Immunology Today, vol. 7, No. 10, 1986
-rtvittus
Natural suppressor (NS) cells Membersof the LGLregulatoryfamily Natural suppressor (NS) activity is defined as the ability of unprimed "null" cells to suppress the response of lymphocytes to immunogenic and mitogenic stimuli. Cells with this ability have been studied for a number of years 1-16. In this review, Tom Maier and his colleagues examine some aspects of non-specific suppression and relate them to NS activity. They also consider NS cells as part of a 'natural' non-specific suppressor-cell family along with natural killer and natural cytotoxic cells. This family has the large granular lymphocyte (LGL) phenotype. NS activity is found in a number of situations and locations which are shown in Table 1. NS cells occur normally in neonatal lymphoid tissue4's and in adult bone marrow (BM) 1'2'6. NS activity is not prominent in normal spleen but can be found there after a variety of manipulations. These include total lymphoid irradiation (TLI)378 ' ' and the induction of chronic graft-versus-host disease (GVHD) across minor histocompatibility barriers9, and the administration of cyclophosphamide (Cy) 1°'11. Although the data are not as complete, we think that NS cells may also be responsible for the suppression seen after treatment with strontium 89 (89Sr)12'13. It is noteworthy that each of these situations involves damage to lymphoid tissue and NS activity is seen in the repopulation which occurs during recovery. An important characteristic of the locations in which NS-cell activity is potentially present is that they are all environments of considerable hematopoiesis. Neonatal spleen and adult bone marrow contain active hematopoietic tissue. Following TLI, the spleen is autotransfused from the shielded bone marrow and quickly becomes a site of rapid stem cell turnover. If GVHD occurs in recipients who have had cytoreductive treatment, there will be proliferation of donor and/or recipient hematopoietic cells. Finally, in treatment with Cy or 89Sr, lymphohematopoietic destruction is followed by cell proliferation and tissue regeneration. Characteristics of NS cells NS cells in vitro can suppress MLR3, inhibit antibody responses 14 or block mitogen activation9. The inhibition is MHC nonrestricted and is not blocked by indomethacin ~s. NS activity is not associated with the lysis of typical natural killer (NK) targets such as YAC-1 3 ' 15 ' 16 . However, Jadusand Parkman 16 report that the suppressive activity of neonatal spleen is associated with lysis of the natural cytotoxic (NC) target WEHI-164 (Ref. 16). NS activity is most pronounced in low density cell populations where LGL are present 1'3'7-9. Phenotypically, the suppressor cells are generally insensitive to anti-Thy-1 + complement treatment 1~'6-13'~s, and to other anti-T-cell marker antibodies such as anti-L3T4 or anti-Lyt 2 (Ref. 4, 9, 11). The suppressor cells are also usually found to lack surface immunoglobulin (slg), although they may express Fc-receptors and therefore
312
Departmentof Medicine,Divisionof ClinicalImmunology,Universityof ColoradoMedicalCenter, Denver,CO80262, USA.
TomMaier,JamesH. Holdaand HenryN. Claman might be counted as slg ÷ under some experimental conditions ~'15. The suppressor cells usually lack la2,4,11,15
Results of studies of the adherence properties of these cells have been equivocal. Some studies showed that at least some of the suppressor activity is lost after passage through G-IO columns or after adherence to plastic7'13, but most experiments indicated that the suppressive activity is unaffected or even enhanced after plastic adherence2~,9,~l In summary, NS cells seem to have a 'null' phenotype, in that they do not generally have markers or characteristics of normal mature T cells, B cells or macrophages. Expression mechanisms of NS cells The actual mechanism used by NS cells to inhibit lymphocyte function is unknown. However, NS cells do not appear to lyse their targets, and thus differ from NK cells 17. As the activity of NS cells is often measured by decreased proliferation of target cells, a possible mechanism of action is release of a soluble mediator. Spleen cells of neonates produce a factor(s) which suppresses in a similar manner to the cells themselves18-2°. Singhal has detected a suppressive glycol/p/d, released from normal cultured BM cells, which is suppressive in vivo and in vitro 2~. More work is needed in this field. NS cell activity is greatly increased by T-cell signals both in vivo and in vitro 22. In vitro, these signals include Con-A (i.e. plant lectin) supernatants, recombinant interferon--/ (rlFN--/) and recombinant interleukin-2 (rlL-2) (Ref. 22 and T. Maier et al., unpublished). Thus, the optimal activity of NS cells is linked to the function of T cells. NS cells and tolerance An important feature of the locations where NS cells are found is that they are all environments where tolerance induction may occur ('windows of tolerance') 15,23. The neonatal state is not only one where self-tolerance is established, but it is also a favorite period for the induction of acquired specific immunological tolerance 24. BM may be a site of some tolerance induction throughout adult life25.26. TL127 or cyclophospham/de treatment 1°'11 are practical means of producing a general immunosuppressive environment where tolerance induction is possible. In BM transplantation, host immunosuppression is essential to the induction of tolerance to the graft 28. The tolerogenic potential of 895r treatment is less clear. Thus, considering the activity of NS cells, their location and their responsiveness to T-cell signals (lymphokines), it seems entireJy possible that NS cells may be involved in the development and maintenance of self-tolerance or of acquired immunological unresponsiveness, at least at the
Imrnunology Today, vol. 7, No. 10, 1986
reviews
-
Possible biological and clinical significance
T cell level 15.23. (This is not meant to imply that NS cell activity is the only means of tolerance induction. We believe this is only one, though maybe the first of what is certainly several overlapping means of developing and maintaining self-tolerance.) According to this model Is.23, when T cells are stimulated by self antigens or alloantigens in an environment rich in NS cells, the lymphokines they release stimulate the suppressive actions of NS cells. This increased NS activity in its turn acts as a negative feedback loop to inhibit the development of fully functional T cells. Interestingly, because of the apparent inability of NS cells to kill target cells, clonal abortion may not occur. This model of NS-mediated tolerance therefore postulates the continued existence of potentially functional auto- and allo-reactive T cells. A corollary of this theory is that diminished NS activity might lead to a loss of self-tolerance, e.g. autoimmunity. The work of Strober et al.,primarily on TLl-induced NS cell activity, indicates that T suppressor cells may be refractory to such NS-mediated down-regulation s.ls,29. Adding this to the model would mean that while the NS cells were down-regulating specific self-reactive T helper clones, specific T suppressor clones to the same antigen may survive and thus supply the specific down-regulation usually seen in the periphery of the mature animal.
As mentioned above, NS activity appears to occur after several immunosuppressive treatment protocols including TLI, cyclophosphamide treatment, and 89Sr treatment. NS activity also occurs in chronic GVHD associated with BM transplantation. NS activity is probably at least partly responsible for the immunosuppression seen in all these situations. This may be especially true of BM transplantation, where the immunosuppressive pretransplant regimen, the BM inoculum, and the frequent development of GVHD all have NS cell activity associated with them. TLI has been proposed as a treatment for several autoimmune diseases such as systemic lupus erythematosus (SLE) and rheumatoid arthritis 27,3°-33. The efficacy of this treatment may be related to TLIinduced NS cell down-regulation of self-reactive clones. NS activity in environments of hematopoiesis, which shows such potent non-specific inhibition may also control cell growth in these situations, perhaps by inhibition of growth factor production 34. They may therefore be an important regulatory component of normal homeostasis in adult BM and during fetal and neonatal life. Recently, Lafferty et al. proposed 3s that some of the anomalies seen with cyclosporin A (CsA) treatment may
Table I. Generalcharacteristicsof LGLnaturalsuppressorsystems NS DEFINITE
NKg
NCh
POSSIBLE
Normal
Induced
Fetaland NeonataP
Adult BMb
TLIc
Cyclod
GVHDe
89Srf
+ +
+ +
+ +
+ +
+ +
+ ?
-+ +-
+ +
NonspecificallySuppress: MLR Ab Response MitogenResponse
+ + +
+ + +
+ + +
+ + ?
+ ? +
? + ?
+ + +
+ ? ?
Lyse: Y A C - I WEHI-164
+-
. ?
?
?
+ -
+
+
+
Location: Environmentof hematopoiesis Environmentof possibletolerance Induction
Markers: LargeMorphology Thy-1+ Ig+ la+ Asial0-GM-1 Plasticor G-IOAdherent ActivityaugmentedbyT-celllymphokines: rlL-2 rlFN-~,
+ +-+
+ +
+ . + -
+ +
.
