BRAIN.

BEHAVIOR,

AND

IMMUNITY

6, 97-116 (1992)

REVIEW Central

Nervous CLIFFORD

*Department

Pathology.

of Medicine, Fuculty

System Influences A. OTTAWAY”,’ Unh,ersity of Medicine.

on Lymphocyte

AND ALAN

of Toronto.

Toronto,

The Utziversity

Migration

J. HussANDt

Ontario,

of Newcastle.

Canadu, and +Discipline NSW. 2308. Australia

oj

The immune response network is only one of many physiologic adaptive responses to environmental change and there is now substantial evidence that adaptive responses involving the central nervous system have an impact on immune outcome. Effective immune function depends upon a highly mobile population of precursor and effector cells of the lymphoid system. In this review it is proposed that many of the alterations in immunity resulting from CNS activity may be explained in terms of changes in lymphocyte migration patterns in response to endocrine signals, neural signals via neurotransmitter release, or direct contacts between nerves and cells of the immune system. #C 1992 Academx Pre\\. Inc.

INTRODUCTION

Effective immune function depends upon a highly mobile population of precursor and effector cells of the lymphoid system. This mobility involves an efficient and selective process of large-scale migration between lymphoid and nonlymphoid tissues via the blood and lymphatic circulatory systems as well as migration on a microscopic scale within tissues. The principal macroscopic factors known to affect the accumulation of migratory lymphoid cells in tissues are the concentration of cells in the blood stream, the delivery of blood to the tissues, and the extraction and retention of cells by the tissue. On a microscopic scale, the principal determinants are the adhesive interaction of lymphoid cells with endothelial cells, the transposition of lymphoid cells across the endothelial basement membrane, and the interaction of the migratory cells with the extracellular and cellular constituents of the tissue. Although there have been many reviews of this topic (Ford, 1975; Woodruff, Clarke, & Chin, 1987; Husband, 1988; Pabst & Binns, 1989; Berg, Goldstein, Jutila, Nakache, Icker, Streeter, Wu, Zhou. & Butcher, 1989: Dustin & Springer, 1991). none has directly addressed the potential for activities of the central nervous system to influence lymphoid cell migration. In recent years, substantial evidence has emerged, from studies by many investigators working with physiologic, psychologic, anatomic, cellular, and molecular approaches, to support the notion that a complex network of interactions occurs between the CNS and the immune system in the intact animal (reviewed in Ader, Felten, & Cohen, 1991; Besedovsky & del Rey, 1986; Sanders & Munson, 1985). Thus, the immune system operates as a specific molecular(antigen)-sensitive mechanism within a spectrum of physiologic adaptive responses with which an organism reacts to perceived changes in its environment (Fig. 1). Within the network of interactions ’ To whom reprint requests should be addressed at % Rm 6360 Medical Sciences Bldg., University of Toronto, Toronto, Ontario. M5.S IA8 Canada. 97 0889-1591193 $5.00 CopyrIght i’ 1992 by Academic Pias. Inc. All rlghts of reproductmn m any form rexwed.

OTTAWAY

AND

Conceptual Physical

HUSBAND Perception

Perception

mune

CNS

b

Hypothalamus

4

Cell Migration Pathways

+

Neural Pathways

==+a

Hormonal Pathways

Perception

Other

Autonomic Nervous System

Hormones

Adrenergic CholinerQic Peptidergic

V

CARDIOVASCULAR

LYMPHATIC

SPACE

c

SPACE

FIG. 1. Opportunities for central nervous system (CNS) effects on lymphoid cell migration. The CNS responds to stimuli which may include physical perception of environmental changes (e.g., by sight, smell, feel, sound, etc.), conceptual thought processes (e.g., stress. conditioning), or even immune responses (e.g., via cytokine effects). CNS activation may lead to release of hormones, adrenal catecholamines. or neurotransmitters. all of which have potential to alter migration patterns.

between the nervous and immune systems some specitic pathways have been identified. Interactions can occur through the direct innervation of lymphoid compartments, by paracrine means through the release of mediators from nerves situated in close proximity to cells, or by neuroendocrine signals in the form of hypothalamic, pituitary, and peripheral endocrine hormones. Since the immune system is an aggregate of specialized cells which are capable of acting in concert to engage in an organized response to antigenic molecules of the environment, the independent mobility of lymphocytes permits functional grouping of these specialized components into various operational clusters and an

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appropriate distribution in time and space of clustered lymphocytic specialists. Lymphocyte migration, therefore, makes possible a reassortment of cells and reconstitution or remodeling of functional immune clusters as forces of change play upon the aggregate system and the organism. The purpose of this review is to examine the proposition that CNS activity can influence the migration and distribution of lymphoid cells on both a macroscopic and microscopic scale and that some of the described effects of CNS activity on immune outcome may be explained on the basis of CNS influence on lymphoid cell migration patterns. INNERVATION

