and emphasizes ongoing research into insect immunity through genetic studies.

1. Physiological
mechanisms in
regulating insect
immunity
SUBMITTED BY:
HEMLATA
Ph.D 2nd year
Dept of Entomology

2. OUTLINE
• What is insect immunity?
• Introduction
• Innate & Adaptive immune system
 Physical barrier (The integument and the Peritrophic membrane)
 Humoral mediated immunity Cell mediated immunity
-Production of Antimicrobial peptides -Nodulation
-Signaling pathways activating genes -Encapsulation
that encode antimicrobial peptides -Phagocytosis
-Melanization
-Example
-The toll pathway
-The IMD pathway
- The JAK-STAT pathway
• Conclusion

3. WHAT IS INSECT IMMUNITY?
•Immunity(derived from Latin term immunis, meaning
exempt),
•Immunity refers to reactions by an animal body to foreign
substances such as microbes and various macro molecules.
( Abbas et al.,1991)
•Immune system- A collection of cells and molecules that
protect the body against infection, malignancy and damaged
cells. ( Abbas et al., 1991)

4. Pathogen attack on insect

5. Bacterial infection in Galleria mellonella

6. INTRODUCTION
• Multicellular organisms are
exposed to different harmful
microorganisms.
• All organisms are equipped with
different systemic potential
protective measures against
pathogens.
• They possess a temporal set of
metabolic pathways.

7. Innate & Adaptive immune system
• Immunity can be categorised into two different types:
1. Innate immunity - Innate immunity is the first line of defence against
infection which is the less specific or non-specific one and eliminates
infection within hours after introduction into body.
2. Adaptive immunity - Adaptive immunity is the second line of defence,
which has more specificity and plays role few days after the initial
infection.
• Insects lack an adaptive immune system, they do have a powerful innate
immune system for fighting infections.

8. 1. Physical barriers
2. Humoral responses
3. Cellular responses (Kanost et.al 2004)
The innate immune system of insects
consists

9. Components of Innate immunity

10. PHYSICAL BARRIER
The Integument and the Peritrophic membrane
• The peritrophic membrane is a layer made of chitin and glycoprotein that
covers the insect midgut and functions as a physical barrier against abrasive
food particles and digestive pathogens (Hegedus et.al 2009).
• This membrane is semipermeable and therefore it is not an efficient barrier
for viruses.
• These structures constitute the initial protection for the hemocele (the insect
body cavity) and the midgut epithelium against microorganisms.
• Integument, the outer surface of an insect, is formed by a single layer of
cells covered by a multilayered cuticle (Ashida and Brey, 1995).

11. Physical barrier

12. Humoral & Cell mediated immunity
• Immune responses can be divided into:
 Humoral mediated immunity -The immunity that is mediated
by macromolecules found in extracellular fluids such as
secreted antibodies, complement proteins, and certain antimicrobial
peptides.
 Cell mediated immunity -Cellular immunity is a
protective immune process that involves the activation of phagocytes,
antigen-sensitized cytotoxic T cells and the release of cytokines and
chemokines in response to antigen.

14. Production of antimicrobial peptides
• Antimicrobial peptides (AMPs) are crucial effectors of the innate immune system.
• They are produced as an early defence mechanism by multicellular organisms.
• AMPs are synthesized in insect fat body and or in some blood cells, and secreted
into haemolymph.
• Activation of humoral immune system to produce AMPs occurs where recognition
of pathogen specific patterns takes place by receptors of some signaling pathways.
• Antimicrobial peptides have mainly antifungal and antibacterial properties .
Humoral mediated immunity

15. ANTIMICROBIAL
PEPETIDES
ACTIVITY INSECTS
Defensins antibacterial and antifungal activity Lepidopteran species
Cecropins antimicrobial activity Hyalophora cecropia,
Bombyx mori ( 13 cecropin)
Moricins activity
against bacterial as well as against yeast and
filamentous fungi
B. Mori (9 moricin),
G. Mellonella (8 moricin)
Drosocin antimicrobial activity D. melanogaster
Attacins inhibit outer‐membrane protein synthesis of
bacteria
H. cecropia
Gloverins and lebocins inhibit bacterial growth by blocking
outer‐membrane protein synthesis
lepidoptera
Diptericin disrupting the cytoplasmic membrane of growing
bacteria
D. melanogaster
Drosomycin exhibits a narrow antimicrobial spectrum and is
only active against some filamentous fungi
D. melanogaster
Metchnikowin active
against both Gram‐positive bacteria and fungi
Drosophila
Different antimicrobial peptides and their activity seen in different

16. Signaling pathways activating genes
that encode antimicrobial peptides
• The Toll pathway
• The Imd pathway
• The JAK‐STAT pathway

17. THE TOLL PATHWAY
• The Toll pathway in insects is mainly responsible
for:
The recognition of fungi and Gram-positive
bacteria
The induction of certain AMPs that are secreted
into the insect hemolymph (Lemaitre and
Hoffmann, 2007; Tsakas and Marmaras, 2010).

