Short and long term immunosuppressive effects of clozapine and haloperidol

Ž .Immunopharmacology 37 1997 75–86

Short and long term immunosuppressive effects of clozapine andhaloperidol

I. Leykin a, R. Mayer b, M. Shinitzky a,)

a Department of Membrane Research and Biophysics, The Weizmann Institute of Science, RehoÕot 76100, Israelb Neurogenic Ltd, Tel-AÕiÕ, Israel

Accepted 27 January 1997

Abstract

In line with the autoimmune hypothesis of schizophrenia we have tested in this study whether the commonly usedneuroleptics, clozapine and haloperidol can also act as systemic immunosuppressants. Twenty one hospitalized chronicschizophrenic patients participated in the study. Five were free of neuroleptic treatment while the other 16 were under

Ž . Ž .chronic treatment with either clozapine ns8 , or haloperidol ns8 . Fourteen age matched normal subjects served as thecontrol group. Conventional in vitro mitogenic stimulation of peripheral blood lymphocytes with phytohaemagglutininŽ .PHA indicated a clear suppression of responsiveness of approximately 50% in all treated patients. The PHA response ofthe untreated patients was virtually identical to that of the control group. The in vitro effect of haloperidol and clozapine onPHA stimulation of lymphocytes from normal subjects was determined by 3H-thymidine uptake and secretion ofinterleukin-2, interleukin-4 and interferon-g . Both clozapine and haloperidol suppressed thymidine incorporation andcytokine secretion at a drug concentration of above 1 mM, reaching full suppression at 50 mM. Similar suppressive effectsof clozapine and haloperidol were also observed in mixed lymphocyte reaction of mouse lymphocytes. Assays withradioactive ligands indicated that clozapine is not incorporated into the lymphocytes but presumably exerts its action bybinding to specific surface sites. The long term immune suppression induced by neuroleptic treatment may inhibit putativeautoimmune responses against neurological sites and could thus act synergistically with the direct antagonistic action onbrain receptors for the overt amelioration of psychotic behaviour. q 1997 Elsevier Science B.V.

Keywords: Neuroleptics; Immunosuppression; Schizophrenia

Abbreviations: ConA, concanavalin A; DA, dopamine; ELISA,enzyme-linked immunosorbent assay; FACS, fluorescence-activated cell sorter; HBSS, Hanks’ balanced salt solution; HEPES,

w x X w xN- 2-hydroxyethyl piperazine-N - 2-ethanesulfonic acid ; 5-HT,5-hydroxytryptamine; IFN-g , interferon g ; IL-2, interleukin-2;IL-4, interleukin-4; MLR, mixed lymphocyte reaction; OD, opticaldensity; PBS, phosphate buffered saline; PHA, phytohaemagglu-tinin; SDS, sodium dodecyl sulfate

) Corresponding author. Tel.: q972-8-9342750; fax: q972-8-9344112; e-mail: [email protected]

1. Introduction

A series of immunological abnormalities havebeen ascribed to schizophrenia. They include ele-

Žvated serum immunoglobulin levels Sugerman et.al., 1982; DeLisi et al., 1985 , decreased mitogenic

response of peripheral blood lymphocytes to phyto-Žhemagglutinin and pokeweed mitogen Vartanyan et

al., 1978; Ganguli et al., 1987; Chengappa et al.,.1995 , the presence of morphologically abnormal

large lymphocytes in the blood and bone marrow

0162-3109r97r$17.00 q 1997 Elsevier Science B.V. All rights reserved.Ž .PII S0162-3109 97 00037-4

( )I. Leykin et al.r Immunopharmacology 37 1997 75–8676

Ž .Vartanyan et al., 1978 , increased serum IL-2 recep-Žtor levels Ganguli and Rabin, 1989; Ganguli et al.,

. Ž1989; Wilke et al., 1996 , decreased IL-2 Ganguli et. Žal., 1989, 1992, 1995 and IFN-g production Wilke

. Žet al., 1996 , and a high serum level of IL-6 Shintani

.et al., 1991 . With respect to lymphocyte sub-popula-Ž .tions Nyland et al. 1980 found a reduced percent-

age of T cells in schizophrenic patients with acuteŽ .relapse. Masserini et al. 1990 found increase of T

suppressor lymphocytes in drug naive schizophrenicpatients, while the drug-treated showed an increaseof T helper lymphocytes. A decrease in the percent-age of T cells in schizophrenic patients during acuteattack and an increase in the helperrsuppressor Tcell ratio which correlated with clinical improvement