OF LYMPHOID

COMPARTMENTS

Lymphoid organs are extensively innervated by the autonomic nervous system (Felten, Ackerman, Wiegand, & Fehen, 1987; Felten, Felten, Bellinger, Carlson, Ackerman, Olschowka, & Livnat, Adden, 1987; Felten, Felten, Carlson, Olschowka, & Livnat, 198.5; Livnat, Felten, Carlson, Bellinger, & Felten, 1983, and noradrenalin has been identified as the postganglionic transmitter associated with the majority of these fibers. Although noradrenergic nerve terminals were originally thought to be associated only with smooth muscle and involved in the regulation of blood flow (Anderson & Anderson, 1975; Ayers, Davies, & Withrington, 1972; Gillespie & Kirpekar, 1965; Reilly, 1985) more recent studies have confirmed that noradrenergic postganglionic sympathetic fibers also innervate the parenchyma of primary (bone marrow and thymus) and secondary lymphoid organs (spleen, lymph nodes, Peyer’s patches, tonsils, and other gut associated lymphoid tissue) (Felton et al., 1985, 1987a,b; Bulloch & Pomerantz, 1984; Giron, Curtcher, & Davis, 1980). These enter the lymphoid organs with the vasculature, but are usually well represented in the traffic areas of the lymphoid organs and ramify well beyond the vascular corridors into the parenchyma. Some of the most detailed studies have concerned the spleen. In developmental studies, Ackerman, Felten, Dijkstra, Livnat, and Felten (1989) have demonstrated that sympathetic noradrenergic nerves develop in synchrony with expansion of the lymphoid compartments and reach a variety of sites where lymphocytes enter, redistribute, and exit. For example, dense noradrenergic innervation is found in the marginal zone (a site of T and B cell entry), the periarteriolar lymphatic sheath (PALS; a site in which T cells predominate and can be activated), the parafollicular and marginal zones (sites of B cell activation), and the outer marginal zone from which lymphocytes exit the spleen. Ultrastructural studies (Feltcn et al., 1987a) have demonstrated noradrenergic fibers in the PALS in close association with T cells of both helper and suppressor phenotypes, and fibers in the marginal sinus and the parafollicular zones have been found in close proximity to T and I3 cells. The intimacy of these associations between nerves and lymphocytes has been considered spatially sufficient to permit direct contacts between the cells at these sites (Felten & Olschowka, 1987), and further study (Felten, Felten, Bellinger, Carlson, Ackerman, & Madden, 1988) has demonstrated the existence of synaptic-like connections between noradrenergic nerve terminals and lymphocytes in these regions. CATECHOLAMINES AND LYMPHOCYTE MIGRATION: THE SYMPATHOADRENAL CONNECTION

These anatomic considerations suggest that there is considerable opportunity for direct contact and paracrine influences of the noradrenergic sympathetic ner-

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system on lymphocytes in lymphoid tissues, and it has been recognized for many years that catecholamines, presented via the blood stream can affect the output of lymphocytes from lymphoid tissues such as lymph node and spleen. Moore (1984) demonstrated that the infusion of phenylephrine into the afferent lymphatics of sheep popliteal lymph nodes increased the output of lymphocytes in efferent lymph. Loeper and Crouzon (1904) demonstrated that adrenalin induces a leukocytosis which, to some extent, is regulated by increased output of cells from the spleen (Frey & Lury, 1914; Miller & Rhoades, 1933). Arteriovenous difference measurements (Ernstrom & Sandberg, 1973) have demonstrated that both noradrenalin and isoproterenol administration result in increased mobilization of lymphocytes and neutrophils from the spleen. Although these agents can also change splenic perfusion, the effect on cell mobilization increases the number of lymphocytes per volume of blood regardless of changes in blood flow, and represents primarily a release of stored cells from the spleen. Ernstrom and Soder (1975) confirmed and extended these observations, and showed that exogenous adrenalin also increases the release of antigen binding cells and antibody forming B cells during an immune response in the spleen. Thus, an adrenergic signal may be able to promote the exit of B cells from the spleen, and Ackerman and coworkers (1989) have argued that the paucity of B cells in the inner PALS may be related to the dense noradrenergic innervation in this region. The potential role of the sympathetic innervation of lymph nodes in cellular dynamics has been highlighted in adoptive transfer experiments (Felton et al., 1987b). When labeled lymph node lymphocytes from normal mice were transferred into syngeneic recipients, which had undergone treatment with 6-hydroxydopamine (60HDA) to achieve functional sympathectomy, their localization in lymph nodes was enhanced. In contrast, the transfer of lymph node lymphocytes from 60HDA-treated donor mice into untreated recipients resulted in decreased accumulation of the lymphocytes in lymph nodes (Felton et al., 1987b). These observations suggest that chemical ablation of the sympathetic innervation of lymphoid tissue can affect both the migratory abilities of lymphocytes and the lymph node environment through which migrating lymphocytes travel. Plasma catecholamine concentrations have been implicated in changes in blood concentrations of lymphocyte subsets in humans in response to simple acute physiologic stressors in the form of physical and psychological exertion. For example, Landmann and colleagues (Landmann, Muller, Perini, Wesp, Erne, & Buhler, 1984) have shown that venous blood concentrations of T and B cells in healthy young adults can change dramatically, in response to standardized bicycle ergometric activity, to 75% of their individually determined maximal work load (Table 1). The experimental activity, which led to marked hemodynamic alterations and increased plasma concentrations of both adrenalin and noradrenalin but no changes in plasma cortisol concentrations, was accompanied by statistically significant changes in the concentration of circulating B cells and CD4+ and CD8+ T cells (Table 1). The effect of physical exertion on the circulating concentrations of these lymphocyte subsets was confirmed in other studies of normal subjects (Landmann, Durig, Gudat, Wesp, & Harder, 1985>, but in subjects who were asplenic, secondary to surgery from remote trauma, increases in either CD8+ T cells or B cells were less marked, and there was no alteration in concentrations of CD4+ T cells in blood response to the standardized cycling activity (Landmann et al., 1985). vous

CNS EFFECTS ON LYMPHOCYTE Catecholamine.

