Download Signal minus 1: A key factor in immunological tolerance to

)^ and Cell Biology {1996) 74, 278-285
Theoretical Article
Signal minus 1: A key factor in immunological
tolerance to tissue-specific self antigens?
CHRISTOPHER R PARISH
Division oj Immunology and Cell Biology. John Curtin School of Medical Research. Australian National University
Canberra. ACT, Australia
Summary Recent data suggest that many autoreactive T cells, particularly to tissue-specific self antigens, can
escape thymic deletion. The current dogma is thai these autoreactive T cells are silenced by the failure of most
tissues to provide co-stimulation (signal 2), antigen alone (signal 1) inducing T cell unresponsiveness.
However. I propose that activation of autoreactive T cells frequently occurs but autodestruction by effector T
cells is tightly regulated. This phenomenon is most evident with lymph node metastasizing tumour cells where
the regional lymph node can mount a vigorous response to the invading tumour cells but tumour growth is
unimpaired. I suggest that autodestruction is prevented by inhibitory receptors on T cells which recognize
class I MHC structures on target cells. These receptors, which I propose deliver 'signal minus T to T cells, were
recently described on NK cells and a subpopulation of peripheral T eells. They are also strikingly similar to a
family of anti-self receptors that my laboratory described on murine T and B cells 15 years ago. In the 'signal
minus 1" model, antigen-activated T cells acquire the inhibitory receptors when they become co-stimulation
independent and gain the ability to exit lymphoid organs and enter non-lymphoid tissues. Thus, if autoreactive effector T cells encounter autoantigen in tissues they are functionally silenced by inhibitory receptor
engagement and signal minus 1 delivery. In contrast. I propose that in response to intracellular infections, cells
down-regulate expression of their ligands for inhibitory receptors. Such a model allows infected cells to be
selectively eliminated by effector T cells. If correct, the model predicts that effector T cells, whether foreignantigen- or autoantigen-specific, can selectively respond to infected cells. This apparent 'usefulness' of
autoreactive T cells may explain their observed persistence even after an encounter with autoantigen. It is also
suggested that signal minus 1 may silence autoreactive B cells specific for tissue-specific cell surface antigens
and lack of signal minus 1 may partially explain the vigorous T cell response to allogeneic MHC. Finally, it is
hypothesized that, in evolutionar>' terms, inhibition of autodestruction by the recognition of a 'self marker'
and delivery of signal minus 1 is an ancient process which probably emerged in early metazoans.
Key words: alloreactivity, autoimmunity, inhibitory receptors, peripheral T cell tolerance, signal minus 1.
Introduction
One of the fundamental questions of immunology is the
means by which peripheral T cell tolerance is maintained
to tissue-specific self antigens. It cannot be assumed that
all autoreactive T cells are eliminated during T cell selection in the thymus.'*-^ There must be considerable numbers of autoreactive T cell clones escaping from the thymus due to the failure of tissue-specific self antigens to be
presented by thymic epithelium to maturing T cells. Furthermore, many developmentally regulated antigens are
expressed after the appearance of a mature immune system. Thus it is reasonable to assume that autoreactive T
cells to tissue-specific self antigens exist in secondary' lymphoid organs. Studies with transgenic mice support this
Correspondence: Christopher R Parish, Division of Immunology and Cell Biology, John Curtin School of Medical Research,
Australian National University. Canberra, ACT 2601, Australia.
Received 31 January 1996; accepted 6 February 1996.
view, autoreactive T cells often appearing in the periphery
when the autoantigen is expressed extrathymically.'How then is reactivity against tissue-specific self antigens
prevented while responsiveness to foreign antigens is
maintained?
The current dogma is that T cell immunity to tissuespecific self antigens is curtailed by the inability of most
cells to constitutively express co-stimulator\' molecules
(signal 2) required for T cell activation.'--^ A corollary of
this model is that antigen alone (signal 1) induces nonresponsiveness in T cells.'"-^ This simple 'two signal"
model has been used extensively to explain many aspects
of T cell immunity. In this paper 1 propose, however, that
the two signal model cannot totally explain T cell tolerance to tissue-specific self antigens, this deficiency of
the model being particularly apparent with metastasizing
tumour cells. An alternative model is presented which, I
believe, can more adequately explain peripheral T cell
tolerance.
279
Tolerance to tissue-specific self antigens
The paradox of lymph node metastasizing tumour cells
In many ways lymph node metastasizing tumour cells are
an excellent model for studying the control of T cell immunity to tissue-specific self antigens. The tumour cells
inevitably carry many tissue-specific self antigens, as well
as tumour-specific antigens."*^ which can be recognized by
substantial numbers of T cell clones that have survived
thymic selection. In fact, it has been known for many
years that the frequency of cytotoxic precursors for a
syngeneic tumour can be high, resembling the precursor
frequency for allogeneic tumour cells.^ Furthermore, by
penetrating the lymph node the tumour cells are accessible to naive T cells which, it is believed, usually do not
recirculate through tissues.^
Despite the existence of tumour-reactive T cells, many
tumours metastasize via lymph nodes without apparently
inducing a destructive immune response. In fact, the regional lymph node is a major route of metastasis for many
epithelial-derived tumours.^ Nevertheless, it has been recognized for some time that substantial T cell proliferation
can occur when tumour cells enter regional lymph nodes."'
