T cells and eosinophils in the pathogenesis of asthma

REVIEW

T cells and e0sinophils in the

pathogenesis of asthma C.J. Corrigan and A.B. Kay Persistent asthma is characterized by chronic inflammation of the bronchial mucosa, where T cells and eosinophils are prominent. This article summarizes the evidence that asthmatic bronchial inflammation is initiated and propagated by cytokines secreted by activated T cells and other cells, and describes how the release of specific cytokines could result in local preferential accumulation and activation of eosinophils. Asthma is a disease characterized by reversible obstruction of the airways, or bronchi. This is accompanied by non-specific bronchial hyperresponsiveness or 'irritability', which is the tendency of the bronchi in asthmatics to constrict in response to a wide range of pharmacological and irritant stimuli. It is now widely accepted that chronic inflammation of the bronchial mucosal lining plays a fundamental role in the genesis of these clinical manifestations. The most striking feature of the histopathology of asthma is the intense infiltration of the bronchial mucosa with eosinophils, macrophages and lymphocytes. In fact, the disease has many of the histopathological features of a chronic, cell-mediated hypersensitivity. The eosinophil appears to be a key cell in producing bronchial mucosal injury ~. This, in turn, is believed to result in bronchial obstruction and irritability, although the precise mechanisms by which this occurs are not clear. Other important pathological features include destruction and desquamation of airway epithelial cells, and collagen deposition below the basement membrane. Asthma is often, though not invariably, associated with atopy, particularly in children. Atopy refers to the genetic predisposition of certain individuals to synthesize, inappropriately, immunoglobulin E (lgE) specific for certain external antigens, particularly inhaled aeroallergens such as grass pollen. This allergen-specific IgE sensitizes mast cells and other cell types with highand low-affinity IgE Fc receptors, causing an energydependent release of preformed, granule-derived and newly formed, membrane-derived pharmacological agents on further allergen exposure. Some atopic asthmatics experience an exacerbation of their disease on exposure to allergens to which they are sensitized. Such patients have traditionally been referred to as 'extrinsic' asthmatics, reflecting the relationship of their disease with external environmental factors. Nonatopic asthmatics, where IgE-mediated mechanisms do not obviously operate, have been labelled 'intrinsic'. Some patients develop asthma after exposure to specific proteins or small molecular weight chemicals at work; these 'occupational' asthmatics form a third clinical category. It is not clear how far IgE-mediated or cell-mediated mechanisms (or both) can be implicated in the various forms of occupational asthma.

The extent to which IgE-mediated mechanisms play a role in asthma pathogenesis is uncertain. Insofar as some non-atopic individuals develop the disease whereas not all atopic individuals do, it would appear that IgEmediated mechanisms are neither necessary nor sufficient for the development of asthma. IgE-mediated mechanisms are clearly important in allergenqnduced short-term exacerbations of asthma in atopic subjects, but the role of IgE in the pathogenesis of the on-going chronic disease is less certain.

Evidence for mucosal inflammation in asthma Many recent studies on the histology of asthma have compared mild asthmatic and normal volunteers, utilizing the techniques of bronchoalveolar lavage (BAL) and bronchial biopsy through the flexible fibreoptic bronchoscope. Elevated numbers of eosinophils, both in the bronchial mucosa and in BAL fluid, were constant features of mild asthma-"L Similarly, elevated numbers of activated lymphocytes, identified either as irregular, atypical lymphocytes by transmission electron microscopy4 or as CD25-bearing cells as shown by immunocytochemistry, were also demonstrated. Most of the CD25-expressing cells in these biopsies were T cells ~. There is evidence that activation of selected "['-cell populations with subsequent eosinophil recruitment and secretion may contribute both to epithelial damage and to bronchial hyperresponsiveness". Although recent studies have not shown significant changes in the numbers of mast cells (or their subtypes), neutrophils or macrophages in the bronchial mucosa of mild asthmatics- ~, this does not exclude the participation of these cell types in more severe disease; furthermore, cell numbers do not necessarily equate with function, and it has been shown for example that the spontaneous release of mediators from mast cells is increased in asthmaticsL The traditional clinical classification of asthma (intrinsic, extrinsic and occupational) implies variability in pathogenesis. One important question, therefore, relates to whether these clinical distinctions are apparent in histopathological terms. Preliminary studies that have addressed this question suggest that they are not: an autopsy study of the bronchial mucosa of a patient who had died with severe occupational asthma showed