.
.
?
+ .
+ .
. . _ -+ -+
? ?
. ?
.
+ . .
? .
. .
.
.
_ -
? -
? ? -+
+ -
_ -
? ?
+ +
? ?
+ -+
+ -
references3-5, 8, 15, 16, 18-20and Maieret al., unpublished b references1, 2, 6, 8, 15, 21,34and Holdaet al., unpublished c references3, 5, 7, 8, 15, 27 and 29 d references10and 11 e references9, 22, 23 and Holdaetal., unpublished f references12 and 13 g references17, 37-40,44-48 h references16, 37-42 +, characteristicispositive; -,characteristicisnegative;-+,characteristicispositiveornegativedependingonreport; ?,characteristicis unknownor resultsarestillincomplete. a
Immunology Today, vol. 7, No. 10, 1986
-reviews be explained in terms of NS cells and their signalling requirements22'23. That is, while CsA is very useful in preventing transplant rejection, many autoimmune reactions are seen after CsA treatment is stopped. This could be explained by the fact that CsA inhibits production of T-cell lymphokines and therefore proliferation of effector-cells. However, this lack of T cell lymphokine production also fails to signal the NS cells which normally may down-regulate self-reactive clones. Therefore, after CsA is discontinued autoimmunity can be the unwanted result. Finally, it has been suggested that local unspecific suppression of the maternal immune system may play an important role in protection of the fetus. A non-T cell accumulates in the uterine decidua of allopregnant mice and nonspecifically inhibits immune responses36. There are intriguing similarities in location and function between this cell and NS cells. One of the purposes of this article is to relate NS cells to other regulatory cells which appear as LGL. The pertinent literature is growing (and is sometimes inconsistent) but a summary description of the other member cells follows.
Natural killer (NK) cells NK activity is defined as selective lysis of certain target cells (classically YAC-1 in the murine system) without deliberate priming or specific immunological memory 37. NK targets are usually transformed lymphoid cells or some (usually immature) normal cells37,38. NK activity occurs in LGL fractions but not all LGL cells have NK activity39. NK cells like NS cells have the 'null' phenotype, being Thy-l-, slg- and non-adherent 37,39. NS cells are usually asialo GM-1- while NK cells are asialo GM-1 + (Ref. 37, 39). NK cells are found in the spleen and BM, but not in the thymus. As with NS cells, their activity is increased by certain T-cell lymphokines4°. These include IL-2 and IFN.
Natural cytotoxic (NC) cells These cells kill certain tumor targets (WEHI-164) in vitro 41 and they are thus similar to NK cells. Neither NC nor NK activities are MHC-restricted and apparently require no priming. Some differences do exist; for example the tumor targets for NC and NK activity are different 38. The targets of NC cells are usually transformed cells that form solid tumors. NC and NK activity also do not correlate with each other in various strains of mice, and NC activity appears earlier in development than does NK activity4~'42. Also, NC and NK activities are found in differing relative amounts throughout the immune system.
