DAMPING T‐CELL ENTHUSIASM
We have frequently reiterated the premise that no self‐respecting organism would permit the operation of an expanding enterprise such as a proliferating T‐cell population without some sensible controlling mechanisms. There are some similarities here with regulations governing corporate takeovers in the business world, where it has been deemed prudent to ensure that no single enterprise is permitted to completely dominate the marketplace. Such monopoly practices, if allowed to occur in an unregulated way, would eventually eliminate all competition. Not a good thing for diversity or overall fitness.
In a similar vein, in order
to preserve immunological diversity and the capacity to rapidly respond to new
challenges of an infectious nature, it is necessary to ensure that T‐cells specific for
particular epitopes are not allowed to proliferate indefinitely and ultimately
dominate the immune compartment. This would inevitably reduce the probability
that responses to freshly encountered antigens would ever get off the ground,
as naive T‐cells would have to compete
for access to DCs with over whelming numbers of previously activated T‐cells, with inevitable
disastrous consequences for immunological fitness. For these reasons, our
highly adapted immune systems have evolved ways of maintaining healthy
competition between T‐cells, which is achieved
through downregulating immune responses upon clearance of a pathogen, along
with culling of the majority of recently expanded T‐cells. This is also
necessary because the immune compartment is of a relatively finite size and
cannot accommodate an infinite number of lymphocytes.
Damping down T‐cell responses occurs via a
number of mechanisms, some of which operate at the level of the activated T‐cell itself, while others
operate via additional T‐cell subsets (regulatory
T‐cells) that use a variety of strategies to rein in T‐cell responses, some of
which are directed at the T‐cell while others are
directed at DCs. Regulatory T‐cells will be discussed at
length in Chapter 8, so here we will focus primarily on molecules present on
activated T‐cells that serve as “off
switches” for such T‐cells. Such molecules
represent important immunological checkpoints, helping to keep T‐cell responses within
certain limits.
Figure 7.18 Downregulation of T‐cell responses. (a) Antigen presentation by a mature
dendritic cell (DC) provides effective antigenic stimulation via peptide–MHC (signal 1) and B7 ligands
(signal 2) that engage the T‐cell
receptor (TCR) complex and CD28 on the T‐cell, respectively. (b) Antigen presentation to a previously
activated T‐cell that is bearing surface CTLA‐4 (a CD28‐related molecule that can also interact with B7 ligands) can lead to T‐cell unresponsiveness owing to inhibitory signals
delivered through CTLA‐4 co‐stimulation (see main text for further details). (c) Whereas naive
T‐cells bearing surface Fas receptor are typically
resistant to ligation of this receptor, activated T‐cells acquire sensitivity to Fas receptor (FasR)
engagement within a week or so of activation. Engagement of FasR on susceptible cells results in activation of the
programmed cell death machinery as a result of recruitment and activation of
caspase‐8 within the FasR complex. Active caspase‐8 the propagates a cascade of further caspase
activation events to kill the cell via apoptosis.
Signals routed through CTLA‐4
downregulate T‐cell responses
Cytotoxic T‐lymphocyte
antigen‐4 (CTLA‐4) is structurally related to
CD28 and also binds B7 (CD80/CD86) ligands. However, whereas CD28–B7 interactions
are co‐stimulatory, CTLA‐4–B7 interactions act in an
opposite fashion and contribute to the termination of TCR signaling (Figure 7.18). Whereas CD28 is constitutively
expressed on T‐cells, CTLA‐4
is not found on the resting cell but is rapidly upregulated within 3–4 hours
following TCR/CD28‐induced activation. CTLA‐4 has a 10‐ to 20‐fold higher affinity for
both B7.1 and B7.2 and can therefore compete favorably with CD28 for binding to
the latter even when present at relatively low concentrations. The mechanism by
which CTLA‐4 suppresses T‐cell activation has been the
subject of lively debate, as this receptor appears to recruit a similar repertoire of proteins (such as PI3K) to its intracellular tail as CD28 does. A
number of mechanisms have been proposed to account for the inhibitory effect of
CTLA‐4 on T‐cell activation. One
mechanism is by simple competition with CD28 for binding of CD80/CD86
molecules on the DC. Another is through recruitment of SHP‐1 and SHP‐2 protein tyrosine
phosphatases to the TCR complex, which may contribute to the termination of TCR
signals by dephosphorylating proteins that are required for TCR signal
propagation. CTLA‐4 may also antagonize the
recruitment ofthe TCR complex to lipid rafts, which is where many of the
signaling proteins that propagate TCR signals reside.
Although conventional T‐cells require CTLA‐4 expression to be induced
after antigen engagement, Tregs constitutively express this receptor and this
appears to play an important role in Treg‐mediated immune suppression. Tregs can use CTLA‐4 to bind CD80/CD86 on APCs,
promoting trans‐endocytosis and removal of
B7 ligands from the APC cell surface, thereby downregulating immune responses.
While this cell‐extrinsic function of CTLA‐4 is becoming widely
recognized, it should be mentioned that Tregs also suppress immune responses in
CTLA‐4 independent ways (as will
be discussed in Chapter 8). Irrespective of its mechanism of action, CTLA‐4 is undoubtedly critical for keeping T‐cells under control and in
this regard is also important for preventing responses to self antigen. CTLA‐4‐deficient mice display a
profound hyperproliferative disorder and die within 3 weeks of birth as a
result of massive tissue infiltration and organ destruction by T‐cells.
PD‐1 also represents an important T‐cell
checkpoint molecule
Another potent T‐cell inhibitory receptor, programmed
death 1 (PD‐1), is currently creating quite a stir because of the
emerging clinical success of antitumor therapies that seek to block its action
and reactivate the immune response against tumors expressing CTL‐inhibitory PD‐1 ligands on their surface.