Despite this vigorous T cell response, the tumour spreads
rapidly to other tissues and eventually kills the host. In
contrast, when the same tumour ceils are injected into
MHC-incompatible hosts they are usually rapidly rejected
by a potent T-cell-mediated immune response.
These observations provide several paradoxes which
cannot be easily explained by the two signal model of T
cell activation. First, despite the lack of co-stimulator\'
molecule expression by the tumour cells^ substantial T
cell activation is induced in the regional lymph nodes.
Second, these activated T cells are very inefficient at eliminating the tumour. Third, if co-stimulation is the limiting
factor why are the tumour cells rejected when they are
transplanted into an allogeneic recipient? Surely increasing the frequency of tumour-specific T cells in the recipient, due to MHC disparity, will not overcome the lack of
co-stimulation provided by the tumour cells.
Clearly additional factor(s) must be operating which
prevent effective T cell immunity against the tumour
associated antigens. In this regard, a model can be developed based on the recently described inhibitory receptors
for class I MHC molecules, which provides an explanation for immunoiogicai tolerance to tissue-specific self
antigens, allows responsiveness to foreign antigens and
also has some implications for alloreaetivity.
Inhibitor) class I MHC receptors and autorosetting
Recent studies with natural killer (NK) cells have revealed
a remarkable family of inhibitory receptors which also
may have profound implications for T cell reactivity.'"-'•*
The receptors recognize MHC class I molecules on
target cells and inhibit NK effector cell function. These
inhibitory receptors are also functionally expressed on T
lymphocytes.
Families of both mouse and human inhibitorv' receptors have been defined and a comparison of their general
properties is presented in Table 1. Although the mouse
and human receptors share striking similarities in their
Table 1 Inhibitory receptors for class I MHC on lymphocytes
Receptor properly
Mouse receptors
Human receptors
Nomenclature
Ly49A-G
p58. NKBl, NKAT(NK
assoeiated transeripts)
Chromosome location
Chromosome 6
Chromosome 19
Cellular distribution
NK cells, T eell subset
NK cells, T eell subset
Family type
C-type lectin
Ig superfamily
Strueture
44kDa disulphide
linked homodimer
p58: 58 kDa. 2 Ig-like domains
NKBl: 70 kDa. 3 Ig-like domains
NKAT: 2-3 Ig-like domains
Ligand(s)
Class 1 MHC ( + speeific
peptide?)
Anionic carbohydrate
Class I MHC + speeifie peptide
Specifieity
Ly49A: H-2D'' and D"^
Ly49C: H-:*'
Ly49G2: H-2D'' and L*^
P58: HLA-A. -B and -C
NKB1:HLA-B
Expression
Clonalty distributed
Exhibit allelie exclusion
Some adaptation to self MHC
Clonally distributed
Function
Inhibit NK (and T?) eell
killing of targets and
lymphokine production
Inhibit NK and T eell
killing oi targets and
tymphokinc production
Based on data reviewed by Yokoyama;'° Raulet and Held;" Lanier and Phillips;'- Bottino <•/ a/," Liebson;''' and Gumperz and
Parham.'*
280
CR Parish
cellular distribution and function surprisingly they have
no structural homology. The murine receptors (Ly49
members) are C-type lectins whereas the human receptors
(p58, NKBl, NKAT) are Ig superfamily members. The
reason for this structural discrepancy is unclear although
it has been suggested that CD94 is the Ly49 equivalent in
humans."^ Nevertheless, the receptors in both species recognize class I MHC molecules containing certain peptides
and discriminate between different MHC alleles. For example, Ly49A, which is expressed on a subset of murine
NK cells, only interacts with H-2D'' and D"^ (Table 1).
Interestingly, some Ly49 molecules have also been demonstrated to bind anionic carbohydrates, an interaction
which inhibits their binding to H-2 molecules.' ''^' ^ Recognition of carbohydrate structures by the human receptors
has not been demonstrated; in fact the NKBi receptor
still recognizes MHC molecules devoid of N-linked
carbohydrate.'^ Currently, at least seven Ly49 genes have
been described and ten cDNA of the p58/NKBI/NKAT
family have been reported. "^"' ^ It is believed that considerable receptor diversity is generated in both species by
alternative splicing of a small family of related genes.
