Download Adaptive Immunity: Activation of naive T cells

ADAPTIVE IMMUNITY: ACTIVATION OF NAÏVE T CELLS
David Straus, Ph.D.
E-mail: [email protected]
Objectives
1. Understand migration of naïve T cells and mechanism of lymph node entry
2. Understand how antigen arrives in lymph nodes and how antigen presenting cell (APC) function is
regulated to enhance activation of naïve T cells.
3. Understand how intercellular interaction between a T cell and APC is initiated and stabilized.
4. Understand how intracellular signals are transmitted from the T cell antigen receptor and result in
changes in gene expression
5. Understand how the outcome of T cell activation is regulated by co-stimulation, antigen density and
cytokines.
References: Parham, P. Chapter 6 p. 145 – 162
Abbreviations:
High endothelial venules: HEV
T cell antigen receptor: TCR
Antigen-presenting cell: APC
Leukocyte function-associated antigen: LFA-1
Intercellular Adhesion Molecule: ICAM
Immunoreceptor tyrosine-based activation motif: ITAM
Nuclear factor of activated T cells: NFAT
Interleukin –2: IL-2
Introduction
An adaptive immune response requires vigorous expansion of naïve T cells having the appropriate
antigen specificity; this is known as “clonal selection”. This increases the number of lymphocytes that can
provide a useful response. Naïve T cells must also differentiate into effector cells in order to provide
regulatory and cytolytic functions required for adaptive immunity. Limiting effector function to only those
T cells necessary for a useful response will limit non-specific cytotoxic damage. The potential for reactivity
to self-antigens demands that this activation process be tightly regulated. This is accomplished by
restricting activation to T cells that receive the appropriate signals from professional antigen presenting cells
in secondary lymphoid tissue.
Overview: Activation of naïve T cells is required for expansion and differentiation of antigen-specific T
cells into effector cells.
Activation of naïve T cells occurs in secondary lymphoid tissue.
Fig 2. Naïve T cells circulate through the blood and lymphatic system, including lymph nodes and other
secondary lymphoid tissue (spleen, Peyer’s patches) where they may encounter antigens.
Fig 3: Naïve T cells enter lymph nodes from the blood stream, and exit through efferent lymphatic vessels.
Fig 4: Once in lymph nodes, if T cells encounter the appropriate antigen they can be stimulated to
proliferate and differentiate.
2. Movement of T cells from the blood into lymph nodes is determined by specific adhesion molecules, and
chemokines.
Fig 5: L-selectin interactions with carbohydrates on the surface of the high endothelial venules (HEV)
promotes rolling of naïve T cells along the venule surface. HEV-bound chemokines interact with specific
chemokine receptors on T cells leading to the transient activation of the integrin LFA-1 (Leukocyte
Function-associated Antigen). LFA-1 binds Intercellular Adhesion Molecule -1 (ICAM-1) on the
endothelium, which stops the rolling of the T cell. Once stopped, the T cells are induced to enter the lymph
node cortex (diapedesis), by following a chemokine gradient.
CCR7
CCL21
CX3CR3
CXCL10
Fig 6: T cell homing to lymph nodes or to sites of infection is determined in part by the selective expression
and activity of specific chemokines/chemokine receptors and adhesion molecules.
The immunosuppressive drug, FTY720 acts to suppress T cell responses by blocking migration out of
lymph nodes.
3. Antigens arrive in lymphoid tissue, and are then presented by Antigen Presenting cells (APCs) to naïve
T cells.
Fig 7: Antigens may be brought to lymph nodes by dendritic cells which encounter pathogens in peripheral
tissue and then migrate to local lymph node through lymphatic vessels.
Fig 8: Pathogens (and their antigens) can also travel independently through the lymph and then be trapped
by macrophages, or B cells, which are resident in the draining lymph node.
While all three cell types, dendritic cells, macrophages and B cells, can serve as APCs, dendritic cells are
the most effective activators of naïve T cells.
Activation requires a highly selective interaction between naïve T cells and APCs.
1. Adhesion molecules mediate the initial interaction between T cells and antigen-presenting cells (APCs).
Fig 9: Once in the lymph node, T cells scan the surface of APCs for the presence of specific antigen. This
scanning depends on transient adhesion interactions: LFA-ICAM , CD2-LFA-3. ICAM3 – DC-SIGN.
Fig 10: If the highly specific antigen receptor on the T cell (TCR) is able to bind antigen presented by the
APC, this enhances LFA-1 binding activity and stabilizes the interaction between the two cells. This has
been termed “inside-out” signaling
2. Engagement of the TCR also leads to specific molecular rearrangements at the site of contact between
the T cell and antigen-presenting cell.
T cell
APC
Fig 11: The TCR and co-receptor are at the center of the interface, while adhesion molecules form an outer
ring. Cytoskeleton and cytoplasmic vesicles are polarized toward APC. This relatively stable structure,
known as the “immune synapse”, can be maintained for several hours.
