Lecture 2 Fungal immunity

1. Lecture 2 Fungal immunity By Prof Dr. M. Refai

3. Local defencemechanisms against mucosal infection

4. Salivary proteins such as lactoferrin, beta-defensins, histatins, lysozyme, transferrin, lactoperoxidase, mucins, and secretory immunoglobulin A. These impair adhesion and growth of fungi in the oropharyngeal cavity. Healthy oral epithelial cells Theseinhibit blastoconidia and/or hyphal growth of fungi

5. if • The fungus succeeds in bridging any of the body surfaces either as a result of • burns, trauma or maceration or • surgical interference or • application of catheter etc., • The fungus will face • non-specific defenses • humoral defenses • cellular host defenses.

7. In general, man and animals are resistant to fungus infection • The intact skin and mucous membranes are not easily invaded by fungi.

8. None specific defence • Secretory Ig A + Complement Opsonization • Neutrophils, Monocytes • Alveolar macrophages • Phagocytosis

9. Phagocytosis

10. Immune cells • Neutrophils • Macrophages • Dendritic cells, • Natural killer cells • T cells • Epithelial and endothelial cells

11. Specific pathogen-associated molecular patterns (PAMPs) pattern-recognition receptor (PRR) immune cells. Pattern recognition of fungi immune cells

12. Signaling pathways are activated to release Transicription factor to regulate expression of Inflammatory cytokines

13. Strategies employed by fungal pathogens to evade host defense mechanisms • 1. Shielding of stimulatory PAMPs, • 2. Modulation of inflammatory signals, • 3. Shedding of decoy components, • 4. Persistence in intracellular environments, • 5. Complement evasion.

14. surface proinflammatory b-(1,3)-glucans or non-stimulatory a-(1,3)-glucan cell wall Shielding of stimulatory PAMPs detection of the invading pathogen

15. C. albicans & dectin-1 • dectin-1 recognizes the ß-glucans at the level of budding scars in the C. albicans, but • it cannot recognize the ß-glucans in the hyphae, where they are shielded by a layer of mannans

16. Histoplasma capsulatum the α-(1,3)-glucan present in its outer layer cell wall contributes to pathogenesis by shielding its immunostimulatoryβ-glucans.

17. Paracoccidioides brasiliensis • Paracoccidioidesbrasiliensis transforms into its pathogenic yeast form, a change in its cell wall glucan polymer linkage occurs from β-(1,3) to α-(1,3)-glucan.

18. Cryptococcus neoformans • Cryptococcus neoformans masks its surface PAMPs through the production of an extracellular capsule of glucuronoxylomannan (GXM)

19. Coccidioides immitis • Coccidioides is recognized by the host by its immunodominant spherule outer wall glycoprotein (SOWgp). • During endospore differentiation, the fungus secretes a metalloproteinase (Mep1) which digests SOWgp • This enables the fungus to evade phagocytosis and killing at its vulnerable endospore stage of development

20. Strategies employed by fungal pathogens to evade host defense mechanisms • 1. Shielding of stimulatory PAMPs, • 2. Modulation of inflammatory signals, • 3. Shedding of decoy components, • 4. Persistence in intracellular environments, • 5. Complement evasion.

21. Modulation of inflammatory signals cytokine production humoral response Preferential activation of TLR2 over TLR4 by certain fungal morphotypic PAMP stimulates a Th2 humoral response over a Th1-dependent antifungal proinflammatory cytokine production.

22. C. albicans • C. albicans induce immunosuppression through TLR2-mediated IL-10 release, • this leads to generation of CD4+CD25+ T-regulatory cells with immunosuppressive potential

23. Aspergillus fumigatus • A. fumigatus evades immune recognition by germination into hyphae with subsequent loss of TLR-4 recognition, • The TLR2-mediated IL-10 pathways remain intact, thus shifting the balance towards a permissive Th2-type profile

24. Strategies employed by fungal pathogens to evade host defense mechanisms • 1. Shielding of stimulatory PAMPs, • 2. Modulation of inflammatory signals, • 3. Shedding of decoy components, • 4. Persistence in intracellular environments, • 5. Complement evasion.

25. Shedding of decoy components Shedding of surface protein gpA by the fungus as decoy to competitively inhibit mannose receptor (MR).

26. Strategies employed by fungal pathogens to evade host defense mechanisms • 1. Shielding of stimulatory PAMPs, • 2. Modulation of inflammatory signals, • 3. Shedding of decoy components, • 4. Persistence in intracellular environments, • 5. Complement evasion.