A particularly intriguing feature of both the mouse and
human inhibitory receptor families is that they are expressed on subpopulations of T lymphocytes. Studies with
NKBl suggest that it is predominantly expressed on T
lymphocytes chronically stimulated by antigen and not by
naive T cells.•^° Thus, the inhibitory receptors regulate the
efFector function of T cells rather than activation of naive
T cells.
A fascinating aspect of the inhibitory receptors on NK
and T cells is that they are strikingly similar to a family of
anti-self receptors that my laboratory described 15 years
ago (Table 2). We were examining a rather bizarre phenomenon, the ability of murine lymphocytes to bind
autologous erythrocytes, a process termed autorosetting.
Our research revealed that lymphocytes preferentially
bind //-2-compatible erythrocytes,-'-- a phenomenon
described independently by two other groups.--^-"* The
autorosetting receptors were found to be H-2L/D restricted and resemble tbe Ly49 family in crossreacting
Table 2 Properties of murine autorosetting receptors on lymphocytes
Mediate binding of murine lymphocytes to autologous
erythrocytes
Receptors present on about 75% of thymocytes,
10-15% of peripheral T cells and 20% of B cells^'-^^'^'
Binding class I MHC restricted,^'-^'' mapping to H-2L/D
regions.-'-^
Recognize anionic carbohydrate structure on erythrocytes
in a H-2L/D restricted manner'^—^
No evidence for 'thymic education", i.e. MHC restriction
of receptors not determined by H-2 haplotype of thymic
epithelium^^
Erythrocyte ligand blocked by the serum protein histidine-rich
glycoprotein
with some H-2 haplotypes.^'-^^ Furthermore, the autorosetting receptors, like Ly49, interact with anionic
carbohydrates,^^"^^ the carbohydrate recognition also being under H-2L/D control.-^ Although MHC-restricted,
analysis of chimaeric animals demonstrated that the thymic epithelium did not alter the MHC specificity of the
autorosetting receptors.^^
The cellular distribution of the autorosetting receptors,
however, differs somewhat from known Ly49 members
being expressed on the majority of thymocytes (75%) and
substantial subpopulations of peripheral T (10-15%) and
B (20%) cells.^'-^^-^'' At the time, the functional relevance
of these receptors was unclear, although it was concluded
that they could not be directly involved in MHCrestricted recognition of foreign antigen by T cells.-^ It
now seems highly likely that the autorosetting receptors
belong to the Ly49 family of inhibitory receptors, presumably being family members which are yet to be characterized. Alternatively, the receptors may represent the
murine equivalent of the p58/NKBl/NKAT receptors in
humans. Further work is clearly required to resolve this
issue and formally establish whether the autorosetting
receptors can deliver an 'inhibitory signal' to lymphocytes. Finally, one other important feature of the autorosetting system should be noted. A unique serum protein
was identified, termed histidine-rich glycoprotein (HRG),
which blocks autorosetting by masking the erythrocyte
ligand.-'^'^'^ Subsequent studies have demonstrated that
HRG exhibits immunoregulatory properties,-^° but the
effect of HRG on T and NK cell effector function has not
been examined.
Hypothesis: does signal minus 1 maintain peripheral T
cell tolerance?
Based on the available information on inhibitory class I
MHC receptors it is possible to propose a plausible model
for peripheral T cell tolerance. In this model the "inhibitory signal' delivered by these receptors will be called, for
simplicity, 'signal minus V.
Inhibition of autodestruction by signal minus 1
Figure I depicts the various stages in T cell activation and
the proposed roles of signal 1, signal 2 and signal minus I
in this process. It is proposed that initial activation of
naive T cells follows the conventional 'two signal' model,
antigen alone (signal I) resulting in T cell inactivation
unless co-stitnulation (signal 2) is provided. The current
dogma is that, due to the availability of appropriate APC,
this signal 2 dependent clonal expansion of naive T cells is
restricted to lymphoid organs.^ However, a key feature of
the new model is that antigen-activated T eells acquire
inhibitory receptors (and, therefore, can receive signal
minus 1) when they gain the ability to exit lymphoid
organs and enter non-lymphoid tissues. Such a proposition ensures that effector T cells specific for tissue-specific
self antigens are functionally silenced if they encounter
antigen in tissues. Certainly, the limited evidence available is consistent with this proposition, only antigen-
Tolerance to tissue-specific self antigens
Naive T cell
I Signal I
LYMPHOID
Signal I
ORGAN
Inaclivalion
Activation
RESTRICTEDi
1
Signal 2
Clonal Expansion
Expression or Inhibitory
Receptors
ABLE TO
Inactivation
Min
Signal I
ENTER
TISSUES
Signal
Signal
EfTector T cell
Inaclivalion
I
Signal I
Cylotoxicily
Lymphokine Release
Figure 1 Stages in T cell activation demonstrating the role of
signal 1 (antigen), signal 2 (co-stimulation) and signal minus 1
(inhibitory receptor engagement) in T cell activation. In this
model it is proposed that initial activation of naive T cells occurs
in lymphoid organs. Acquisition of inhibitory receptors occurs
when activated T cells become signal 2 (co-stimulation) independent and gain the ability to exit lymphoid organs and migrate
into tissues. 'Inaetivation' refers to functional silencing of T cells,
not their deletion.