101

MIGRATION

TABLE 1 Hemodynamic, and Lymphocyte Changes with Experimental

Stressors

Experimental stressor Measure

Baseline

8 min cognitive

Cycling

Adrenaline (pgiml) Noradrenaline (pgiml) Heart rate (min ‘) Blood pressure (mm Hg) Systolic Diastolic Lymphocytes (X 10 -‘/ml) CD4 CD8 B

46 (5) 279 (21) 65 (2)

52 (5) 288 (20) 73 (2P

109 (ll)* 1094 (107)* 165 (5)*

III (3) 71 (3)

124 (3)* 81 (3)*

150 (4)* 58 (4)*

3.1 (0.4) I .8 (0.3) 1.4 (0.1)

3.1 (0.3) I.9 (0.2) I .8 (0.2)

4.9 (0.6)* 3.7 (0.5)* 4.5 (0.4)*

Nole. Data are adapted from Landmann et al. (1984). * Statistically significant change from baseline, p < .Ol.

The effect of acute psychologic stress, in the form of an 8-min cognitive conflict test, was also studied by those investigators (Landmann et al., 1984). During this experimental stress there was more modest alteration of hemodynamic measures and plasma adrenalin than occurred during cycling (Table 1) and lymphocyte subset concentrations were not disturbed in the same way. Manuck, Cohen, Rabin, Muldoon, and Bachen (1990) studied the circulating concentrations of lymphocytes in healthy young subjects and their alterations during a somewhat different form of cognitive exertion (Table 2). These subjects were assigned 20 min of two alternating types of standardized frustrating cognitive tasks and a variety of physiologic measurements were collected at baseline and after the task. The sympathoadrenal activation of the subjects was judged by changes in heart rate, systolic and diastolic blood pressure, and concentrations of circulating catecholamines, and the sample was divided into “high” responders and “low” responders based upon these indices. Those showing a sympathoadrenal response

Catecholamine,

TABLE 2 Hemodynamic, and Lymphocyte Changes in High and Low Responders following a 20-min Cognitive Stress Exercise High responders

Measure

Baseline

Low responders

Poststress

Baseline

Poststress

Adrenaline (pgiml) 24 (2) 48(8) Noradrenaline (pgiml) 235 (36) 268 (34) Heart rate (min ‘) 59 (2) 77 (3) Blood pressure (mm Hg) Systolic 116 (2) 132 (2) Diastolic 63 (2) 71 (2) Lymphocytes (X lO~‘/ml) CD4 7.4 (0.8) 7.2 (0.6) CD8 5.5 (0.3) 6.8 (0.4)* B 2.1 (0.3) 2.0 (0.3) Note. Data are adapted from Manuck et al. (1990). * Statistically significant change from baseline. p < .Ol.

18 (3) 244 (35) 52 (2)

26 (3) 238 (29) 59 (3)

115 (2) 58 (3)

120 (2) 62 (3)

6.6 (0.6) 5.4 (0.7) 2.5 (0.3)

6.3 (0.4) 5.4 (0.5) 2.3 (0.3)

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had more marked cardiovascular changes, and elevated concentrations of plasma adrenalin, but neither group showed a change in plasma noradrenalin or cortisol. The high responders, however, had statistically significant elevations in numbers of circulating CD8+ T cells without detectable changes in CD4+ or B cells (Table 2). The mechanisms by which catecholamines or noradrenergic sympathetic nervous activity may modulate lymphocyte distributions are not established, but could involve both macroscopic and microscopic migration features. It is well documented that circulating catechoiamines or sympathetic innervation may affect both vascular smooth muscle to produce changes in regional blood flow, and thereby change the delivery of lymphocytes to postcapillary venules of tissues, and the opportunity for lymphocytes to enter tissues (Ottaway & Parrott, 1979; Ottaway, Manson-Smith, Bruce, & Parrott, 1980: Hay & Hobbs, 1977; Ottaway, 1988a). Perhaps it is less well known that lymph flow is also under sympathetic control. Electrical stimulation of regional sympathetic nerves (Harty et al., 1988), and emotional stress (Shannon et al., 1976; McHale et al., 1983) have been shown to result in increased lymphatic pumping, which in turn has marked effects on lymphocyte output (McHale and Thornbury, 1989). The effects of alterations in perfusion are likely to differ in different tissues, especially in iymphoid compared to nonlymphoid tissues. The efficiency of lymphocyte extraction across the endothelium of nonlymphoid tissues is normally low, and overall migration rates in these sites may be strongly delivery dependent (Ottaway, 1988a). In lymphoid tissues, however, the situation is more complex. Blood borne lymphocytes enter lymph nodes and Peyer’s patches by adhering to and subsequently penetrating between the specialized endothelial cells lining the PCV of these tissues. Complementary interactions between surface molecules (adhesins) on lymphocytes and endothelial cells permit selective attachment of subsets of blood borne lymphocytes to the endothelium of particular lymphoid tissues (Gallatin, Weissman, & Butcher, 1983; Woodruff et al., 1987; Streeter, Rouse, & Butcher, 1988; Streeter. Berg, Rouse, Bargatze, & Butcher, 1988; Holzmann. McIntyre, & Weissman, 1989). Attachment of circulating lymphocytes in the PCV of Peyer’s patches has been visualized in intact mice, and is an extremely rapid and efficient process (Bjerknes, Cheng, & Ottaway, 1986). An interesting feature of this lymphocyteendothelial cell interaction is that the majority of lymphocytes collide with the endothelial surface multiple times before achieving a sufficiently secure attachment to be able to leave the vasculature. Although this process amplifies the overall efficiency with which lymphocytes can be collected at these sites, changes in the hemodynamics of these vessels would be expected to modulate the efficiency with which lymphocytes are extracted (Ottaway, 1988a). Another potentially important feature of the migration of lymphoyctes into lymphoid tissues is that the rate at which lymphocytes can be extracted from the blood varies with the subpopulation concerned. For example, a consistent difference between the rate at which CD4+ and CD8+ T cells gain entry to lymphoid tissues occurs in a variety of animals (Table 3). Thus, we would expect that hemodynamic changes that increase the rate of blood flow to particular lymphoid tissues, and/or the collision frequency of lymphocytes with the endothelium, would have a more pronounced effect on the rate of redistribution of CD4+ cells than that of the CD8+ subpopulation.