Ž .was reported by Coffey et al. 1983 .As a whole, these series of findings point to a

serious aberration in immune function ofschizophrenic patients, which could promote an au-toimmune reaction which may contribute to the psy-

Ž .chotic state Noy et al., 1994 . The presence ofantibrain autoantibodies in schizophrenic patientsŽHeath et al., 1989; Knight et al., 1990; Henneberg et

.al., 1994; Yang et al., 1994 which can induceŽbehavioural changes upon injection to animals Pan-

.dey et al., 1981 , supports this possibility. On thisground it was proposed that the etiology ofschizophrenia is associated in part with autoimmune

Ž .elements Knight, 1984; Noy et al., 1994 . Indeed,schizophrenia shares several features with recog-nized autoimmune diseases, such as Graves’ disease,insulin-dependent diabetes and rheumatoid arthritisŽ .Knight, 1984 . These features include genetic pre-

Ždisposition Stater and Cowie, 1971; Kessler, 1980;.Knight and Adams, 1982 , variation in identical

Ž .twins Knight and Adams, 1982 , cycling of remis-sion and relapse, possible environmental triggers,

Žsuch as viral infections Adams, 1969; Pert et al.,.1988 , as well as associations between histocompati-

Žbility antigens and disease susceptibility Adams and.Knight, 1980; Roberts and Kinnell, 1981 . We re-

cently reported on marked psychiatric improvementsin a schizophrenic patient treated with azathioprine, adrug commonly used for autoimmune and inflamma-

Ž .tory diseases Levine et al., 1994 .The dopamine hypothesis of schizophrenia

ŽMeltzer and Stahl, 1976; Snyder, 1976; Seeman,.1992 , the current basis of neuroleptic design, asserts

that binding to the dopamine receptors is the domi-nant factor in the activity of neuroleptic drugs. How-ever, this hypothesis does not explain comprehen-sively the therapeutic profile of neuroleptics. Morethan 20% of schizophrenic patients are refractory totreatment with conventional antidopaminergic drugsŽ .Davis et al., 1980 , while others show slow clinicalbenefit, although the blockage of the D dopamine2

Ž .receptor is in principle fast Freed, 1988 . In clozap-ine, the currently highest rated neuroleptic, a com-plex mechanism of action is displayed. Clozapinehas been proven to be more effective in nonrespon-

Ž .ders to conventional neuroleptics Kane et al., 1988 ,Žthan in responders Van Praag et al., 1976; Fischer-

Coruelssen and Ferner, 1976; Shopsin et al., 1979;Leon, 1979; Gladhorn et al., 1987; Pickar et al.,

.1992 . Furthermore, the affinity of clozapine to theD receptor is substantially weaker than the affinity2

Žof less effective conventional neuroleptics Creese etal., 1976; Peroutka and Snyder, 1980; Richelson,

.1984 .Explanation for the clinical superiority of clozap-

ine may be present in a putative immunosuppressiveproperties of this drug. Evidence that antipsychoticagents can act as immunosuppressants appear spo-

Ž .radically in the literature. Descotes and Evreux 1981reported that major tranquilizers can suppress hyper-sensitivity in mouse immune reactions. Nahas et al.Ž .1979 reported inhibitory effect of psychotropicdrugs on DNA synthesis in cultured lymphocytes.

Ž .Similarly, Baker et al. 1977 found that both pheno-thiazine and nonphenothiazines inhibit the transfor-mation of T-lymphocytes in an in vitro proliferationassay. These effects were suggested to be mediatedthrough inhibition of IL-2 production by the acti-

Žvated T-lymphocytes Boukhris et al., 1988; Schle-.uning et al., 1989 . Chlorpromazine has been shown

to have an immunoregulatory action similar to thatof cyclosporin A, a potent immunosuppressive agentŽ . ŽSchleuning et al., 1989 . Finally, Maes et al. 1994,

.1995 have recently reported on immunological al-terations in schizophrenic patients induced by neu-roleptic treatment, including clozapine. It should benoted, however, that converse reports also appear in

Ž .the literature. Pisciotta and Konings 1994 foundthat at a concentration of 10 mM clozapine, as wellas some of it’s metabolites, do not inhibit prolifera-tion of PHA stimulated lymphocytes. These authors

( )I. Leykin et al.r Immunopharmacology 37 1997 75–86 77

also proposed that serum antibodies to polymor-phonuclear leukocytes are responsible for a clozap-

Ž .ine-induced agranulocytosis. Rapaport et al. 1990assayed mitogen-induced T lymphocyte response inpatients treated with various neuroleptics, includingclozapine, and did not detect a substantial effect ofthe treatment.