experienced T cells appearing to express the inhibitory
receptors.'--^
Another important aspect of the T cell activation pathway depicted in Fig. 1 is that acquisition of inhibitory
receptors oeeurs when T cell clones become costimulation (signal 2) independent. Obviously T cells do
not need to recognize co-stimulatory molecules to mediate their effector function. If co-stimulation was required,
target cell recognition would be extremely limited. However, co-stimulation independence poses serious problems
of autoreactivity, a problem ameliorated by the expression of inhibitory receptors by effector T cells.
Thus, based on the above model, the immune system
can tolerate considerable autoreactivity. What is tightly
controlled is autodestruction by effector T cells. In fact, it
could be argued that thymic deletion is primarily aimed at
eliminating T cell clones which recognize self antigens expressed by 'professional' APC, a point already highlighted
by Matzinger.^ In contrast, the bulk of autoreactive T cells
are regulated extrathymically by signal minus 1.
How is selective recognition of foreign antigen
achieved?
So far this discussion has concentrated on the functional
silencing of autoreactive T cells by signal minus 1. If one
28
accepts this concept how does the immune system discriminate between self and foreign antigens? For this discrimination to occur I propose that, in response to intracellular infections, cells down-regulate expression of their
]igand(s) for inhibitor^' receptors. Such a response allows
infected cells to be selectively eliminated by effector T
cells. At this level the model somewhat resembles the
'danger hypothesis" proposed by Polly Matzinger.-^
Loss of the ligand for inhibitory receptors on infected
cells is a fundamental feature of the 'signal minus V
hypothesis and warrants further discussion. In the case of
NK cells it is clear that target cells become susceptible to
lysis if class I MHC is down-regulated and, therefore,
inhibitory receptors are unable to engage."^"'^ However,
displacement of self peptides from class I MHC molecules
can also render targets susceptible to lysis by human NK
cells.-^' Thus it appears that subtle changes in the inhibitory ligand on target cells following intracellular infection
could prevent signal minus I delivery. In Figs 2 and 3 the
inhibitory ligand is depicted as class I MHC + self peptide
associated with an oligosaccharide structure, thus incorporating features of both the human and murine inhibitory receptor systems (Table 1). Following viral infection,
it is proposed that the oligosaccharide structure is lost and
an inappropriate (self or viral) peptide is bound to class I
MHC. Whether the inhibitors' ligand is modified as depicted, or in some other way, remains to be determined.
Certainly, such molecular details are not required for the
development of the hypothesis. Nevertheless, loss of the
oligosaccharide component of the inhibitory ligand is an
attractive possibility which requires investigation. By selectively down-regulating oligosaccharide expression inhibitory receptor engagement is prevented but class I
MHC expression is retained for viral peptide presentation
to T cells. Farlier studies from my laboratory suggesting
that oligosaccharide structures are associated with MHC
molecules may have relevance here.^The behaviour of autoantigen-specific and viral
antigen-specific effector T cells will now be considered. In
the case of autoantigen-specitic T cells (Fig. 2), presentation of the autoantigen by uninfected cells results in engagement of the inhibitory' receptor and no response. In
contrast, presentation of the autoantigen by virus infected
cells allows a response to occur, the presence of a modified
inhibitory ligand resulting in signal minus I not being
received. This is an intriguing prediction as it implies that
autoreactive T eells could selectively eliminate infected
target cells. Certainly the frequently reported appearance
of autoreactive T cells at early stages during a viral
infection^-'' may be a manifestation of this phenomenon.