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REVIEW Eosinophil mediators

Cytokines IL-1 IL-3 IL-5 IL-6 GM-CSF TGF-c( TGF-~

Newly synthesized membrane-derived mediators Leukotriene C4/D4 Platelet-activating factor 15-HETE Prostaglandin E~, E2 Thromboxane B2

eosinophils preferentially accumulate in the inflamed mucosa. Established eosinophil chemoattractants, such as platelet activation factor (PAF) and C5a, while highly potent, are nonspecific in the sense that they also attract neutrophils. On the other hand, IL-5 can specifically prime eosinophils for enhanced locomotor responses to PAF, leukotriene B4 (LTB4) and IL-8 (Ref. 15). Eosinophil migration from the vascular space into the tissues is initiated by an interaction between receptors on the cell surface with their ligands on the surface of vascular endothelial cells. Eosinophils and Granule-derived basic proteins Major basic protein (MBP) neutrophils both express the ~2 integrins LFA-1 Eosinophil cationic protein (ECP) (CD18-CDlla) and Mac-1 (CD18-CD11b) as well as Eosinophil derived neurotoxin (EDN) Eosinophil peroxidase (EPO) the receptor for E-selectin. However, eosinophils appear to be unique insofar as (1) IL-3 and IL-5 upreguFig. 1. A diagrammatic representation of the cytokines, mediators and basic late eosinophil, but not neutrophil, adhesion to proteins produced by human eosinophils. Many of these have proinfiamma- unstimulated endothelial cells 16, and (2) eosinophils, but not neutrophils, express the f31 integrin 0~4~1 tory properties relevant to the asthma process. (CD49a-CD29, VLA-4), which is a ligand for VCAM1 on the surface of stimulated endothelial cellsEv.These histological changes similar to those seen in fatal non- mechanisms, together with the properties of IL-3, IL-5 occupational asthma 8, while a recent immunocyto- and GM-CSF in prolonging eosinophil survival, might chemical study 9 comparing bronchial biopsies from offer a partial explanation as to how eosinophils extrinsic and occupational asthmatics showed that accumulate preferentially in the asthmatic bronchial these were indistinguishable in terms of their inflam- mucosa. matory cell infiltrate. A similar situation pertained with 'intrinsic' asthmatics, although there was in this Pro-inflammatory properties of eosinophils Eosinophils can secrete a number of lipid mediators case some evidence of an additional macrophage infiltrate ~°. Examination of BAL fluid obtained from a and proteins which may have a role to play in the group of 'intrinsic' asthmatics ~ showed increased pathophysiology of asthma (Fig. 1). They elaborate numbers of activated T cells, eosinophils and neu- eicosanoids derived from the 5- and 15-1ipoxygenase trophils as compared to normal controls. These obser- pathways, especially LTC4/D4, as well as substantial vations suggest many similarities in the bronchial quantities of PAF (Refs 18,19). Both LTC4 and PAF histopathology in patients with asthma, regardless of cause bronchoconstriction whilst LTC 4 is a mucus the nature of identifiable provoking agents, and lend secretagogue. PAF also increases vascular permeability. support to the hypothesis that the pathogenesis of It has been difficult to demonstrate the presence of LTC4/D4 and PAF in asthma, possibly because of their asthma is independent of coexisting atopy. low concentrations and rapid metabolism, although Eosinophils and asthma pathogenesis clinical trials have suggested that LTD4 receptor antagonists and 5-1ipoxygenase inhibitors may be of some Preferential accumulation of eosinophils in asthma Eosinophils are non-dividing, granular cells that benefit for the therapy of chronic disease2°. Eosinophils store four basic proteins in their granarise principally in the bone marrow. Eosinophil differentiation, like that of all leukocytes, is influenced by ules: major basic protein (MBP), eosinophil-derived lymphokines. Interleukin 3 (IL-3), IL-5 and granulo- neurotoxin (EDN), eosinophil cationic protein (ECP) cyte-macrophage colony stimulating factor (GM-CSF) and eosinophil peroxidase (EPO). MBP is toxic for promote eosinophil differentiation12. Whereas IL-3 and human respiratory epithelial cells and pneumocytes2l. GM-CSF act on the precursors of a number of leuko- Inhalation of MBP, albeit at high concentrations, cytes, IL-5 appears to be specific for eosinophils. IL-5 caused bronchial hyperresponsiveness in a primate may be the most important cytokine for terminal dif- model of asthma 22. EPO is also toxic for respiratory ferentiation of the committed eosinophil precursor, epithelium and pneumocytes, particularly in combisince it is released principally by T cells and the nation with peroxide and halide ions 2~. Degranulation eosinophilia associated with parasitic infections is of eosinophils with release of these proteins follows T-cell dependent~3. This hypothesis is further supported engagement of their IgG, IgA and IgE receptors 24,2~, by the observation that transgenic mice that constitut- and by direct stimulation with soluble mediators such ively express the gene for IL-5 develop a marked as PAF and LTB4 (Ref. 26). Despite these observations, peripheral blood eosinophilia E4. If IL-5 alone were suf- it is not clear precisely what produces eo~inophil ficient to mediate the eosinophilia associated with degranulation in vivo, although the observation that asthma, this might explain why there is a consistent adherence of eosinophils to fibronectin enhances LTC4 expansion of eosinophils and not other leukocyte production suggests that extracellular matrix proteins lineages. may serve as one extravascular activating stimulus One of the fundamental problems in investigating (Anwar, A.R.E., Walsh, G.M., Cromwell, O. et al., the pathogenesis of asthma has been to explain why submitted).

