14 Overview and future perspectives G O R D O N M. S T I R R A T JAMES R. SCOTT
INTRODUCTION Our understanding of the immune system has increased dramatically over the last two decades predominantly as a result of the introduction of monoclonal antibody technology and protein sequencing. An explosion of knowledge is beginning, brought about by molecular cloning and sequencing of genes coding for surface molecules. By the time of the 4th International Workshop on Human Leucocyte Differentiation Antigens in 1989 almost 90 distinct human leukocyte surface molecules had been identified by monoclonal antibodies (Knapp et al, 1989). They have been given the prefix CD for 'cluster determinant' (sometimes CDw denoting provisional designation by the workshop). It is clear that many of them are broadly distributed over a variety of types of leukocyte and some are present on seemingly totally unrelated cell types, e.g. platelets, epithelial cells and brain cells. Thus 'immunology now finds itself within the mainstream of classical molecular biology, its central phenomena providing a fertile hunting ground for clues to the basis of cell signalling, differentiation and growth control' (Nossal, 1988). CELL ADHESION MOLECULES---FAMILIES AND SUPERFAMILIES
Other strands of evidence point ineluctably in the same direction as suggested by Nossal (1988). For example, at least four families of ceU-surface molecules regulate the migration of lymphocytes and their interaction during immune responses (Springer, 1990; Mallett and Barclay, 1991). Several members of these families also subserve non-immunological functions within other systems in the body. The first of these families to be described was the immunoglobulin superfamily (IgSF). Some of its members are described in Table 1. The most significant feature of this family is that its members share 90-100 amino acids ('the immunoglobulin domain') suggesting that, despite their varied functions, they originally sprang from a common primeval gene. It is, therefore, likely that the IgSF are related in evolution and share the functional property of involvement in recognition Baillidre's Clinical Obstetrics and Gynaecology-657 Vol. 6, No. 3, September 1992 Copyright© 1992, by Bailli~re Tindall ISBN 0-7020-1634-9 All rights of reproduction in any form reserved
658
G. M. STIRRATAND J. R. SCOTT Table 1. Some members of the immunogiobulin supergene family (IgSF).
Immunoglobulins Major histocompatibility antigens [3z-microglobulin Several T cell antigens T cell receptor Macrophage Fc receptor Carcinoembryonal antigens Colony stimulating factor I receptor Platelet derived growth factor receptor Neural cell adhesion molecule Myelin-associated glycoprotein Major glycoprotein of peripheral myelin Lymphocyte-related function antigen 2 Lymphocyte-related function antigen 3 Intercellular cell adhesive molecule 1 Intercellular cell adhesive molecule 2
IgG, IgM, IgA, IgD, IgE MHC class I & class II B2m CD1,2,4&8 Tcr (CD3) FcR CLA CSF-IR PDGFR N-CAM MAG Po LFA-2 LFA-3 ICAM-1 ICAM-2
Self/non-self recognition Viral infection
6U!lleU6!S IleO-IlaO Growth • development Figure 1. Immunoglobulinsupergene family.
OVERVIEW AND FUTURE PERSPECTIVES
659
events at the cell surface (Williams, 1984). Williams suggested that molecules corresponding to a single immunoglobulin domain were first expressed at cell surfaces at an early stage in the evolution of multicellular organisms and that these molecules interacted with receptors on other cells as an integral part of cellular interactions. Increasing diversity and sophistication of both arms of this recognition system allowed specificity of cell interactions in development and tissue formation. It is suggested that it was from these primitive recognition systems that self/non-self recognition evolved into the immune system. Figure 1 provides a paradigm for this process. The second major cell-adhesion family and among the most versatile, is that of the integrin adhesion receptors (Springer, 1990). Like many receptors, integrins transduce information from the outside to the inside of the cell. Growth and differentiation of a variety of connective tissues and nervous system cells are dependent on the integrins. They contain or- and J3-subunits of between 750 and 1100 amino acids. The o~-subunits are up to 65% identical and the J3-subunits up to 45% identical in amino acid sequence. These subfamilies of integrins are distinguished by their ~3-subunits known as 13i-, [~2- and J33-integrins. The 131-subfamily includes receptors which bind to the extracellular matrix components fibronectin, laminin and collagen found within the tissues and basement membranes of, for example, muscle, the nervous system and epithelium including endothelium. Among the integrins with immune regulatory functions are lymphocyte-related function antigen 1 (LFA-1) and very late activation molecules VLA-4, 5 and 6. LFA-1, restricted to leukocytes, is involved in T helper and B-lymphocyte responses, natural killer (NK) cell function, antibody-dependent mediated cytotoxicity, and adherence of leukocytes to endothelial cells, fibroblasts and epithelial cells (Springer, 1990). The counter-receptors or ligands for LFA-1 on the target cell are ICAM-1 or 2 (for intercellular cell adhesive molecules), both of which are members of the IgSF. In contrast to the restricted expression of LFA-1, ICAM-1 is found on a wide variety of cells particularly during inflammatory responses in which it is intimately involved. LFA-2 and 3 are also members of the IgSF, the latter being the counter-receptor on the target cell for the former on the lymphocyte (the idiosyncrasy and inconsistency of the nomenclature must soon be sorted out!). Figure 2 illustrates how the IgSF and LFA-1 function as recognition molecules during antigen-specific T cell interaction. This is discussed in greater detail in Chapter 1. Less is known about a third family known as 'selectins' which are involved in cell interactions in the vascular tree and are expressed on leukocytes and endothelial cells (Springer, 1990). The selectins have sequences of amino acids in common with several lectins (proteins which activate lymphocytes), epidermal growth factor (EGF) and many complement regulatory proteins (see Chapter 3). So far, three selectins have been identified and found to regulate leukocyte binding to endothelium at inflammatory sites (Springer, 1990). Mel-14 and endothelial leukocytes adhesion molecule (ELAM-1) bind neutrophils
660
G. M. STIRRAT AND J. R. SCOTI"
-'3
Members of IgSF Figure 2. Antigen-specific T cell interaction (adapted from Springer, 1990). This shows the important role of members of the IgSF and the integrin LFA-1 as receptors on T cells and counter-receptors on target or antigen-presenting cells. The main purpose of these adhesion receptors is cell-cell signalling.