Also, it should be noted that inactivation of NK and T cell
cytotoxicity by inhibitory receptors is reversible and
transient.'-' The apparent 'usefulness" of autoreactive T
cells also may explain the observed persistence of these
cells in the periphery.'--^-* Based on the current dogma
deletion of autoreactive T cells would seem the safest
option rather than their observed persistence in an anergic
state requiring the continual presence ofautoantigen.'--^"
In order to explain this paradox it has been proposed that
persistent autoreactive T cells maintain self tolerance by
suppressor meehanisms.^^-^* However, if autoreactive T
282
(a)
CR Parish
TCR
Autoantigen
Specific
TCell
Uninfected
Cell
[nhibilory
Receptor
Uninrected
Cell
Viral Antigen
Specific
TCell
Peptide
NO RESPONSE
(b)
NO RESPONSE
(b)
TCR
TCR
Virus
Infected
Cell
Autoanligen
Specific
TCell
No Signal
Virus
Viral Antigen
Specific
TCell
Infected
Cell
I Signal
Inhibitory
Receptor
RESPONSE
Figure 2 The role of inhibitory receptors in the interaction of
autoantigen-specific T cells with (a) uninfected cells or with (b)
virus-infected cells carrying the autoantigen. The MHC inhibitory ligand recognized by the inhibitory receptor is depicted, for
convenience, as class 1 MHC-^ self peptide associated with an
oligosaccharide structure. On virus infected cells the MHC inhibitory ligand is modified by loss of the oligosaccharide and an
inappropriate peptide being presented by class I MHC. Engagement of the autoreactive TCR results in signal 1 ( + ), whereas
engagement of the inhibitory receptors results in signal minus 1
( - ) . 'Response' refers to cytotoxicity., lymphokine release and T
cell proliferation. Note that autoantigen-specific T cells can respond to virus infected cells. In the figure the TCR is depicted as
interacting with MHC + self peptide. The class of the MHC is not
specified but. in theory, could be either class 1 or class II MHC
molecules.
cells can be harnessed to eliminate infected cells in tissues
their persistence can be justified on these grounds as well.
In the case of viral antigen-specific T cells (Fig. 3),
uninfected cells are not recognized due to the lack of
MHC + viral peptide on these cells. When the viralantigen-specific T cells encounter virus-infected cells,
however, a prompt response occurs due to TCR recognition in the absence of inhibitory receptor engagement.
Incidentally, it is presumed throughout this discussion
that inhibitory receptor engagement and signal minus I
delivery does not occur unless the TCR interacts with
antigen (see Fig. 3a).
In Figs 2 and 3 the TCR is depicted as interacting with
RESPONSE
Figure 3 The role of inhibitory receptors in the interaction of
viral antigen-specific T cells with (a) uninfected cells or with (b)
virus-infected cells. For more of the interaction details see Fig. 2.
Note that inhibitory receptors do not deliver a negative signal
(signal minus 1) unless TCR is engaged (compare uninfected cells
in Figs 2 and 3).
MHC + self or viral peptide. The class of the MHC is not
specified but, in theory, could be either class I or class II
MHC molecules. However, it would be anticipated that
the signal minus 1 hypothesis, as proposed, would predominantly control recognition of uninfected or infected
tissue cells by class I MHC restricted T cells. Since class II
MHC has a restricted cellular distribution and is involved
in the presentation of extracellular processed antigen to T
cells by 'professional' APC. inhibitory receptors would
only control the response of class II MHC restricted T
cells under special circumstances. For example, one can
envisage situations where tissue cells are induced to express class II MHC molecules following stimulation by
various pro-inflammator\' cytokines. In such cases, the
inhibitory receptors could control autoreactive T cells
which are class II MHC restricted. In fact, it has been
shown that the inhibitory receptors can prevent T cell
cytolysis triggered by class II MHC recognition.-'^
Implications for alloreactivity and autoreactive B cells
The signal minus I hypothesis presented above also may
have relevance to two other types of immune responses;
Tolerance to tissue-specific self antigens
the activation of T cells by allogeneic MHC and the response of B cells to tissue-specific self antigens.
It is generally believed that the unusually violent response of T cells to allogeneic MHC molecules is due to
the high frequency of alloreactive T cells.^^ Since a high
frequency of cytotoxic precursors has been reported
against both syngeneic and allogeneic tumour cells'^ this
explanation may be overly simplistic. On the other hand,
since inhibitory receptors on T cells are MHC-restricted
(Table 1), the failure of these receptors to recognize allogeneic MHC may partially explain the violent T cell response observed. Certainly, it appears likely that the rejection of allogeneic tumours may be due to the inability of
the tumour cells to inactivate effector T cells by signal
minus 1.
So far this essay has concentrated on the control of T
cell autoreactivity. However, as with T cells, one would
expect the continual appearance in the periphery of
autoreactive B cells.•'••'^ Of particular importance would
be B cells specific for tissue-specific self antigens expressed on cell surfaces. These B cells would not be
deleted in the bone marrow during development,^* and
yet could produce undesirable blocking antibodies
against important cell surface molecules. Do inhibitory
class I MHC receptors control these autoreactive B cells?
At present there is no evidence for inactivation of B cells
by inhibitory receptors. It is clear, however, that the
autorosetting receptors detected on a subpopulation of
peripheral B cells are strikingly similar to the murine
inhibitory receptors, being class I MHC-restricted and
anionic carbohydrate specific (Table 2). Thus, one could
envisage a situation where autoreactive B cells could
interact with tissue-specific cell surface antigens via their
surface Ig and subsequently be inactivated by inhibitory
receptor engagement.
Autoimmunity
Here I have proposed that autoreactivity is common but
autodestruction is rare due to signal minus I intervention.