The purpose of this work was to assess immuno-suppressive properties of haloperidol and clozapineas a possible arm in their clinical effectiveness.

2. Materials and methods

2.1. Patients

Twenty one schizophrenic patients of various cat-egories and stages participated in the study. These

Žwere 12 males and 9 females age 21–65 34.3"11.6.years; mean"SE . The patients were hospitalized in

the Beer-Yakov Mental Health Center, Israel. Diag-nosis of schizophrenia was established by the DSM-

Ž .IV criteria American Psychiatric Association, 1995 .Five of them were either newly diagnosed or free ofneuroleptic treatment for at least 4 months. The other16 patients were over 6 months under chronic treat-

Ž .ment with either clozapine ns8 , or haloperidolŽ .ns8 . All patients were free of acute or chronicmedical disorders which could affect immune func-tion.

2.2. Control subjects

Healthy volunteers, mostly of the hospital staff,constituted the control group. These were 6 males

Žand 8 females age 24–60 32.3"10.2 years; mean."SE .

2.3. Chemicals and radiochemicals

Clozapine and haloperidol were purchased fromSigma Chemical Co., USA. 3H-Clozapine was pur-chased from DuPont NEN, USA. 3H-methyl-cloza-pine was synthesized by methylation, using 3H-

Ž .methyl-iodide Amersham, UK and subsequentdemethylation. 3H-thymidine was purchased fromAmersham, UK.

2.4. Lymphocytes

Human peripheral blood, 10–12 ml with heparinas anti-coagulant, was drawn in the morning beforebreakfast and drug intake. The blood was processedwithin 4 h. Platelet rich plasma was collected by

Žslow centrifugation 100 g for 20 min at room.temperature which afforded sedimentation of ery-

throcytes and leukocytes. The platelet rich plasmawas separated, HBSS was added to form a 40 ml ofHBSS suspension of peripheral blood cells. The cells

Žwere layered gently on 10 ml Ficoll-Paque Phar-.macia in a 50 ml falcon tube and centrifugated at

900 g for 20 min. at room temperature. The interfacelayer of mononuclear cells, consisting mostly oflymphocytes, was collected and contained 7–9=106

cells.Mouse lymphocytes were prepared from spleens

Ž k . Ž b.of C3H H-2 and C57BL H-2 mice.

2.5. In Õitro stimulation of lymphocytes

Tests were carried out in 96 microwell platesŽ .Nunc, Denmark in triplicate in a humidified atmo-sphere with 5% CO at 378C. Each of the assays2

contained 250,000 lymphocytes suspended in 200 mlof RPMI 1640 medium containing 10% heat inacti-vated fetal calf serum for mouse lymphocytes or 2%heat inactivated human AB serum for human lym-

Žphocytes, 2 mM L-glutamine, penicillin 100. Ž .unitsrml , streptomycin 100 mgrml , gentamycin

Ž .sulfate 50 mgrml , 1% non-essential amino acids, 1mM sodium pyruvate, 5=10y5 M b-mercapto-ethanol, in addition to 20 mM HEPES buffer.

The stimulants used were: PhytohaemagglutininŽ . Ž .PHA Wellcome Diagnostics, England for human

Ž . Žlymphocytes and Concanavalin A ConA Sigma.Chemical Co., USA for mouse lymphocytes.

Ž .For one-way mixed lymphocyte reaction MLRŽ k .lymphocytes from C3H H-2 mouse served asŽ . Ž b.responders, and irradiated 2000 rads C57BL H-2

mouse lymphocytes were used as stimulants. Theratio of responder to stimulant was 1:2.

Stimulation of lymphocytes in the above assayswas scored by 3H-thymidine incorporation after puls-ing with 1 mCi per well of 3H-thymidine for 18 h.Supernatants were stored at y208C and subse-quently used for cytokine profile determination.