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REVIEW (a) Eosinophils have the capacity to elaborate certain cytokines including transforming growth factor c( (TGF-00, TGF-[~, GM-CSF, IL-1, IL-3, IL-5, IL-6 and IL-8 (Fig. 2) 2~-~2. The GM-CSF secreted by eosinophils was shown to exert an autocrine effect on cell survival 29. These extensive pro-inflammatory properties of eosinophils, along with their abundance in the asthmatic broncial mucosa, have strengthened the current consensus that eosinophils are prime inflammatory effector cells in this disease. Eosinophils in asthma It is well known that a blood and sputum eosinophilia is often, though not invariably, observed in association with asthma of diverse aetiology. Both longitudinal and cross-sectional studies of asthmatic patients suggest that blood eosinophil counts correlate with the degree of bronchial hyperresponsiveness. Immunostaining of the bronchial mucosa of patients who died with severe asthma revealed the presence of large numbers of activated eosinophils 3~ and considerable amounts of MBP deposited in the airways ~. In atopic asthmatics developing a late-phase bronchoconstrictor response following allergen challenge, the degree of associated bronchial hyperresponsiveness correlated with the peripheral blood eosinophil count 34. The numbers of eosinophils in peripheral blood, BAL fluid and bronchial biopsies in a group of asthmatics were elevated as compared to normal controls, and it was possible to demonstrate an increasing degree of eosinophilia with clinical severity3s. Increased concentrations of MBP were found in BAL fluid from atopic asthmatics as compared to normal controls ~. In this study, correlations were observed between the concentrations of MBP, the numbers of desquamated epithelial cells in BAL fluid and the degree of bronchial hyperresponsiveness. In two studies employing allergen bronchial challenge of atopic asthmatics with BAL six hours later, the late-phase airways obstruction was accompanied by an influx of eosinophils and neutrophils into BAL fluid, which was not observed in subjects developing an isolated earlyphase response 3<~7. Similar observations were made in a study using local allergen challenge through the bronchoscope 38. In subjects with red cedar asthma, plicatic acid inhalation elicited a BAL eosinophilia together with sloughing of bronchial epithelial cells > . In a placebo-controlled double-blind study of asthmatics, sodium cromoglycate therapy suppressed the accumulation of eosinophils in bronchial mucus and BAL fluid, coinciding with clinical improvement4°.

