Immunology Today, Vol. 10, No. 11, 1S89 Q

Cytoklnes as communication signals between ieukocytes and endothelial ceils

Hemostasis, inflammatory reactions and immunity involve close

Alberto Mantovani and ElisabeUa Dejana

increasing the levels of the enzyme cyclooxygenase and the availability of the substratum, arachidonate. PGI2 is a platelet anti-aggregating agent and a potent vaso- dilator. It is interesting that IL-1 also directly inhibits the contraction of vascular smooth muscle 6. IL-l-induced PGI2 is probably involved in the vasodilation that occurs at sites of inflammation and cell-mediated immune reactions, as well as in the hypotension associated with the systemic inoculation of this cytokine and of the functionally related cytokine tumor necrosis factor (TNF)7. 8

interactions between immunocompetent cells and the vascular endothelium. Cytokines, produced by and acting on endo- thelial cells, are mediators of the complex bidirectional interac- tions between leukocytes and vascular cells. Cytokines affect endothelial cell function in inflammation, thrombosis and angiogenesis, in addition to their role as accessory cells. As well as acting as targets for the action of cytokines, endothelial cells are important producers of polypepTide mediators that regulate hematopoiesis, the differentiation and proliferation of T and B cells and the extravasation of leukocytes. In this review, Alberto Mantovani and Elisabetta Dejana discuss endothelial cells as important participants in the induction and regulation of coagulation, inflammation and immunity and cytokines as crucial mediators of the symbiotic i.qteractions between

vascular cells and leukocytes.

370

In 1926, in what may have been the first observation of lymphokine activity, Zinsser and Tamiya 1 described how a soluble product released by leukocytes affected the function of vessel wall elements. With the availability of pure recombinant cytokines, the area of soluble po!ypeptide mediators produced by leukocytes and acting on endothelial cells (ECs) has become the object of intensive investigation. As a result of these studies the vascular endothelium has ceased to be viewed as a passive lining of blood vessels. It is now evident that hemostasis, inflammatory reactions and immunity in- volve close interactions between immunocompetent cells - - = . - I . . . . . . . I . . . . J - - ~ L _ L ' _ _ _ _

- , f lu Vd~UUldr ~ r luo i . r lo l lum. Cytokines are mediators of the complex bidirectional interactions between leuko- cytes and vascular elements. Here the available infor- mation on the role of ECs as targets and producers of cytokines is summarized.

Modulation of EC function by oltokines Interleukin-1

Interleukin 1 (IL-1)is a pleiotropic cytokine produced by many cell types, most notabiy by mononuclear phagocytes 2.3. The effects of IL-1 on vascular endo- theliurn have been extensively analyzed and this mediator can serve as a prototype of cytokines that activate ECs in a prothrombotic, proinflammatory man- ner (Table 1). IL-1 alters the functional properties of vascular cells, including their arachidonate metabolism, thrombogenic properties, leukocyte recruitment and cytokine production (Table 1).

Arachidonate metabolism. IL-1 induces production of the arachidonate metabolite prostacyclin (PGI2) by vascular cells 4.s. The PGI2 response elicited by IL-1 differs from that of 'classical stimuli' (for example, thrombin) because it is slow, lasts for at least 24 to 48 hours and depends on protein synthesis. IL-1 stimulates PGI2 production by

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Changes in EC antithrombotic properties. The antithrombotic properties of ECs are profoundly altered by exposure to II.-1, which induces tissue-type procoagulant activity in them 9. Moreover, IL-1 suppresses the cell surface anti- coagulant activity mediated by the thrombomodulin protein C pathway lo. IL-1 shifts the fibrinolytic propert;es of EC by decreasing tissue-type plasminogen activator and augmenting production of an inhibitor of plas- minogen activator, both in vitro and in vivo 1~-~4. The relevance of tnese observations to the prothrombotic changes induced by IL-1 in ECs has been established by infusing IL-1 in rabbits and monitoring functional par- ameters and fibrin strand formation on ECs (Ref. 10). IL-1 increases the production of piateiet-activating factor (PAF) by ECs by an increase in acetyltransferase activity 15. This phospholipid is a potent activator of platelets and leukocytes and is a vasoconstrictor.