2.6. Cytokine determination

Cytokine secretion was scored by an ELISAŽ .Abrams et al., 1992 . Purified anti-cytokine mono-

Žclonal antibodies mouse anti-human IL-4, rat anti-mouse IL-2, rat anti-mouse IL-4, rat anti-mouseIFN-g , from Pharmingen, USA; mouse anti-humanIL-2, mouse anti-human IFN-g , from Genzyme diag-

.nostics, USA were diluted in coating buffer andcoated in microtiterplates by overnight incubation at48C. After washing with PBSrTween 20, wells wereblocked by incubation with PBSr10% fetal calfserum for 2 h at room temperature. After additionalwashing standards and supernatants from the cellcultures were added and incubated for 4 h at roomtemperature. Following washings with PBSrTween

Ž20, biotinylated anti-cytokine detecting mAb Phar-.mingen, USA; Genzyme diagnostics, USA were

added. After additional incubation for 45 min atroom temperature and washing, streptavidin-

Žperoxidase conjugate Jackson ImmunoResearch.Laboratories, Inc., USA was added and incubated

for 30 min at room temperature. After washing aX Žmixture of H O and 2,2 -azino-bis 3-ethylbe-2 2

. Žnzthiazoline-6-sulfonic acid Sigma Chemical Co.,.USA was added. Color reaction was developed at

Ž .room temperature 10–80 min and stopped byŽadding SDSrN, N-dimethyl formamide Sigma

.Chemical Co., USA . Plates were scored by ODmeasurement at 405 nm. Standards of human andmouse rIFN-g , rIL-2 and rIL-4, purchased fromPharmingen, were used as reference.

3. Results

Two conventional neuroleptic drugs, clozapineand haloperidol, were examined in parallel. Prelimi-nary experiments have indicated that these drugs caninhibit mitogenic stimulation of lymphocytes at con-centrations above 1 mM. Yet, as typical hydrophobiccompounds, their solubility in water is low. Testingof the concentration at saturation under the condi-tions used in the lymphocyte stimulation assay, wastherefore undertaken. Stock solutions of 5 mM inmethanol of clozapine or haloperidol were dilutedinto HBSS or methanol-water 1:1 at 378C to formfinal solutions of up to 50 mM. The solutions were

Ž . Ž .Fig. 1. Solubility of clozapine A and haloperidol B in HBSS at378C measured by the dependence of OD on the drug concentra-tion. Measurements were taken immediately upon solubilizationŽ . Ž .o and after 72 h at 378C q . Identical values at the linear range

Ž .were obtained in methanol-water 1:1 not shown .

Ž .immediately centrifuged in a microfuge gf5000Ž .for 20 s and the optical density OD at 378C was

recorded at 297 nm for clozapine and at 247 nm forhaloperidol. The final HBSS solutions were kept at378C for 72 h to allow aggregation, then centrifugedand their OD values were recorded. The results ofthe solubility experiment are presented in Fig. 1.Both drugs exhibited a clear linear dependence ofOD on concentration with slopes corresponding to

Ž . 4extinction coefficients e of 1.33=10 and 1.05=

104 My1 cmy1 for clozapine and haloperidol, re-spectively. Identical linear dependencies were alsorecorded in the methanol-water solutions. The


Ž . Ž .Fig. 2. Clozapine A and haloperidol B inhibition of proliferation of human lymphocytes, stimulated with 90 mgrml PHA, in the absenceŽ .and presence of different concentrations of neuroleptics 1, 5, 10, 20 and 50 mM . N: nonstimulated lymphocytes. The results are presented

3 Ž) .in units of cpm of incorporated H-Thymidine, measured in triplicate. Drug concentration is above the solubility limit.


Ž .recorded values of e see Fig. 1 are virtually identi-Žcal with those registered The Merck Index, 11th

.Ed., 1989, items 2417 and 4511 and indicate truemonomeric solution. However, after 72 h of incuba-tion at 378C deviation from linearity was observed atconcentrations above 47 mM and 25 mM of clozap-

Ž .ine and haloperidol, respectively see Fig. 1 . Theseresults indicate that under the experimental condi-tions used the saturated solution of monomericclozapine was 47 mM and that of monomerichaloperidol was 25 mM.

Lymphocytes of patients treated with neurolepticdrugs responded to PHA stimulation approximately50% less than lymphocytes of untreated patients or

Ž .control subjects Fig. 2 . This observation clearlysupports the possibility that a chronic treatment withneuroleptics can lead to an overt immune suppres-sion. We have then tested the effects of these drugsin various in vitro lymphocyte stimulation assays. Asshown in Fig. 2, both neuroleptics exhibited suppres-sion of mitogenic stimulation in a dose dependentmanner. The suppression was already apparent under1 mM drug concentration and reached completion atan apparent concentration of 50 mM drug. Thisconcentration is above the solubility limit and part of

Ž .the drug there is actually ineffective see above .The potency of clozapine and haloperidol to in-

Fig. 3. Clozapine and haloperidol inhibition of proliferation ofmouse lymphocytes, stimulated with 2.5 mgrml ConA. The re-sults are presented in units of cpm of incorporated 3H-Thymidine.Ž) . Drug concentration is above the solubility limit.

