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Chapter 7 Brain and immune system: a one-way conversation or a genuine dialogue?
E. R. de Kloet. V. M. Wiegant and D. de Wied (Eds.)
Prapess in Brain Research, Val. 12
71
0 1987 Elsevier Science Publishers B.V. (Biomedical Division)
CHAPTER 7
Brain and immune system: a one-way conversation or a genuine dialogue? Rudy E. Ballieux“ and Cobi J. Heijnenb “Departmentof Clinical Immunology. University Hospital, Utrecht and bDeparrment of Immunology. University Hospitalfor Children and Youth ‘Her Wilhelmina Kinderziekenhuis., Utrecht. The Netherlands
stem cell
Introduction The nervous system and the immune system show a certain degree of congruence in that they both mediate the interaction of the individual with the often hostile and threatening ‘Urnwelt’. The responses of both physiological systems have several characteristics in common: communication at a distance, capability to develop memory and the use of chemical messengers (neurotransmitters, neuroendocrine peptides and lymphokines) to transmit messages. Actually, at a recent meeting on psychoneuroimmunology, the immune system was referred to as a ‘mobile brain’. This is because new and exciting findings, which will be discussed later in this paper, support the concept of cells of the immune system serving as ‘free-floating nerve cells’. What cells are these? It is now generally known that the ‘work horses’ of the immune system are the lymphocytes. These cells are rather small (approximately 10pm in diameter), possess little cytoplasm and are in a resting state for the greater part of their lifespan. They circulate via lymph and bloodstream and home-in on lymphoid organs, such as tonsils, lymph nodes, spleen and gut-associated lymphoid tissues. Two major subsets of lymphocytes can be distinguished which develop from a stem cell localized in the bone marrow (Fig. 1). One subset
\
committed lymphoid
/t
T lymphocyte (cellular immunity )
marrow
B lymphocyte
(humoral immunity)
\
2
+
T-effector
CY toioxic lymphokines
T-muppresaor
I-helper
I
plasma cell
Fig. 1. The development of the T and B cell subsets. The NK cell is not included in this figure.
comprises lymphocytes which come to full maturation in the bone marrow itself and hence are
12
called B lymphocytes. Another subset of lymphocytes leaves the bone marrow at an immature stage to spend a (short) period of their life in the thymus. Here they develop into fully differentiated, mature (thymus-derived) T lymphocytes before entering the bloodstream to populate the various lymphoid organs. A third minor subset of mononuclear white blood cells lacks the surface and functional characteristics of T and B lymphocytes. This subset includes cells which kill certain tumor cells and virus-infected cells without any obvious immunological specificity. These cells are known as natural killer cells or NK cells. They most probably play an important role in the surveillance against tumors and viral infections. The B lymphocytes are the precursors of antibody forming cells: plasma cells. The transition of a B cell into a plasma cell involves the binding of antigen to the corresponding receptor at the cell surface of the B cell. This receptor-mediated signal results in activation of the B cell. The B cell then becomes sensitive towards non-antigen specific growth and differentiation factors (lymphokines) many of which are derived from T lymphocytes. For most antigens to stimulate B cells successfully an additional antigen-specific ‘helper-signal’ derived from T cells is required. The T cells which deliver this signal after being activated by antigen are referred to as helper T cells (Th-cells). In analogy to other biological processes, an ongoing immune response is subject to (feedback) regulation. Part of this immunoregulatory function is exerted by suppressor T cells (Ts-cells). Ts lymphocytes, in general, down-regulate immune reactivity. Apart from these regulatory T cell subsets, effector T cells can be distinguished functionally. This latter subset includes T lymphocytes that kill target cells (e.g. tumor cells or virus-infected cells) by direct cytotoxic action. Furthermore, there are T effector cells which, upon interaction with antigen, produce a variety of lymphoid factors (lymphokines) which mediate immunopharmacological reactions (e.g. delayed type hypersensitivity reactions).
Many of the various T and B cell functions can be measured in vitro in cell culture systems or in vivo in animal and human models. It is beyond the scope of this short review to discuss the various possibilities of analyzing immune capacity and the limitations of some of these experimental models. Relevant information can be found in several recent publications related to this aspect (Ballieux, 1984; Rose et al., 1986). The immune system and the nervous system not only share certain characteristics, they also have been shown to be functionally interconnected. There is ample evidence that stressful environmental conditions can modulate immune functions. This implies that the brain can influence the immune system. Indeed, it has been demonstrated in several studies, initiated by the work of Ader and Cohen (1973, that, e.g. a classical conditioning type of situation can alter T and B cell-mediated immune responses. On the other hand, experiments reported by Besedovski and his colleagues a decade ago (Besedovski et al., 1977; Besedovski et al., 1979) show clearly that activation of the immune system by antigenic stimulation induces changes in brain function and in neuroendocrine profiles. Some aspects of the bilateral interaction of central nervous system (CNS) and immune system will be discussed below.