Since inactivation of effector T cells is mediated by the
overriding of signal 1 by signal minus 1 (Figs 1 and 2). it is
relatively easy to envisage situations where autoimmunity
could occur. For example, T cell clones with high affinity
TCR for tissue-specific self antigens could receive sufficient signal 1 to overcome inhibitory receptor inactivation.'^ Another, possibly more likely, scenario is
that effector T cells, although co-stimulation independent
are still co-stimulation responsive. Thus, the abnormal
expression of co-stimulator molecules by target cells may
provide sufficient positive signals to override signal minus
1. A clear example of this possibility is the ability of B7
transfected tumour cells to be killed by NK cells, despite
the engagement of inhibitory receptors.^**
At the genetic level, predisposition to autoimmune disease could occur in individuals where signal minus 1
delivery is impaired. This genetic effect could occur either
syslemically or at the organ-specific level. An interesting
possibility, for example, is that a predisposition to organspecific autoimmunity could be due to low level expres-
283
sion of MHC class I inhibitory ligands in the target organ
prone to autoimmune attack.
Evolutionary significance of signal minus I
It is highly likely that the active recognition of self (i.e.
signal minus 1) in order to prevent autoreactivity is an
evolutionarily ancient process. In fact, it has been proposed on several occasions that specific recognition of self
is the basis of self/non-self discrimination in all multicellular animals.**^-*^ In these models it has been suggested
that a 'self marker', present on all cells of the individual, is
recognized by an anti-self receptor on phagocytes. Engagement of the anti-self receptor prevents phagocytosis of
self, whereas the lack of the self marker on foreign cells
allows them to be ingested. In other words, phagocytes
will ingest any cell or particle (presumably recognized by
promiscuous receptors) unless self is specifically recognized. A common suggestion has been that histocompatibility antigens represent the self marker.•''^"•*^ Antigenspecific recognition of the MHC by T cells is, therefore, a
sophisticated variation which has evolved in higher vertebrates. Interestingly, a number of authors have also suggested that a carbohydrate structure may be the self
marker."*^""' When one considers the general properties
of the inhibitory' receptors for class I MHC on mammalian lymphocytes (Table 1) these ideas are remarkably
prophetic.
There is some experimental evidence in invertebrates
which supports the concept of active recognition of self.
Colonial tunicates only fuse if they share one allele of the
fusion gene. Active rejection occurs if all alleles are
incompatible.""^ Similarly, gorgonians accept autografts
but reject all xenografts and most allografts by a mechanism which appears to involve positive recognition of
self.'*'' The capacity to discriminate between self and nonself may also exist in protozoa. Studies over 70 years ago
showed that amoebae do not ingest their own or another
amoeba's pseudopodia,'''^' despite the considerable phagocytic potential of these organisms. Observations such as
this suggest that the emergence of active recognition of
self may have even been essential for speciation to occur
in early metazoans in some ecosystems.
Concluding remarks
At this stage it is worth re-examining the immunological
paradoxes raised earlier by lymph node metastasizing
tumour cells. Can these paradoxes be explained by the
signal minus 1 hypothesis? Certainly the failure of the
host to reject a tumour despite a vigorous T cell response
in lymph nodes can be explained by the action of inhibitory receptors. As depicted in Fig. 1, autoreactive T cell
clones only express inhibitory receptors following substantial clonal expansion and the acquisition of the ability
to exit the node and enter tissues. Since metastasizing
tumour cells are already present in the node and express
inhibitory receptor ligands they could rapidly inactivate
these T cells immediately they expressed inhibitory receptors (Fig. 2). A very different situation applies when the
tumour cells are implanted in an allogeneic recipient. In
284
CR Parish
this case tumour rejection ensues as the inhibitory receptors on T ceils are class I MHC-restricted and, therefore,
unable to recognize inhibitory ligands on the allogeneic
tumour cells.
One interesting paradox is the ability of lymph node
metastasizing tumour cells to induce a vigorous T cell
response despite the tumour cells lacking co-stimulator
molecules. This observation implies that bystander costimulation can be provided by professional APC in
lymph nodes. I am unaware of in vitro studies demonstrating that naive T cells can respond to tumour cells when
bystander co-stimulation is provided. Nevertheless, the
juxtaposition of T cells, tumour cells and professional
APC in the microenvironment of the lymph node appears
to allow T cell activation to proceed.
In conclusion, in this essay I have attempted to address
one of the most tantalizing mysteries of immunology, the
manner by which the immune system is tolerant to tissuespecific self antigens but retains responsiveness to foreign
antigens. A fundamental feature of the model is that autoreactivity is common but autodestruction is rare. The
recent explosion of information on inhibitory receptors
on NK cells and T cells suggests that regulation of autoimmunity at the effector cell level may be a common feature
of peripheral T cell tolerance. If this concept is correct it
has profound implications for the treatment of autoimmune disease and for the immunotherapy of cancer.