to the endothelium, but the former is quickly released from the cell surface and the latter rapidly inactivated. Integrins, which remain permanently upregulated on the activated neutrophil, then take over to achieve transendothelial migration. A fourth superfamily of cell surface proteins has recently been described (Mallett and Barclay, 1991). Its members are related to the nerve growth factor receptor (NGFR) found on neural cells and within it are the B cell antigen designated CD40, the activated T cell antigen OX-40, and two receptors for tumour necrosis factor (TNF) called TNFR-I and TNFR-II and found on a variety of cell types. Most of the current functional information exists about NGFR and TNFR-I and II which are cytokine growth factor receptors.
661
OVERVIEW AND FUTURE PERSPECTIVES
THE IMMUNE SYSTEM AS PART OF A GREATER CELL-CELL
SIGNALLING SYSTEM It is, therefore, clear that the 'immune system' is not as discrete an entity either functionally or in terms of its effectors and receptors as had been originally believed. On one hand, polypeptides produced by activated cells initially named interleukins then lymphokines (because they were thought to be produced solely by leukocytes), are now called cytokines because the same polypeptides are produced by a variety of cell types. In addition the same cytokine can function within several cells and tissues. For example, cytokines are crucial mediators between vascular cells and leukocytes (Mantovani and Dijana, 1989). Endothelial cells which function like mononuclear phagocytes in their production and response to various cytokines are active participants in the induction and regulation of coagulation, inflammation and immunity. In addition there is such a strong link between immunology, growth and differentiation, and oncology that categorization becomes meaningless. Several proto-oncogenes code either for cytokine growth factors or their receptors (see Table 2) (Adamson, 1987). Similarly, some cytokines cause transcription of cellular proto-oncogenes. Table 2. Some proto-oncogenes (c-onc) and their products. c-onc
Product
Putative function
c-sis
Platelet derived growth factor (PDGF) [3-chain
Growth factor in embryogenesis (?) Mitogenesis Proliferation Regulation of DNA synthesis Regulator of growth and metabolism Signal transduction in mitogenesis and differentiation Signal transduction in mitogenesis and differentiation
c-myc c-erb-A c-erb-B
c-frns
Thyroid hormone receptor Epidermal growth factor receptor (EGFR) Colony stimulating factor 1 receptor (ISF-IR)
Exactly the same diversity of expression and function applies to the families of cell adhesion receptor molecules such that the conceptual boundaries between, for example, the immune system, the vascular system, and the nervous system are now at least blurred if not non-existent. Janeway (1988) has pointed out that the immune and nervous systems are often compared. Both react in a very specific manner to signals, both have memory, and the cellular organization of both is highly complex in which cells with similar outward appearance have widely different functions. The main feature which distinguishes them is their very different anatomical organization. The immune system is highly mobile whereas the nervous system is relatively static. However, even this difference is lessening because it has now been demonstrated that one particular subset of lymphocytes has a very precisely defined anatomy. Lymphocytes have classically been described as having receptors with variable chains designated tx and 13. A small subset has now been discovered with different variable chains now called ~/and 8. These lymphocytes are only found in epithelia and two of the
662
G. M, STIRRAT AND J. R. SCOTT
most important are the dendritic epithelial cells (DEC) in skin and the intestinal epithelial lymphocytes (IEL). The ot/13T cells are the dominant immunological effector cells characterized by their recirculation capacity and specialization for MHC recognition. The ~//~ T cells seem to be less sophisticated in their antigen specificity and memory. This has led to the speculation (Mantovani and Dijana, 1988) that epithelial surfaces were the site at which cell-mediated immunity originally developed in evolutionary terms. The ~/~ T cell system may, therefore, be the progenitor of the call3 antigen-specific T cell. SOME OTHER EXAMPLES OF SEQUENCE HOMOLOGY AND THEIR CONSEQUENCES Structural homology is not only a feature of the families and superfamilies of cell adhesion molecules. Many major antigens recognized during a wide variety of bacterial and parasitic diseases belong to conserved protein families which share extensive amino acid sequences identical with the surface molecules on the host's cells (Cohen and Young, 1991). For example, the induction of heat shock proteins (hsp) is brought about by cellular 'stress' in any type of cell, from prokaryotic to human. Infection of the body by bacteria and many protozoan and helminthic parasites results in antibody responses to hsps. The protein structure of members of the hsp family is remarkably similar: more than 50% of the amino acid sequences are shared by, for example, bacterial and mammalian hsps. This produces a problem for 'classical' immunology because bacterial hsps are strongly immunogenic to the mammalian immune system despite containing a series of epitopes which are also 'self' to that animal. One might predict that immune reactions to hsps might be associated with autoimmune disease. This is in fact the case (reviewed in Cohen and Young, 1991): immunity to mycobacterial hsp 65 is associated with autoimmune arthritis and diabetes. Immunity to hsps 70 and 90 is linked with systemic lupus erythematosus (SLE). The puzzle is not that this happens but why it occurs so infrequently. Cohen and Young (1991) pose the questions--what is the point of the immune system focusing on the very molecules that the infective agent shares with the host: how is this focus of attention encoded within the system: and how is the chance of autoimmune disease reduced or eliminated? In trying to answer these questions about 'self' and 'non-self' they draw an analogy with the nervous system 'homunculus' to be found in all physiology textbooks. The neurological homunculus draws a functional picture of the body in terms of the dominance of the neural networks within different organs or structures. Thus the tongue and fingers are much more prominent in the homunculus than the trunk. The 'self' seen by the nervous system is different from that described anatomically. They invoke the presence of immune networks centred around a selected few 'self-antigens' which encode the dominance of these antigens as a result of preformed lymphocyte networks. This produces an alternative 'homunculus' in which the immune system 'sees' some parts of self as being dominant. This is illustrated in Figure 3. Each dominant
663
OVERVIEW AND FUTURE PERSPECTIVES
neT ork
. ~ )
cell
.......
