dendritic cells are part of innate immune system, antigen presenting cells in skin, activation of t cells and inducing and maintaining immune tolerance, 4 types- langerhans cells, dermal dendritic cells, merkel cells, melanocytes

1. JAI KUMAR PILLAI
KIMS, BANGALORE
DENDRITIC CELLS
1

2.  Innate immunity-Immune mechanisms that are
used by the host to immediately defend itself
 Comprises of bariers, complements, antimicrobial
peptides, cytokines, macrophages, DCs , NK cells,
PMNs
 Dendritic cells (DCs) are component of innate
immune system. Their main function is to process
antigen material and present it on the surface to T-
cells, thus functioning as antigen-presenting cells.
2

3. 3

4. Location
 Dendritic cells -skin ( Langerhans cells,dermal
dendritic cells)
 inner lining of the nose, lungs, stomach and
intestines.
 They can also be found in an immature state in
the blood.
4

5. FUNCTIONS OF DENDRITIC
CELLS
 Antigen presentation and activation of T cells.
 Inducing and maintaining immune tolerance.
 Maintain immune memory in tandem with B cells.
5

6. TYPES OF DENDRITIC CELLS
 Langerhans cells
 Dermal dendritic cells
 Melanocytes
 Merkel cells
6

7. LANGERHANS CELLS
 First described by Paul langerhans
in 1868
 They are bone marrow derived , dendritic ,antigen
presenting cells situated in middle of epidermis
 They are present in
epidermis,dermis,thymus,tonsils,lymph nodes,
epithelia of oral and genital mucous membranes.
7

8. Embryology
 They appear by fourteenth week of fetal life
 Derived from mesenchymal precursors in bone
marrow
• until the 12th wk, cd1a- & lack birbecks granules
not present
8

9.  They are distributed in basal ,spinous and
granular cell layers,
 They constitute 2% to 8% of total epidermal cell
population.
 They vary in distribution according to their
anatomical sites, their number ranging between
460 and 1000/sqmm of epidermis.
9

10.  Within the epidermis LCs are anchored to
surrounding keratinocytes by E-cadherin mediated
homotypic adhesions
 Dermal CD14/Cd11c have the potential to
differentiate into LCs under TGF-B1
10

11.  Plasmacytoid dendritic cells- In conditions SLE,
virus infectn, psoriasis, allergic CD. Non-indigenous
DCs precursors characterized by highly dev
ER(plasma cell like appearance) enters the skin
 Produce natural IFNs in respone to TLR ligand
 Hence named principal type IFN producing cells
11

12.  In conditions like Atopic dermatitis, contact eczema
another non indigenous DC precursors originate
from myeloid precursors
 Inflammatory dendritic epidermal cells
 Characterized by expression of CD1a, CD1b, CD23
& CD36
 Immune response triggered by these is in TH112

13. Danger signals
TLR ligands, chemical haptens, hypoxia
 LCs enlarges,downregulation of Fc receptors & E-
cadherin, increased MHC 2
 Migrate downward into dermis-enter afferent
lymphatics- reach T cell zones of lymph node
 TNF-a & IL-1B stimulates this process
 To penetrate BM, LCs attach to it via a6 containing
integrin receptors producing type 4
collagenase(MMP9)
 Osteopontin
13

14. 14

15. Histology
 Light microscopy : pale staining & convoluted nuclei
15

16. On EM: lobulated nucleus
• Absence of tonofilaments, desmosomes
• Flat plate like structure with hemispherical blebs at
one end (birbecks granule)
• C/S app: tennis racquet shape.
`
 arise from infolding of plasma membrane function as
phagosomes that digest extracellular material
16

17. 17

18.  Identified by special techniques
like impregnation by gold
chloride ,staining by ATPase
and alpha-D-mannose.
Immunohistochemical Markers
 ATPase +ve , DOPA negative
 S100, CD1a +ve
 HLA-DR,DP,DQ antigen +ve
 Recently identified
Langerin(CD207)-single best
marker
18

19. Other importance
 Membrane receptors for C3b & Fc portion of IgG
 Recognition of antigens& cell mediated immunologic
reactions as allergic CD
 Allograft rejection(1a antigens)
 Immune surveillance against UV neoplasia(UV
decreases no. of langerhans cells)
19