Leukocyte recruitment. Local injection of IL-1 into tissues results in rapid recruitment of leukocytes from the blood compartment 16. IL-1 is not directly chemotactic, but elicits leukocyte extravasation indirectly by changing the adhesive properties of ECs and inducing production of chemotactic cytokines (Fig. 1). IL-l-treated ECs provide a better substratum for adhesion of 13olymorphonuclear leukocytes, monocytes and lymphocytes 17. Leukocyte adhesion involves various EC surface sti'uctures. Although PAF induced by IL-1 could in principle play a role in leukocyte adhesion, its role appears to be marginal 18. ECs constitutively have intercellular adhesion molecules (ICAM-1 and ICAM-2). ICAM-1 is increased by IL-1 (Ref. 19) and in addition, IL-1 induces expression of a novel leukocyte adhesion molecule, endothelial leukocyte adhesion molecule 1 (ELAM-1) 20. The cDNA encoding ELAM-1 has recently been cloned and shows it to be a member of an emerging family of surface structures involved in leukocyte-endothelium ad- hesion 2°. ELAM-1 has been identified in vivo on micro- vascular ECs at sites of delayed-type hypersensitivity re- actions and in pathological tissues in man 21.

IL-l-treated ECs also provide a better substratum for adhesion of human tumor cells 22. As predicted on the

© 1989, Elsevier Science Publishers Lid, UK. 0167-49191891503.50

Immunology Today, Vol. 10, No. 11, 1989

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basis of in-vitro findings, in-vivo administration of IL-1 (and TNF) augmented lung metastasis from human tumor cells injected intravenously into nude mice (R. Giavazzi. et al., unpublished). These observations provide an explanation for the preferential secondary localization of tumors at sites of inflammation and suggest that metastasis should be carefully considered in the evaluation of the potential of cytokines in cancer therapy.

IL-1 elicits production of various cytokines in ECs (see below) including colony-stimulating factors (CSFs) and chemotactic cytokines23, 24 in addition to expression of adhesion structures. The recruitment and activation of leukocytes can be highly disruptive of the integrity of the vessel wall and of the underlying tissue. Recently, it has been shown that IL-1 induces production of cytokines that inhibit leukocyte adhesion and chemotaxis 2s.26. These inhibitors probably represent negative circuits induced by IL-1 that serve to tune the recruitment and activation of leukocytes with the potential to be highly disruptive to the integrity of the vessel wall and to the underlying tissue.

Ceil shape and proliferation. IL-1 induces release of yon Wille- brand factor from cultured ECs and a decrease in its accumulation in the extracellular matrix 27.28. After a prolonged interaction (at least 2-5 days) with IL-1 alone, or even more effectively in combination with gamma- interferon (IFN-~/), ECs show altered morphology and abnormal matrix structure 29-31. The cells become elon- gated with a dendriticlfibroblastic-like appearance. The matrix is highly organized and mainly composed of glycosaminoglycans 30.

The growth of vascular cells is modulated by IL-1, which inhibits the proliferation of ECs but promotes the growth of smooth muscle cells and fibrob!asts to some extertt 32. The activity of IL-1 on smooth muscle cells, but not on ECs, is influenced by the induction of inhibitory prostaglandins: after inhibition of arachidonate metab- olism stimulation of cell proliferation was easily observed 33. IL-1 induces platelet-derived growth factor (PDGF) release by ECs and smooth muscle cells. This could account for its effeG in increasing proliferation of smooth muscle cells and fibroblasts 32. Thus, by a series of cell--cell interactions, the release of IL-1 may play an important role not only in the vascular responses to inflammation and thrombosis but may also contribute to atherogenesis.

IL-I is also reported to have a growth-promoting effect on EC-like Kaposi sarcoma cultures 34. The long- term growth of these cells is sustained by a growth factor released from retrovirus-infected CD4 + T cells.

There are two molecular species of iL-I (oL and 13) that evoke an overlapping spectrum of responses in ECs (Ref. 35). However, dissociations of the actions on ECs of the two IL-I molecular species have been reported 3s.36, but the molecular basis of these differences is unknown.