Fig. 4. Inhibition of mouse lymphocyte proliferation by clozapineand haloperidol in a mixed lymphocyte reaction. The results are

3 Ž) .presented in units of cpm of incorporated H thymidine. Drugconcentration is above the solubility limit.

hibit lymphocyte proliferation was confirmed in aseries of tests with mouse lymphocytes. Stimulationof mouse lymphocytes with ConA was also inhibitedby different concentrations of clozapine andhaloperidol, as presented in Fig. 3. In mixed lympho-cyte reaction with two allogeneic mouse lympho-

Ž .cytes see Section 2 these neuroleptics were foundto inhibit lymphocyte proliferation in a dose-depen-dent manner, as shown in Fig. 4.

For additional confirmation of the immuno-suppressive activity of clozapine and haloperidol aseries of ELISA assays were performed with theculture media removed from the lymphocyte stimula-tion assay to detect the influence of clozapine andhaloperidol on the secretion of IL-2, IL-4 and IFN-gafter stimulation with PHA and ConA. As shown inFigs. 5 and 6, both drugs inhibited the secretion ofIL-2, IL-4 and IFN-g in stimulated lymphocytes in adose dependent manner. A summary of the datapresented above is given in Table 1.

A series of pilot experiments described belowwere carried out for a qualitative assessment of themechanism underlying the immunosuppressive ac-tion of clozapine and haloperidol. Cultivation oflymphocytes with 50 mM clozapine where full sup-pression of lymphocyte activation was reached didnot reveal any membrane defect or increased mem-brane permeability, which was confirmed by trypan


blue exclusion tests and FACS analysis in the pres-Ž .ence of propidium iodide data not shown .

The measured radioactivity of human lympho-

cytes after incubation with 3H-clozapine or 3H-methyl-clozapine for 1, 5, 10, 30 and 60 min wasfound in all cases to remain at the basal level which

Ž . Ž .Fig. 5. Clozapine A and haloperidol B inhibition of IFN-g , IL-2 and IL-4 release following human lymphocyte stimulation with 90Ž) .mgrml PHA. Drug concentration is above the solubility limit.


indicated that clozapine was not incorporated intothe cells. This observation suggests that the immunesuppression of lymphocytes described above presum-

ably originated by the binding of the tested drug tosurface receptors. Along this possibility, preliminaryexperiments with clozapine indicated inhibition of

Ž . Ž .Fig. 6. Clozapine A and haloperidol B inhibition of IFN-g , IL-2 and IL-4 release following mouse lymphocyte stimulation with 2.5Ž) .mgrml ConA. Drug concentration is above the solubility limit.


Table 1Ž .Summary of 50% inhibition doses ID 50

Lymphocyte activity ID 50 of ID 50 ofclozapine haloperidolŽ . Ž .mM mM

Mitogen stimulationhuman lymphocytes with PHA f11 f17mouse lymphocytes in MLR f15 f24mouse lymphocytes with ConA f6 f7

Interferon-g release followinghuman lymphocytes stimulated with PHA f20 f21mouse lymphocytes stimulated with ConA f21 f28

Interleukin-2 release followinghuman lymphocytes stimulated with PHA f20 f32mouse lymphocytes stimulated with ConA f20 f31

Interleukin-4 release followinghuman lymphocytes stimulated with PHA f10 f18mouse lymphocytes stimulated with ConA f17 f14

dopamine and serotonin binding to lymphocytes,which awaits verification by further experiments.