20. FUNCTIONS
 cutaneous immune mechanism,especially in
antigen presentation ,
 stimulation of T cell response
 phagocytosis
 regulation of epidermal differentiation
20

21. APPLIED ASPECTS
Implicated in various immune processes
 allergic contact dermatitis,
 HIV,
 allograft rejection,
 immune tolerance and
 surveillance against neoplasia.
 Psoriasis, sarcoidosis, CD their no. in skin is reduced
 Because of their antigen presenting ability they have
become prospective vehicle for tumor therapy and
tumor vaccines
21

22. Dermal Dendritic Cells
 Bone marrow derived leukocytes having migratory and
AP property
 Markers include CD1b, CD1c, subunitA of clotting factor
X111
 Differentiated from macrophages
• expression of MHC 2
• CD205
• absence of phagolysosome
 DDC are located in the vicinity of superficial plexus
 DDC proliferate constitutively in human dermis
22

23. 23

24. MELANOCYTES
 Dendritic cells that synthesize and secrete
melanin contaning organelles called
melanosomes.
 Appearance of melanocytes in epidermis takes
place in craniocaudal direction
 in accordance with the development of neural
crest from which they are derived.
24

25. EMBRYOLOGY
 Neural crest, they migrate to epidermis, various
mucous epithelia, hair follicle, dermis,
leptomeninges inner ear, uveal tissue
 In retina originate from optic cup of primitive
forebrain
 Primitive melanocytes in skin are first found
during eighth week of fetal life
 At 5mon melanosomes begin to transfer pigment
to keratinocytes
25

26. 26

27. `
 They express integrin receptors and use them to
migrate to epidermis during development
 Melanoblast migration and differentiation into
melanocytes
• Wnt , ET3, BMPs
• Steel factor, c-kit , HGF
27

28. MITF affects melanoblast differentiation by
inducing tyrosinase, TRP1& dopachrome
tautomerase(TRP2)
28

29.  BMPs- disulfide linked dimeric proteins
 belongs to TGF-B family
 Signaling suppresses neural crest differentiation
into melanoblast
 Functions as Wnt antagonists
29

30.  Endothelins bind & activate transmembrane G
protein linked receptors EdnrA , B
 Ednr B receptors imp for melanoblast migration
and proliferation
 Defect causes waardenburg synd &
Hirschsprung synd
30

31.  SF-early melanoblast development requires
cytokine SF and its TK transmembrane receptor
c-kit
Mutn of c-kit/Sf causes Piebaldism
 HGF is ligand for transmembrane TK receptor
met, also essential for melanoblast proliferation &
differentiation
31

32. Types
 Dendritic :- these are capable of pigment transfer to
other cells
 Non-dendritic :- they retain the melanosomes
synthesized
Eg: uvea, retina , leptomeninges
32

33. Site specific melanocytes
1]Cutaneous Melanocytes
• Largest no. of melanocytes
• Skin & hair follicle
• Part of “epidermal melanin
unit”
2] Melanocyte stem cells
• present in hair follicle bulge
• Express TRP2 & nestin
• Other transcription factr-
SOX10 & Pax5
33

34. 3]Ocular melanocyte
uveal tract; they do not transfer their
melanosomes
required for proper routing of I/L & C/L neural
fibres in optic chiasm
Imp: visual abn in pts with albinism
34

35.  4]Otic Melannocyte
• reside in cochlea & imp for hearing
• Maintainence of endolymph through regulation of
K transport
• Imp: Waardenburg synd
 5]CEPHALIC Melanocyte
distributed through out the meninges & more
dense in leptomeninges above pons & medulla
oblongata.
35

36. Epidermal melanin unit
 Assn between dendritic melanocytes and keratinocytes
within epidermis
 One melanocyte is in contact with 36 basal and
suprabasal keratinocyte
 Ratio of melanocyte to keratinocyte in basal layer of
epidermis varies frm 1:4 to 1:10
36

