THE DEVELOPMENT OF T LYMPHOCYTES

1
THE DEVELOPMENT OF T LYMPHOCYTES
2
DEVELOPMENT OF T LYMPHOCYTES

• Development of T and B cells have similarities
  and differences
• Similarities
• Derivation from stem cells in BM
• Antigen receptors produce by gene rearrangements
• Differences
• B cells rearrange receptor genes in BM while T
  cells rearrange receptor genes in thymus
• T cell receptors have MHC restriction

3
DEVELOPMENT OF T LYMPHOCYTES

• T cells originate from stem cells in bone marrow
  and migrate to thymus to mature
• Thymus
• Lymphoid organ in upper anterior thorax
• Primary lymphoid organ
• Fully developed before birth
• Increases in size until puberty then degenerates
• Degeneration has no discernible impact on T cell
  immunity

4
(No Transcript)
5
CELLULAR ORGANIZATION OF THE THYMUS

• Consists of two symmetrical lobes each enclosed
  by capsule from which trabeculae extend and
  divides each lobe into lobules
• Lobule consists of
• Cortex
• Immature thymocytes, cortical epithelial cells
  and macrophages
• Medulla
• Mature thymocytes, medullary epithelial cells,
  dendritic cells and macrophages

6
(No Transcript)
7
(No Transcript)
8
(No Transcript)
9
DEVELOPMENT OF T CELLS IN THYMUS

• Immature cells enter thymus at subcapsular region
  of outer cortex
• Express neither CD4 nor CD8
• Called double-negative thymocytes
• Double negative thymocytes initially express
• CD44 and CD25
• CD44 expression declines
• Rearrangement of genes for beta, gamma and delta
  chains

10
(No Transcript)
11
(No Transcript)
12
DEVELOPMENT OF T CELLS IN THYMUS

• Lineages of T cell receptors
• Alphabeta
• Gammadelta
• Thymocytes, start your rearrangements
• Productive rearrangement of gamma and delta genes
• Gammadelta receptor
• Productive rearrangement of beta gene
• Rearranged beta chain assembled with surrogate
  alpha chain (pT-alpha) to form pre T cell
  receptor
• Alphabeta receptor

13
(No Transcript)
14
GENE REARRANGEMENT IN ALPHABETA T CELLS

• Beta chain rearranges first with potential 80
  success rate
• Locus on homologous chromosome
• Two sets (beta 1 and 2) of D, J and C gene
  segments
• Beta chain combines with
• Surrogate alpha chain (pT-alpha), CD3 and zeta
• Pre T cell receptor
• Beta chain rearrangement stops and CD4 and CD8
  expressed
• Double-positive thymocytes
• Alpha chain rearranged and pairs with beta chain

15
(No Transcript)
16
(No Transcript)
17
(No Transcript)
18
POSITIVE AND NEGATIVE SELECTION OF DOUBLE
POSITIVE T-LYMPHOCYTES

• Alphabeta T cells are screened
• Positive selection
• Selection of immature T cells which recognize
  self MHC molecules
• Negative selection
• Elimination of immature T cells which are
  activated by self peptides
• Gammadelta T cells are not screened

19
POSITIVE SELECTION OF ALPHABETA T CELLS

• Takes place in cortex of thymus
• Mediated by self peptideself MHC molecules
  presented on surface of cortical epithelial cells
• Cortical epithelial cells express
• MHC class I and MHC class II molecules
• T cells which recognize self peptideself MHC
  continue maturation
• T cells which do not recognize self peptideself
  MHC commit apoptosis

20
POSITIVE SELECTION CONTROLS EXPRESSION OF CD4 AND
CD8 CO-RECEPTORS

• Double-positive thymocyte interacts through its
  alphabeta receptor with either
• MHC I or MHC II
• Interaction results in single-postive thymocyte
• Interaction with MHC I results in CD8 T cells
• Interaction with MHC II results in CD4 T cells
• Mechanism by which interaction selects for
  single- positive thymocytes is unknown

21
(No Transcript)
22
BARE LYMPHOCYTE SYNDROMES

• Immunodeficiency showing importance of MHC
  molecules in selection of co-receptors and T cell
  development
• Disease characterized by
• Severe immunodeficiency
• Lack of expression of either MHC I or MHC II by
  lymphocytes and thymic epithelial cells
• Persons who lack expression of MHC I
• CD8 (-) and CD4 ()
• Persons who lack expression of MHC II
• CD8 () and CD4 (-)

23
REMOVAL OF T CELLS SPECIFIC FOR SELF ANTIGENS IN
THYMUS BY NEGATIVE SELECTION

• T cells which strongly bind self peptideself MHC
  molecules are potentially autoreactive
• Negative selection mediated by
• Dendritic cells and macrophages at
  cortico-medullary junction of thymus
• Autoreactive T cells undergo apoptosis
• Mechanisms of positive and negative selection
  unknown
• Positive and negative selection results in highly
  personalized T cell immunity

24
(No Transcript)
25
DEVELOPMENT OF T CELLS AFTER LEAVING THYMUS

• Small percentage of alphabeta T cells survive
  positive and negative selection
• Mature naïve T cells recirculate between blood
  and secondary lymphoid tissues
• Mature T cells are longer liver than mature B
  cells
• Encounter with antigen
• T cell rich areas of secondary lymphoid tissues