The influence of psychosocial factors on immunocompetence Many studies, in man and in animal models, have shown that stress can alter immune reactivity. One of the classical examples in man has been the effect of bereavement on immune function as reported originally by Bartrop et al. (1977) and in later years in a series of studies by Stein and colleagues (Schleifer et al., 1983; Stein et al., 1986). These authors found decreased proliferative responses after stimulation of peripheral blood lymphocytes in cell cultures with mitogens. Both T and B cell responses were lowered compared to pre-bereavement levels. These well con-
13
trolled longitudinal studies of a prospective nature clearly demonstrated that suppression of mitogen-induced lymphocyte activity is a direct consequence of the bereavement event. An important element in psychosocial stress concerns the ability of the individual to cope with the situation. This was clearly demonstrated in a study by Samuel et al. (1986) on the effect of abortion or miscarriage on immune function. The results of their study show clearly that women who had not accepted the loss of their (unborn) child (poor coping) had a significant reduction in mitogen-induced T cell activity in comparison with women who could handle this event. A marked correlation was found between immunosuppression and degree of psychic depression. In a prospective study of less dramatic nature, Locke et al. (1984) examined healthy college students for an association between N K cell function, stressful conditions and rate of individual psychic distress which developed from the long-term stress. It was found that not stress itself, but stress in association with poor coping behavior (relatively high psychic distress), was significantly associated with low NK cell activity. It is obvious from these studies in humans that the ability to cope with the demanding situation greatly reduces the deleterious effect on the immune response. Although it is very difficult to translate these human studies into their animal equivalents, a unique animal stress model which allows for the analysis of individual coping behavior has been introduced by Croiset etal. (1986). These investigators adopted the one-trial learning passive avoidance test, described by Ader et al. (1972). In this model rats after habituation to the experimental environment receive a single and mild, but unavoidable electric footshock (learning trial). The effect of this aversive experience is tested 24 h or 120 h later in the so-called retention test, in which the rat is placed in a conflict situation. It can avoid being exposed again to the formerly aversive environment, but at the cost of remaining in an uncomfortable situation on a platform in bright light. The passive
avoidance behavior of the animals has been studied extensively in relztion to memory consolidation, retrieval processes and various (neur0)endocrine and physiological functions. The significance of these findings for the understanding of the observed changes in immune responsiveness, induced by the retention test, have recently been reviewed (Veldhuis et al., 1986). It was found by Croiset and her colleagues that splenic lymphocytes of rats that had been subjected to the retention test responded less well after stimulation in vitro with concanavalin A (Con A) than cells of rats of the apparatus control group (no shock received). Preliminary data disclosed that animals which showed a long entrance latency (effective avoidance behavior) had a more decreased immune reactivity than animals that exhibited a short entrance latency (manuscript in preparation). This observation touches upon the intriguing issue of whether effective avoidance behavior is in one way or another a reflection of poor coping ability. Apart from the above-mentioned studies investigating the effects of differences in personality traits regarding coping behavior, it is also possible to investigate the influence on the immune system of a stressful situation which is presented in either a controllable or an incontrollable way. Laudenslager and coworkers (1983) subjected rats to a restraint stress in combination with multiple electric shocks. The experimental group of animals was divided into animals given inescapable shocks and animals which received escapable shocks. The rats in the latter group had learned how to turn off the electric current; a paired animal received a shock of identical strength that it could not control. T cell function was measured in vitro by measuring the proliferative response of spleen cells after stimulation with the mitogen phytohaemagglutinin (PHA) and Con A. It was found that mitogen-induced T cell proliferation was suppressed in the ‘inescapable shock‘ group as compared to the ‘escapable shock’ group. Animals of the latter group did not differ from unshocked controls in their T cell response.
14
The same ‘yoked’ testing procedure had previously been applied to assess the effect of stressor controllability on tumor susceptibility. Sklar and Anisman (1979) as well as Visintainer et al. (1982) reported that experiencing uncontrollable exposure to electric shocks increased tumor growth and impaired tumor rejection. It therefore seems that controllability of the stressful situation may be one of the important factors that determines to what extent the immune response is modulated by the stressor. The results of a study by Koolhaas et al. (1986, pers. commun.) are interesting in this respect. These authors analyzed several immune parameters in rats living in long-term colonies. Under such conditions a social hierarchy is formed in which the leader of the colony can be distinguished next to a few potential leaders. The remaining animals show a more or less subordinate behavior. It was found that the number of peripheral blood lymphocytes of the individual rats correlated significantly with flight and escape behavior in the colony which was most prominent in the dominant animal. The Con Ainduced T cell response also showed a correlation with the social status of the animal in the colony. Although the experimental situation in the colonies differs basically from that in the ‘yoked situation’, one could argue that the social status relates to the degree of controllability of the environmental conditions in the colony. It is remarkable that social status also seems to be associated with baseline immune function.