Early gene expression. In conclusion, IL-1 alters the function of ECs in a number of ways. Similar changes are induced by the functionally related cytokine TNF (see below) and by bacterial lipopolysaccharides (LPS). Most of these effects have a delayed onset, last for several hours and are Gependent on protein synthesis, suggesting an effect at the I~vel of DNA. Activation of ECs by IL-, and TNF is

0

' E VI WS-- Table 1. Functional responses elicited by o/tokines in endothelial cells

Cytokine EC In-vivo Selected functional significance refs response

IL-I; TNF; LT Production of PGI2

Production of PCA, PAl, TM, PAF, vW release

Increased expression of ICAM-1 and 2; expression of ELAM-1

Production of chemota~ic factors (CSFs; IL-8; MCP-1)

Production of CSFs

Production of IL-1; IL-6

Production of PDGF

Tumor cell adhesion

Vasodilation

Thrombosis

Leukocyte adhesion and extravasation; virus infection

Leukocyte recruitment

Hematopoiesis

Pleiotropic local and systemic effects

Smooth muscle cell proliferation (atherosclerosis)

Metastasis

IFN-~ MHC class II antigen and Antigen ICAM-1 expression presentation

G-CSF; GM-CSF; EC proliferation; Angiogenesis; 13FGF; ~FGF; migration and PA hematopoietic TGF-13 microenvironment

5

9, 10-15, 27

19,20

23, 24, 26

23, 24, 40

47, 55, 81

32, 33

22

19, 52

62, 67, 72

PCA: procoagulant activity; PAl: plasminogen activator; TM: thrombomodulin; vW: yon Willebrand factor

associated with induction of the immediate early genes c-fos and c-jun 37. Interestingly, IFN-~/which induces a distinct set of changes in EC, does not induce expression of these early genes. The products of the proto- oncogenes c-fos and c-]un transregulate gene transcrip- tion and are therefore probably part of the network of molecular events that mediate the changes in EC func- tions by monokines.

Tumo: necrosis factor Tumor necrosis factor (TNF) (Ref. 38) elicits a set of

proinflammatory, prothrombotic responses in ECs which largely overlap with those induced by IL-1. Thus, TNF induces expression of aahesion structures for leukocytes 39, of chemota~ic cytokines and CSFs (Ref. 40), stimulates PGI2 production al, procoagulant activity 42, tissue type plasminogen activator 43, PAF (Ref. 44); and decreases the thrombomodulin-protein C anticoagulation pathways 4s.46. Reduction of thrombo- modulin by TNF is caused by accelerated internalization and reduction of gene transcription 4s.46. Lymphotoxin (LT or TNF-13) is a related cytokine produced by T and B lymphocytes, which acts via the same receptor and has activities similar to those of TNF. It has been reported that LT is less active than TNF in inducing CSF production, expression of leukocyte adhesion molecules and IL-1 release 47.48, though this point remains contentious 49. TNF induces migration, but inhibits proliferation, of ECs. 371

o

r-f ytdl#$ T i s s u e E n d o t h e l i u m

~ ICAM{~ Blood

IL-1 C S F IL -8

MCPol i

~ CSF • I L - 8 -

MCP-1

C S F

IL-8 P ~ MCP-1

©

Fig. 1. Cytokine-endothelial cell interactions in the regulation of leukocyte recruitment from the blood compartment, ii.-1 is not directly chemotactic but elicits leukocyte extravasation by inducing the expression of adhesion structures (ELAM-I and ICAM) and the production of chernoattractants (CSFs, 11.-8, MCP-I) by ECs and tissue cells. IL-l-stimulated ECs also produce inhibitors of adhesion and chemotaxis. Monocyte chemotactic protein 1 (MCP-I) refers to a monocyte chemotactic factor originally identified in the culture supematant of tumor cells s3-Ss. The negative sign indicates cytokines that inhibit leukocyte adhesion and chemotaxis 25.26.

312

In vivo, TNF is angiogenicS°,S~; it augments the ex- pression of major histocompatibility complex (MHC) class I antigens in ECs, whereas IL-1 has little effe~ on MHC expression 31. TNF and IL-1 are additive at optimal con- centrations in inducing various EC responses including procoagulant activity (PCA), neutrophil adhesion mol- ecules and CSF production 31.42. As for IL-1, the actions of TNF on ECs are probably important under several physiological and pathological conditions - i n particular TNF causes hemorrhagic necrosis of tumors that requires the presence of a vascular bed 38. Tumor tissue ECs are probably involved in this phenomenon.

6aroma-interferon The action of IFN-~/on ECs is unique and distinct from

those of other cytokines (Table 1). Unlike other polypeptide mediators, IFN--y induces ECs to act as accessory cells without inducing proinflammatory/ prothrombotic or proliferative/migratory activities.