26
DEVELOLPMENT OF T CELLS AFTER LEAVING THYMUS

• Following activation by antigen T cells
  differentiate into different effector cells
• CD8 to cytotoxic T cells
• CD4 to
• TH1
• TH2
• Healthy individuals
• Twice number of CD4 as CD8 cells
• Flow cytometry
• Identification and enumeration of T cells

27
(No Transcript)
28
REFERENCE RANGES FOR T LYMPHOCYTES IN ADULTS

• Absolute Percent
• CD3 782-2204 60-87
• CD4 443-1345 31-58
• CD8 171-914 13-40
• CD4/CD8 ratio gt 1.0

29
T LYMPHOCYTES AND MALIGNANT DISEASES

• T cell cancers primarily associated with
• Early and late stages of development
• Classification of malignant T-cells diseases
• Leukemia
• Malignant disease with origin in bone marrow
• Chronic lymphocytic leukemia (CLL)
• Lymphoma
• Malignant disease with origin in lymph nodes or
  lymphatic tissue
• Mycosis fungoides and Sezary syndrome

30
Figure 5-16
31
ACUTE LYMPHOBLASTIC (LYMPHOCYTIC) LEUKEMIA (ALL)

• Approximately 4,000 new cases each year in US
• Most common cancer of children
• Majority resemble immature B cells
• B-ALL or C (common)-ALL
• Minority resemble immature T cells
• T-ALL

32
ACUTE LYMPHOBLASTIC (LYMPHOCYTIC) LEUKEMIA (ALL)

• Causes
• Translocation of genes
• Philadelphia translocation involving 9 and 22
  (20)
• Acquired mutational event
• Effects
• Turns on oncogene
• Turns off tumor suppressor gene
• Results
• Large numbers of immature, abnormal lymphocytes
  produced in bone marrow
• Decrease production of RBC, WBC and platelets
  with resulting clinical manifestations

33
ADULT T-CELL LEUKEMIA / LYMPHOMA (ATLL)

• Cutaneous T-cell lymphoma (CTCL)
• CD4 origin
• Incubation period
• 20 to 40 years
• Clinical courses
• Acute (aggressive)
• Chronic (indolent)
• Etiology
• Human T-Cell Lymphotrophic Virus I (HTLV-I)

34
ADULT T-CELL LEUKEMIA / LYMPHOMA (ATLL)

• Human T-cell Lymphotrophic virus type 1 (HTLV-1)
• First human retrovirus identified
• Endemic in southern Japan, Caribbean and central
  Africa
• Uses CD25 to enter lymphocytes
• Virus is immunosuppressive and oncogenic
• Mechanism of immunosuppression
• Stimulation of CD4 TH1 cells
• Mechanism of oncogenicity
• Not well illucidated
• Tax protein central molecule

35
(No Transcript)
36
MYCOSIS FUNGOIDES (MF)

• Cutaneous T-cell lymphoma (CTCL)
• Most common (50)
• CD4 T cells
• Etiology is unknown
• Clinical course
• Indolent
• Cutaneous patches to plaques to tumors
  (mushroom-like) w/wo pruritis

37
(No Transcript)
38
MYCOSIS FUNGOIDES (MF)

• Laboratory diagnosis
• Histopathology of skin biopsy
• Hematoxylin and eosin stain
• Immunohistochemical stains
• Treatment depends on stage
• Topical steroids and nitrogen mustards
• PUVA photochemotherapy
• Psoralens
• UVA radiation

39
SEZARY SYNDROME (SS)

• Cutaneous T-cell lymphoma (CTCL)
• CD4 origin
• Considered terminal stage of MF
• Clinical manifestations and course
• Skin is bright red, scaly and very itchy
• Entire skin involved, lymph nodes, viscera and
  blood
• Aggressive course
• Death by OI secondary to immunosuppression

40
(No Transcript)
41
SEZARY SYNDROME (SS)

• Clinical pathology
• Sezary cells (gt 1,000/uL) in peripheral blood
• CD4/CD8 ratio of gt 10
• Loss of normal T cell antigens
• CD2, CD5, CD7
• Anatomic pathology
• Skin biopsy shows
• Cerebriform lymphocytes in epidermis and upper
  dermis with Pautriers microabscess formation

42
(No Transcript)
43
(No Transcript)
44
(No Transcript)
45
(No Transcript)
46
(No Transcript)
47
(No Transcript)
48
CASE STUDY

• 49 year WM presents with 3 month history of
• Lesions located on buttocks and hips
• Flat, pinkish-red patchy lesions
• Sometimes itchy
• Differential diagnosis of
• Eczema, psoriasis, contact dermatitis
• Treatment
• Topical and oral steroids

49
CASE STUDY

• Cutaneous rash and itch intermittent for 5 years
• Presents at age 54 with more extensive rash and
  itch
• Flat, pinkish-red patchy lesions (25)
• Dry, slightly raised scaling plaques which are
  very itchy (75)
• Specimens
• Punch biopsy of skin for histopathology
• Blood for CBC with diff and flow cytometry

50
CASE STUDY

• Histopathology
• Hematoxylin / Eosin stain
• Dense infiltrate of medium sized cerebriform
  lymphocytes in epidermis, upper dermis and
  perivascular. Pautriers microabscesses present.
• Immunohistochemical stains
• Strong reactivity with CD3 and CD4 T cell
  antigens
• CBC showed lt 1,000 Sezary cells/uL
• Flow cytometry showed CD4/CD8 ratio of 1.0

51
(No Transcript)