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TABLE 3 Estimated Clearance Rates (ml Blood Clearediminig Tissue) for T Cell Subsets from Blood into Lymphoid Tissues of Different Species Species Tissue

T cell subset

Mouse”

Lymph node

CD4 CD8 CD4

0.2 0.1 0.2 0.1

Peyer’s patch

CD8

Spleen

CD4 CD8

1.3

Rath

Sheep’

0.3

0.2 0.1 -

1.6

-

0.7

0 Data are adapted from Fisher & Ottaway (1991). ’ Data are adapted from Smith & Ford (1983) and Ottaway & Fisher (1990). c Data are adapted from Washington et al. (1988).

In addition to changes in vascular dynamics more direct mechanisms exist by which sympathetic neural activity may affect migratory cells. This can occur through close associations between nerve endings and lymphoid cells, or by paracrine interactions via catecholamines released within short distances of lymphoid cells at some time during their migration through a particular compartment. Whether adrenergic or noradrenergic signals might be able to alter the expression of lymphocyte or vascular addressin molecules has not, to our knowledge, been determined, but could, in principle, be a potent means by which catecholamines could influence the accumulation of lymphocyte populations. It is known, however, that lymphocytes have adrenoreceptors and that subsets of lymphocytes differ with respect to the distribution of (Y- and B-adrenergic receptors. In mice, for example, splenic B cells express more /3-adrenergic binding capacity than do T cells (Miles, Atweh, Otten, Arnason, & Chelmicka-Schorr, 1984). T lymphocytes in peripheral blood express fewer B receptors than B cells, but CD8+ cells express more than CD4+ T cells (Landmann, Burgisser, West, & Buhler. 1985). Ernstrom and Sandberg (1973) demonstrated that stimulation of B-adrenergic receptors does not induce the same degree of mobilization as does stimulation of the (Y receptors, indicating that the extent of mobilization of cells from the spleen is also dependent on whether (Yor B receptors are stimulated. Receptor density may be modulated, in part, by noradrenalin availability since denervation of the murine spleen leads to enhanced expression of B-adrenergic receptors (Miles et al., 1984). Further, splenic lymphocytes from the pig have been found to have increased expression of adrenergic receptors in response to T cell activation in vitro, although increased receptor expression is abrogated in the presence of B-adrenergic agonists (Westley & Kelley, 1987). Thus, it is likely that different functional subpopulations of lymphocytes have varying sensitivities to catecholamine effects, whether the effects of the these mediators are produced via direct interactions with sympathetic nerves or indirectly through hemodynamic alterations. NEUROPEPTIDES

AND LYMPHOCYTE

MIGRATION

In addition to the potential for modulation of lymphocyte traffic through noradrenergic nerves and sympathoadrenal pathways there are a wide range of neuropeptide transmitters released by the central and peripheral nervous system. A

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number of these neuropeptides have been shown to have immunomodulating properties. Because of the abundance of peptidergic nerves in the enteric nervous system, potential interactions between local neuropeptides and lymphocytes in this tissue have been a special focus of attention (O’Dorisio, Wood, & O’Dorisio, 1986; Stead, Tomioka, Pezzati, Marshall, Croitoru, Perdue, Stanisz, & Bienenstock, 1991; Ottaway, 1991a,b). It needs to be recalled, however. that these neuropeptides are also found in many other tissue sites and local interactions need not be restricted to the intestine. Two of the commonest enteric neuropeptides, vasoactive intestinal peptide (VIP) and substance P, have been shown to have properties relevant to the migration of lymphoid cells. VIP is a 28 amino acid neuropeptide which has been implicated as a modulator of a great variety of biological processes (Said, 1984, 1986). It is found in neurons throughout the central and peripheral nervous system and is especially well represented in neurons of the enteric nervous system (Costa, Furness, & LlewellynSmith, 1987). VIP has also been isolated from mast cells and eosinophils (Cutz, Chart, Track, Goth, & Said, 1978; Goetzl, Gromtol, Van Dyke, et al., 1990), and may be available in tissues from both neural and inflammatory activities. VIP has been implicated as a potent regulator of a wide variety of immune functions in vitro (Ottaway, 1991a) and several lines of evidence suggest that this neuropeptide can modulate lymphoid cell migration in vivo. VIP is a potent stimulator of human monocyte chemotaxis (Sacerdote, Ruff, & Pert, 1988), and direct infusion of the peptide into the afferent lymphatic of cannulated lymph nodes in sheep markedly decreases the output of lymphocytes in the efferent lymphatic of the nodes (Moore, 1984; Moore, Spruck, & Said, 1988). For example, the infusion of approximately 0.1 nmol of VIP decreases the efferent flow of lymphocytes by about 75% and this reduction is sustained for many hours (Moore et al., 1988). This effect demonstrates subpopulation selectivity, in that B cells are more affected than T cells, and CD8+ T cells are more affected than are CD4+ cells (Moore et al., 1988). In mice, the local interaction of VIP with T cells has been implicated in the migration of blood borne T cells into mesenteric lymph nodes and Peyer’s patches (Ottaway, 1984, 1985). These tissues contain VIP-immunoreactive nerves in the T cell corridors and in close proximity to the specialized endothelium of the PCV (Ottaway, Lewis, & Asa, 1987). Murine T cells have receptors for VIP, and CD4+ cells bind the neuropeptide more effectively than CDS+ cells (Ottaway, 1988b). The density of VIP receptors on these cells can be decreased by exposure of the cells in vitro to the peptide and decreased expression of VIP binding capacity leads to decreased localization of the cells into mesenteric nodes and Peyer’s patches on subsequent transfer of the treated cells to syngeneic animals (Ottaway, 1984). This effect is selective for those tissues, and time course studies of the migration showed that the altered migration of the receptor depleted cells is due to a decrease in the rate at which the cells are extracted from blood (Ottaway, 1985). Taken together, these studies support the notion that the local release of VIP can alter the migration of T cells capable of recognizing the molecule, and that this effect is produced at or near the level of the endothelium. The precise mechanism is unknown, but may involve alteration of the activity of the cells through activation of cyclic AMP. Similarly. the mechanism by which VIP produces decreased output of lymphocytes is not yet established. It is known that the vast