Mechanisms involved in stress-induced immunomodulation Two main pathways can be distinguished for mechanisms involved in neuroimmunomodulation. One pathway is of a humoral nature and involves the hypothalamus/pituitary/adrenal axis and corresponding hormones. These include enkephalins and endorphins, ACTH, vasopressin, prolactin, corticosteroids and sex hormones. The second pathway is represented by the autonomic nervous system, including the innervation of lymphoid
tissues such as thymus, spleen, lymph nodes and even bone marrow (Bullock and Pomerantz, 1984; Felten et al., 1985). Neurotransmitters may locally modulate immune reactivity in this circuit. Receptors for substances like catecholamines and VIP have been identified on lymphocytes and interaction of these neurotransmitters with corresponding receptor sites has indeed been reported to modulate lymphocyte functions. The wealth of information available on the effect of hormones, brain peptides and neurotransmitters on cells of the immune system (Tecoma and Huey, 1985) has not yet yielded a clear picture. This is because the experimental conditions used in many studies differ essentially regarding the read-out systems used and the doses of (peptide) hormones applied. Furthermore, the level of complexity of the in vivo processes can hardly be mimicked by in vitro assays. The effect of an integrated action of the various hormones, brain peptides and transmitters on the immune system, as it takes place in the intact organism, will be especially difficult to analyse in test tubes. This important methodological issue will not be elaborated further here although it is an essential element in studies on stress-induced changes in immune reactivity (see, e.g. Guillemin et al., 1985). It is now clear however that, under certain conditions, hormones different from corticosteroids can modulate immune functions in vivo (Berczi, 1986). Shavit and his colleagues (1984) reported that exposure of rats to inescapable, intermittent electric footshock (which causes learned helplessness and opioid-mediated analgesia) results in suppressed NK cell activity (see also Laudenslager et al., 1983 as discussed earlier). It was found that this suppression could be blocked by the opioid antagonist naltrexone and that the stress-induced suppression of NK cell function could be mimicked by morphine administration. Although several mechanisms can account for these findings, one possibility is a direct action of opioid peptides released by stress on NK cells. This result, obtained in vivo,
15
is interesting since endorphins seem to potentiate NK cell activity in man (Plotnikoff et al., 1986a). There are a number of recent reports, sometimes with conflicting results, on the influence of opioid peptides on the immune system (see, e.g. Plotnikoff et al., 1986a). A few studies have been directed towards immunomodulation of human lymphocyte function by these peptides. These studies include clinical trials on the effect of enkephalins and endorphins in cancer and AIDS patients (Plotnikoff et al., 1986b) and the effect of pendorphin (1-31) and several of its fragments on T cell proliferation and antibody synthesis in vitro (Heijnen et al., 1986a; Heijnen et al., 1986b).It is apparent from the data available that endorphins may modulate immune function by interaction with sites on the (human) lymphocyte other than the opioid receptor. It should be mentioned in this context that the effect of bendorphin fragment 10-16 on certain lymphocyte functions completely mirrors the effect of the intact 1-31 peptide (Heijnen et al., 1986b). Research regarding the cellular effects of the various hormones on lymphocytes and macrophages is still in its infancy but interesting results are becoming available (Munck et al., 1984; Tecoma and Huey, 1985). These include the inhibition of production of 11-2 (T cell growth factor) by glucocorticosteroids, thus preventing clonal expansion of activated T cells (Arya et al., 1984) and the change in recirculation and homing pattern of gut-derived lymphocytes caused by VIP (Ottaway, 1984). Lymphokines: mediators in the communication between immune system and CNS As mentioned earlier, Besedovski et al. (1977) were the first to show that, in rats, an ongoing immune response generates signals which are received by the brain. This results in an increased firing rate of neurons in the ventromedial nucleus of the hypothalamus and a decreased turnover in hypothalamic noradrenaline. Furthermore the glucocorticosteroid levels in blood were
increased when the immune response was at its peak. In subsequent studies, Besedovski and coworkers (1985) could demonstrate that two hours after administration to rats of a factor(s), produced in vitro by lymphoid cells after stimulation (with a mitogen), the effects observed in the hypothalamus during an in vivo immune response were mimicked. This result strongly suggests that a messenger, present in the lymphokine preparation, can influence CNS activity (Fig. 2). These STRESS
1 1
neuroendocrine system hormone: feeddack
and/or
innervation
neuropeptides hormones
The interaction of the CNS and the immune system: a two-way encounter. Fig.2.
observations promoted investigations on the nature of the lymphoid cell-derived ‘neurohormone(s)’. Blalock et al. (1985) published a series of papers which showed that, on the basis of immunoreactivity towards hormone specific antisera, human as well as rodent lymphocytes produce a number of ‘classical‘ pituitary hormones (ACTH, growth hormone, TSH, hCG and endorphin-like peptides). These results have now been given a solid molecular basis, since several groups have demonstrated messenger RNA for POMC in spleen lymphocytes and macrophages (Lolait et al., 1986; Westly et al., 1986). Activation of cloned mouse Th-cells with Con A yielded
76
mRNA coding for preproenkephalin (Zurawski et al., 1986). Interestingly enough, Blalock and his colleagues have shown that the processing of POMC might differ depending on the nature of the stimulus. Thus, activation of leukocytes by Newcastle disease virus causes the production of ACTH 1-39 and Pendorphin 1-31, whereas stimulation with bacterial lipopolysaccharide (LPS) results in the synthesis of ACTH 1-24 and Q or pendorphin (Pendorphin 1-17, 1-16) (Blalock, 1985). Not only does the processing of POMC appear to depend on the stimulus used to activate the leukocytes, but the production of the peptide hormones also seems to be sensitive to hormonal regulation. It was found that corticotrophin releasing factor (CRF) induces de novo synthesis and release of leukocyte-derived ACTH 1-39 and Fendorphin whereas dexamethasone blocks the production of these peptide hormones (Smith et al., 1986). Finally, to complete the dialogue as concerns POMC-derived peptides, both Blalock (1986) and Besedovski et al. (1985) have reported that supernatants of activated lymphocytes, containing lymphokines, induce the release of CRF in the hypothalamus and the subsequent rise in blood cortisol levels. The nature of this glucocorticoid increasing factor (GIF) is still unknown. According to recent data a well-defined factor derived from macrophages, interleukin 1 (11- l), induces the release of ACTH from the pituitary gland and subsequent increase in serum level of cortisol when it is given to mice. Even 0.1 pg recombinant 11-1 injected in a mouse resulted in a significant increase in ACTH and cortisol (Del Rey, 1986). The observed activity of the putative G I F (Besedovski et al., 1985) on ACTH release cannot be blocked by anti-11-1 (Del Rey, 1986). The process of regulation of hypothalamic and pituitary activities by lymphokines is far from being fully understood. Exciting new insights, however, have recently been obtained which allow the statement that the interaction between the brain and the immune system is a genuine dialogue (Fig. 2). It is not impossible that even-
tually a new therapeutic avenue in clinical medicine will involve interfering with this conversation by influencing the chemical vocabulary used in the brain-body dialogue.