/ ,// ",,,,
(2), ," ........ .:""............
"
i .........
Controlled autoimmune
response
®
MHC Class U
presentation Conserved antigen on microbe
®
(~) AutoimmuneB / ~ Self epitope Microbial epitope
response
Microbe- (~) specificT cell
Figure 3. Immune homunculus lymphocyte network (derived from Cohen and Young, 1991). 1, The conserved antigen on the microbe is recognized by a natural autoimmune B cell (or macrophage); 2, the self and microbial epitopes are processed by the B cell which presents them on its cell surface; 3/4, the response to the self epitope is regulated by the lymphocyte network; 5, this results in a controlled autoimmune response; 6/7, T cells recognize the microbial antigen and are free to respond aggressively.
self-antigen is, they suggest, served by an interacting set of T and B cells some of which suppress and others stimulate. As a result of connections between various interacting lymphocytes in the network, some lymphocytes can become activated even without being driven by contact with a specific immunogen. They argue that this immunological homunculus hypothesis would allow the immune system to deal with self molecules efficiently and predictably. The dominance of a few selected self-antigens 'blinds' the system to other competing 'recessive' self-antigens to which it is, therefore, tolerant. It also links immunological dominance with control. Autoimmune responses are channelled and controlled so that autoimmunity is graded, guarded and usually transient. Hsp molecules have been conserved because they are essential to both the host and parasite. In immune terms the body has made a virtue out of necessity. If, of course, the immune homunculus errs by omission or commission frank autoimmune disease can result. The ideal therapy for autoimmune disease is not, therefore, general suppression of the immune system or inflammatory responses but restoration of the connections between the
664
6 . M. STIRRAT A N D J. R. SCOT'I"
natural T and B cell networks and the autoimmune effector cells, e.g. by vaccination using attenuated autoimmune cytotoxic T cells (reviewed in Cohen and Young, 1991). Biochemical similarities between the mechanisms within the human body and lower orders is not confined to hsps. There is clear peptide sequence homology between the MHC system of vertebrate and invertebrate chordates (Reinisil and Litman, 1989). This has been a neglected area of study but from it has arisen the observation that cell surface adhesion molecules involved in MHC recognition (Figure 2) may share characteristics with those involved in sperm recognition of the envelope of the ovum. In addition, it is now becoming clear that MHC associated immune functions in all classes of vertebrates derive from genes and gene products similar to those in mammals. Chicken MHC class IIB genes are related to mouse and human class II two genes to the same extent. Gene duplications probably occurred independently in the avian and mammalian lineages and subsequently in different groups of placental mammals.
OVERVIEW
As a result of these developments in our understanding of the immune system, the genes controlling it, its products and their receptors, it is apparent that: 1.
The immune system is an integral part of a generalized cell-cell signalling system which, among its functions, has developed the ability to recognize self and non-self. 2. The genes coding for the immune system developed from a primitive progenitor, developing antigen specificity in the process. In doing so the humoral immune system became linked opportunistically with the classical pathway of complement activation (Atkinson and Farries, 1987). The so-called 'alternative' pathway is probably phylogenetically older than the 'classical' pathway (see Chapter 3). The immune system has also turned potentially serious phylogenetic problems, such as the sharing of heat shock proteins with pathogens, into advantageous mechanisms for controlling autoimmunity while still retaining the ability to eliminate the infective agents. 3. The fundamental purpose of cell adhesion molecules, originally described in relation to immune regulation, is to do with the regulation of development and differentiation (Cunningham, 1991). One of the outstanding features is the dynamic pattern of their expression during development (for references see Cunningham, 1991). Their expression and that of their receptors is often coded for by proto-oncogenes (c-onc) which are also crucially involved in growth regulation and differentiation (Adamson, 1987) (see Table 2). The roles of the c-oncs may vary with the stage of development. Eades et al (1988) found tumour necrosis factor receptor (TNFR) in a purified plasma membrane preparation of human villous trophoblast. Among the proto-oncogenes expressed on
OVERVIEW A N D FUTURE PERSPECTIVES
665
extravillous trophoblast are c-sis and c-erb (see Table 2), which code for PDGF J3-chain and EGFR respectively (Gousti et al, 1985; Venter et al, 1987). Tavare and Holmes (1989) observed receptors for epidermal growth factor (EGFR) and insulin throughout gestation on syncytiotrophoblast. On first-trimester cytotrophoblast, however, the expression of EGFR decreased as the cytotrophoblast columns invade the maternal decidua whereas the expression of insulin receptors remained constant. This differential expression is probably relevant to placental development. CONSEQUENCES FOR IMMUNE DISEASES IN OBSTETRICS AND GYNAECOLOGY
1.