CD4 ÷ T cells and asthma pathogenesis CD4 ÷ T cells and eosinophils CD4 ÷ T cells are clearly an important source of IL-5, IL-3 and GM-CSF. The roles of these agents in enhancing eosinophil maturation, survival, activation4~3 and local eosinophil accumulation are well documented and have been discussed. In addition to T cells, other cell types, such as mast cells, macrophages, epithelial cells, fibroblasts and neutrophils, as well as eosinophils themselves, are potential sources of cytokines which

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Fig. 2. bxprcssion of GM-CSF mRNA in human eosinophils using the techtuque,,/in situ hybridization. Silver grains concentrated in the cell cytoplasm represent autoradiographs of radiolabelled GM-CSF anti-sense riboprobes. C~)unterstammg was with carbol chromotrope 2R. (a) Conventional illuminat,m: (fi! dark ground illumination. Photomicrographs courtesy of Dr Q. Harold and Dr R. Maqbef '~. may influence eosinophil function'S'2"44-4% Nevertheless, T cells are unique amongst inflammatory cells in the sense that they can recognize and respond to processed antigens directly (rather than through passively adsorbed surface immunoglobulins such as IgE), and it is our belief that they play a pivotal role in initiating and orchestrating on-going immunologicallydriven chronic asthma, particularly in situations where the IgE response is absent or minimal. T cells and asthmatic inflammation Recent immunocytochemical studies of bronchial biopsies taken from patients with asthma have shown that activated (CD25 +) T cells can be detected in the bronchial mucosa, and that their numbers correlate both with the numbers of local activated eosinophils and with disease severity. Activated (CD25 ÷ HLA-DR÷) CD4 ÷ T cells, but not CD8 ÷ T cells, were also detected in the peripheral blood of patients with acute severe asthma 47,4~ and their numbers were reduced following therapy, to a degree which correlated with clinical improvement. Peripheral blood T cells from asthmatics clinically resistant to corticosteroid therapy were shown to express these activation markers in v i v o 4~ and to be refractory to the inhibitory effects of corticosteroids in vitro ~°. It was demonstrated in mild asthmatics that, while both CD4 ÷ and CD8 ÷ T cells in BAL fluid express activation markers, only the numbers of activated CD4 ÷ T cells correlated with the numbers of BAL eosinophils and disease severity ~. A selective increase in CD4 ÷ T cells in BAL fluid was observed 48 hours after allergen challenge in those asthmatics who had previously been shown to develop a late-phase reaction 5-', suggesting that selective recruitment of CD4 ÷ T cells to the lung may occur in association with this experimental model of asthma. These observations do not necessarily mean that CD4 ÷ T cells and their products actually cause the late-phase asthmatic