References Ader, R. and Cohen, N. (1975) Behaviorally conditioned immunosuppression. Psychosom. Med., 37: 333-340. Ader, R., Weijnen, J.A.W.M. and Moleman, P. (1972) Retention of a passive avoidance response as a function of the intensity and duration of electric shock. Psychon. Sci., 26: 125-127. Arya, S . K., Wong-Staal, F. and Gallo, R. C. (1984) Dexamethasone-mediated inhibition of human T cell growth factor and gamma interferon messenger RNA. J. Immunol., 133: 273-276. Ballieux, R. E. (1984) Stress and immune response: parameters and markers. In: R. E. Ballieux, J. F. Fielding and A. L'Abbate (Eds.), Breakdown in Human Adaptation to 'Stress'. Towards a Multidirciplinary Approach, Vol. 2. Martinus Nijhoff Publishers, The Hague, pp. 732-739. Bartrop, R. W., Luckhurst, E., Lazarus, L., Kiloh, L. B. and Penny, R. (1977) Depressed lymphocyte function after bereavement. Lancet, i: 834-836. Berczi, I. (Ed.) (1986) Hormones and Immunity, Elsevier, New York, in press. Besedovski, H. 0.. Sorkin, E., Felix, D. and Haas, H. (1977) Hypothalamic changes during the immune response. Eur. J . Immunol., 7: 323-325. Besedovski, H. O., Del Rey, A., Sorkin, E., Da Prada, M. and Keller, H. H. (1979) Immunoregulation mediated by the sympathetic nervous system. Cell. Immunol., 48: 346-355. Besedovski, H.O., Del Rey, A., Sorkin, E., Lotz, W. and Schwulera, U. (1985) Lymphoid cells produce an immunoregulatory glucocorticoid increasing factor (GIF) acting through the pituitary gland. Clin. Exp. Immunol., 59: 622-628. Blalock, J. E. (1985) Proopiomelanocortin-derived peptides in the immune system. Clin. Endocrinol., 22: 823-827. Blalock, J. E. (1986) Production and action of lymphocytederived neuroendocrine peptide hormones - Summary. Prog. Immunol., 6: in press. Blalock, J. E., Harbous-McMenamin, D. and Smith, E. M. (1985) Peptide hormones shared by the neuroendocrine and immunologic systems. J. Immunol., 135: 858s-861s. Bulloch, K. and Pomerantz, W. (1984) Autonomic nervous system innervation of thymic-related lymphoid tissue in wild-type and nude mice. J. Comp. Neurol., 228: 57-63. Croiset, G., Veldhuis. H. D., Ballieux, R. E., De Wied, D. and Heijnen, C. J. (1986) The impact of mild emotional stress induced by the passive avoidance procedure on immune reactivity. Ann. N . Y. Acad. Sci., in press.