4. Discussion

The currently employed neuroleptic drugs wereby and large designed to act as antagonists of thedopamine receptors. The most widely used is

Ž .haloperidol a butyrophenone for which the potencyto reduce psychotic symptoms was found to correlatewith the affinity of this drug to the D dopamine2

Žreceptor Janicki and Ko, 1980; Beresford and Ward,.1987 . The mechanism of therapeutic action of

haloperidol was therefore attributed to the blockingof binding of endogenous dopamine to the D2

Ž .dopaminergic receptors Janicki and Ko, 1980 . Mostof the neurological and endocrinological adverse ef-fects of haloperidol could also be accounted for bysuch mechanism. The conventional per os dose ofhaloperidol for the treatment of psychoses and asso-ciated behavior disorders is 15 mg per day. How-ever, in severe psychoses or resistant symptoms daily

Ždoses of up to 100 mg are used Beresford and.Ward, 1987 . Under these doses the steady state

plasma concentrations of haloperidol is in the rangeŽof 10–100 nM Lane et al., 1995; Ulrich et al.,

.1995 .

Clozapine is considered as the most effectiveŽneuroleptic drug currently available Pickar et al.,

.1992 . It is prescribed mostly to refractory patientsŽwhere its efficiency is clearly noticed Kane et al.,

.1988 . Clozapine is classified as an "atypical" an-tipsychotic drug, since it possesses antiserotoniner-gic, antiadrenergic, anticholinergic, and antihistamin-ergic activities which are even more potent than its

Ž .antidopaminergic activity Fitton and Heel, 1990 .The range of treatment doses of up to 900 mg ofclozapine per day is reflected in plasma concentra-

Žtion at steady state of 0.5–1.5 mM Hasegava et al.,.1993; Centorrino et al., 1994 .

As shown in this study, clozapine and haloperidolhave immunosuppressive activities in vitro whenpresent at concentrations above 1 mM. This could bedemonstrated in inhibition of lymphocyte stimulationŽ . 3Figs. 2–4, Table 1 , measured by H-thymidineincorporation into newly synthesizing DNA, and spe-

Žcific suppression of cytokine secretion Figs. 5 and.6, Table 1 . The in vitro immune suppression pro-

files of haloperidol and clozapine were very similarand were apparent already at a drug concentration of1 mM. The degree of suppression of both drugs wasdose dependent and at 50 mM, which is above the

Ž .saturation level of both drugs see Fig. 1 , it reachedthe basal level of the tested immune functions. Inline with the plasma steady state concentrations pre-sented above, only in the case of clozapine, an invivo decline of up to 10% in immune responsivenessis expected shortly after initiation of drug treatment.For haloperidol treatment, the steady-state plasmaconcentration is 1 to 2 orders of magnitude lowerthan that of clozapine and is substantially below thelevel of immediate immune suppression.

The in vitro immune responsiveness of untreatedschizophrenic patients was essentially identical to

Ž .that of normal subjects Fig. 2 . However, chroni-cally treated patients with haloperidol or clozapinedisplayed in vitro immune responsiveness which wassubstantially below that observed in normals. It isplausible that in these patients a long term immunesuppression is slowly developed to reach the levelobserved here. Such a long term effect is probablyalso concentration dependent and therefore is morerelevant to clozapine treatment where the drug dosageis markedly higher than, for example, in the case ofhaloperidol.


The antipsychotic effect of clozapine andhaloperidol is in most cases noticed only after sev-eral weeks. Therefore, their mechanism of actioncannot be accounted for merely by specific interac-

Ž .tion with receptor s which should have been re-Ž .flected in a fast h action as, for example, in the

Ž .case of benzodiazepines e.g. valium . Along thisrationale, a slow systemic process seems to be asso-ciated with the action of these drugs which togetherwith the specific response related to receptor bindingexerts the overt antipsychotic action. In principle,this putative slow process could be mediated by thechronic occupancy of neurologic receptors which isexpected to divert the receptor assembly and activityinto a new state of homeostasis associated withamelioration of mental dysfunction. Yet, the slowadaptation to clozapine and other neuroleptics mightbe associated with the long term effect on the im-mune system which was described here.

As suggested above, immune suppression by neu-roleptics presumably builds up slowly to reach asteady state after a period of weeks. At this immunesuppressed state the inherent cellular and humoralautoimmune responses against neurological entitiesŽ .e.g. the D dopamine receptor may subside which2

could act synergistically with the direct effect in-duced by binding of the drug to brain receptors. Inline with this hypothesis part of the clinical superior-

Ž .ity which is claimed for clozapine Kane et al., 1988might be due to its somewhat unexpected action asan immunosuppressant.

Acknowledgements

We thank Dr. Alpha Peled and Mrs. Anna Gelfandfor their technical assistance. This study was sup-ported by a grant from the Mayer Family Founda-tion. MS is the Olin-Sang Professor of LeukemiaResearch.

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