Transplant. Rev. (1975), Vol. 26 Published by Munksgaard, Copenhagen, Denmark No part may be reproduced by any process without written permission from the author(s)

A Disquisition on Suppressor T Cells RICHARD K . GERSHON

After a slow start the concept of the suppressor T cell has picked up steam, such that many of those who attended a recent symposium on 'Suppressor Cells in Immunity' in London, Ontario came away with a feeling of wonderment that, in the face of so much suppression, an immune response could occur at all. Several reviews covering the historical aspects of the subject and categorizing classes of suppressor T cell activity have recently been published (Gershon 1974a, 1974b, in press; Gershon et al. 1974). In addition, the proceedings of the above mentioned symposium are to be bublished (Singhal & Sinclair, in press), making the production of further compendia seem redundant In spite of the plethora of reports on T cell dependent immunosuppression, the suppressor T cell has not passed from a concept to an entity. The term 'suppressor T cell' was originally coined as an operational one, simply to emphasize the negative side of T cell regulation (Gershon et al. 1972). The term still remains operational in that we do not know whether there is a T cell which is programmed during differentiation to act as an obligatory suppressor cell after triggering with antigen. Despite this limitation there are some important questions concerning suppressor T cell activity to which answers can be reasonably deduced from present evidence. I will discuss some of these in this article. I. IS SUPPRESSION TOO MUCH HELP?

This is basically a siUy question which might just as well be posed as 'Is help too little suppression?' Nonetheless, it often arises, for several reasons: A. A common way of demonstrating T dependent suppression is to give supraoptimal doses of antigen, which suggests that the excess antigen may be turning on excess numbers of helper cells. That this is not the case is suggested by studies in which antigen dose has been held constant and the numAssociate Professor of Pathology, Department of Pathology, Yale University School of Medicine, 333 Cedar Street, New Haven, Conecticut 06510, U.SA.

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ber of T cells being stimulated has been varied. Under these conditions suppression is most easily induced with fewer T cells, which also give less helper activity at optimal doses of antigen (Gershon 1974c and unpublished observations). Other examples of T cell suppression which are not readily explainable by too much help include 1) suppression produced by low doses of thymus independent antigens, such as Sni, which do not elicit significant amounts of helper activity at any dose (Baker et al. 1974), 2) suppression produced by deaggregated gammaglobulins which likewise fail to induce appreciable help (Taylor & Elson 1974, Basten et al. 1974, Benjamin 1975, Gershon et al. 1974), 3) suppression by antigens which are normally non-immunogenic because of histocompatibility linked genetic unresponsiveness (Gershon et al. 1973, Kapp et al. 1974), and 4) suppression induced by sub-immxmogenic doses of thymus dependent antigens (Kolsch et al. 1974, Askenase et al. 1975). B. Another reason the question arises is that the suppression produced by adoptive transfer of cells often turns into help when the transferred cells are sufficiently diluted. There is no clearcut explanation for this observations (although I will attempt one below, see next to last paragraph in II-C) but the 'too much help' theory is not a likely candidate as the amount of help seen at the lower dilutions is usually small and nowhere near the theoretical maximum which can be produced when the cells used for adoptive transfer are immxmized in a fashion which is optimal for raising helper cells. Furthermore, there is a report of suppression being effected at low cell multiplicities and disappearing when the number of 'suppressor' cells is increased (Haskill & Axelrad 1972). Durkin et al. (to be published) have similar findings. We have also repeated this finding and have been able to obliterate the suppression with an anti-0 serum (Naidorf & Gershon, to be published). Thus, there are some dose dependent peculiarities determining the net effect of the interaction between mixed cell populations, but the notion of 'too much help' is not of too much help in elucidating the basis for these vagaries, although of course it might apply in some isolated instances. In summary, the answer to (Question I is; almost surely not always, probably not usually, and possibly not ever. n. WHAT IS THE ROLE OF CELL INTERACTIONS IN THE GENERATION OF SUPPRESSOR T CELLS?

A) Interactions between sub-populations of non-immune cells T cells can and have been separated into subpopulations according to various criteria (Raff & Cantor 1971, Asofsky et al. 1971). One commonly used mode is to separate them by means of their differential migratory characteristics.