The clear placement of the immune system within a generalized system regulating cell-cell signalling and fundamentally involved in development gives obstetricians and gynaecologists and scientists involved in research on reproductive biology and immunology a highly privileged position for advancing understanding of these crucial areas. The messages which pass between mother and fetus as a result of cell-cell signalling from the earliest point in implantation throughout the whole of pregnancy are highly relevant to the normal fetomaternal immune relationship (Chapter 2) and pregnancy disorders consequent on pathology within the placental bed (Chapter 4). This could also play a role in pregnancy loss (Chapter 5). Now that the lack of classical MHC antigens on syncytiotrophoblast and the non-classical nature of the MHC antigens on other populations of trophoblast has been established beyond peradventure (see Chapter 2), more attention can be focused on developmental questions posed by the possible functions of the nonclassical HLA-G found there, and of the profusion of cell adhesion molecules and their receptors on trophoblast. Holmes and Simpson (Chapter 3) have already given new evidence for the importance of complement regulatory protein expression at the fetomaternal interface and this story will be developed over the next few years. The model of sperm-egg interactions as an illustration of the involvement of cell surface adhesion molecules in MHC recognition (Reinisil and Litman, 1989) (and vice versa) may be initially useful as reproductive biologists try to understand the former and immunologists the latter. 2. The immunological homunculus hypothesis of Cohen and Young (1991) is of considerable relevance to further developments in our understanding of autoimmune conditions in pregnancy (Chapter 9), and the anticardiolipin antibody (ACA) syndrome discussed in Chapter 6. Because of the 'partial self' nature of the fetus in respect of the mother (and vice versa) it may also shed light on Rhesus disease and such 'alloimmune' conditions as thrombocytopenia (Chapter 8). In addition, the attempts to develop contraceptive vaccines (Chapter 12) may be facilitated by some of the concepts outlined by Cohen and Young (1991).
666 3. 4.
5.
G. M. STIRRAT AND J. R. SCOTT
The cytokine link between leukocytes and endothelial cells raises interesting questions about the pathogenesis of pre-eclampsia addressed in Chapter 10. Regrettably, several cell adhesion receptors are subverted as virus receptors (White and Littman, 1989). Of more than 100 distinct rhinovirus variants, 90% bind to the IgSF member, ICAM-1. Rhinoviruses use the immune response to induce sneezing and the secretion of mucus which facilitates spread to other individuals. CD4, a member of the IgSF, acts as a receptor for HIV which then infects and kills or renders functionless CD4+ T helper cells and downregulates CD4 expression. It also spreads by fusion of infected with uninfected cells to form syncytia by involving the integrin LFA-1. This is of relevance to the discussion of HIV infection in pregnancy in Chapter 11. The increasingly obvious overlap between immunology and oncology in the context of recognition systems which are fundamentally to do with cell differentiation and development (Nossal, 1988) are highly relevant to the discussions on gestational trophoblastic tumours (Chapter 7) and gynaecological cancer (Chapter 13). Malik and Epenetos (Chapter 13), for example, have elegantly presented the role of cytokines in pathophysiology and potential for treatment of gynaecological cancers.
CONCLUSION The 'state of the art' consideration of the immune system in health and disease in obstetrics and gynaecology contained in this volume illustrates well the value of an interdisciplinary approach to a subject. This will become increasingly important now that the artificial barriers between physiological systems is being broken down. Our understanding of what is scientifically and clinically relevant to obstetricians, gynaecologists, reproductive biologists or immunologists can only increase if unhelpful barriers with other clinicians and scientists are also removed. Cassius says in Shakespeare's Julius Caesar: 'There is a tide in the affairs of men which, taken at the flood leads on to fortune.' The biochemical and molecular biological technology now (and increasingly) at our disposal means that, by interdisciplinary and, sometimes, lateral thinking, we are at a watershed in the scientific understanding of the pathophysiological processes involved in development and differentiation. Who knows what scientific fortunes could result if these opportunities are taken; or to what intellectual or clinical 'shallows and miseries' we will be bound if we do not. REFERENCES Accolla RS, Auffray C, Singer DS & Guardiola J (1991) The molecular biology of MHC genes. Immunology Today 12: 97-99. Adamson ED (1987) Oncogenes in development. Development99: 449--471. Atkinson JP & Farries T (1987) Separation of self from non-self in the complement system. Immunology Today8: 212-215.
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Cohen IR & Young DB (1991) Auto-immunity, microbial immunity and the immunological homunculus. Immunology Today 12: 105--110. Cunningham BA (1991) Cell adhesion molecules and the regulation of development. American Journal of Obstetrics and Gynecology 164: 939-948. Eades DK, Cornelius P & Fekala PH (1988) Characterisation of the tumour necrosis factor receptor in human placenta. Placenta 9: 247-251. Gousti AS, Betsholtz C & Pfeifer-Chisson S (1985) Co-expression of the s/s and myc protooncogenes in developing human placenta suggests autocrine control of trophoblast growth. Cell 41: 301-312. Janeway CA (1988) Frontiers of the immune system. Nature 333: 804-806. Knapp W, Rieber P, Dorken B e t al (1989) Towards a better definition of human leucocyte surface molecules. Immunology Today 10: 253-261. Mallett S & Barclay AN (1991) A new superfamily of cell-surface proteins related to the nerve growth-factor receptor. Immunology Today 12: 220-223. Mantovani A & Dijana E (1989) Cytokines as communication signals between leucocytes and endothelial cells. Immunology Today 10: 370-375. Nossal GJ (1988) Triumphs and trials of immunology in the 1980's. Immunology Today 9: 286--291. Reinisil CL & Litman GW (1989) Evolutionary immunobiology. Immunology Today 10: 278--281. Springer TA (1990) Adhesion receptors of the immune system. Nature 346: 425-434. Tavare JM & Holmes CH (1989) Differential expression of the receptors for epidermal growth factor and insulin in the developing human placenta. Cell Signalling 1" 55--64. Venter DJ, Tusi NK, Kumar S & Gullick WJ (1987) Over expression of the c-erbB-2 oncoprotein in human breast carcinomas: immunohistological assessment correlates with gene amplification. Lancet if: 69-72. White JM & Littman DR (1989) Viral receptors of the immunoglobulin superfamily. Cell 56: 725-728. Williams AF (1984) The immunoglobulin superfamily takes shape. Nature 308: 12-13.