27. Persistence in intracellular environments Upon internalization by immune or non-phagocytic cells, Candida or Aspergillus can resist intracellular killing, germinate and eventually escape

28. Aspergillus fumigatus Alveolar macrophages • The first defensive cells that inhaled conidia • The conidia are internalized by the macrophages and prevented from growth for several hours until the macrophage begins to destroy them. • At 24 h after internalization, 90% of the conidia are killed. • The conidia then germinate to hyphae • The hyphae are too large to be engulfed

29. Aspergillus fumigatusneutrophils • Resting conidia are • relatively resistant to killing by either reactive oxygen intermediates or neutrophil cationic peptides and • their ingestion triggers neutrophil degranulation and the respiratory burst only weakly. • Resting conidia • activate the alternative pathway • induce neutrophil chemotaxis

30. Aspergillus fumigatus Polymorphonuclear neutrophils • bind the surface without the need for complement or immunoglobulin • This binding triggers secretion of reactive oxidative intermediary agents that rapidly damage the hyphae; • 50% of hyphae are destroyed in 2 h

31. Histoplasma capsulatum Macrophages • represent the first line of defense during infection withH. capsulatum, • they rapidly phagocytose the inhaled conidia and transforming yeast cells, • the infected macrophage subsequently activate effector T cells and enhance the release of Th1-associated proinflammatory cytokines (IL-12, IFN-γ, and TNF-α(

32. Histoplasma capsulatum • the main cytokines involved in Histoplasma clearance from the host are IL-12, IFN-γ, and TNF-α • IL-12 through its ability to regulate IFN-γ production is critical in inducing a protective immune response in primary infection with the pathogen. • IFN-γ is pivotal for the host's innate resistance to systemic infection with H. capsulatum.

33. Strategies employed by fungal pathogens to evade host defense mechanisms • 1. Shielding of stimulatory PAMPs, • 2. Modulation of inflammatory signals, • 3. Shedding of decoy components, • 4. Persistence in intracellular environments, • 5. Complement evasion.

34. Complement evasion Binding of complement-inhibitory C4 binding protein (C4BP)and Factor H on fungal surface.

35. Immune response to fungi: • Innate immunity II • Humoral immunity HI • Cell-mediated immunity CMI

36. Innate immunity • Physical barriers in the form of: • Skin, • Mucous membranes, • Cell membranes, • Cellular receptors • Humoral factors.

37. HI The main recognized functions of antibodies in fungal infections include • prevention of adherence, • toxin neutralization, • antibody opsonization and • antibody-dependent cellular cytotoxicity

38. CMI • The type of CMI induced is critical in determining resistance or susceptibility to fungal infection. • In general, • Th1-type CMI is required for clearance of a fungal infection, • Th2 immunity usually results in susceptibility to infection or allergic responses

39. CMI • Th1 cells produce predominantly cytokines such as IFN-g, and promote cell-mediated immunity and phagocyte activation. • Th2 cells produce predominantly cytokines such as interleukins 3 and 4 (IL-3 and IL-4) and tend to promote antibody production

40. Cell mediated immunity • Resistance to several mycoses is associated however with cell mediated immunity, particularly phagocytosis and killing of invading fungi by • peripheral blood monocytes, • alveolar and peritoneal macrophages • polymorphonuclear leukocytes.

41. i. Alveolar macrophage defense • The first phagocytic cells to encounter the inhaled or aspirated fungal particles • have the capacity to kill Aspergillus conidia by both oxidative and non-oxidative mechanisms. • have relatively weak oxidative candidicidal activity • fungi characterized by hyphal forms, are too large to be ingested completely by phagocytic cells.

42. the alveolar macrophages Histoplasmacapsulatum: • Macrophages are the primary host cells • The yeast form survive within macrophages and multiplies intracellularly. • Thus the macrophage, which usually eliminates organisms from the primary site of infection, becomes a site of continued fungal growth and aids in dissemination of the yeast to other tissues.

43. ii. Peritoneal macrophages • can phagocytize, inhibit intracellular growth or inhibit germination of conidia of Aspergillus or Candida. • Activated macrophages appear to play a role in resistance against disseminated candidosis.

44. iii. Normal peripheral blood monocytes • Monocytes can damage and apparently kill Aspergillus hyphae, even though hyphae are too large to be ingested completely. • Monocytes and macrophages have been shown to be important cells in host defense against dimorphic fungi. When activated they have increased phagocytosis and decrease growth of intracellular yeasts.

45. iv. Polymorphnuclear leukocytes (PMN) • PMN play a major role in host defense against fungi. • PMN can kill Candida blastoconidia, Coccidioidesimmitisarthroconidia and Blastomyces conidia. • can also damage the hyphal forms of Aspergillus and Candida.

46. The important role of the PMN in the host defense against fungus infection becomes obvious after the fungus escapes control by macrophage and monocyte cell lines and start mycelial growth, where an impressive ability of PMN to damage these organism is evident

47. Application of immune response • Vaccination • Serodiagnosis

48. Important points to consider in antifungal immunityand its relevance to vaccination • usually fungi display only moderate virulence • antifungal immune responses are usually redundant. • almost all pathogenic fungi have mechanisms to evade or intoxicate immune responses

49. Most fungal diseases pose obstacles to the concept and practice of vaccination • coccidiomycosis, histoplasmosis, blastomycosis, and paracoccidioidomycosis • are geographically limited, and • low-incidence deep-seated diseases • widespread illnesses e.g. aspergillosis, cryptococcosis, and candidiasis • typically occur in the immunocompromised • theoretically—ineligible for active immunisation

50. Antibodies used for passive immunity