Comparison of NS cell activity with NK and NC suppressor activity
314
We have used Table 1 to compare the three LGL cells, NS, NK and NC cells, which appear to have at least some suppressor activity. Although the major activity associated with both NK and NC cells is cytotoxicity, these two cell types also have suppressive potential. NK cells inhibit the in-vitro responses of both T and B cells as well as inhibiting hematopoiesis 17 •4 3 - 4 9 . Howeyer, unlike NS cells the suppressive activity of NK cells may be associated with a lytic activity ~7. Although most of this work has been done with NK cells at least one recent paper
reports the suppressive potential of an NC population found in neonatal spleens 16. One very interesting area where there seems to be a difference between NS-cell activity and NK- or NC-cell activity is the enhancing effects that T-cell lymphokines (especially IFN-~/) have on them. NS cell activity is tremendously responsive to both rlL-2 and rlFN-% with suppressive activity often being enhanced 10 fold or greater (Ref. 24, 25 and T. Maier et aL, unpublished), This is at rlFN-~, concentrations of as little as 1 IU/ml. NK suppressive activity is only somewhat enhanced with rlFN-~, at concentrations of 100 IU/ml 17. Also, while early work indicated that less pure preparations of IFN--/were potent activators of NK c~otoxic activity, more recent work with rlFN--y has indicated that this lymphokine has little if any enhancing effect on NK cytotoxic activitys°. Thus, in contrast to NS cells, NK cells may not be very responsive to IFN-~,. This also seems to be the case with NC cells where the suppression caused by these cells is actually lost with the addition of rlFN-~/~6. It is important to realize that because of the relative scarcity of individual markers which might distinguish these cells from one another, it is difficult to assign one particular activity to an individual cell type. In other words, when working with bulk populations containing LGL with both cytotoxic and suppressor activity, it is difficult to say that both the c~otoxicity and suppression are carried out by the same cell; only that both activities are found in the same population. On the other hand, cloned cell lines carried in vitro for long periods with strong lymphokine and/or antigen stimulation, might be more homogeneous but may have developed some properties which are not necessarily those of 'native' LGL. In conclusion there are several cells within the LGL family that appear to have significant suppressive ability. These are NS, NK, and possibly also NC cells. These cells all suppress a number of immune responses without apparent priming and in a genetically unrestricted manner. The precise lineages and developmental pathways of these cells are not yet clear. Because of their phenotypic and functional similarities, however, we believe that it is unlikely that they are completely distinct entities. Rather, we believe that they may be related to each other. The biological and clinical significance of these suppressor cells is still somewhat speculative, but it is safe to assume that their relevance to tolerance and unresponsiveness will be elucidated soon.
References 1 Duwe, A.K. and Singhal, S.K. (1979) Cell. Immunol. 43, 372-381 2 Dorshkind, K. and Rosse,C. (1982)Am. J. Anat. 164, 1-17 30seroff, A., Okada, S. and Strober, S. (1984) J. Immunol. 132, 101-110 4 Rodriguez,G., Andersson, G., Wigzell, H. et. al. (1979) Eur. J. ImmunoL 9, 737-746 50kada, S. and Strober, S. (1982)J. Immunol. 129, 1892-1897 6 deFazio,S., Hartner, W.C. and Monaco, A.P. (1985) J. Immunol. 135, 3034-3038 7 May, R.D., Slavin,S. and Vitetta, E.S.(1983)J. Immunol. 131, 1108-1114 8 Weigensberg, M., Morecki, S., Weiss, L. etal. (1984)
Immunology Today, vol. 7, No. 10, 7986
reviews J. Immunol. 132,971-978
9 Maier, T., Holda, J.H. and Claman, H.N. (1985)J. Immunol. 135, 1644-1651 10 Greeley, E.H., Segre, M. and Segre, D. (1985)J. ImmunoL 134, 847-851 11 Segre, M, Tomei, E, and Segre, D. (1985) Cell. ImmunoL 91, 443-454 12 Merluzzi, U.J., Levy, EM., Kumar, V. etal. (1978) J. Immunol. 121,505-512 13 Levy, E.M., Bennett, M., Kumar, V. etaL (1980)J. Immunol. 124, 611-618 14 Layton, J.E., Uhr, J.W., Pure, E. etal. (1983)J. Immunol. 130, 2502-2504 15 Strober, S. (1984)Annu. Rev. Immunol. 2,219-237 16 Jadus, M.R. and Parkman, R. (1986)J. Immunol. 136, 783-792 17 Targan, S., Brieva, J,, Newman, W. et aL (1985)J. ImmunoL 134, 666-669 18 Argyris, B. (1981) Cell. Imrnunol. 62,412-424 19 Peeler, K., Wigzell, H. and Peck, A.B. (1983) Scand. J. Immunol, 17,443-453 20 Jadus, M.R. and Peck, A.B, (1986)Scand. J. Immunol. 23, 35--44 21 Saffran, D., Mortari, F., Atkinson, M. et al. (1986) Fed. Proc. 45, 494 22 Maier, T., Holda, J.H. and Claman, H.N. (1985)J. Exp. Med. 162,979-992 23 Holda, J.H., Maier, T. and Claman, H.N. (1985) Immunol. Rev. 88, 87-105 24 Billingham, R.E., Brent, L. and Medawar, P.B. (1953) Nature (London) 172,603-606 25 Kast, W.M., deWaal, L.P. and Melief, C,J.M. (1984) J. Exp. Med. 160, 1752-1766 26 Chervenak, R., Moorhead, J.W. and Cohen, J.J. (1985) J. ImmunoL 134, 695-698 27 Kotzin, B.L. and Strober, S. (1984) Clin. Immunol. Allergy4, 331-358 28 Storb, R. (1983) Transplant. Proc. 15, 1379-1384 29 Okada, S. and Strober, S. (1982) J. Exp. Med. 156, 522-538 30 Kotzin, B.L. and Strober, S. (1979)J. Exp. Med. 150,