INTRODUCTION Our understanding of the immune system has increased dramatically over the last two decades predominantly as a result of the introduction of monoclonal antibody technology and protein sequencing. An explosion of knowledge is beginning, brought about by molecular cloning and sequencing of genes coding for surface molecules. By the time of the 4th International Workshop on Human Leucocyte Differentiation Antigens in 1989 almost 90 distinct human leukocyte surface molecules had been identified by monoclonal antibodies (Knapp et al, 1989). They have been given the prefix CD for 'cluster determinant' (sometimes CDw denoting provisional designation by the workshop). It is clear that many of them are broadly distributed over a variety of types of leukocyte and some are present on seemingly totally unrelated cell types, e.g. platelets, epithelial cells and brain cells. Thus 'immunology now finds itself within the mainstream of classical molecular biology, its central phenomena providing a fertile hunting ground for clues to the basis of cell signalling, differentiation and growth control' (Nossal, 1988). CELL ADHESION MOLECULES---FAMILIES AND SUPERFAMILIES
Other strands of evidence point ineluctably in the same direction as suggested by Nossal (1988). For example, at least four families of ceU-surface molecules regulate the migration of lymphocytes and their interaction during immune responses (Springer, 1990; Mallett and Barclay, 1991). Several members of these families also subserve non-immunological functions within other systems in the body. The first of these families to be described was the immunoglobulin superfamily (IgSF). Some of its members are described in Table 1. The most significant feature of this family is that its members share 90-100 amino acids ('the immunoglobulin domain') suggesting that, despite their varied functions, they originally sprang from a common primeval gene. It is, therefore, likely that the IgSF are related in evolution and share the functional property of involvement in recognition Baillidre's Clinical Obstetrics and Gynaecology-657 Vol. 6, No. 3, September 1992 Copyright© 1992, by Bailli~re Tindall ISBN 0-7020-1634-9 All rights of reproduction in any form reserved
658
G. M. STIRRATAND J. R. SCOTT Table 1. Some members of the immunogiobulin supergene family (IgSF).
Immunoglobulins Major histocompatibility antigens [3z-microglobulin Several T cell antigens T cell receptor Macrophage Fc receptor Carcinoembryonal antigens Colony stimulating factor I receptor Platelet derived growth factor receptor Neural cell adhesion molecule Myelin-associated glycoprotein Major glycoprotein of peripheral myelin Lymphocyte-related function antigen 2 Lymphocyte-related function antigen 3 Intercellular cell adhesive molecule 1 Intercellular cell adhesive molecule 2
IgG, IgM, IgA, IgD, IgE MHC class I & class II B2m CD1,2,4&8 Tcr (CD3) FcR CLA CSF-IR PDGFR N-CAM MAG Po LFA-2 LFA-3 ICAM-1 ICAM-2
Self/non-self recognition Viral infection
6U!lleU6!S IleO-IlaO Growth • development Figure 1. Immunoglobulinsupergene family.
OVERVIEW AND FUTURE PERSPECTIVES
659
events at the cell surface (Williams, 1984). Williams suggested that molecules corresponding to a single immunoglobulin domain were first expressed at cell surfaces at an early stage in the evolution of multicellular organisms and that these molecules interacted with receptors on other cells as an integral part of cellular interactions. Increasing diversity and sophistication of both arms of this recognition system allowed specificity of cell interactions in development and tissue formation. It is suggested that it was from these primitive recognition systems that self/non-self recognition evolved into the immune system. Figure 1 provides a paradigm for this process. The second major cell-adhesion family and among the most versatile, is that of the integrin adhesion receptors (Springer, 1990). Like many receptors, integrins transduce information from the outside to the inside of the cell. Growth and differentiation of a variety of connective tissues and nervous system cells are dependent on the integrins. They contain or- and J3-subunits of between 750 and 1100 amino acids. The o~-subunits are up to 65% identical and the J3-subunits up to 45% identical in amino acid sequence. These subfamilies of integrins are distinguished by their ~3-subunits known as 13i-, [~2- and J33-integrins. The 131-subfamily includes receptors which bind to the extracellular matrix components fibronectin, laminin and collagen found within the tissues and basement membranes of, for example, muscle, the nervous system and epithelium including endothelium. Among the integrins with immune regulatory functions are lymphocyte-related function antigen 1 (LFA-1) and very late activation molecules VLA-4, 5 and 6. LFA-1, restricted to leukocytes, is involved in T helper and B-lymphocyte responses, natural killer (NK) cell function, antibody-dependent mediated cytotoxicity, and adherence of leukocytes to endothelial cells, fibroblasts and epithelial cells (Springer, 1990). The counter-receptors or ligands for LFA-1 on the target cell are ICAM-1 or 2 (for intercellular cell adhesive molecules), both of which are members of the IgSF. In contrast to the restricted expression of LFA-1, ICAM-1 is found on a wide variety of cells particularly during inflammatory responses in which it is intimately involved. LFA-2 and 3 are also members of the IgSF, the latter being the counter-receptor on the target cell for the former on the lymphocyte (the idiosyncrasy and inconsistency of the nomenclature must soon be sorted out!). Figure 2 illustrates how the IgSF and LFA-1 function as recognition molecules during antigen-specific T cell interaction. This is discussed in greater detail in Chapter 1. Less is known about a third family known as 'selectins' which are involved in cell interactions in the vascular tree and are expressed on leukocytes and endothelial cells (Springer, 1990). The selectins have sequences of amino acids in common with several lectins (proteins which activate lymphocytes), epidermal growth factor (EGF) and many complement regulatory proteins (see Chapter 3). So far, three selectins have been identified and found to regulate leukocyte binding to endothelium at inflammatory sites (Springer, 1990). Mel-14 and endothelial leukocytes adhesion molecule (ELAM-1) bind neutrophils
660
G. M. STIRRAT AND J. R. SCOTI"
-'3
Members of IgSF Figure 2. Antigen-specific T cell interaction (adapted from Springer, 1990). This shows the important role of members of the IgSF and the integrin LFA-1 as receptors on T cells and counter-receptors on target or antigen-presenting cells. The main purpose of these adhesion receptors is cell-cell signalling.