37. 37

38. DISTRIBUTION
 Melanocytes are more abundant in skin of face
and male genitalia upto 2900/mm2 than on trunk
upto 1250/mm2
 As a rule greater amount of melanin is present in
basal keratinocytes than in melanocytes
 basal cells at tip of rete ridges are more38

39. MELANIN SYNTHESIS AND
MELANOSOMES
 Melanin is produced in melanosomes(pigment
granules or organelles)
 A key protein involved in melanosome assembly is
NCKX5 encoded by SLC24A5.
 The major hormone controlling melanin synthesis is
MSH from the pitutary gland.
39

40. 40

41. Melanin Synthesis
 Synthesis of melanin starts with the conversion
tyrosine to dihydroxyphenyalanine(dopa) by
tyrosinase, a copper containing, aerobic enzyme
 Critical, rate limitting step {Raper-mason pathway}
 Then dopa is oxidized to dopaquinone by tyrosinase
41

42. 42

43.  These reactions occurs in melanosomes
 then passed on to surrounding keratinocytes
 undergo series of developmental and biochemical
stages;
STAGES OF MELANIZATION
 Stage1:melanosomes are spherical, membrane bound
vesicles 0.3um composed of longitudinally oriented
concentric lamellae.
No melanin.
43

44.  Stage2: melanosomes are ellipsoidal 0.5um
Melanin deposited within cross-linked long filaments .
 Stage 3: melanin deposition increases by enzymatic
and non-enzymatic polymerizatn
 Stage 4: melanosomes are fully developed and electron
opaque because of dense deposits of melanin.
mainly by polymerization
44

45. 45

46. 46

47.  Between these electron dense melanized cores and
their outer membranes mature melanosomes house
distinct vesicles 40nm in diameter called
vesiculoglobular bodies.
 Inv in internal organization & melanization of eu- and
pheomelanosomes
 During progression from stage 1 to stage 4 tyrosinase
decreases and acid phosphatase increases( helping in
degradation)
47

48. FACTORS INFLUENCING MELANIN
SYNTHESIS
 UV LIGHT : Increases melanocytes
 HORMONES:MSH,ACTH, oestrogen,
progestrone,Androgens,lipotropins,thyroxine
all have melanocyte stimulating property
 OTHERS:- pgd2,pge2,arachidonic acid,oleic acid
 AGEING : Causes decrease in follicular
melanocytes
48

49. TYPES OF MELANIN
 Eumelanin
• insoluble
• produced in
eumelanosomes
• Large,elliptical
• Highly structured
fibrillar glycoprotein
matrix
• black and brown
colour of skin and
hair
 Pheomelanin
• Sulfur containing,
soluble
• Pheomelanosomes
• Smaller, spherical
• Disorganised and
loose glycoprotein
matrix
• lighter colour of hair
49

50. Melanosome transport
 Once melanosomes are formed, melanized and reach
tips of dendrites,
 they are transported from melanocytes to
keratinocytes by apocopation
 Keratinocytes phagocytize the melanosome laden tips
of melanocytic dendrites
 In the epidermis melanosomes become concentrated
in a umbrella like array above nuclei of keratinocytes
on the side towards skin surface i.e. on the sunny side
of nuclei
50

51. 51

52.  Following transfer to keratinocytes
 fully melanized melanosomes are conveyed
upwards as basal keratinoctes mature
 are eventually degraded by lysosomal enzymes
and
 shed as cornified cells are desquamated.
52

53. COLOUR OF SKIN
 Absolute number of melanocytes in human skin
is same for both sexes and all races.
 Differences in color among the races result
from differences in the
1.number
2.size
3.degree of melanization
4.distribution
5.rate of degradation of melanosomes
within keratinocytes
53

54. NORMAL MICROANATOMY
 On staining with
H&E they contain
round to oval dark
stained nuclei and
clear halo of
surrounding
cytoplasm
54

55.  On electron microscopy,these cells characteristically
‘hang down’ from basal cell layer and are devoid of
tonofilaments or desmosomes.
 abundant melanosomes in varying stages of
melanisation.
 numerous mitochondria , well developed RER Golgi55