-
371-378 31 Theofilopoulos, A.N., Balderas, R., Shawler, D.L. etaL (1980)J. Immunol. 125, 2137-2142 32 Strober, S., Tanay, A., Feild, E. etal. (1985)Ann. Intern. Med. 102, 441--449 33 Strober, S., Feild, E., Hoppe, R.T. etal. (1985)Ann. Intern. Med. 102, 45C~458 34 Soderberg, LS.F. (1985) Cell. Imrnunol. 92, 313-320 35 Lafferty, K.J., Gill, R. and Babcock, S. (1986) Prog. Allergy 38, 247-257 36 Clark, D.A., Slapsys, R., Croy, A. etal. (1984)Am. J. Reprod. Immunol. 5, 78~33 37 Herberman, R.B. (ed) (1982) NK Cells and Other Natural Effector Cells. Academic Press, New York 38 Lin, Y., Collins, J.L., Patek, P.Q. etaL (1983) J. Immunol. 131, 1154-1159 39 Callewaert, DM. (1985)Nat. Immun. Cell Growth Regul. 4, 61-77 40 Shau, H. and Golub, S.H. (1985) Nat. Immun. Cell Growth Regul. 4, 113-119 41 Stutman, O., Paige, C.J., Figarella, E.F. (1978)J. Immunol. 121, 1819-1826 42 Stutman, O. and Cuttito, M.J. (1981)Nature (London) 290, 254-257 43 Hansson, M., Kiessling, R. and Andersson, B. (1979) Nature (London) 278, 174-176 44 Brieva, J., Targan, S. and Stevens, R.H. (1984) J. Immunol. 132, 611-615 45 Brieva, J. and Stevens, R.H. (1984) J. Immunol. 133, 12881292 46 Kiessling, R., Hochman, P.S., Hailer, O. etal. (1977)Eur. J. Immunol. 7, 655-663 47 Holmberg, L.A., Miller, B.A. and Ault, K.A. (1984) J. Immunol. 133, 2933-2939 48 Abruzzo, L.V. and Rowley, D.A. (1983) Science 222, 581-585 4g Abruzzo, L.V., Mullen, C.A and Rowley, D.A. (1986) Cell. Immunol. 98, 260-278 50 Sayers, T.J., Mason, A.T. and Ortaldo, J.R. (1986) J. ImmunoL 136, 2176-2180
315
-rtvittus
Natural suppressor (NS) cells Membersof the LGLregulatoryfamily Natural suppressor (NS) activity is defined as the ability of unprimed "null" cells to suppress the response of lymphocytes to immunogenic and mitogenic stimuli. Cells with this ability have been studied for a number of years 1-16. In this review, Tom Maier and his colleagues examine some aspects of non-specific suppression and relate them to NS activity. They also consider NS cells as part of a 'natural' non-specific suppressor-cell family along with natural killer and natural cytotoxic cells. This family has the large granular lymphocyte (LGL) phenotype. NS activity is found in a number of situations and locations which are shown in Table 1. NS cells occur normally in neonatal lymphoid tissue4's and in adult bone marrow (BM) 1'2'6. NS activity is not prominent in normal spleen but can be found there after a variety of manipulations. These include total lymphoid irradiation (TLI)378 ' ' and the induction of chronic graft-versus-host disease (GVHD) across minor histocompatibility barriers9, and the administration of cyclophosphamide (Cy) 1°'11. Although the data are not as complete, we think that NS cells may also be responsible for the suppression seen after treatment with strontium 89 (89Sr)12'13. It is noteworthy that each of these situations involves damage to lymphoid tissue and NS activity is seen in the repopulation which occurs during recovery. An important characteristic of the locations in which NS-cell activity is potentially present is that they are all environments of considerable hematopoiesis. Neonatal spleen and adult bone marrow contain active hematopoietic tissue. Following TLI, the spleen is autotransfused from the shielded bone marrow and quickly becomes a site of rapid stem cell turnover. If GVHD occurs in recipients who have had cytoreductive treatment, there will be proliferation of donor and/or recipient hematopoietic cells. Finally, in treatment with Cy or 89Sr, lymphohematopoietic destruction is followed by cell proliferation and tissue regeneration. Characteristics of NS cells NS cells in vitro can suppress MLR3, inhibit antibody responses 14 or block mitogen activation9. The inhibition is MHC nonrestricted and is not blocked by indomethacin ~s. NS activity is not associated with the lysis of typical natural killer (NK) targets such as YAC-1 3 ' 15 ' 16 . However, Jadusand Parkman 16 report that the suppressive activity of neonatal spleen is associated with lysis of the natural cytotoxic (NC) target WEHI-164 (Ref. 16). NS activity is most pronounced in low density cell populations where LGL are present 1'3'7-9. Phenotypically, the suppressor cells are generally insensitive to anti-Thy-1 + complement treatment 1~'6-13'~s, and to other anti-T-cell marker antibodies such as anti-L3T4 or anti-Lyt 2 (Ref. 4, 9, 11). The suppressor cells are also usually found to lack surface immunoglobulin (slg), although they may express Fc-receptors and therefore