to the endothelium, but the former is quickly released from the cell surface and the latter rapidly inactivated. Integrins, which remain permanently upregulated on the activated neutrophil, then take over to achieve transendothelial migration. A fourth superfamily of cell surface proteins has recently been described (Mallett and Barclay, 1991). Its members are related to the nerve growth factor receptor (NGFR) found on neural cells and within it are the B cell antigen designated CD40, the activated T cell antigen OX-40, and two receptors for tumour necrosis factor (TNF) called TNFR-I and TNFR-II and found on a variety of cell types. Most of the current functional information exists about NGFR and TNFR-I and II which are cytokine growth factor receptors.
661
OVERVIEW AND FUTURE PERSPECTIVES
THE IMMUNE SYSTEM AS PART OF A GREATER CELL-CELL
SIGNALLING SYSTEM It is, therefore, clear that the 'immune system' is not as discrete an entity either functionally or in terms of its effectors and receptors as had been originally believed. On one hand, polypeptides produced by activated cells initially named interleukins then lymphokines (because they were thought to be produced solely by leukocytes), are now called cytokines because the same polypeptides are produced by a variety of cell types. In addition the same cytokine can function within several cells and tissues. For example, cytokines are crucial mediators between vascular cells and leukocytes (Mantovani and Dijana, 1989). Endothelial cells which function like mononuclear phagocytes in their production and response to various cytokines are active participants in the induction and regulation of coagulation, inflammation and immunity. In addition there is such a strong link between immunology, growth and differentiation, and oncology that categorization becomes meaningless. Several proto-oncogenes code either for cytokine growth factors or their receptors (see Table 2) (Adamson, 1987). Similarly, some cytokines cause transcription of cellular proto-oncogenes. Table 2. Some proto-oncogenes (c-onc) and their products. c-onc
Product
Putative function
c-sis
Platelet derived growth factor (PDGF) [3-chain
Growth factor in embryogenesis (?) Mitogenesis Proliferation Regulation of DNA synthesis Regulator of growth and metabolism Signal transduction in mitogenesis and differentiation Signal transduction in mitogenesis and differentiation
c-myc c-erb-A c-erb-B
c-frns
Thyroid hormone receptor Epidermal growth factor receptor (EGFR) Colony stimulating factor 1 receptor (ISF-IR)
Exactly the same diversity of expression and function applies to the families of cell adhesion receptor molecules such that the conceptual boundaries between, for example, the immune system, the vascular system, and the nervous system are now at least blurred if not non-existent. Janeway (1988) has pointed out that the immune and nervous systems are often compared. Both react in a very specific manner to signals, both have memory, and the cellular organization of both is highly complex in which cells with similar outward appearance have widely different functions. The main feature which distinguishes them is their very different anatomical organization. The immune system is highly mobile whereas the nervous system is relatively static. However, even this difference is lessening because it has now been demonstrated that one particular subset of lymphocytes has a very precisely defined anatomy. Lymphocytes have classically been described as having receptors with variable chains designated tx and 13. A small subset has now been discovered with different variable chains now called ~/and 8. These lymphocytes are only found in epithelia and two of the
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G. M, STIRRAT AND J. R. SCOTT
most important are the dendritic epithelial cells (DEC) in skin and the intestinal epithelial lymphocytes (IEL). The ot/13T cells are the dominant immunological effector cells characterized by their recirculation capacity and specialization for MHC recognition. The ~//~ T cells seem to be less sophisticated in their antigen specificity and memory. This has led to the speculation (Mantovani and Dijana, 1988) that epithelial surfaces were the site at which cell-mediated immunity originally developed in evolutionary terms. The ~/~ T cell system may, therefore, be the progenitor of the call3 antigen-specific T cell. SOME OTHER EXAMPLES OF SEQUENCE HOMOLOGY AND THEIR CONSEQUENCES Structural homology is not only a feature of the families and superfamilies of cell adhesion molecules. Many major antigens recognized during a wide variety of bacterial and parasitic diseases belong to conserved protein families which share extensive amino acid sequences identical with the surface molecules on the host's cells (Cohen and Young, 1991). For example, the induction of heat shock proteins (hsp) is brought about by cellular 'stress' in any type of cell, from prokaryotic to human. Infection of the body by bacteria and many protozoan and helminthic parasites results in antibody responses to hsps. The protein structure of members of the hsp family is remarkably similar: more than 50% of the amino acid sequences are shared by, for example, bacterial and mammalian hsps. This produces a problem for 'classical' immunology because bacterial hsps are strongly immunogenic to the mammalian immune system despite containing a series of epitopes which are also 'self' to that animal. One might predict that immune reactions to hsps might be associated with autoimmune disease. This is in fact the case (reviewed in Cohen and Young, 1991): immunity to mycobacterial hsp 65 is associated with autoimmune arthritis and diabetes. Immunity to hsps 70 and 90 is linked with systemic lupus erythematosus (SLE). The puzzle is not that this happens but why it occurs so infrequently. Cohen and Young (1991) pose the questions--what is the point of the immune system focusing on the very molecules that the infective agent shares with the host: how is this focus of attention encoded within the system: and how is the chance of autoimmune disease reduced or eliminated? In trying to answer these questions about 'self' and 'non-self' they draw an analogy with the nervous system 'homunculus' to be found in all physiology textbooks. The neurological homunculus draws a functional picture of the body in terms of the dominance of the neural networks within different organs or structures. Thus the tongue and fingers are much more prominent in the homunculus than the trunk. The 'self' seen by the nervous system is different from that described anatomically. They invoke the presence of immune networks centred around a selected few 'self-antigens' which encode the dominance of these antigens as a result of preformed lymphocyte networks. This produces an alternative 'homunculus' in which the immune system 'sees' some parts of self as being dominant. This is illustrated in Figure 3. Each dominant
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OVERVIEW AND FUTURE PERSPECTIVES
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Figure 3. Immune homunculus lymphocyte network (derived from Cohen and Young, 1991). 1, The conserved antigen on the microbe is recognized by a natural autoimmune B cell (or macrophage); 2, the self and microbial epitopes are processed by the B cell which presents them on its cell surface; 3/4, the response to the self epitope is regulated by the lymphocyte network; 5, this results in a controlled autoimmune response; 6/7, T cells recognize the microbial antigen and are free to respond aggressively.