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majority of the lymphocytes leaving via efferent lymphatics are cells which were extracted from the blood stream (Hall & Morris, 1965); thus, if the neuropeptide were acting only as a local vasodilator, one would anticipate increased lymphocyte traffic rather than the observed decrease (Ottaway, 1988b). There have been no reported studies of the ability of neuropeptides such as VIP to affect the expression or distribution of lymphocyte and/or endothelial cell adhesion molecules, but effects of neural signals at this molecular level or on the cellular processes that permit lymphocytes to cross the basement membrane of the venule may be possible. Substance P is an 11 amino acid peptide produced by sensory neurons and may be released from peripheral nerve terminals as part of an axon reflex (Pernow, 1983). Its physiologic effects include contraction of intestinal smooth muscle, arteriolar vasodilation, increased secretion of salivary glands and nasal epithelium alteration of microvascular permeability, and mast cell degranulation (Pernow, 1983). The effects of substance P on vascular permeability may have important implications on cell traffic, increasing the probability of cells extravasating from the circulation into sites at which the neuropeptide is released. Acute infusion of substance P into the afferent lymphatics of sheep lymph nodes increases the volume of efferent lymph and the output of lymphocytes from the node by a means that appears to be independent of alteration in prostaglandin synthesis (Moore, 1984). In further studies of this effect, it has been shown that the output of CD4’ T cells was selectively depressed at early times (l-2 h) after substance P infusion, but that there was a selective enhancement of CD4+ cells in the efferent lymph at longer intervals (~24 h) after infusion (Moore, Whitley, Lami, & Said, 1990). Substance P has also been shown to modulate the function of a number of cells involved in inflammatory reactions. For example, it promotes chemotaxis and phagocytosis of neutrophils (Bar-Shavit, Goldman, Stabinsky, Gottlieb, Fridkin, Teichberg, & Blumberg 1980; Marasco, Showell, & Becker, 1981), and is a potent chemotactic stimulus for human monocytes (Ruff. Wakl, & Pert, 1985). In addition to effects of substance P on lymphocyte mobility through changes in vascular perfusion and permeability and the release of inflammatory mediators, substance P may interact directly and selectively with lymphocytes through cell surface receptors. In mice, most T and B lymphocytes express substance P receptors, and cells expressing the receptors are particularly frequent among lymphocytes from the spleen and Peyer’s patches (Stanisz, Scicchitano, Dazin, Bienenstock, & Payan, 1987). In human peripheral blood however, receptors appear to be expressed more on CD4+ T cells than on CD8+ T cells (Payan, Brewster, & Goetzl, 1984; Payan, Brewster, Missirian-Bastian, & Goetzl, 1984). While lymphocyte activation via these receptors has been shown to modify functional activities (e.g., IgA synthesis from B cells (Stanisz, Befus, & Bienenstock, 1986)) there is as yet no direct evidence that the effects on lymphocyte traffic are also mediated via receptor-ligand interactions. NEUROENDOCRINE HORMONES AND LYMPHOCYTE THE PITUITARY-ADRENAL CONNECTION

MIGRATION:

Peripheral nerves provide the “hard wiring” for potential connections between the brain and the immune system that may influence lymphoid cell migration through direct contacts, through paracrine mechanisms, or through secondary

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catecholamine mediated endocrine effects. Another major pathway by which the CNS may be capable of altering lymphocyte distribution, however, is through neuroendocrine secretions, and the hypothalam+pituitary-adrenal axis may be particularly potent in this regard. Selye’s original observation (1936) that animals exposed to acute stressors display lymphoid tissue involution has been shown to be correlated with the production of pituitary adrenocorticotrophic hormone (ACTH) and its transduction via the adrenal cortex into the release of circulating corticosteroids (Axelrod & Reisine, 1984). One of the first cellular links between stress-induced immunosuppression and corticosteroids was the demonstration by Fauci and Dale (1974) that corticosteroid administration results in a transient, but profound, lymphocytopenia, involving T cells in particular, but all lymphocyte populations to varying degrees. It is now clear that this reflects an acute redistribution of circulating lymphocytes into various lymphoid compartments. Fauci (1975) reported a redistribution of circulating lymphocytes into bone marrow following the administration of corticosteroids to mice, and Zatz (1975) has shown that while labeled lymphocytes from untreated donorg survive normally in cortisone treated recipients, their accumulation in lymph nodes and spleen was decreased while that in the marrow was increased. ACTH infusion in rats disrupts the normal recirculation pattern of thoracic duct lymphocytes (TDL) (Spry, 1972) such that labeled lymphocytes do not reach the thoracic duct of recipient animals until the ACTH infusion is stopped, although their subsequent recirculation was normal (Spry, 1972). Cox and Ford (1982) studied the tissue localization of rat TDL in response to bolus infusions of ACTH or prednisolone and demonstrated transiently enhanced localization of lymphocytes in the marrow. Continuous infusion of prednisolone, however, had a more profound effect on the accumulation of lymphocytes in a variety of organs. Prolonged infusion inhibited the entry of lymphocytes into lymph nodes, but the degree of inhibition varied in nodes situated in different regions of the body. The retention of the cells in the lymph nodes was also altered, and lymphocytes that accumulated were not released normally into the lymph. It is noteworthy that the transferred cells in these animals survived in the recipients, and after cessation of the corticosteroid treatment their subsequent migration was normal. Although corticosteroids can affect the accumulation of lymphocytes in many tissues, including mucosal sites (Walzer, LaBine, Redington, & Cushion, 1984), a feature of the effect of these hormones is the impairment of the ability of cells to migrate from blood into lymph nodes, with a concomitant increase in endothelial transit time (Cox & Ford, 1982). Chung, Samlowski, and Daynes (1986) observed that adoptively transferred lymphocytes from untreated donors into corticosteroid treated recipients localized more readily in the bone marrow at the expense of lymph nodes, but treatment of lymphocytes in vitro with corticosteroids has no effect on their subsequent in vivo migration. Using the Stamper and Woodruff (1976) in vitro assay to observe the ability of specialized lymphoid epithelia to bind lymphocytes, these authors confirmed that the redistribution of lymphocytes associated with elevated corticosteroid is mediated, at least in part, by steroids effects on vascular endothelial cells. The nodal venules of corticosteroid treated animals displayed a reduced ability to bind normal lymphocytes, but the ability of lymphocytes from treated animals to bind to the specialized PCV’s of untreated animals was unaffected.