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There is a recircxilating T cell population which is highly concentrated in the lymph. Another population of T cells fails to recirculate and thus is found predominantly in the spleen. The recirculating T cell can be preferentially ablated by treating animals with low doses of anti-lymphocyte serum (ALS). The nonrecirculating T cell is preferentially removed by splenectomy and also is diminished 3-4 weeks after adult thymectomy. All three of these modes of T cell removal have been reported to lead to augmented immune responses (reviewed in Gershon 1974a, b, in press; Gershon et al. 1974). The augmented response produced by removal of a recirculating T cell by treatment with ALS can be further augmented by returning to the treated animals the lymph node cells which are rich in the cells that were removed by the ALS treatment (Baker et al. 1970). Paradoxically, returning a population rich in non-recirculating T cells leads to a suppression of the augmented response. A similar paradox can be foimd in a study of the in vivo effect of treatment with anti-0 serum (Gelfand & Paul, in press). T cells taken from mice inoculated with anti-0 serum suppressed the immune response of adoptive recipients. Pretreatment of the donor mice with either low doses of ALS, or with thymectomy 3-4 weeks prior to the injection of the anti-0 serum, abrogated the induction of the suppressor T cells. These types of results defy simple explanation and suggest strongly that the generation of suppressor T cell activity is dependent upon a complex series of interactions between subpopulations of T cells. We have foimd that a similar series of complex interactions takes place between cortisone sensitive and cortisone resistant thymocytes both in the generation of GVH activity (Cohen & Gershon 1975) and in the production of killer cells (Frank et al. in press). Thus, when the activity of an iinfractionated population of thymocytes is compared to the activity of the cortisone resistant fraction of that population, it is found that at high cell multiplicities the presence of cortisone sensitive cells in the inoculum is suppressive. At lower cell multiplicites the cortisone sensitive cells are augmentative. Similar bidirectional effects can be seen in the DNA synthetic response to SRBC when the cell multiplicites are held constant and the antigen dose is varied. Another interesting form of regulatory interaction between T cell populations is seen in GVH reactions performed in lethally irradiated mice. We have found that the activity of the responding inoculimi of parental T cells is regulated by host (Fi) T cells (Gershon et al. 1974). When the activity of the parental cells is high, the addition of Fi T cells to the irradiated host suppresses the response. The addition of the same Fi population to host in which the parental T cells are responding less well leads to augmentation of the response. These studies are particularly interesting because the Fi T cells seem to be regulating a response in which they are not directly participating. By

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that I mean that they are acting in a situation where there is no antigen for them to respond to, since they are incapable of immunological recognition of the parental T cell and they are syngeneic with the host. For several reasons it is highly unlikely that they are efiEecting their regulation by serving as a source of antigen to the parental T cells. One is that the GVH reactions are taking place in a sea of host antigens and the added small numbers of parental thymocytes would represent only a minor fraction of the antigenic stimulus. Furthermore, it has been seen that thymocytes do not carry significant amoxmts of stimulatory antigens for GVH type reactions (Simpson, in press). It therefore seems most likely that the Fi T cells regulate the response by recognizing feed-back signals from the responding parental cells and in turn making products wliich regulate that response. Since products of a GVH reaction can cause Fi T cells to synthesize DNA (Harrison & Paul 1973, Gershon et al. 1974) it shoxild not be surprising that the Fi T cells can also make products which affect the response of the cells which are turning them on. B) Interactions between non-immune T cells and immune T or B cells Celada (1966) has shown that the adoptive transfer of immune cells to normal adult mice often fails to confer immunity upon the recipients, although the same immune cells are capable of conferring high levels of adoptive immunity when transferred to newborn or irradiated mice. This finding has recently been confirmed and extended (Bell & Shand 1975). The usual type of explanation offered for these findings is that the adoptively transferred cells fail to localize in the appropriate lymphoid tissue of the host because there is not enough room for them. However, studies with marked T cells have shown that large numbers of adoptively transferred cells join the circulation of the host and come to rest in the host lymphoid tissues (Lance & Taub 1969, Bell & Shand 1975), where they continue to react for the lifetime of the host (A. J. S. Davies, personal communication). Since lymphoid organs draining antigen can increase their size two to three fold within 24-48 hours of antigenic stimulation by means of recruitment of cells from the circulation rather than by proliferation (Taub & Gershon 1972), it would seem that the only limit to the number of injected cells which can join the host lymphoid tissue would be the nximber which can pass through the lungs without causing embolization. Therefore, an alternative explanation to the 'Celada phenomenon' must be sought. We have found that when 5 X 10' carrier immime T cells are put into a normal host and the host is then immunized with DNP attached to that carrier, the anti-hapten response of the recipient is significantly less than that of controls (Eardley & Gershon, in press). This is a

