Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

T CELLS AND MHC PROTEINS
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Effector Helper T Cells Help Activate Other Cells of the Innate and
Adaptive Immune Systems
In contrast to T C cells, helper T cells (T H cells) are crucial for defense against both
extracellular and intracellular pathogens, and they express CD4 rather than CD8
co-receptors and recognize foreign peptides bound to class II rather than class
I MHC proteins. Once naïve T H cells are induced on activated dendritic cells to
become effector cells, they can help activate other cells: they help activate B cells
to become antibody-secreting cells and later to undergo Ig class switching and
somatic hypermutation; they help activate macrophages to destroy any intracellular
pathogens multiplying within the macrophage’s phagosomes; they help
induce naïve T C cells to become effector cells that can kill infected target cells;
and they stimulate the activated dendritic cell that activated them to maintain the
dendritic cell in an activated state. In each case, the effector T H cell recognizes the
same complex of foreign peptide and class II MHC protein on the target-cell surface
that it initially recognized on the activated dendritic cell. As discussed later,
the T H cell stimulates the target cell both by secreting a variety of cytokines and by
displaying co-stimulatory proteins on its surface.
Naïve Helper T Cells Can Differentiate Into Different Types of
Effector T Cells
When activated by binding to a foreign peptide bound to a class II MHC protein
on an activated dendritic cell, a naïve T H cell can differentiate into several distinct
types of effector T cells, depending on the nature of the pathogen and the cytokines
they encounter. These cells include four subtypes of helper cells—T H 1, T H 2,
T FH , and T H 17 cells—and regulatory (suppressor) T cells. Figure 24–44 summarizes
both the cytokines that induce these effector T cells and some of the cytokines
the effector cells secrete, as well as the master transcription regulators that
control the effector cell’s development.
Naïve T H cells activated by dendritic cells secreting the cytokine interleukin‐12
(IL12) develop into T H 1 cells. These effector cells produce interferon-γ (IFNγ),
TYPE OF EFFECTOR
T CELL
SOME OF MAIN
CYTOKINES PRODUCED
naïve helper
T cell
IL12
T-bet
IFNγ
T H
DIFFERENTIATION
T H 1 cell
Gata3
IL4, IL5, IL13
TCR
co-stimulatory
protein
CD4
activated dendritic cell
class II
MHC protein
INDUCING
CYTOKINES
IL4
IL6 +
IL21
TGFβ +
IL6
TGFβ
T H 2 cell
Bcl6
T FH cell
Rorγt
T H 17 cell
Foxp3
regulatory
T cell
IL4, IL21
IL17
TGFβ, IL10
Figure 24–44 Differentiation of naïve
helper T cells into different types of
effector helper cells or regulatory
T cells in a peripheral lymphoid organ.
The cytokines produced by the activating
dendritic cell (and by other cells in the
environment) mainly determine which type
of effector T cell develops, as indicated.
Some of the main cytokines produced by
each type of effector cell are also shown,
and the master transcription regulator for
each subset is indicated in the nucleus.
There is increasing evidence that some
of the effector cells are plastic and can
change the cytokines they produce in
response to changes in their environment
(not shown).