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The cellular and molecular mechanisms by which corticosteroids alter lymphocyte-endothelial cell interactions are not known. The effect, however, is not restricted to the specialized endothelium of lymph nodes and Peyer’s patches because the ability of lymphocytes and monocytes to reach sites of acute inflammation is also reduced by steroid administration (Thompson & Van Furth, 1970; Chung, Samlowski, & Daynes, 1986). It is possible that interference with monocyte/macrophage migration may be a control point in inflammatory sites since Samlowski, Chung, Burnham, and Daynes (1988) have demonstrated that macrophage-mediated signals can initiate increased lymphocyte circulation through lymph nodes draining acute inflammatory lesions by increasing the frequency and density of specialized PCV in the lymph node. A regulatory role for local macrophages or dendritic cells (or their cell products) has also been suggested by the work of Hendriks and Eestermans (1983) which showed that interruption of afferent lymphatic flow into popliteal nodes decreased the density of specialized PCV’s in the node. Thus, the effects of corticosteroids may involve mechanisms mediated through other lymphoid cells as well as direct effects on the endothelial cells themselves. Another mechanism by which the pituitary-adrenal signals may influence lymphocyte migration is through the ability of ACTH to suppress the production of interferon-y (IFNy), through direct effects on CD4’ T cells (Johnson, Ton-es, Smith, Dion, & Blalock, 1984. IFNy has been shown to affect the expression of MHC class II determinants by endothelial cells (Pober, Gimbrone. Cotran, Reiss, Burakoff, Fiers, & Ault, 1983), to increase the binding of T cells to endothelia (Yu, Haskard, Cavender, Johnson, & Ziff, 1985), and to enhance the recirculation of lymphocytes through lymph nodes (Kimber, Sparshott, Bell, & Ford, 1987). In pharmacologic doses, IFNy induces a marked lymphocytopenia in rats and can enhance the retention of lymphocytes within lymph nodes as well as increasing their entry rate. In sheep, IFNy markedly decreases the efferent output of lymphocytes from lymph nodes, and inducers of IFNy such as poly 1:C or inactivated vaccinia virus induce a substantial decrease in the thoracic duct output of lymphocytes (Korngold, Blank, & Murasko, 1983). The molecular mechanisms underlying these effects are not yet defined, but the ACTH-IFNy path might involve alterations in the ability of lymphocytes to interact with both endothelial cells and extracellular tissue determinants. Other gene products of the proopiomelanocortin (POMC) complex may also affect lymphoid cell migration. B-Endorphin and met-enkephalin can both enhance IFNy production by mononuclear cells (Brown & Van Epps. 1986), and B-endorphin can alter the expression of the CD4 determinant on T cells and HLA-DR determinants on mononuclear cells (Puppo, Corsini, Mangini, Bottaro, & Barreca, 1985), both of which play a role in defining lymphocyte traffic and distribution patterns. It now appears that, given appropriate stimuli, lymphocytes themselves can produce POMC gene products. Lymphocytes can process the POMC product to produce ACTH in response to corticotrophin releasing hormone (CRH) (Smith, Morrill, Meyer, & Blalock, 1986), specific virus infection (Smith & Blalock, 1981; Westley, Kleiss, Kelley, Wong, & Pick-Hoong, 1986), and lipopolysaccharide endotoxin (Harbour-McMenamin, Smith, & Blalock, 1985). Lymphocytes can also produce various endorphins as well as other endocrine peptides under specific conditions (Weigent & Blalock, 1987; Smith & Blalock, 1981; Lolait, Clem-

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ents, Markwick, Cheng, McNally, Smith, & Funder, 1986; Carr & Blalock, 1991). This suggests an additional avenue by which lymphocyte migration and distribution patterns might be altered in response to inflammatory and/or immunologic and/or endocrinologic events. If lymphocyte-derived ACTH and B-endorphin can also affect local IFNy production, a scenario by which lymphocytes might regulate the migration of their fellows, in a paracrine fashion, can, at least in principle, be envisaged. EFFECTS