Acknowledgements
I thank Hilary Warren, Phil Hodgkin and Meredith Bradbury for reading the manuscript and providing honest and
constructive comments.
References
1 Fowlkes BJ, Ramsdell F. T cell tolerance. Curr. Opin. Immunol. 1993; 5: 873-79.
2 Arnold B, Schbnrich G, Hammerling GJ. Multiple levels of
peripheral tolerance. Immunol. Today. 1993; 14: 12-14.
3 Matzinger P. Tolerance, danger, and the extended family.
Annu. Rev. Immunol. 1994; 12: 991-1045.
4 Boon T, Cerottini, J-C, Van den Eynde B, van der Bruggen P,
Van Pel A. Tumor antigens recognized by T lymphocytes.
Annu. Rev. Immunol. 1994; 12: 337-65.
5 Hellstrom, KE, Hellstrom I, Chen L. Can co-stimulated
tumor immunity be therapeutically efficacious? Immunol.
Re^'. 1995; 145: 123-45.
6 Warren HS, Lafferty KJ. A high frequency of cytotoxic precursor cells for a syngeneic tumour. Seand. J. Immunol.
1979; 10: 349-52.
7 Mackay CR. Homing of naive, memory and effector lymphocytes. Curr. Opin. Immunol. 1993; 5: 423-7.
8 Hart IR. The spread of tumours. In: Franks LM, Teich N
(eds) Introduction to the Cellular and Molecular Biology of
Cancer. Oxford: Oxford University Press, 1987; 27-39.
9 Tachibana T, Yoshida K. Role of the regional lymph node in
cancer metastasis. Cancer Metast. Rev. 1986; 5: 55-66.
10 Yokoyama WM. Right-side-up and up-side-down NK-cell
receptors. Curr. Biol. 1995; 5: 982-5.
I1 Raulet DH, Held W. Natural killer cell receptors: The offs
and ons of NK cell recognition. Cell 1995; 82: 697-700.
12 Lanier LL, Phillips JH, Molecular and cell biology of inhibitory MHC class I receptors on NK cells and T cells. Immunol. Today 1996; 17:86-91.
13 Bottino C. Vitale M, Pende D, Biassoni R, Moretta A. Receptors for HLA class I molecules in human NK cells. Semin.
Immunol. 1995; 7: 67-74.
14 Liebson PJ. MHC recognizing receptors: They're not just for
T cells anymore. Immunity. 1995; 3: 5-8.
15 Gumperz JE, Pamham P. The enigma of the natural killer
cell. Nature 1995; 378: 245-8.
16 Moretta A, Vitale M, Sivori S el al. Human natural killer cell
receptors for HLA-class I molecules. Evidence that the Kp43
{CD94) molecule functions as a receptor for HLA-B alleles. /
Exp. Med 1994; 180: 545-55.
17 Daniels BF. Nakamura MC, Rosen SD, Yokoyama WM,
Seaman WE. Ly-49A, a receptor for H-2D*', has a functional
carbohydrate recognition domain. Immunity 1994; 1:
785-92
18 Brennan J, Takei F. Wong S, Mager DL. Carbohydrate recognition by a natural killer cell receptor, Ly 49C. / Biol.
Chem. 1995; 270: 9691-94.
19 Gumperz JE, Litwin V, Phillips JH, Lanier LL. Pamham P.
The Bw4 public epitope of HLA-B molecules confers reactivity with natural killer cell clones that express NKB1, a putative HLA receptor. / E.xp. Med. 1995; 181: 1133-44.
20 Phillips JH, Gumperz JE, Pamham P, Lanier LL. MHC class
I receptors on T lymphocytes inhibit superantigendependent cell-mediated cytotoxicity. Science 1995; 268,
403-5.
21 Sia DY. Parish CR. Anti-self receptors. I. Direct detection of
H-2L region-restricted receptors on murine thymocytes. /.
E.xp. Med. 1980; 151: 553-65.
22 Sia DY. Parish. CR. Anti-self receptors. II. Demonstration of
H-2L region-restricted receptors on subpopulations of peripheral Tand B lymphocytes. / Immunol. 1980; 124: 236671.
23 Primi D, Lewis GK, Triglia R. Goodman JW. Rosette formation between murine lymphocytes and erythrocytes. A new
locus in the H'2 region. / Exp. Med. 1979; 149: 1349-59.
24 Charreire J, Camaud C, Bach JF. Studies on mouse autoreactive cells. I. Role of H-2 antigens in mouse autologous
rosette formation. Celt Immunol. 1980; 49: 372-8.
25 Sia DY. Parish CR. Anti-self receptors. IV //-^restricted
receptors on thymocytes recognize carbohydrate structures
on target cells. Immunogenetics 1981; 12: 587-99.