self-antigen is, they suggest, served by an interacting set of T and B cells some of which suppress and others stimulate. As a result of connections between various interacting lymphocytes in the network, some lymphocytes can become activated even without being driven by contact with a specific immunogen. They argue that this immunological homunculus hypothesis would allow the immune system to deal with self molecules efficiently and predictably. The dominance of a few selected self-antigens 'blinds' the system to other competing 'recessive' self-antigens to which it is, therefore, tolerant. It also links immunological dominance with control. Autoimmune responses are channelled and controlled so that autoimmunity is graded, guarded and usually transient. Hsp molecules have been conserved because they are essential to both the host and parasite. In immune terms the body has made a virtue out of necessity. If, of course, the immune homunculus errs by omission or commission frank autoimmune disease can result. The ideal therapy for autoimmune disease is not, therefore, general suppression of the immune system or inflammatory responses but restoration of the connections between the
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natural T and B cell networks and the autoimmune effector cells, e.g. by vaccination using attenuated autoimmune cytotoxic T cells (reviewed in Cohen and Young, 1991). Biochemical similarities between the mechanisms within the human body and lower orders is not confined to hsps. There is clear peptide sequence homology between the MHC system of vertebrate and invertebrate chordates (Reinisil and Litman, 1989). This has been a neglected area of study but from it has arisen the observation that cell surface adhesion molecules involved in MHC recognition (Figure 2) may share characteristics with those involved in sperm recognition of the envelope of the ovum. In addition, it is now becoming clear that MHC associated immune functions in all classes of vertebrates derive from genes and gene products similar to those in mammals. Chicken MHC class IIB genes are related to mouse and human class II two genes to the same extent. Gene duplications probably occurred independently in the avian and mammalian lineages and subsequently in different groups of placental mammals.
OVERVIEW
As a result of these developments in our understanding of the immune system, the genes controlling it, its products and their receptors, it is apparent that: 1.
The immune system is an integral part of a generalized cell-cell signalling system which, among its functions, has developed the ability to recognize self and non-self. 2. The genes coding for the immune system developed from a primitive progenitor, developing antigen specificity in the process. In doing so the humoral immune system became linked opportunistically with the classical pathway of complement activation (Atkinson and Farries, 1987). The so-called 'alternative' pathway is probably phylogenetically older than the 'classical' pathway (see Chapter 3). The immune system has also turned potentially serious phylogenetic problems, such as the sharing of heat shock proteins with pathogens, into advantageous mechanisms for controlling autoimmunity while still retaining the ability to eliminate the infective agents. 3. The fundamental purpose of cell adhesion molecules, originally described in relation to immune regulation, is to do with the regulation of development and differentiation (Cunningham, 1991). One of the outstanding features is the dynamic pattern of their expression during development (for references see Cunningham, 1991). Their expression and that of their receptors is often coded for by proto-oncogenes (c-onc) which are also crucially involved in growth regulation and differentiation (Adamson, 1987) (see Table 2). The roles of the c-oncs may vary with the stage of development. Eades et al (1988) found tumour necrosis factor receptor (TNFR) in a purified plasma membrane preparation of human villous trophoblast. Among the proto-oncogenes expressed on
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extravillous trophoblast are c-sis and c-erb (see Table 2), which code for PDGF J3-chain and EGFR respectively (Gousti et al, 1985; Venter et al, 1987). Tavare and Holmes (1989) observed receptors for epidermal growth factor (EGFR) and insulin throughout gestation on syncytiotrophoblast. On first-trimester cytotrophoblast, however, the expression of EGFR decreased as the cytotrophoblast columns invade the maternal decidua whereas the expression of insulin receptors remained constant. This differential expression is probably relevant to placental development. CONSEQUENCES FOR IMMUNE DISEASES IN OBSTETRICS AND GYNAECOLOGY
1.