IFN--y was the first molecularly defined cytokine shown to be active on ECs (Ref. 52). It augments the expression of MHC class I antigens and induces class II antigens and the invariant chainS2, s3. Class II induction was observed both with umbilical vein and microvascular ECs. Some evidence supports an in-vivo role for IFN-~/-induced EC class II expression s4. IFN-~/augments the production of IL-1 by LPS-stimulated ECs (Ref. 55). MHC class II antigen expression, as well as IL-1 and IL-6 preduction, underlies the ability of ECs to act as antigen-presenting cells s6. IFN-~ differs from IL-1 and TNF with regard to

Immunology Today, Vol. 10, No. 11, 1989

effects on coagulation and fibrinolysis. IFN-~ (but not IFN-~/), reportedly induces PGI2 production in ECs (Ref. 57) - an activity also reported for IL-2 (Ref. 58).

In addition to i,:duction of MHC cla3s II antigens, which play a crucial role in integrating ECs into immuno- logical circuits, IFN-~/induces other changes whose sig- nificance is as yet unknown. Upon prolonged exposure to IFN-~/and TNF, a transition from an epitelioid to a fibroblastoid shape occurs 31 and IFN-~/ inhibits EC growth by down-regulating fibroblast growth factor (FGF) receptors s9. This cytokine also induces a slow increase in ICAM-I expression ~9. Finally, IFN-~/protects ECs from the in-vitro cytotoxicity of IL-2-induced lympho- kine-activated killer cells 6°.

Granulocyte and granulocyte-macrophage colony-stimulating factors Granulocyte (G-) and granulocyte--macrophage col-

ony-stimulating factors (GM)-CSF regulate the prolifer- ation and differentiat ion of hematopoiet ic precursors of the myelomonocyt ic lineage and modulate several func- tions of terminally differentiated polymorphonuclear cells and monocytes 61. These factors were initially con- sidered to be restricted to the hematopoiet ic system but it was recently observed that G- and GM-CSF induce migration and proliferation of ECs. ECs have receptors for G- and GM-CSF that are similar in number and affinity to those present on myelomonocytic cells 62. Also, Kojima and co-workers 63 reported that G-CSF enhances plasminogen activator in ECs. In general, proteinases are induced in ECs by chemotactic stimuli and their activity is required for cell migration and new vessel formation. The si~,~ificance in vivo of these observations in the regu- lation of the hematopoiet ic microenvironment of which ECs are an essential constituent, and in the pathophysiol- ogy and pharmacology of CSFs, remain to be estab- lished. G- and GM-CSF do not stimulate the pro- inf lamrnatory/prothrornbotic properties of ECs, nordo they induce functions associated with accessory activity (F. Breviario et aL, unpublished). Thus G- and GM-CSF induce a pattern of responses in EC similar to that induced by FGFs'-- L_,_ .~ ~bee u~uwl.

Transforming growth factor ~ and fibroblast growth factors Activated mononuclear phagocytes produce trans-

forming growth factor ~ (TGF-13) and FGF (Ref. 64). Therefore, these cytokines should be considered in the context of polypeptide mediators involved in leukocyte- EC interactions.

TGF-13 has a unique modulatory activity on ECs, which sets it apart from the patterns identified so far; it inhibited growth when added to cells in monolayer 6s-~9 but stimulated capillary tube formation when ECs were grown in a three-dimensional collagen gei ~7. The inhi- bition of EC growth was related to a decrease in the high-affinity receptors of EGF, and to an inhibition of EGF-induced expression of specific competence genes (c-myc, JE, KC) 68. TGF-13 also inhibits EC chemotaxis and EC proteinase activity 69, and inhibits the effect of IL-1 and TNF on neutrophil adhesion to ECs (Ref. 70).

" "4":0, Tr:.~_~ in,4,,,-~ angiogenesis and, when in- jected subcutaneously into mice, causes formation of granulation tissue 71. In general, it appears that in cells of mesenchymal origin, whether TGF-[~ stimulates or in- hibits proliferation and/or activation is a function of the entire set of growth factors operant in that cell.

Immunology Today, VoL 1 O, No. 11, 1989 I

Acidic and basic FGF (also called heparin-binding growth factor I and II) share most activities on EC (for re,,iew see Ref. 72). They are potently mitogenic and induce EC chemotaxis and proteinase synthesis 69. Thesc types of EC responses are a common feature of growth- factor stimulation and, as reported above, the same type of activities are induced by G- and GM-CSF. Acidic and basic FGF do not change the response of ECs to coagu- lation, ;ibrinolysis and tl-,ay do not increase leukocyte adhesion (E. Dejana et al., unpublished). Thus their effects are apparently distinct from those of IL-1 or TNF. In contrast, it was found that prolonged exposure of ECs to FGF and heparin induces a decrease in the ability of the cells to synthesize PGI2 (Ref. 73) and plasminogen activator 74.