26 Kolb H. A ganglioside receptor on lymphocytes mediates
recognition of self. Biochem. Biophys. Res. Comm. 1982; 85;
678-83.
27 Parish CR, Rylatt DB. Snowden JM. Demonstration of lymphocyte surface lectins that recognize sulphated polysaccharides. / Celt Sci. 1984; 67: 145-58.
28 Sia DY, Parish CR. Anti-self receptors. III. Lack of allelic
exclusion and thymic epithelium dependence of H-2L
region-restricted receptors on lymphocytes. Scand. J. Immunol. 1981; 13; 535-40.
29 Rylatt DB, Sia DY, Mundy JP, Parish CR. Autorosette inhibition factor; isolation and properties of the human plasma
protein. Eur. J. Biochem. 1981; 119: 641-6.
30 Shatsky M, Saigo K, Burdach S, Leung LLK, Levitt U .
Histidine-rich glycoprotein blocks T cell rosette formation
and modulates both T cell activation and immunoregulation.
J. Biol. Chem. 1989; 264: 8254-9.
31 Malnati MS, Peruzzi M, Parker KC el al. Peptide specificity
Tolerance to tissue-specific self antigens
32
33
34
35
36
37
38
39
40
in the recognition of MHC class I by natural killer cell clones.
Science 1995; 267; 1016-18.
Parish CR. The paradox of carbohydrate histocompatibility
antigens. In; Hoffmann GW, Levy JG, Nepom GT (eds)
Paradoxes in Immunology, Boca Raton. CRC Press, 1986;
187-98.
Kesson AM, Blanden RV, Mullbacher, A. The secondary in
vitro murine cytotoxic T cell response to the flavivirus. West
Nile. Immunol. Cell Biol. 1988; 66: 23-32.
Alferink J, Schittek B, Schonrich G. Hammerlmg G, Arnold
B. Long life span of tolerant T cells and the role of antigen in
maintenance of peripheral tolerance. Int. Immunol. 1995; 7:
331-6.
Coutinho A, Bandcira A. Tolerize one, tolerize them all:
tolerance is self assertion. Immunol. Today 1989; 10: 264-6.
McCullagh P. The significance of immune suppression in
normal self tolerance. Immunol. Rev. 1996 (in press).
MacDonald HR, Cerottini J-C. Ryser J-E et al. Quantitation
and cloning of cytolytic T lymphocytes and their precursors.
Immunol. Rev. 1980: 51: 93-120.
Goodnow CC. B-cell tolerance. Curr. Opin. Immunol. 1992;
4: 703-10.
Gcldhof AB. Racs G. Bakkus M. Devos S, Thielcmans K,
Debaetsclier P. Expression of B7-1 by highly metastatic
mouse T lymphomas induces optimal natural killer cellmediated cytotoxicity. Cancer Res. 1995; 55; 2730-3.
Hildcmann WH. Specific immunorecognition by histocompatibility markers: The original polymorphic system of immunoreactivity characteristic of all multicellular animals.
Immunogenetics \911\ 5: 193-202.
285
41 Kolb H. On the phylogenetic origm of the immune
system-a hypothesis. Dev. Comp. Immunol. 1977; 1; 193206.
42 Langman RE. Cell-mediated immunity and the major histocompatibility complex. Rev. Physiol. Biochem. Pharmacol.
1978; 81: 1-37.
43 Cohn M. The T-cell receptor mediating restrictive recognition of antigen. Cell 1983; 33: 657-69.
44 Coombe DR, Ey PL, Jenkin CR. Self/non-self recognition in
invertebrates. Q. Rev. Biol. 1984; 59: 231-55.
45 Coombe DR, Parish CR. Sulfated polysaccharide-mediated
sponge cell aggregation: The clue to invertebrate self/nonselfrecognition? In: Grosberg RK, Hedgccock D. Nelson K. (eds)
Invertebrate Historecognition. New York. Plenum Publishing
Corp., 1988; 31-54.
46 Parish CR. A simple model for self non-self discrimination in
invertebrates. Nature 1911; 267; 711-12.
47 Rothenberg BE. The self recognition concept: an active function for the molecules of the major histocompatibility complex based on the complementary interaction of protein and
carbohydrate. Dev. Comp. Immunol. 1978; 2: 23-38.
48 Scofield VL, Schlumpberger JM. Weissman IL. Colony specificity in the colonial tunicate Botryltus and the origins of
vertebrate immunity. .4m. Zooi 1982; 22; 783-94.
49 Theodor JL. Histo-incompatibility in a natural population of
gorgonians. Zooi J. Linn. Soc. 1976; 58: 173-6.
50 Reynolds BD. Interactions of protoplasmic masses in relation to the study of heredity and environment in .Arcella
polypora. Biol. Bull. 1924; 46; 106-40.