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controls ~, reflecting the increased concentrations of the corresponding protein found in concentrated asthmatic BAL fluid 53. Cultured peripheral blood CD4 ÷ and CD8 ÷ T cells from both atopic and non-atopic asthmatics spontaneously secrete factors that prolonged the life of eosinophils in vitro, to an extent which correlated with the numbers of peripheral blood eosinophils in the same subjects57; antibody neutralization experiments suggested that this activity was attributable partly to GM-CSF and partly to IL-5. Taken together, these studies support the general hypothesis that, in atopic and non-atopic asthma, actiFig. 3. In situ hybridization study of asthmatic bronchial biopsy showing vated CD4 ÷ T cells secrete lymphokines that are relexpression of IL-5 mRNA. Silver grains represent autoradiographs of lym- evant to the accumulation and activation of eosinophils phokine anti-sense riboprobe. Counterstaining was with haematoxylin. in the bronchial mucosa. Photomicrograph courtesy of Dr Q. Hamid ~4. Are asthmatic CD4 ÷T cells of T,2 phenotype? There has been considerable interest recently in the responses, especially since the peak of this reaction usually occurs 6-9 hours after allergen challenge. classification of T cells according to their profiles of However, they do suggest that the T-cell recruitment lymphokine secretion. Mouse T cells can be subdivided sets the scene for the intense inflammation and persist- into TH1 and TH2 types on the basis of selective secreent airway narrowing that accompanies repeated aller- tion of IL-2 and IFN-7 for the TH1 response and IL-4 gen exposure. It should be mentioned as an aside that and IL-5 for the TH2 category. TH2 cells are of particuobservations made from models of provoked asthma lar interest with regard to the pathogenesis of asthma after a single challenge are not necessarily representa- because of the eosinophil-specific effects of IL-5 and tive of on-going chronic disease. because IL-4 enhances IgE synthesis. TH1 cells, in conMeasurement of lymphokines in vivo is problemati- trast, appear to participate in delayed-type hypersensical because of their low concentration, rapid metab- tivity reactions and inhibit IgE synthesis through their olism and unquantifiable degree of dilution. secretion of IFN-y. In both atopic and non-atopic Furthermore, 'physiological' concentrations of lym- human subjects, TH1 and TH2 type T-cell clones can be phokines have, in general, not been defined, and so it distinguished in vitro5S; furthermore, there is increasing is often unknown whether a specific assay such as an evidence that these cell types may also be involved in ELISA assay is sensitive enough. Bioassays are more inflammatory processes in vivo. For example, recent relevant in this regard, provided it can be ensured that studies using in situ hybridization 59,~° demonstrated a they are specific for the particular lymphokine being THl-like pattern of cytokine mRNA expression in measured. The problem has been emphasized in a cutaneous tuberculin reactions and a TH2-1ike pattern recent study of BAL fluid from asthmatics ~3, where in allergen-induced late-phase cutaneous reactions. T cytokines were detectable only after considerable con- cells in the BAL fluid of mild, atopic asthmatics centration of the BAL fluid. Clearly, such a procedure showed elevated expression of IL-4 and IL-5 mRNA as might result in variable loss of specific proteins. One compared with non-atopic controls, reflecting a TH2alternative to the direct measurement of lymphokines like patternSL However, it would appear that IL-4 and is the detection of their messenger RNA using the tech- IL-5 expression need not always be co-ordinately regunique of in situ hybridization with lymphokine-specific lated in vivo: T cells purified from the peripheral blood cDNA probes or riboprobes. Although this is not a of non-atopic asthmatics spontaneously secreted elevstrictly quantitative technique, and with the proviso ated quantities of IL-5, but not IL-4, as compared that mRNA synthesis does not always equate with with normal controls, whereas those from atopic secretion of the corresponding protein, it does have the patients secreted elevated quantities of both IL-4 and advantage that it can localize the secretion of lym- IL-5 (Ref. 61). It is possible, therefore, that increased phokines within cells and tissues. Using this technique IL-4 synthesis is associated with the atopic state and is it was recently demonstrated that IL-5 mRNA was not a prerequisite for the development of asthma. elaborated by cells in the bronchial mucosa of a ma- Irrespective of whether or not T cells in asthma can be jority of mild asthmatics but not normal controls (Fig. regarded as TH2-1ike cells, characterization of their 3) 54. The numbers of mRNA signals correlated broadly profiles of cytokine secretion in asthma in various cliniwith the numbers of activated T cells and eosinophils cal settings may allow a pathophysiological classifiin biopsies from the same subjects. In another study~5, cation of the disease. it was shown that significantly higher numbers of BAL cells expressed mRNA encoding IL-2, IL-3, IL-4, IL-5 T cells, IgE and asthma and GM-CSF but not IFN-7 in mild atopic asthmatics, As stated, asthma and atopy are closely linked, and as compared with non-atopic normal controls. some investigators have suggested that all asthma is Increased numbers of cells expressing mRNA encoding IgE-mediated thus casting doubt upon the concept of tumour necrosis factor ix (TNF-ix) were observed in the 'intrinsic' form of the disease. For example, in a BAL cells from asthmatics as compared with normal recent epidemiological survey62 a direct correlation was

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REVIEW Smooth muscle contraction (bronchospasm)

Mast cell Pharmacological mediators Histamine, LTs, PGs, PAF y

Immediate symptoms: episodic wheeze

/

N~gE production

\

Allergen (or antigen)