77 Del Rey, A. (1986) Immune-feedback regulatory signals. In: I. Berczi (Ed.) Hormones and Immunity. Elsevier, New York, in press. Felten, D. L., Felten, S.Y., Carlson, S.L., Olschowka, J.A. and Livnat, S. (1985) Noradrenergic and peptidergic innervation of lymphoid tissue. J. Immunol., 135: 755s165s. Guillemin, R., Cohn, M. and Melnechuk, T. (Eds.) (1985). Neural modulation of immunity. Raven Press, New York. Heijnen, C. J., Bevers, C., Kavelaars, A. and Ballieux, R.E. ( l986a) Effect of a-endorphin on the antigen-induced primary antibody response of human blood B cells in vitro. J. Immunol., 136: 213-216. Heijnen, C.J., Croiset, G., Zijlstra, J. and Ballieux, R.E. ( 1986b) Modulation of lymphocyte function by endorphins. Ann. N. Y. Acad. Sci., in press. Laudenslager, M. L., Ryan, S. M., Drugan, R.C., Hyson, R. L. and Maier, S . F. (1983) Coping and immunosuppression: inescapable but not escapable shock suppresses lymphocyte proliferation. Science, 221: 568-570. Locke, S.E., Kraus, L., Leserman, J., Hurst, M. W., Heisel, J.S. and Williams, R.M. (1984) Life change stress, psychiatric symptoms, and natural killer cell activity. Psychosom. Med., 46: 441-453. Lolait, S.J., Clements, J.A., Markwick, A.J., Cheng, C., McNally, M., Ian Smith, A. and Funder, J. W. (1986) Proopiomelanocortin messenger ribonucleic acid and posttranslational processing of beta endorphin in spleen macrophages. J . Clin. Invest., 77: 1776-1779. Munck, A., Guyre, P. M. and Holbrook, N. J. (1984) Physiological functions of glucocorticoids in stress and their relation to pharmacological actions. Endocrine Rev., 5: 25-44. Ottaway, C.A. (1984) In vitro alteration of receptors for vasoactive intestinal peptide changes the in vivo localization of mouse T cells. J. Exp. Med., 160: 1054-1069. Plotnikoff, N.P., Faith, R.E., Murgo, A. J. and Good, R.A. (Eds.) (1986a) Enkephalins and Endorphins. Stress and the Immune System. Plenum, New York. Plotnikoff, N.P., Miller, G.C., Nimeh, N., Faith, R.E., Murgo, A.J. and Wybran, J. (1986b) Enkephalins and endorphins, behavioral stress, and immunomodulators in normal volunteers and cancer patients. Ann. N. Y. Acad. Sci.,in press.
,
Rose, N. R., Friedman, H.and Fahey, J. L. (1986) Manual of Clinical Laboratory Immunology. American Society (Microbiology) Washington DC. Samuel, D., Naor, S . , Pecht, M. and Trainin, N. (1986) Bereavement and the Immune system: the Effect ofthe Loss of an Unborn Child on Mitogenic Stimulation of T cells. Abstract No. 64, International Workshop on Neuroimmunomodulation, Dubrovnik, June 1-6, 1986. Schleifer, S. J., Keller, S. E., Camerino, M., Thorton, J. C. and Stein, M. (1983) Suppression of lymphocyte stimulation following bereavement. J . Am. Med. Assoc., 250: 374-377. Shavit, Y., Lewis, J.W., Terman, G.W., Gale, R.P. and Liebeskind, J. C. (1984) Opioid peptides mediate the suppressive effect of stress on natural killer cell cytotoxicity. Science, 223: 188-190. Sklar, L. S.and Anisman, H. (1979) Stress and coping factors influence tumor growth. Science, 205: 513-515. Smith, E.M., Morrill, E.M., Meyer 111, W.J. and Blalock, J. E. (1986) Corticotropin releasing factor induction of leukocyte-derived immunoreactive ACTH and endorphins. Nature (London), 321: 881-882. Stein, M., Schleifer, S. J. and Keller, S. E. (1986). Stress, depression and immunity. In: R.C.A. Frederickson, H.C. Hendrie, J.N. Hingtgen and M.H. Aprison (Eds.), Neuro-Regulation of Autonomic, Endocrine and Immune Systems. Martinus Nijhoff Publ., Boston, pp. 367-388. Tecoma. E. S. and Huey, L. Y. (1985) Psychic distress and the immune response. L f e Sci., 36: 1799-1812. Veldhuis, H. D., Croiset, G., Ballieux, R. E., Heijnen, C.J. and De Wied, D. (1986) In: D. Nerozzi and G. Frajese (Eds.), Proceedings of the conference on Hypothalamic dysjirncrion in neuropsychiam'c disorders. Raven Press, New York, in press. Visintainer, M. A,, Volpicelli, J. R. and Seligman, M. E. P. (1982) Tumor rejection in rats after inescapable or escapable shock, Science, 216: 437439. Westly, H. J., Kleiss, A. J., Kelley, K. W.,Wong, P. K.Y. and Yuen, P.-H. (1986) Newcastle disease virus-infected splenocytes express the pro-opiornelanocortin gene. 3. Exp. Med., 163: 1589-1594. Zurawski, G., Benedik, M., Kamb, B.J., Abrams, J.S., Zurawski, S. M. and Lee, F. D. (1986) Activation of mouse T helper cells induces abundant preproenkephalin mRNA synthesis. Science, 232: 712-775.
Prapess in Brain Research, Val. 12
71
0 1987 Elsevier Science Publishers B.V. (Biomedical Division)
CHAPTER 7
Brain and immune system: a one-way conversation or a genuine dialogue? Rudy E. Ballieux“ and Cobi J. Heijnenb “Departmentof Clinical Immunology. University Hospital, Utrecht and bDeparrment of Immunology. University Hospitalfor Children and Youth ‘Her Wilhelmina Kinderziekenhuis., Utrecht. The Netherlands
stem cell
Introduction The nervous system and the immune system show a certain degree of congruence in that they both mediate the interaction of the individual with the often hostile and threatening ‘Urnwelt’. The responses of both physiological systems have several characteristics in common: communication at a distance, capability to develop memory and the use of chemical messengers (neurotransmitters, neuroendocrine peptides and lymphokines) to transmit messages. Actually, at a recent meeting on psychoneuroimmunology, the immune system was referred to as a ‘mobile brain’. This is because new and exciting findings, which will be discussed later in this paper, support the concept of cells of the immune system serving as ‘free-floating nerve cells’. What cells are these? It is now generally known that the ‘work horses’ of the immune system are the lymphocytes. These cells are rather small (approximately 10pm in diameter), possess little cytoplasm and are in a resting state for the greater part of their lifespan. They circulate via lymph and bloodstream and home-in on lymphoid organs, such as tonsils, lymph nodes, spleen and gut-associated lymphoid tissues. Two major subsets of lymphocytes can be distinguished which develop from a stem cell localized in the bone marrow (Fig. 1). One subset
\
committed lymphoid
/t
T lymphocyte (cellular immunity )
marrow
B lymphocyte
(humoral immunity)
\
2
+
T-effector
CY toioxic lymphokines
T-muppresaor
I-helper
I
plasma cell
Fig. 1. The development of the T and B cell subsets. The NK cell is not included in this figure.