Similar to CTLA‐4, PD‐1 also belongs to the CD28
family of co‐receptors, and mediates its
inhibitory effect subsequent to antigen binding through recruitment of the
phosphatase SHP‐2, which dephosphorylates
and inactivates proximal signaling adaptors such as ZAP‐70 in T‐cells and Syk in B‐cells. Prior to antigen
stimulation, T‐cell PD‐1 expression is upregulated
then triggered by either of its two receptors: PD‐L1, which is expressed mainly on nonlymphoid cells,
and PD‐L2, expressed on APCs.
Thus, like CTLA‐4, PD‐1 is involved in the
suppression of T‐cell‐driven immune responses.
Unlike CTLA‐4 however, deficiency of
which leads to fatal autoimmune disease in mice, loss of PD‐1 has a less drastic
outcome, resulting in the development of a range of different autoimmune
diseases depending on the genetic background of the mice. This difference
between PD‐1 and CTLA‐4 function seems to reflect
a propensity for PD‐1 activation to drive
responses only in PD‐1‐expressing cells (cell
intrinsic responses), whereas CTLA‐4 responses are more far‐reaching, not only through
intrinsic processes but also through cell extrinsic T‐cell‐driven CTLA‐4‐mediated down regulation of
CD28 on APCs and effector T‐cells.
Importantly, PD‐L1 is expressed at
significantly high levels on many tumor types, which is correlated with poor
clinical prognosis. This indicates that tumor cells may aggressively
express PD‐L1 on their surface to block CTL‐mediated
killing. Indeed, preclinical animal studies using blocking antibodies directed
against either PD‐1 or PD‐L1 have shown promising
effects in re‐stimulating the T‐cell‐mediated immune response to
promote tumor regression. Many PD‐1/PD‐L1
blocking therapies are now in advanced phase clinical trials and have shown impressive
clinical responses in multiple tumor types, including a 38% response rate by
the anti‐PD‐1 drug MK‐3475 in patients with
advanced melanoma. Because PD‐1 action is primarily cell
intrinsic, immune‐associated side‐effects with PD‐1‐blocking therapies have been
considerably less severe than with CTLA‐4‐inhibitory therapies, which
have also proved successful in the clinic. Therapies designed at re-stimulating T‐cell‐mediated antitumor immunity
are particularly desirable, as activating the adaptive immune system to target
tumors not only offers an exquisite layer of precision, because of the
generation of highly specific antigen receptors against tumor antigens, but
also generates long‐lived memory, which may
significantly lesson the chances of tumor relapse.
Cbl family ubiquitin ligases restrain TCR signals
A number of other molecules
have been identified that may be involved in reigning in T‐cell activation and these
include the Cbl family of proteins: c‐Cbl, Cbl‐b, and Cbl‐c. Membersof the Cbl family
are protein ubiquitin ligases that can catalyze the degradation
of proteins through attaching polyubiquitin chains to such molecules, thereby
targeting them for destruction via the ubiquitin‐proteasome
pathway. The ζ chain of the CD3 co‐receptor complex has been
identified as a target for c‐Cbl‐mediated ubiquitination and
this can result in internalization and degradation of the TCR complex. Thus, c‐Cbl proteins may raise the
threshold for TCR‐induced signals through
destabilizing this complex. Mice doubly deficient in c‐Cbl and Cbl‐b (which appear to exert
somewhat redundant functions) exhibit hyperresponsiveness to TCR‐induced signals, resulting
in excessive proliferation and cytokine production in naive as well as
differentiated effector T‐cells; such mice die from
autoimmune disease as a consequence. This appears to be due to a defect in downmodulation of the TCR
complex in activated T‐cells. Whereas TCR complexes
are normally internalized and degraded after stimulation via cognate peptide–MHC complexes (an event
which contributes to the termination of TCR signals), TCR complexes fail to be
internalized in c‐Cbl/Cbl‐b‐deficient cells, leading to
greatly extended TCR signaling and runaway T‐cell expansion.
Cbl family proteins can also
exert their influence on TCR signaling in other ways and may have an especially
important role in maintaining the requirement for CD28 co‐stimulation for proper T‐cell activation.
Surprisingly, mice deficient in Cbl‐b lose the normal requirement for CD28 co‐stimulation (i.e., signal 2) for T‐cell proliferation; such
cells make large amounts of IL‐2 and proliferate vigorously
in response to TCR stimulation alone. This implies that Cbl‐b plays a major role in
maintaining the requirement for signal 2 for activation of naive T‐cells. Although it is not
yet clear exactly how this operates, activation of Vav, which occurs downstream
of TCR as well as CD28 receptor stimulation, appears to be suppressed by Cbl‐b in wild‐type cells. Thus, for
effective Vav activation, signals 1 and 2 are normally required. However, in
the absence of Cbl‐b, a sufficient amount of
Vav activation is achieved through TCR stimulation alone, bypassing the need
for CD28 co‐stimulation.
T‐cell death occurs through stimulation of membrane Fas
receptors
Another important way of
standing down T‐cells from active duty is to
kill such cells through programmed cell death (Figure
7.18). Naive T‐cells, as well as recently
activated T‐cells, express the membrane Fas (CD95) receptor, but
are insensitive to stimulation via this receptor as these cells contain an
endogenous inhibitor (FLIP) of the proximal signaling molecule caspase‐8 that is activated as a
result of stimulation through the Fas receptor. However, upon several rounds of
stimulation, experienced T‐cells become sensitive to
stimulation via their Fas receptors, most likely owing to loss of FLIP
expression, and this situation results in apoptosis of these cells. Mice
defective in expression of either Fas or FasL manifest a lymphoproliferative
syndrome that results in autoimmune disease due to a failure to cull recently expanded lymphocytes.
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