312
Departmentof Medicine,Divisionof ClinicalImmunology,Universityof ColoradoMedicalCenter, Denver,CO80262, USA.
TomMaier,JamesH. Holdaand HenryN. Claman might be counted as slg ÷ under some experimental conditions ~'15. The suppressor cells usually lack la2,4,11,15
Results of studies of the adherence properties of these cells have been equivocal. Some studies showed that at least some of the suppressor activity is lost after passage through G-IO columns or after adherence to plastic7'13, but most experiments indicated that the suppressive activity is unaffected or even enhanced after plastic adherence2~,9,~l In summary, NS cells seem to have a 'null' phenotype, in that they do not generally have markers or characteristics of normal mature T cells, B cells or macrophages. Expression mechanisms of NS cells The actual mechanism used by NS cells to inhibit lymphocyte function is unknown. However, NS cells do not appear to lyse their targets, and thus differ from NK cells 17. As the activity of NS cells is often measured by decreased proliferation of target cells, a possible mechanism of action is release of a soluble mediator. Spleen cells of neonates produce a factor(s) which suppresses in a similar manner to the cells themselves18-2°. Singhal has detected a suppressive glycol/p/d, released from normal cultured BM cells, which is suppressive in vivo and in vitro 2~. More work is needed in this field. NS cell activity is greatly increased by T-cell signals both in vivo and in vitro 22. In vitro, these signals include Con-A (i.e. plant lectin) supernatants, recombinant interferon--/ (rlFN--/) and recombinant interleukin-2 (rlL-2) (Ref. 22 and T. Maier et al., unpublished). Thus, the optimal activity of NS cells is linked to the function of T cells. NS cells and tolerance An important feature of the locations where NS cells are found is that they are all environments where tolerance induction may occur ('windows of tolerance') 15,23. The neonatal state is not only one where self-tolerance is established, but it is also a favorite period for the induction of acquired specific immunological tolerance 24. BM may be a site of some tolerance induction throughout adult life25.26. TL127 or cyclophospham/de treatment 1°'11 are practical means of producing a general immunosuppressive environment where tolerance induction is possible. In BM transplantation, host immunosuppression is essential to the induction of tolerance to the graft 28. The tolerogenic potential of 895r treatment is less clear. Thus, considering the activity of NS cells, their location and their responsiveness to T-cell signals (lymphokines), it seems entireJy possible that NS cells may be involved in the development and maintenance of self-tolerance or of acquired immunological unresponsiveness, at least at the
Imrnunology Today, vol. 7, No. 10, 1986
reviews
-
Possible biological and clinical significance
T cell level 15.23. (This is not meant to imply that NS cell activity is the only means of tolerance induction. We believe this is only one, though maybe the first of what is certainly several overlapping means of developing and maintaining self-tolerance.) According to this model Is.23, when T cells are stimulated by self antigens or alloantigens in an environment rich in NS cells, the lymphokines they release stimulate the suppressive actions of NS cells. This increased NS activity in its turn acts as a negative feedback loop to inhibit the development of fully functional T cells. Interestingly, because of the apparent inability of NS cells to kill target cells, clonal abortion may not occur. This model of NS-mediated tolerance therefore postulates the continued existence of potentially functional auto- and allo-reactive T cells. A corollary of this theory is that diminished NS activity might lead to a loss of self-tolerance, e.g. autoimmunity. The work of Strober et al.,primarily on TLl-induced NS cell activity, indicates that T suppressor cells may be refractory to such NS-mediated down-regulation s.ls,29. Adding this to the model would mean that while the NS cells were down-regulating specific self-reactive T helper clones, specific T suppressor clones to the same antigen may survive and thus supply the specific down-regulation usually seen in the periphery of the mature animal.