comprises lymphocytes which come to full maturation in the bone marrow itself and hence are
12
called B lymphocytes. Another subset of lymphocytes leaves the bone marrow at an immature stage to spend a (short) period of their life in the thymus. Here they develop into fully differentiated, mature (thymus-derived) T lymphocytes before entering the bloodstream to populate the various lymphoid organs. A third minor subset of mononuclear white blood cells lacks the surface and functional characteristics of T and B lymphocytes. This subset includes cells which kill certain tumor cells and virus-infected cells without any obvious immunological specificity. These cells are known as natural killer cells or NK cells. They most probably play an important role in the surveillance against tumors and viral infections. The B lymphocytes are the precursors of antibody forming cells: plasma cells. The transition of a B cell into a plasma cell involves the binding of antigen to the corresponding receptor at the cell surface of the B cell. This receptor-mediated signal results in activation of the B cell. The B cell then becomes sensitive towards non-antigen specific growth and differentiation factors (lymphokines) many of which are derived from T lymphocytes. For most antigens to stimulate B cells successfully an additional antigen-specific ‘helper-signal’ derived from T cells is required. The T cells which deliver this signal after being activated by antigen are referred to as helper T cells (Th-cells). In analogy to other biological processes, an ongoing immune response is subject to (feedback) regulation. Part of this immunoregulatory function is exerted by suppressor T cells (Ts-cells). Ts lymphocytes, in general, down-regulate immune reactivity. Apart from these regulatory T cell subsets, effector T cells can be distinguished functionally. This latter subset includes T lymphocytes that kill target cells (e.g. tumor cells or virus-infected cells) by direct cytotoxic action. Furthermore, there are T effector cells which, upon interaction with antigen, produce a variety of lymphoid factors (lymphokines) which mediate immunopharmacological reactions (e.g. delayed type hypersensitivity reactions).
Many of the various T and B cell functions can be measured in vitro in cell culture systems or in vivo in animal and human models. It is beyond the scope of this short review to discuss the various possibilities of analyzing immune capacity and the limitations of some of these experimental models. Relevant information can be found in several recent publications related to this aspect (Ballieux, 1984; Rose et al., 1986). The immune system and the nervous system not only share certain characteristics, they also have been shown to be functionally interconnected. There is ample evidence that stressful environmental conditions can modulate immune functions. This implies that the brain can influence the immune system. Indeed, it has been demonstrated in several studies, initiated by the work of Ader and Cohen (1973, that, e.g. a classical conditioning type of situation can alter T and B cell-mediated immune responses. On the other hand, experiments reported by Besedovski and his colleagues a decade ago (Besedovski et al., 1977; Besedovski et al., 1979) show clearly that activation of the immune system by antigenic stimulation induces changes in brain function and in neuroendocrine profiles. Some aspects of the bilateral interaction of central nervous system (CNS) and immune system will be discussed below.