Signals from the T cell receptor and co-receptors alter gene transcription
1. Receptor clustering leads to phosphorylation of Immunoreceptor Tyrosine-based Activation Motifs
(ITAMs) of the cytoplasmic domains of the TCR. The ZAP-70 tyrosine kinase binds to phosphorylated
ITAMs and is then activated by phosphorylation. Co-receptor (CD4, CD8) binding to MHC helps recruit
the Lck tyrosine kinase which enhances phosphorylation of ITAMs and ZAP-70.
Fig 12: TCR and co-receptor binding to MHC+antigen leads to receptor phosphorylation and
recruitment of tyrosine kinases.
2. Tyrosine kinases mediate the activation of the phosphatidylinositol and mitogen-activated kinase (MAP
kinase) signaling pathways that lead to the activation of transcription factors.
TCR
CD4
CD28
PO4-YPO4-Y-Y-PO4
-Y-PO4
Protein tyrosine kinases
Phosphatidyl inositol
pathway
MAP kinase
pathways
Transcription factor activation
Fig 13: Intracellular signaling pathways couple the TCR to activation of transcription factors
The NFAT transcription factor (Nuclear Factor of Activated T cells) is regulated by intracellular calcium
levels and the calcineurin phosphatase. Increases in intracellular calcium increase the phosphatase
activity of calcineurin which dephosphorylates NFAT, promoting nuclear translocation and the
transcription of immune response genes such as interleukin-2 (IL-2).
Fig 14: Nuclear localization of the NFAT transcription factor is controlled by the calcium regulated
phosphatase, Calcineurin.
Other transcription factors, including NFκB and AP1 are activated following T cell receptor stimulation and
induce changes in gene expression. NFkB is regulated by signals initiated by Protein Kinase C. AP-1 is
regulated by MAP kinase pathways.
IL-2 acts in an autocrine fashion to promote proliferation.
1. T cell activation induces the production of both IL-2 and an additional subunit of the IL-2 receptor,
IL-2Ralpha, which generates a high affinity form of the receptor.
Fig 15: IL-2 is an autocrine growth factor for T cells
2. Immunosuppressive drugs that target T cells block the production and function of IL-2: Cyclosporin
A and FK506 (tacrolimus) block IL-2 production, and rapamycin blocks signaling through the IL-2
receptor.
Outcome of antigen recognition by naïve T cells depends upon the conditions of activation.
Fig 16: T cell activation can lead to distinct effector cell types, or suppression.
1. Responsiveness requires co-stimulation. Activation of naïve T cells requires both signals from the TCR
and a co-stimulatory signal provided by CD28. A)`The ligand for CD28 is B7, found on APCs. Signals
from CD28 increase IL-2 production and cell survival.
Fig 17: CD28 provides a second signal necessary for activation of naïve T cells.
B). Recognition of an antigen in the absence of a CD28 co-stimulatory signal leads to T cell nonresponsiveness: anergy. Future encounter with this antigen, even with co-stimulatory signals present, will
not lead to T cell activation.
Fig 18: A TCR signal without a co-stimulatory signal leads to non-responsiveness.
C). Professional antigen-presenting cells provide the B7 ligands for the CD28 costimulatory receptor on
naïve T cells. However, B7 expression is regulated. Without a “danger” signal provided by pathogens,
APCs don’t have co-stimulatory function. Pattern recognition receptors, such as the Toll-like receptors
(TLR), allow recognition of pathogen-specific products, e.g. bacterial lipopolysaccharide, and lead to
increased antigen presentation function of APCs.
Fig 19 & 20: Pathogen products induce the expression of the co-stimulatory ligand, B7, and stimulate migration
of dendritic cells to nearby lymph nodes.
D). Note that if APCs lack costimulatory function, then they induce non-responsivness in T cells that
recognize the antigens that they present. This helps limit reaction to self-antigens and prevents development
of autoimmunity. This means of inducing tolerance to self-antigens is one form of peripheral tolerance.
2. Naïve CD4 T cells can mature into distinct types of effector cells, which differ in the cytokines they
produce and the type of immune responses that they promote. Most is known about the differentiation and
function of Th1 and Th2 cells.
Fig 21
The outcome of naïve CD4+ T cell activation is influenced by the specific activation environment.
T reg
Th17
TGFß
APC
CD4
IL-6
+
TGFß
IL-12
Th1
IL-4
Th2
Fig 22: The presence of specific cytokines can bias differentiation toward Th1 (IL-12) or Th2 (IL-4)
developmental fates. More recent work has shown that T reg (TGFß) and Th17 (TGFß+IL-6) effector cell
differentiation is also regulated by the presence of specific cytokines during T cell activation.
In addition to cytokines, the strength of the MHC/antigen –TCR interaction also influences Th differentiation.
Fig 23: High affinity antigen favors Th1 differentiation, while
low affinity interactions favor Th2 differentiation.