Macrophage~

TH2 cell

//' os,no0h,, Bronchial inflammation PAF, LTs r

Basic proteins

IL-3 .d GM-CSF . ~ IL-8 ~ TNF " "

Epithelial cell

Chronicsymptoms: bronchialirritabilityand chronicwheeze

Mast cell

Fig. 4. Hypothesis fi)r the pathogenesis of asthma. Eosinophils are believed to be prime pro-inflammatory effector cells causing bronchial damage, which, in turn, leads to chronic asthma symptoms. Although many cells may secrete cytokines (including mast cells, epithelial cells, macrophages) that influence eosinophil differentiation, survival and function, the T,2-type T cell is seen as having a central role since it is capable of direct antigen recognition. The putative 'driving' antigen for asthmatic inflammation is suggested here to be allergen, although other antigens (viral, epithelial) are also possible candidates. Although T cells also influence the synthesis of lgE, lgE-mediated mechanisms are seen as playing a secondary role only in atopic subjects, where they may be responsible fi~r acute, short-lived symptoms superimposed on the chronic, on-going cell-mediated inflammatory disease.

observed in all asthmatics between serum IgE concentrations (corrected for age and sex) and clinical severity. On the other hand, some of the clinical and pathological distinctions between atopic and nonatopic asthma, such as the presence of both IL-4 and IL-5 in concentrated BAL fluid from atopic asthmatics but IL-5 and not IL-4 in fluid from n o n - a t o p i c subjects "~, are impressive and indisputable. Similarly, in occupational asthma caused by sensitization to small chemicals such as toluene diisocyanate, subjects may be non-atopic with negative skin prick tests and yet have a profound peripheral blood eosinophilia with evidence of activated T-cells and eosinophils in bronchial biopsies l°. While it is possible that localized IgE-mediated reactions, which are not detectable systemically, may occur in such patients, a reasonable alternative hypothesis is that asthma is a chronic cellmediated disease which may occur independently of the presence or absence of IgE or other antibody-mediated immune mechanisms. What, then, is the antigen(s) that drives T cells in the chronic non-atopic form of the disease? A number of speculative suggestions might be made. Viral antigens, particularly adenoviruses and rhinoviruses, are strong candidates, particularly as infections by these agents are common triggers in exacerbations of asthma. Autoantibodies formed from damaged bronchial mucosa is another possibility,

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although organ- and tissue-specific antibodies have been difficult to detect in this disease. The ability of common aeroallergens, such as the house dust mite, to elicit a CD4 + T-cell response in non-atopics is well documented 63. That such cells could escape normal control at damaged bronchial mucosal surfaces in nonatopic asthmatics is a testable hypothesis. Conclusion Asthma is clearly a complex disorder with many variants and several possible mechanisms apart from the events associated with bronchial mucosal inflammation - for example, the influences of local neuronal and hormonal networks may be operative - but these are outside the scope of this article. Here, the focus is on T cells, eosinophils and mucosal damage in asthma and the evidence supporting the hypothesis that the disease is 'immunologically driven' even though in many instances the driving antigen(s) or allergen(s) are unknown. Asthma and atopy are closely linked but the disease can apparently occur in the absence of an abnormal IgE response or excessive local production of IL-4. The cytokine profile of cells in the bronchial mucosa of atopic asthmatics (who have been the most intensely studied) corresponds to that of TH2 type T cells. IL-3, IL-5 and GM-CSF appear to be critical for eosinophil mobilization and activation, and

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eosinophil products may be responsible for the mucosal damage typical of the disease. IL-5 is, possibly, particularly important, since it has several eosinophil-specific associated biological activities. It is equally clear that other inflammatory cells such as mast cells, fibroblasts, macrophages and epithelial cells may also produce cytokines, particularly GM-CSF, which are relevant to the asthma process. However T cells, through their unique capacity for primary antigen recognition, may play a central role in orchestrating the asthmatic inflammatory response. These points have been summarized diagrammatically (Fig. 4). C.J. Corrigan and A.B. Kay are at the Dept o f Allergy and Clinical Immunology, National Heart and Lung Institute, Dovehouse Street, London, UK S W 3 6L Y. References