The influence of psychosocial factors on immunocompetence Many studies, in man and in animal models, have shown that stress can alter immune reactivity. One of the classical examples in man has been the effect of bereavement on immune function as reported originally by Bartrop et al. (1977) and in later years in a series of studies by Stein and colleagues (Schleifer et al., 1983; Stein et al., 1986). These authors found decreased proliferative responses after stimulation of peripheral blood lymphocytes in cell cultures with mitogens. Both T and B cell responses were lowered compared to pre-bereavement levels. These well con-
13
trolled longitudinal studies of a prospective nature clearly demonstrated that suppression of mitogen-induced lymphocyte activity is a direct consequence of the bereavement event. An important element in psychosocial stress concerns the ability of the individual to cope with the situation. This was clearly demonstrated in a study by Samuel et al. (1986) on the effect of abortion or miscarriage on immune function. The results of their study show clearly that women who had not accepted the loss of their (unborn) child (poor coping) had a significant reduction in mitogen-induced T cell activity in comparison with women who could handle this event. A marked correlation was found between immunosuppression and degree of psychic depression. In a prospective study of less dramatic nature, Locke et al. (1984) examined healthy college students for an association between N K cell function, stressful conditions and rate of individual psychic distress which developed from the long-term stress. It was found that not stress itself, but stress in association with poor coping behavior (relatively high psychic distress), was significantly associated with low NK cell activity. It is obvious from these studies in humans that the ability to cope with the demanding situation greatly reduces the deleterious effect on the immune response. Although it is very difficult to translate these human studies into their animal equivalents, a unique animal stress model which allows for the analysis of individual coping behavior has been introduced by Croiset etal. (1986). These investigators adopted the one-trial learning passive avoidance test, described by Ader et al. (1972). In this model rats after habituation to the experimental environment receive a single and mild, but unavoidable electric footshock (learning trial). The effect of this aversive experience is tested 24 h or 120 h later in the so-called retention test, in which the rat is placed in a conflict situation. It can avoid being exposed again to the formerly aversive environment, but at the cost of remaining in an uncomfortable situation on a platform in bright light. The passive
avoidance behavior of the animals has been studied extensively in relztion to memory consolidation, retrieval processes and various (neur0)endocrine and physiological functions. The significance of these findings for the understanding of the observed changes in immune responsiveness, induced by the retention test, have recently been reviewed (Veldhuis et al., 1986). It was found by Croiset and her colleagues that splenic lymphocytes of rats that had been subjected to the retention test responded less well after stimulation in vitro with concanavalin A (Con A) than cells of rats of the apparatus control group (no shock received). Preliminary data disclosed that animals which showed a long entrance latency (effective avoidance behavior) had a more decreased immune reactivity than animals that exhibited a short entrance latency (manuscript in preparation). This observation touches upon the intriguing issue of whether effective avoidance behavior is in one way or another a reflection of poor coping ability. Apart from the above-mentioned studies investigating the effects of differences in personality traits regarding coping behavior, it is also possible to investigate the influence on the immune system of a stressful situation which is presented in either a controllable or an incontrollable way. Laudenslager and coworkers (1983) subjected rats to a restraint stress in combination with multiple electric shocks. The experimental group of animals was divided into animals given inescapable shocks and animals which received escapable shocks. The rats in the latter group had learned how to turn off the electric current; a paired animal received a shock of identical strength that it could not control. T cell function was measured in vitro by measuring the proliferative response of spleen cells after stimulation with the mitogen phytohaemagglutinin (PHA) and Con A. It was found that mitogen-induced T cell proliferation was suppressed in the ‘inescapable shock‘ group as compared to the ‘escapable shock’ group. Animals of the latter group did not differ from unshocked controls in their T cell response.
14
The same ‘yoked’ testing procedure had previously been applied to assess the effect of stressor controllability on tumor susceptibility. Sklar and Anisman (1979) as well as Visintainer et al. (1982) reported that experiencing uncontrollable exposure to electric shocks increased tumor growth and impaired tumor rejection. It therefore seems that controllability of the stressful situation may be one of the important factors that determines to what extent the immune response is modulated by the stressor. The results of a study by Koolhaas et al. (1986, pers. commun.) are interesting in this respect. These authors analyzed several immune parameters in rats living in long-term colonies. Under such conditions a social hierarchy is formed in which the leader of the colony can be distinguished next to a few potential leaders. The remaining animals show a more or less subordinate behavior. It was found that the number of peripheral blood lymphocytes of the individual rats correlated significantly with flight and escape behavior in the colony which was most prominent in the dominant animal. The Con Ainduced T cell response also showed a correlation with the social status of the animal in the colony. Although the experimental situation in the colonies differs basically from that in the ‘yoked situation’, one could argue that the social status relates to the degree of controllability of the environmental conditions in the colony. It is remarkable that social status also seems to be associated with baseline immune function.
Mechanisms involved in stress-induced immunomodulation Two main pathways can be distinguished for mechanisms involved in neuroimmunomodulation. One pathway is of a humoral nature and involves the hypothalamus/pituitary/adrenal axis and corresponding hormones. These include enkephalins and endorphins, ACTH, vasopressin, prolactin, corticosteroids and sex hormones. The second pathway is represented by the autonomic nervous system, including the innervation of lymphoid