56. 56

57.  Melanin can be bleached by strong oxidizing
agent such as H2O2 or KMNO4
DOPA reaction- unfixed tissue of enzymatically
seperated epidermal sheets are incubated in
0.01% soln of 3-4 dihydroxyphenylalanine this
stains functional active melanocyte as dark brown
or black.
57

58. 58

59. IMMUNOHISTOCHEMICAL
TECHNIQUE
ANTIGEN ANTIBODY POSITIVE
CELLS
ANTIGEN
LOCATION
S100 PROTEIN Anti-S100
PROTEIN
Melanocytes,lan
gerhans
cells,schwann
cells,adipocytes,
myoepithelial
cells of sweat
glands
Nuclear and
cytoplasmic
gp100 HMB-45 Fetal
melanocytes,acti
vate adult
melanocytes
Cytoplasmic
Melan-A/MART-1 A103/M2-7C10 Melanocyte Cytolplasmic
Tyrosinase T311 Melanocytes Cytoplasmic
PNL2 ANTIGEN PNL2
ANTIBODY
Melanocytes Cytoplasmis
59

60. FUNCTIONS OF MELANIN
 Absorption of UV (phototoxic and photosensitize)
 Thermoregulation
 Free radical quencher
 Protection against lipid peroxidation
 Photoprotection, photoageing
 Protection from photocarcinogenesis,
 Regulation of vit D synthesis.
 Camouflage and sexual appeal.
60

61. APPLIED ASPECTS
 In vitiligo melanocytes are destroyed.
 In albinism melanocytes are normal in number but
unable to synthesize fully pigmented
melanosomes because of defective enzymatic
formation of melanin.
 Freckles result from increase in production of
pigment by normal number of melanocytes.
 Nevi are benign proliferations of melanocytes
 Malignant counterpart - melanoma
61

62. MERKEL CELLS
 In 1875,FRIEDRICH MERKEL identified at
base of rete ridges cells that were in
contact with nerve fibrils and named them
tastzellen or touch cells.
 Slow adapting type1 mechanoreceptors in
sites of high tactile sensitivity
62

63. Embryology
 Appear in fetal skin by sixteenth week of
gestation.
 They originate in the epidermis itself, presumably
from germinative keratocytes
63

64.  Found in basal layer of epidermis
 In hairy skin & glabrous skin of digits, lips, regions
of oral cavity , outer root sheath of hair follicle
 They are arranged in groups at tips of rete ridges.
64

65. 65

66. STRUCTURE
 Electron lucent cytoplasm rich in organelles
 Lobulated nuclei
 Margins of cells project cytoplasmic “spines” towards
keratinocytes
 Poorly developed desmosomes,delicate cytoplasmic
microfilaments.
 Characteristic spherical granules which are
membrane limited with a dense central core
66

67. 67

68.  Merkel cells are supplied by myelinated nerves
 as they near epidermis lose their myelin sheaths and
continue as unmelinated axons
 surrounded by cytoplasm and basement membranes
of schwann cells.
 The nerve fibres terminate in flat,meniscus like
contacts that are studded along basal aspects of
merkel cells.
68

69. 69

70.  On silver impregnated sections, the meniscoid
nerve terminal that covers basal portion of each
merkel cell can be seen as merkel disk.
 Immunohistochemical markers :-
 K8, K18, K19 & K20
 K20 is highly specific for merkel cells.
70

71.  Merkel cells express neuroendocrine markers
such as chromogranin A and synaptophysin.
 Neurosecretory substances in particular
neuropeptides that are stored in densecore
granules include VIP , CGRP, Serotonin,
substanceP.
71

72. FUNCTIONS
 They are slowly adapting , low threshold type 1
mechanoreceptors.
 They may enhance or induce the excitability of
sensory nerve endings via release of
neuropeptides.
72

73. APPLIED ASPECTS
 Merkel cell hyperplasia with keratinocyte
hyperproliferation is seen in adnexal tumors such
as naevus sebaceus, tricoblastomas,
trichoepitheliomas, nodular hidradenomas.
 Merkel cell hyperplasia with hyperplasia of nerve
endings neurofibromas
neurilemmomas,
nodular prurigo
neurodermatitis
73

74. THANK YOU
74