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58 Wierenga, E.A., Snoek, M., de Groot, C. et al. (1990)

J. Immunol. 144, 4651-4656 59 Kay, A.B., Ying, S., Varney, V. et al. (1991)J. Exp. Med. 173,775-778 60 Tsicopoulos, A., Hamid, Q., Varney, V. et al. (1992) J. Immunol. 148, 2058-2061

61 Walker, C., Bode, E., Boer, L. etal. (1992) Am. Rev. Respir. Dis. 146, 109-115 62 Burrows, B., Martinez, F.D., Halonen, M., Barbee, R.A. and (line, M.G. (1989) New Engl. J. Med. 320, 271-277 63 O'Hehir, R.E., Bal, V., Quint, D. et al. (1989) Immunology 66,499-504

Cervical lymphatics, the blood-brain barrier and the immunoreactivity of the brain: a new view Helen E Cserr and Paul M. Knopf This new view of the immunoreactivity of the normal brain is based on three key components. First, there is an active and highly-regulated communication between the brain and the central immune organs. Secondly, the connection from the brain to the draining nodes is much larger than previously appreciated. And third, the blood-brain barrier, by virtue of its selective permeability properties, contributes to the regulation of immunoregulatory cells and molecules in the brain cell microenvironment. The brain has been characterized immunologically as a site of limited reactivity. This concept, termed immune privilege, was developed from classical studies which showed that allografts usually fare better in the brain than in more conventional sites. Immune rejection of transplants is usually a celbmediated immune reaction. Mechanisms of humoral immunity in the normal central nervous system (CNS) and their relation to the concept of immune privilege were not defined. The biological significance of immune privilege in the brain and the eye, which includes an extracranial projection of the CNS (the retina), seems clear. As expressed by Leslie Brent ~, 'it may be supposed that it is beneficial to the organism not to turn the anterior chamber or the cornea of the eye, or the brain, into an inflammatory battlefield, for the immunological response is sometimes more damaging than the antigen insult that provoked it'. While there may be agreement as to the benefits of immune privilege, mechanisms contributing to this immune state have been a subject of debate. Classical explanations emphasized the isolation of the brain from the immune system2"3. The absence of a conventional lymphatic system within the brain was believed to interfere with the afferent arm of the immune response to CNS antigen, due to the perceived lack of drainage of immunogenic material to the regional nodes, whereas the presence of the blood-brain barrier was held to block the efferent arm, by preventing the entrance of effector cells and molecules of the immune system into the brain. Paucity of antigen-presenting cells (APCs) has also been emphasized, again as evi-

dence for a deficient immune environment 4. More recent explanations have begun to seek active regulatory mechanisms contributing to immune privilege, including the presence of local factors in the brain and eye which regulate the expression of major histocompatibility complex (MHC) class II molecules ~ or induce antigen-specific suppression <-. Our interest in brain-immune system interactions grew out of experiments on the turnover of cerebral interstitial fluid and its outflow to lymph. Despite the absence of typical lymphatic channels in brain tissue, a significant fraction (14-47%) of protein injected into brain could be collected from the cervical lymphatics of common laboratory animalsL On the basis of this finding, we began to explore connections between the brain and the immune system in a rat model with normal brain-barrier permeability. Our results support a new view of the immunoreactivity of the CNS which includes continuous and highly regulated communication between the brain and the immune system via cervical lymphatics and the blood-brain barrieF '"~. Specific characteristics of the response to antigen injected into the brain are basically similar to those described by Streilein and colleagues for the anterior chamber of the eye l~,12. They include antigen-specific suppression of cell-mediated immunity, both of cytotoxic lymphocytes (CTL) and of delayed-type hypersensitivity (DTH), plus a normal or enhanced antibody response. Since the concentration of complement components is low in the normal CNS, antibody responses will not be inflammatory. Thus, this new characterization of CNS immunoreactivity remains consistent

O 1992, Elsevier Scien, e Publishers 1 td. UK

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