tissues such as thymus, spleen, lymph nodes and even bone marrow (Bullock and Pomerantz, 1984; Felten et al., 1985). Neurotransmitters may locally modulate immune reactivity in this circuit. Receptors for substances like catecholamines and VIP have been identified on lymphocytes and interaction of these neurotransmitters with corresponding receptor sites has indeed been reported to modulate lymphocyte functions. The wealth of information available on the effect of hormones, brain peptides and neurotransmitters on cells of the immune system (Tecoma and Huey, 1985) has not yet yielded a clear picture. This is because the experimental conditions used in many studies differ essentially regarding the read-out systems used and the doses of (peptide) hormones applied. Furthermore, the level of complexity of the in vivo processes can hardly be mimicked by in vitro assays. The effect of an integrated action of the various hormones, brain peptides and transmitters on the immune system, as it takes place in the intact organism, will be especially difficult to analyse in test tubes. This important methodological issue will not be elaborated further here although it is an essential element in studies on stress-induced changes in immune reactivity (see, e.g. Guillemin et al., 1985). It is now clear however that, under certain conditions, hormones different from corticosteroids can modulate immune functions in vivo (Berczi, 1986). Shavit and his colleagues (1984) reported that exposure of rats to inescapable, intermittent electric footshock (which causes learned helplessness and opioid-mediated analgesia) results in suppressed NK cell activity (see also Laudenslager et al., 1983 as discussed earlier). It was found that this suppression could be blocked by the opioid antagonist naltrexone and that the stress-induced suppression of NK cell function could be mimicked by morphine administration. Although several mechanisms can account for these findings, one possibility is a direct action of opioid peptides released by stress on NK cells. This result, obtained in vivo,
15
is interesting since endorphins seem to potentiate NK cell activity in man (Plotnikoff et al., 1986a). There are a number of recent reports, sometimes with conflicting results, on the influence of opioid peptides on the immune system (see, e.g. Plotnikoff et al., 1986a). A few studies have been directed towards immunomodulation of human lymphocyte function by these peptides. These studies include clinical trials on the effect of enkephalins and endorphins in cancer and AIDS patients (Plotnikoff et al., 1986b) and the effect of pendorphin (1-31) and several of its fragments on T cell proliferation and antibody synthesis in vitro (Heijnen et al., 1986a; Heijnen et al., 1986b).It is apparent from the data available that endorphins may modulate immune function by interaction with sites on the (human) lymphocyte other than the opioid receptor. It should be mentioned in this context that the effect of bendorphin fragment 10-16 on certain lymphocyte functions completely mirrors the effect of the intact 1-31 peptide (Heijnen et al., 1986b). Research regarding the cellular effects of the various hormones on lymphocytes and macrophages is still in its infancy but interesting results are becoming available (Munck et al., 1984; Tecoma and Huey, 1985). These include the inhibition of production of 11-2 (T cell growth factor) by glucocorticosteroids, thus preventing clonal expansion of activated T cells (Arya et al., 1984) and the change in recirculation and homing pattern of gut-derived lymphocytes caused by VIP (Ottaway, 1984). Lymphokines: mediators in the communication between immune system and CNS As mentioned earlier, Besedovski et al. (1977) were the first to show that, in rats, an ongoing immune response generates signals which are received by the brain. This results in an increased firing rate of neurons in the ventromedial nucleus of the hypothalamus and a decreased turnover in hypothalamic noradrenaline. Furthermore the glucocorticosteroid levels in blood were
increased when the immune response was at its peak. In subsequent studies, Besedovski and coworkers (1985) could demonstrate that two hours after administration to rats of a factor(s), produced in vitro by lymphoid cells after stimulation (with a mitogen), the effects observed in the hypothalamus during an in vivo immune response were mimicked. This result strongly suggests that a messenger, present in the lymphokine preparation, can influence CNS activity (Fig. 2). These STRESS
1 1
neuroendocrine system hormone: feeddack
and/or
innervation
neuropeptides hormones
The interaction of the CNS and the immune system: a two-way encounter. Fig.2.
observations promoted investigations on the nature of the lymphoid cell-derived ‘neurohormone(s)’. Blalock et al. (1985) published a series of papers which showed that, on the basis of immunoreactivity towards hormone specific antisera, human as well as rodent lymphocytes produce a number of ‘classical‘ pituitary hormones (ACTH, growth hormone, TSH, hCG and endorphin-like peptides). These results have now been given a solid molecular basis, since several groups have demonstrated messenger RNA for POMC in spleen lymphocytes and macrophages (Lolait et al., 1986; Westly et al., 1986). Activation of cloned mouse Th-cells with Con A yielded
76
mRNA coding for preproenkephalin (Zurawski et al., 1986). Interestingly enough, Blalock and his colleagues have shown that the processing of POMC might differ depending on the nature of the stimulus. Thus, activation of leukocytes by Newcastle disease virus causes the production of ACTH 1-39 and Pendorphin 1-31, whereas stimulation with bacterial lipopolysaccharide (LPS) results in the synthesis of ACTH 1-24 and Q or pendorphin (Pendorphin 1-17, 1-16) (Blalock, 1985). Not only does the processing of POMC appear to depend on the stimulus used to activate the leukocytes, but the production of the peptide hormones also seems to be sensitive to hormonal regulation. It was found that corticotrophin releasing factor (CRF) induces de novo synthesis and release of leukocyte-derived ACTH 1-39 and Fendorphin whereas dexamethasone blocks the production of these peptide hormones (Smith et al., 1986). Finally, to complete the dialogue as concerns POMC-derived peptides, both Blalock (1986) and Besedovski et al. (1985) have reported that supernatants of activated lymphocytes, containing lymphokines, induce the release of CRF in the hypothalamus and the subsequent rise in blood cortisol levels. The nature of this glucocorticoid increasing factor (GIF) is still unknown. According to recent data a well-defined factor derived from macrophages, interleukin 1 (11- l), induces the release of ACTH from the pituitary gland and subsequent increase in serum level of cortisol when it is given to mice. Even 0.1 pg recombinant 11-1 injected in a mouse resulted in a significant increase in ACTH and cortisol (Del Rey, 1986). The observed activity of the putative G I F (Besedovski et al., 1985) on ACTH release cannot be blocked by anti-11-1 (Del Rey, 1986). The process of regulation of hypothalamic and pituitary activities by lymphokines is far from being fully understood. Exciting new insights, however, have recently been obtained which allow the statement that the interaction between the brain and the immune system is a genuine dialogue (Fig. 2). It is not impossible that even-
tually a new therapeutic avenue in clinical medicine will involve interfering with this conversation by influencing the chemical vocabulary used in the brain-body dialogue.
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