18. The Toll pathway

19. THE IMD PATHWAY
• The Imd pathway is a broadly-conserved NF-κB immune signalling
pathway of insects and some arthropods (Palmer et.al, 2015) that
regulates a potent antibacterial defence response.
• The Imd pathway was first discovered in 1995 using Drosophila
fruit flies by Bruno Lemaitre and colleagues, who also later
discovered that the Drosophila Toll gene regulated defence against
Gram-positive bacteria and fungi (Hoffmann et al,1995).

20. THE IMD PATHWAY

21. The Toll and Imd pathway in
Drosophilla

22. THE JAK (Janus‐Kinase) STAT
PATHWAY
• The JAK/STAT pathway is activated by infection or septic
injury, which are detected by a variety of immunological
effectors in insects and by the release of cytokines in mammals
(Agaisse and Perrimon, 2004).
• JAK/STAT-dependent immune genes should display an
inducible expression upon immune challenge as well as be
constitutively expressed in flies carrying a gain-of-function
mutation in the JAK/STAT pathway (Ekengren et.al 2001).

23. THE JAK-STAT PATHWAY

24. Cellular mediated immunity
• Cellular immune responses are immediately after
an invasion of the hemocele. Cellular immune
responses include
1. Nodulation
2. Encapsulation
3. Phagocytosis (Browne,2013)
4. Melanization

26. Nodulation
• Nodule formation is a prompt response to clear microorganisms
from the haemocoel (Arai et al, 2013).
• In nodule formation, pathogens are at first recognised by specific
molecules (such as PGRPs, LPs) and haemocytes aggregate
surround the pathogen.
• The haemocytes release a flocculent material around foreign
substances and these aggregated substances entrap foreign material.
• Begin to melanise in that region within 30 minutes. After
melanisation, the entire substance act like foreign body and is
removed from the body by encapsulation process.

27. Nodulation

28. Encapsulation
• The process in which haemocytes aggregate around the large foreign bodies
and initially form a consolidated capsule.
• This capsule is composed of two or more layers to block the gaseous
exchange.
• Complete encapsulation leads to death of foreign bodies due to lack of
oxygen large metazoan parasite.
• It is a major defensive strategy adapted by hymenopteran eggs, larvae etc.
• This process is ineffective in case of tachinid larvae because they possess
respiratory funnel that remains directly connected with host’s respiratory
system.

29. Encapsulation

30. Phagocytosis
• Phagocytosis is a process in which virus, bacteria and different
microorganisms engulfed or phagocytized by specific cells in
order to protect the body from infection.
• The phagocytosis is carried out by engulfing foreign bodies
with pseudopodia or by making close contact with their plasma
membrane.
• Phagocytosis is a simple defensive mechanism in insects
against foreign particles and also an effective means of
removal of autolyzed tissues.

31. Phagocytosis

32. Melanization
• Melanization is the process of melanin formation.
• It is activated during wound healing and also in nodule and capsule
formation against large pathogens or parasites in several
insects(Vlisidou and wood, 2015).
• The enzyme phenoloxidase (PO) is a key in this process. Then
active PO binds to foreign surfaces including hemocyte membrane
(Ling and Yu,2005), where it initiates melanin formation.

33. Example
• The Drosophila hemocyte population displays three major cell types:
plasmatocytes, lamellocytes, and crystal cells (15, 16).
• The plasmatocytes are devoted to removing dead cells (during
embryogenesis and metamorphosis) and bacteria (during infection).
• The function of lamellocytes is to encapsulate dead tissues during
metamorphosis and foreign bodies that are too large to be phagocytosed,
such as parasite eggs.
• The crystal cells contain the precursors to the prophenoloxidase cascade
involved in the melanization process that is activated during wound healing
or encapsulation

35. CONCLUSION
• Another fascinating aspect of insect defense mechanisms against
infections is the current view that insects depend only on its innate
immune response to fight invading microorganisms.
•Microbial challenges generate this specific primed response, the
fungus Beauveria bassiana, a natural fly pathogen, can also induce
specific protection against a second exposure to the fungus
(Pham,2007). These results indicate that insect immune responses
can indeed adapt and suggest that insect hemocytes may also present
an activation response similar to the one known in mammalian
leukocytes.

36. • The insect innate immunity comes from studies of
Drosophila, where genetics analysis has been instrumental
in elucidating the antimicrobial peptide response, as well as
to open the door for the study of Toll‐like receptors, which
are essential for the innate immune response of mammals.
• Similarly, future genetic screens will help identifying novel
host antiviral genes and also the receptor molecules that
sense viral infection. Yet, it is important to keep in mind
that insect‐pathogen interactions have coevolved.