The clear placement of the immune system within a generalized system regulating cell-cell signalling and fundamentally involved in development gives obstetricians and gynaecologists and scientists involved in research on reproductive biology and immunology a highly privileged position for advancing understanding of these crucial areas. The messages which pass between mother and fetus as a result of cell-cell signalling from the earliest point in implantation throughout the whole of pregnancy are highly relevant to the normal fetomaternal immune relationship (Chapter 2) and pregnancy disorders consequent on pathology within the placental bed (Chapter 4). This could also play a role in pregnancy loss (Chapter 5). Now that the lack of classical MHC antigens on syncytiotrophoblast and the non-classical nature of the MHC antigens on other populations of trophoblast has been established beyond peradventure (see Chapter 2), more attention can be focused on developmental questions posed by the possible functions of the nonclassical HLA-G found there, and of the profusion of cell adhesion molecules and their receptors on trophoblast. Holmes and Simpson (Chapter 3) have already given new evidence for the importance of complement regulatory protein expression at the fetomaternal interface and this story will be developed over the next few years. The model of sperm-egg interactions as an illustration of the involvement of cell surface adhesion molecules in MHC recognition (Reinisil and Litman, 1989) (and vice versa) may be initially useful as reproductive biologists try to understand the former and immunologists the latter. 2. The immunological homunculus hypothesis of Cohen and Young (1991) is of considerable relevance to further developments in our understanding of autoimmune conditions in pregnancy (Chapter 9), and the anticardiolipin antibody (ACA) syndrome discussed in Chapter 6. Because of the 'partial self' nature of the fetus in respect of the mother (and vice versa) it may also shed light on Rhesus disease and such 'alloimmune' conditions as thrombocytopenia (Chapter 8). In addition, the attempts to develop contraceptive vaccines (Chapter 12) may be facilitated by some of the concepts outlined by Cohen and Young (1991).
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The cytokine link between leukocytes and endothelial cells raises interesting questions about the pathogenesis of pre-eclampsia addressed in Chapter 10. Regrettably, several cell adhesion receptors are subverted as virus receptors (White and Littman, 1989). Of more than 100 distinct rhinovirus variants, 90% bind to the IgSF member, ICAM-1. Rhinoviruses use the immune response to induce sneezing and the secretion of mucus which facilitates spread to other individuals. CD4, a member of the IgSF, acts as a receptor for HIV which then infects and kills or renders functionless CD4+ T helper cells and downregulates CD4 expression. It also spreads by fusion of infected with uninfected cells to form syncytia by involving the integrin LFA-1. This is of relevance to the discussion of HIV infection in pregnancy in Chapter 11. The increasingly obvious overlap between immunology and oncology in the context of recognition systems which are fundamentally to do with cell differentiation and development (Nossal, 1988) are highly relevant to the discussions on gestational trophoblastic tumours (Chapter 7) and gynaecological cancer (Chapter 13). Malik and Epenetos (Chapter 13), for example, have elegantly presented the role of cytokines in pathophysiology and potential for treatment of gynaecological cancers.
CONCLUSION The 'state of the art' consideration of the immune system in health and disease in obstetrics and gynaecology contained in this volume illustrates well the value of an interdisciplinary approach to a subject. This will become increasingly important now that the artificial barriers between physiological systems is being broken down. Our understanding of what is scientifically and clinically relevant to obstetricians, gynaecologists, reproductive biologists or immunologists can only increase if unhelpful barriers with other clinicians and scientists are also removed. Cassius says in Shakespeare's Julius Caesar: 'There is a tide in the affairs of men which, taken at the flood leads on to fortune.' The biochemical and molecular biological technology now (and increasingly) at our disposal means that, by interdisciplinary and, sometimes, lateral thinking, we are at a watershed in the scientific understanding of the pathophysiological processes involved in development and differentiation. Who knows what scientific fortunes could result if these opportunities are taken; or to what intellectual or clinical 'shallows and miseries' we will be bound if we do not. REFERENCES Accolla RS, Auffray C, Singer DS & Guardiola J (1991) The molecular biology of MHC genes. Immunology Today 12: 97-99. Adamson ED (1987) Oncogenes in development. Development99: 449--471. Atkinson JP & Farries T (1987) Separation of self from non-self in the complement system. Immunology Today8: 212-215.
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Cohen IR & Young DB (1991) Auto-immunity, microbial immunity and the immunological homunculus. Immunology Today 12: 105--110. Cunningham BA (1991) Cell adhesion molecules and the regulation of development. American Journal of Obstetrics and Gynecology 164: 939-948. Eades DK, Cornelius P & Fekala PH (1988) Characterisation of the tumour necrosis factor receptor in human placenta. Placenta 9: 247-251. Gousti AS, Betsholtz C & Pfeifer-Chisson S (1985) Co-expression of the s/s and myc protooncogenes in developing human placenta suggests autocrine control of trophoblast growth. Cell 41: 301-312. Janeway CA (1988) Frontiers of the immune system. Nature 333: 804-806. Knapp W, Rieber P, Dorken B e t al (1989) Towards a better definition of human leucocyte surface molecules. Immunology Today 10: 253-261. Mallett S & Barclay AN (1991) A new superfamily of cell-surface proteins related to the nerve growth-factor receptor. Immunology Today 12: 220-223. Mantovani A & Dijana E (1989) Cytokines as communication signals between leucocytes and endothelial cells. Immunology Today 10: 370-375. Nossal GJ (1988) Triumphs and trials of immunology in the 1980's. Immunology Today 9: 286--291. Reinisil CL & Litman GW (1989) Evolutionary immunobiology. Immunology Today 10: 278--281. Springer TA (1990) Adhesion receptors of the immune system. Nature 346: 425-434. Tavare JM & Holmes CH (1989) Differential expression of the receptors for epidermal growth factor and insulin in the developing human placenta. Cell Signalling 1" 55--64. Venter DJ, Tusi NK, Kumar S & Gullick WJ (1987) Over expression of the c-erbB-2 oncoprotein in human breast carcinomas: immunohistological assessment correlates with gene amplification. Lancet if: 69-72. White JM & Littman DR (1989) Viral receptors of the immunoglobulin superfamily. Cell 56: 725-728. Williams AF (1984) The immunoglobulin superfamily takes shape. Nature 308: 12-13.