Endothelial cells as a source of cvtokines in addition to acting as targets for the action of

cytokines, ECs are important producers of various mediators that regulate the hematopoietic system, the differentiation and proliferation of T and B cells and the recruitment of leukocytes at sites of inflammation (Table 2). ECs are known to release CSFs (Refs 23, 24, 40, 41): G-, GM- and M-CSF have been identified in these cells at the protein and mRNA level. Various stimuli including IL-1, TNF and LPS induce production of CSFs. Interest- ingly, rat microvascular ECs have been reported to produce GM-CSF constitutively. The regulation of GM- CSF gene expression is reportedly different in ECs and macrophages 4°. The above stimuli induce transcription of CSF genes in ECs, whereas expression of GM-CSF in macrophages was found to be mainly regulated at the post-transcriptional level. Dexamethasone did not affect GM-CSF induction in ECs, while it strongly suppressed its production in macrophages. By producing CSFs (includ=

w r l i E r l I L - I ,,,y and IU6, have t.bl- acl:lvi~), ECs partici- pate in the regulation of hematopoiesis. The finding that ECs respond to G- and GM-CSF suggests the possibility of autocrine or paracrine circuits involving ECs and CSFs in the maintenance of the bone marrow microenviron- ment.

Various agents including LPS, TNF and IL-I itself induce production of IL-1 in ECs and in vascular smooth mdsc!e celIsd7.SS,7S-78; it has been identified in both its released and cell-associated form. Interestingly, there is evidence of differential regulation of IL-1 in ECs compared with mononuclear phagocytes. Zuckerman and co-workers 79 reported that dexamethasone inhibited IL-1 production Jr/macrophages but not in ECs. Exposure to polyl:C or virus induces production of IFN-~ and -13 in ECs (Ref. 80). ECs also produce large amounts of IL-6 (IFN-132) identified at the mRNA and protein level 81. Appreciable levels of spontaneous IL-6 production were detected, though one cannot exclude the possibility that this re- flected exposure to as yet unidentified signals present in the in-vitro culture system. IL-1, TNF and LPS enhanced IL-6 mRNA and protein in ECs. IL-6 does not induce the functional changes in ECs elicited by IL-1, ,ncluding pro- duction of PGI2, procoagulant activity and neutrophil adhesion molecules, nor does it synergize with IL-1 (Ref. 81). Thus IL-1 does not act on ECs via IL-6. IL-6 is a pleio- tropic lymphokine that affects the proliferation of T and B cells, has CSF activity and regulates the production of acute phase proteins in the liver 82. Hence, by producing high levels of both this cy*okine and IL-1, ECs participate

Q

. . . . . r Vl J$ I Table 2. Cytokines produced by endothelial cells

Cytokine Stimulus Function Refs

IL-1 LPS Lymphocyte activation; 47, 55, 75, 78 IL-1 local and systemic TNF inflammation; acute phase IFN-~ responses; hematopoiesis

IL-6

CSFs

'spontaneous' LPS II.-1 TNF

LPS IL-1 TNF

Lymphocyte activation; acute 81 phase response; hematopoiesis

Leukocyte recruitment and activation; hematopoiesis; EC proliferation

Chemotactic LPS Leukocyte recruitment and 26 factors IL-1 activa[ion (MDNCF/IL-8; TNF MCP-1)

PDGF IL-1

23,24,40

Smooth muscle cell proliferation 33 (atheroscierosis)

in the regulation of specific immunity and acute phase responses.

ECs release chemoattractants that induce the directional locomotion of polymorphonuclear ceils and monocytes 26. G- and GM-CSF have chemotactic activity 83.84 but antiDodies directed against these factors only partially inhibit the chemotactic activity of EC super- natants (Ming et al., unpublished). ECs produce mem- bers of the novel, recently identified family of chemo- tactic factors homologous to platelet factor 4 (for example, see Refs 85, 86). Upon stimulation with IL-1, TNF or LPS, ECs express monocyte-derived neutrophi! chemotactic factor (MDNCF)/IL-8 activity and mRNA. Nuclear run off experiments showed transcriptional acti- vation of the MDNCF/IL-8 gene. Similarly, ECs express activity and mRNA of a novgl member of a related gene family 87-89 chemotactic for monocytes (A. Sica et al., unpublished). Induction of these chemotactic factors in ECs as well as expression of leukocyte adhesion mol- ecules is probably important for induction of leukocyte extravasation (Fig. 1).