OF OTHER

HORMONES

ON LYMPHOCYTE

MIGRATION

There is considerable literature on the effects of sex hormones on immune function, many of which are related to changes in migration and circulation of cells, particularly in mucosal regions (reviewed by Sullivan (1986)). These hormones, particularly estrogen and testosterone, have been shown to alter the intrinsic localization abilities of B lymphoblasts in the gut (McDermott, Clark, & Bienenstock, 1980; Mirski, Befus, & Bienenstock, 1983), uterus (McDermott et al., 1980; Wira & Sandoe, 1980), and lacrimal gland (Hann, Allansmith, & Sullivan, 1988). They also regulate T lymphoblast entry into the mammary gland (Manning & Parmely, 1980) and the content of T helper cells in lacrimal glands (Gudmundsson, Bjornsson, Olafsdottir, Bloch, Allansmith, & Sullivan, 1988). Other pituitary hormones have been implicated in modulation of cell migration potential. Arginine vasopressin (AVP) is a neurohypophyseal nonapeptide and, in addition to its anti-diuretic and vasopressor effects, has been shown to modulate the stress response by acting in concert with CRH to stimulate ACTH release (Gibbs, 1986). It also has direct effects on lymphocyte migration by providing a helper signal for the induction of IFN production by lymphocytes (Johnson, Farrar, & Torres, 1982: Johnson & Torres, 1985). Growth hormone release has also been reported to have immunostimulatory effects by promoting lymphocyte activation with consequent effects on their migratory potential (Snow, 1985; Kass, Kendrick, & Finland, 1953). The anti-inflammatory and antipyretic effect of another POMC derivative, a-melanocyte stimulating hormone (a-MSH) (Cannon, Tatro, Reichlin, & Dinarello, 1986), is a reflection of its effects on influx of inflammatory cells to sites of inflammation by blocking neutrophil accumulation in response to IL-I (Mason & Van Epps, 1989). RHYTHMS

OF LYMPHOCYTE

MIGRATION

Significant daily rhythmic variations in the blood concentrations of circulating T and B cells have been identified in humans. In general, this circadian variation is greater for T cells than for B cells, and the lowest concentrations of both populations occur in the early morning hours. Circadian variation of major T cell subsets has also been reported in several studies (Table 4). The largest variations appear to occur in the circulating concentration of CD4+ cells. Although smaller amplitude variations of the CD8+ subpopulation have been identified by some workers (Bertouch, Roberts-Thomson, & Bradley, 1983; Ritchie, Oswald, Micklem, Boyd, Elton, Hazwinska, & James, 1983), others have found either no significant temporal variation (Yato, Yokoi, Takano, Kanegane, Yachie, Miyawaki, & Taniguchi, 1990) or a statistically significant biphasic (i.e., 12 h) rhythm (Levi, Canon, Touitous, Sulon, Mechkouri, Ponsart, Touboul, & Vannetzel, Mowzowicz, Reinberg, & Mathe, 1988). Daily rhythms of T and B cells have also been demonstrated in mice (Kawate,

CNS EFFECTS

ON LYMPHOCYTE

109

MIGRATION

TABLE 4 Circadian Variation in Concentrations of Human Peripheral Blood T and B Cells in Relation to Peak Cortisol Levels Time of peak concentration

(h:min)

T cell subsets

Ref.

No. subjects

Total T cells

CD4

CD8

B cells

Cortisoi

Abo et al., 1981 Bertouch et al., 1983 Ritchie et al., 1983 Miyawaki et al., 1984” Levi et al.. 1985 Levi et al., 1988

4 9 IO 8 7 5

0o:oo 21:oo 22:20 nd 01:20 nd

nd 2l:OO 22:30 07:oo nd 01:20

nd 21:oo 22:oo nsv nd 20:30 & 08:30

0o:oo 18:00 23:05 nd 20:oo nd

08:30 06:OO 09:50 07:oo nd 09:40

Note. nd, not done; nsv, no significant variation. “ Subjects were not tested between 22:00 and 07:OO.

Abo, Hinuma, & Kumagai, 1981). These animals are more active during darkness and the peak time of circulating lymphocyte numbers occurs near midday when the mice are housed under normal conditions, the reverse of the human situation. Periodicity of lymphocyte populations also occurs in the spleen of mice (Kawate, Abo, Hinuma, & Kumagai, 1981) and appears to be phase shifted with respect to blood. In the spleen, peak numbers of B cells occur approximately 6 h after the blood maximum, whereas the number of splenic T cells are approximately 2 h out of phase with variations in the blood stream (Kawate et al., 1981). The rhythmic variations of lymphocyte populations in blood are probably accounted for by compartmental shifts of circulating lymphocytes between blood and spleen, lymph nodes, bone marrow, and other tissues. There have been no reported studies of the circadian variation of specific T cell subpopulations in mice, but we would expect that differences exist between the tissue phases for CD4+ and CD8+ T cells since the mean residence time of T cells in murine lymphoid tissues varies with the T cell phenotype (CD4 vs CD8) and the lymphoid tissue in which the cells accumulate (Fisher & Ottaway, 1991). The carcadian variation of plasma corticosteroids has been implicated as one factor involved in diurnal variations in circulating lymphocyte concentrations, but whether this is a complete or necessary explanation is not clear. In mice adapted to an inversion of the light-dark cycle there is a reversal of both plasma corticosterone and blood T and B cell concentrations, and the rhythm of circulating lymphocytes is severely dampened by adrenalectomy (Kawate et al., 1981). In human studies, some investigators (Abe, Kawate, Itoh, & Kumagai, 1981; Ritchie et al., 1983), but not others (Bertouch et al., 1983; Levi et al., 1988), have demonstrated a significant inverse covariation of the concentrations of circulating lymphocyte populations and serum cortisol. It is possible, however, that some lymphoid subpopulations may respond to other endocrine rhythms. The circadian rhythm of NK cell activity described in humans (Abo et al., 1981) and rats (Fernandez, Caradente, Halber, Halberg, & Good, 1979) is disrupted by surgical removal of the rat pineal gland, but this occurs in the absence of a disruption of the rhythm of plasma corticosterone (McNulty, Relfson, Fox, Kus, Handa, & Schneider, 1990). The responsiveness of the pineal gland and melatonin secretion