As mentioned above, NS activity appears to occur after several immunosuppressive treatment protocols including TLI, cyclophosphamide treatment, and 89Sr treatment. NS activity also occurs in chronic GVHD associated with BM transplantation. NS activity is probably at least partly responsible for the immunosuppression seen in all these situations. This may be especially true of BM transplantation, where the immunosuppressive pretransplant regimen, the BM inoculum, and the frequent development of GVHD all have NS cell activity associated with them. TLI has been proposed as a treatment for several autoimmune diseases such as systemic lupus erythematosus (SLE) and rheumatoid arthritis 27,3°-33. The efficacy of this treatment may be related to TLIinduced NS cell down-regulation of self-reactive clones. NS activity in environments of hematopoiesis, which shows such potent non-specific inhibition may also control cell growth in these situations, perhaps by inhibition of growth factor production 34. They may therefore be an important regulatory component of normal homeostasis in adult BM and during fetal and neonatal life. Recently, Lafferty et al. proposed 3s that some of the anomalies seen with cyclosporin A (CsA) treatment may
Table I. Generalcharacteristicsof LGLnaturalsuppressorsystems NS DEFINITE
NKg
NCh
POSSIBLE
Normal
Induced
Fetaland NeonataP
Adult BMb
TLIc
Cyclod
GVHDe
89Srf
+ +
+ +
+ +
+ +
+ +
+ ?
-+ +-
+ +
NonspecificallySuppress: MLR Ab Response MitogenResponse
+ + +
+ + +
+ + +
+ + ?
+ ? +
? + ?
+ + +
+ ? ?
Lyse: Y A C - I WEHI-164
+-
. ?
?
?
+ -
+
+
+
Location: Environmentof hematopoiesis Environmentof possibletolerance Induction
Markers: LargeMorphology Thy-1+ Ig+ la+ Asial0-GM-1 Plasticor G-IOAdherent ActivityaugmentedbyT-celllymphokines: rlL-2 rlFN-~,
+ +-+
+ +
+ . + -
+ +
.
.
.
?
+ .
+ .
. . _ -+ -+
? ?
. ?
.
+ . .
? .
. .
.
.
_ -
? -
? ? -+
+ -
_ -
? ?
+ +
? ?
+ -+
+ -
references3-5, 8, 15, 16, 18-20and Maieret al., unpublished b references1, 2, 6, 8, 15, 21,34and Holdaet al., unpublished c references3, 5, 7, 8, 15, 27 and 29 d references10and 11 e references9, 22, 23 and Holdaetal., unpublished f references12 and 13 g references17, 37-40,44-48 h references16, 37-42 +, characteristicispositive; -,characteristicisnegative;-+,characteristicispositiveornegativedependingonreport; ?,characteristicis unknownor resultsarestillincomplete. a
Immunology Today, vol. 7, No. 10, 1986
-reviews be explained in terms of NS cells and their signalling requirements22'23. That is, while CsA is very useful in preventing transplant rejection, many autoimmune reactions are seen after CsA treatment is stopped. This could be explained by the fact that CsA inhibits production of T-cell lymphokines and therefore proliferation of effector-cells. However, this lack of T cell lymphokine production also fails to signal the NS cells which normally may down-regulate self-reactive clones. Therefore, after CsA is discontinued autoimmunity can be the unwanted result. Finally, it has been suggested that local unspecific suppression of the maternal immune system may play an important role in protection of the fetus. A non-T cell accumulates in the uterine decidua of allopregnant mice and nonspecifically inhibits immune responses36. There are intriguing similarities in location and function between this cell and NS cells. One of the purposes of this article is to relate NS cells to other regulatory cells which appear as LGL. The pertinent literature is growing (and is sometimes inconsistent) but a summary description of the other member cells follows.