As discussed above, IL-1 also stimulates ECs to pro- duce cytokines that inhibit leukocy;.e adherence and chemotaxis; these probably represent negative feedback signals of leukocyte recruitment 26,27.

Concluding remarks In this review, we have summarized our current under-

standing of the role of cytokines as communication signals between vascular ECs and leukocytes. Various cytokines are produced by and act on ECs; the responses elicited by these cytokines seem to follow discrete, essentially non-overlapping patterns (Table 1): IL-1 is the prototype of cytokines which induce proinflammatory/ prothrombotic changes in ECs; IFN-.y activates their accessory cell functions; molecules of the CSF and FGF families stimulate the migration/proliferation pathway; finally, TGF-i3, in addition to novel recently identified 373

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cytokines, downregulates several proinflammatory activi- ties of ECs.

From these and other studies ECs have emerged as active participants in the induction and regulation of coagulation, inflammation and immunity. Cytokines are crucial mediators of the symbiotic bidirectional interac- tion between.vascular cells and leukocytes. Generally pleiotropic mediators such as IL-1, which are produced by and act on diverse cells and tissues, may function as communication signals between the vessel wall and underlying tissues.

In considering the role of cytokines, it should be emphasized that several of these molecules are present concomitantly or in sequence in vivo, with possible reciprocal amplification or inhibition. Moreover the occu,~-ence of cascades of release and action involving ECs and cytokines should be considered when trying to relate in-vitro observations to the complexity of in-vivo phenomena.

In their production and response to various cytokines and in the complexity of the activation patterns elicited, ECs are reminiscent of mononuclear phagocytes. This similarity and interrelationship has long been recognized and expressed in the now largely abandonea definition of the 'reticuloendothelial system'.

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CD3 + T cells mediate relatively promiscuous patterns of major histocompatibility complex (MHC)-unrestricted target cell lysis following activation. Cell-cell contact between target and effector cells is essential in this form of cytotoxicity. Although the T-cell receptor (TCR}-CD3 molecular complex can transmit signals that inifiate MHC unrestricted T-cell killing, recognition of targets by the TCR is nor essential for this form of cytotox- icity. In this review by Dwain Thiele and Peter Lipsky, a model of the triggering of T cells to effect MHC-unrestricted cytotoxicity

is proposed.

role of ceil surface recognilion struc|ure$ in ihe • OnO mO L 0 .Q. ~ - . - , 9

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killing by T cells

Cell-mediated cytotoxicity is one of the important effec- tor functions produced during cellular immune re- sponses. Cytotoxicity by various effector cells shows different degrees of specificity. Antigen-specific T-cell- mediated cytotoxicity restricted by T-cell receptor (TCR) recognition of antigenic peptide fragments, presented in the context of target cell MHC-encoded surface proteins,

ILiver Unit, 2Rheumatic Diseases Division, Department of Internal Medicine, The University of Texas Southwestern Medical Center, Dallas, TX 75235, USA.

Dwain L iele and Peter E. Ups z

is the most focused 1. While 'classical' natural killer (NK) cell-mediated cytotoxicity is not MHC restricted, such killing also entails a certain degree of target specificity. In addition to mediating antibody-dependent cell-mediated cytotoxicity, NK cells distinguish virally-infected from non-virally-infected targets 2. Moreover, NK cells can discriminate between self bone marrow cells and par- ental hemopoietic histocompatibility antigen disparate, but MHC antigen compatible, bone marrow stem cells3; they also lyse certain 'NK sensitive' cultured tumor cell lines with far areater efficiency than they can kill other, 'NK-resistant',-tumor cell lines or any of a variety of freshly isolated malignant or nonmalignant cell ty, pes 4.s. In contrast to the fine specificity of target cell lysis by antigen-specific, MHC-restricted cytotoxic T lymphocytes (CTL) and the relative specificity of target cell lysis by NK cells, more non-specific patterns of T-cell-mediated cyto- toxicity have also been noted.

~) 1989, Elsevier Science Publishers Ltd, UK. 0167-4919189/$03.50

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