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to light-dark perception, therefore, may be relevant to the migration populations of lymphoid cells (Maestroni & Conti. 1991).

of some

CONCLUSIONS

The effectiveness of the immune system can be assessed in a variety of ways: its productivity, efficiency, durability, and responsiveness. Many factors contribute to an effective immune response, but a principal determinant of outcome is whether an environmental antigen will meet antigen-specific lymphocytes within a particular location in a timely fashion. Another determinant is the opportunity for encounter with combinations, or operational clusters, of antigen-specific cells (e.g., B cells, CD4+ and/or CD8+ T cells). The migration of lymphoid cells throughout the body simultaneously provides stability and responsiveness of immune function but it is also a primary means through which immunological responses become intimately linked with other physiologic processes throughout the body. The investigations which we have reviewed here suggest that there are at least five general means through which CNS events might influence macroscopic and microscopic lymphocyte migration. These include (1) Catecholamine release from either adrenal medulla or sympathetic nerve terminals mediating the mobilization of lymphocytes from secondary lymphoid tissues, increased concentrations of circulating lymphocytes, altered hemodynamics, and changes in the rate of cell delivery to tissues; (2) The activation of the peptidergic arm of the autonomic nervous system with local release of neuropeptides such as VIP and substance P leading to local vasculature alteration, and/or local interactions with receptor-bearing populations of lymphocytes: (3) The activation of the pituitary-adrenal axis leading to corticosteroidmediated alterations of lymphocyte-endothelial interactions and/or lymphocytestromal interactions, and modulation of the effects of the cytokine IFNy: (4) The actions of other endocrine signals; (5) Circadian patterns of CNS activity, expressed through the pituitary-adrenal axis and other hypothalamic and/or pineal endocrine signals, which mediate rhythmic alterations in the availability of subpopulations of circulating lymphocytes. These neurally initiated processes need not be restricted to bulk changes in the transit of lymphocytes into and out of tissues, but differential effects on the migration of subpopulations of lymphocytes can occur. Subpopulation selectivity can arise in three ways: (I) The ability of lymphoid tissues to extract and retain lymphocytes can vary for major subpopulations of lymphocytes; (2) Different lymphocyte subpopulations have varying abilities to recognize and respond to adrenergic and neuropeptide signals to which they may be exposed during their migration through tissues; (3) Qualitative and quantitative differences exist in the local innervation of lymphoid tissues in different regions of the body. The ability of experimental stress paradigms and various conditioning paradigms to modulate immune responses in intact animals is now well documented. We propose that alterations of immune function observed in these investigations may be related, at least in part, to changes in the compartmental distribution.

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EFFECTS

ON

LYMPHOCYTE

MIGRATION

111

localization, and migration of lymphocytes mediated by neurophysiologic and neuroendocrine pathways. Further, the demonstrated impact of neuroendocrine activities on cell distribution may translate in real terms to patterns of disease in man and animals. We suggest that these issues deserve further attention in understanding the mechanisms of behaviorally mediated alterations in immune function, and that rigorous testing of this proposition will advance our understanding of immune physiology and the nature of the interactions among behavior, the brain, and immune responses. REFERENCES Abo, T., Kawate, T., Itoh, K., & Kumagai, K. (1981). Studies on the bioperiodicity of the immune response. I. Circadian rhythm of human T, B and K cell traffic in the peripheral blood. J. lmmuno/.

126, 1360-1363.

Ackerman, K. D., Felten, S. Y., Dijkstra, C. D., Livnat, S., & Felten, D. L. (1989). Parallel development of noradrenergic innervation and cellular compartmentation in the rat spleen. Exp. Neurd. 103, 239-255. Ader, R., Felten, D. L., Cohen, N. (1991). Psychoneuroimmunology, second ed. Academic Press: New York. Anderson, A. 0.. & Anderson, N. D. (1975). Studies on the structure and permeability of the microvasculature in normal rat lymph nodes. Am. J. Pathol. 80, 387418. Axelrod, J., & Reisine, T. D. (1984). Stress hormones: Their interactions and regulation. Science 224, 452-459. Ayers, A. B., Davies, B. N., & Withrington, P. G. (1972). Responses of the isolated perfused human spleen to sympathetic nerve stimulation, catecholamines and polypeptides. Br. J. Pharmaco/. 44, 17-30. Bar-Shavit, Z., Goldman, R., Stabinsky, T., Gottlieb. P., Fridkin, M., Teichberg, V. I., & Blumberg, S. (1980). Enhancement of phagocytosis-A newly found activity of substance P residing in its N-terminal tetrapeptide sequence. Biochem. Biophys. Res. Commun. 94, 1445-1451. Berg, E. L., Goldstein, L. A., Jutila, M. A., Nakache, M.. Icker, L. J., Streeter, P. R., Wu, N. W., Zhou. D., & Butcher, E. C. (1989). Homing receptors and vascular addressins: Cell adhesion molecules that direct lymphocyte traffic. Immunol. Rev. 108, 5-18. Bet-touch, J., Roberts-Thomson, P. J., & Bradley, J. (1983). Diurnal variation of lymphocyte subsets identified by monoclonal antibodies. Er. Med. J. 286, 1171-l 172. Besedovsky, H. O., & de1 Rey, A. (1986). Immune-neuroendocrine network. In B. Cinader & R. G. Miller (Eds.), Progress in immunology VI. pp. 57. Academic Press: New York. Bjerknes, M., Cheng, H., & Ottaway, C. A. (1986). Dynamics of lymphocyte