Natural killer (NK) cells NK activity is defined as selective lysis of certain target cells (classically YAC-1 in the murine system) without deliberate priming or specific immunological memory 37. NK targets are usually transformed lymphoid cells or some (usually immature) normal cells37,38. NK activity occurs in LGL fractions but not all LGL cells have NK activity39. NK cells like NS cells have the 'null' phenotype, being Thy-l-, slg- and non-adherent 37,39. NS cells are usually asialo GM-1- while NK cells are asialo GM-1 + (Ref. 37, 39). NK cells are found in the spleen and BM, but not in the thymus. As with NS cells, their activity is increased by certain T-cell lymphokines4°. These include IL-2 and IFN.
Natural cytotoxic (NC) cells These cells kill certain tumor targets (WEHI-164) in vitro 41 and they are thus similar to NK cells. Neither NC nor NK activities are MHC-restricted and apparently require no priming. Some differences do exist; for example the tumor targets for NC and NK activity are different 38. The targets of NC cells are usually transformed cells that form solid tumors. NC and NK activity also do not correlate with each other in various strains of mice, and NC activity appears earlier in development than does NK activity4~'42. Also, NC and NK activities are found in differing relative amounts throughout the immune system.
Comparison of NS cell activity with NK and NC suppressor activity
314
We have used Table 1 to compare the three LGL cells, NS, NK and NC cells, which appear to have at least some suppressor activity. Although the major activity associated with both NK and NC cells is cytotoxicity, these two cell types also have suppressive potential. NK cells inhibit the in-vitro responses of both T and B cells as well as inhibiting hematopoiesis 17 •4 3 - 4 9 . Howeyer, unlike NS cells the suppressive activity of NK cells may be associated with a lytic activity ~7. Although most of this work has been done with NK cells at least one recent paper
reports the suppressive potential of an NC population found in neonatal spleens 16. One very interesting area where there seems to be a difference between NS-cell activity and NK- or NC-cell activity is the enhancing effects that T-cell lymphokines (especially IFN-~/) have on them. NS cell activity is tremendously responsive to both rlL-2 and rlFN-% with suppressive activity often being enhanced 10 fold or greater (Ref. 24, 25 and T. Maier et aL, unpublished), This is at rlFN-~, concentrations of as little as 1 IU/ml. NK suppressive activity is only somewhat enhanced with rlFN-~, at concentrations of 100 IU/ml 17. Also, while early work indicated that less pure preparations of IFN--/were potent activators of NK c~otoxic activity, more recent work with rlFN--y has indicated that this lymphokine has little if any enhancing effect on NK cytotoxic activitys°. Thus, in contrast to NS cells, NK cells may not be very responsive to IFN-~,. This also seems to be the case with NC cells where the suppression caused by these cells is actually lost with the addition of rlFN-~/~6. It is important to realize that because of the relative scarcity of individual markers which might distinguish these cells from one another, it is difficult to assign one particular activity to an individual cell type. In other words, when working with bulk populations containing LGL with both cytotoxic and suppressor activity, it is difficult to say that both the c~otoxicity and suppression are carried out by the same cell; only that both activities are found in the same population. On the other hand, cloned cell lines carried in vitro for long periods with strong lymphokine and/or antigen stimulation, might be more homogeneous but may have developed some properties which are not necessarily those of 'native' LGL. In conclusion there are several cells within the LGL family that appear to have significant suppressive ability. These are NS, NK, and possibly also NC cells. These cells all suppress a number of immune responses without apparent priming and in a genetically unrestricted manner. The precise lineages and developmental pathways of these cells are not yet clear. Because of their phenotypic and functional similarities, however, we believe that it is unlikely that they are completely distinct entities. Rather, we believe that they may be related to each other. The biological and clinical significance of these suppressor cells is still somewhat speculative, but it is safe to assume that their relevance to tolerance and unresponsiveness will be elucidated soon.
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