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direct confirmation of similar experiments performed by Tada & Takemori (1974). However, the same T cells which suppress the response of normal animals cooperate very nicely with normal B cells in irradiated hosts. The adoptive inunime response in the irradiated host can be markedly suppressed by the addition of as few as 1 X 10' normal T cells. Addition of normal B cells has no such effect. Thus, this is an example where immune and normal T cells, both of which act as helpers on their own, generate high degrees of suppression when intermixed. Relating this to the feedback model we have been discussing, it would seem that the normal T cells recognize the feedback signals from the immune T cells and, as a consequence, react to the antigen in a highly suppressive manner. We have demonstrated similar interactions between immime B cells and normal T cells, both in vivo and in vitro, using sheep RBC as antigen (Gershon et al. 1974). We foimd that immune B cells and immune T cells could cooperate quite nicely; but when non-immune T cells were added to the reactions the specific response was almost totally suppressed. On the other hand, non-immune T cells had no effect on mixtures of immune T cells and non-immune B cells. We offer the following explanation for these results. The higher the level of activity of immune cells, the more feedback products they make. These products are recognized by regulatory T cells which can either suppress or augment the immime response. When the level of activity of the regulatee is high, it is likely that the regulator cell will act as a suppressor. The fact that normal and immune T cells behave differently in this regulatory function suggests that antigen produces a differentiational event in T cells which causes them to be more rresistant to suppression and/or causes them to accept a higher level of feedback signals before producing suppression. This is probably why it is extremely difficult to overcome tolerance by the adoptive transfer of normal cells, which can be done quite efficaciously when preimmunized cells are used (Billingham et al. 1956, Mitchison 1971). There is additional evidence that immime T cells are qualitatively different than normal T cells in their differential reading of feedback signals. For example, an actively immunized mouse with a sheep RBC antibody titer of Iog2 7 will exhibit a good T cell mitotic response when challenged with an appropriate dose of antigen (Kriiger & Gershon 1974). A normal mouse with a passively acquired antibody titer of 7 will have no mitotic T cell response to the same dose of antigen (Davies 1969). Thus, it is clear that immune T cells are better than normal T cells at responding to antigen in the face of large amounts of feedback product. It would seem that presentation of too much feedback product to T cells before they have a chance to differentiate and set the 'thymostaf (Gershon 1974b) higher is an excellent method for activating suppressor cells. This mode of suppressor T cell generation is probably an

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aberrant or exaggerated form of normal regulation produced by manipulations which alter the normal ratios of regulators and regulatees, and thus upset the system. Based on this evidence one might modify the answer to Question I above and say that too much, too soon is as bad for the immime response as it was for Dianna Barrymore (1957). C) Speculations on how modification of feedback interactions may affect attempts to demonstrate suppressor T cell activity Antigen or mitogen stimulated T cells release, or cause to be released, substances which have non-specific immunosuppressive effects (reviewed in Gershon 1974a, in press). Addition of normal, or sometimes even immune, cells to suppressed animals fails to overcome the anergic state, which indicates that the added cells are themselves 'infected' by the suppressive milieu in the antigen stimulated host. Limitation of space for the adoptively transferred cells to react in is an unlikely explanation for these findings, as discussed above (see II-B). In addition, since lethal irradiation of the host does not prevent suppression of the adoptively transferred cells, space limitation arguments are ruled out directly because the adoptively transferred cells can reconstitute the response of lethally irradiated non-suppressed animals. The ease with which suppression can be demonstrated by transferring normal cells into suppressed animals contrasts with the difficulty in demonstrating the suppression by transferring cells out. In fact, several workers have noted that cells transferred out of suppressed animals into either tissue culture or normal animals respond as well as, or sometimes even better than, normal cells. I will describe two experiments which illustrate this phenomenon particularly well. Waterston (1970) showed that 4 days after immunization of mice with pig RBC there was a marked impairment of the response to sheep RBC, which would not be overcome by lethal irradiation and reconstitution with normal cells. However, cells taken from the pig RBC treated mice, which failed to respond to sheep RBC in the intact animal, gave an augmented response to sheep RBC when put into Mishell-Dutton culture. CMler & Weigle (1973) noted tiiat 86 days after induction of tolerance to human gammaglobulin, spleen cells of tolerant mice were able to cooperate quite nicely with normal thymocytes, cortisone resistant thymocytes, or normal spleen cells, when the two populations were admixed and transferred into lethally irradiated recipients. They interpreted these results to indicate that there was no B cell tolerance in the tolerant mice at the time of transfer. They also suggested that there were no suppressor cells involved because the tolerant cells did not diminish the adoptive response of normal cells. How-

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ever, their attempts to terminate the unresponsive state of the tolerant spleen cell donors, by inoculating them with normal thymocytes, cortisone resistant thymocytes, or 100 X 10* normal spleen cells, failed to overcome the imresponsive state. If one accepts the arguments presented above, that there is adequate space in which the adoptively transferred cells can lodge and respond, this then becomes another example in which the adoptive transfer of cells into an animal suppresses those cells, while cells transferred out of the suppressed animal fail to exhibit any suppressive characteristics. This interpretation is supported by the findings of Benjamin (1975) who, by using a more sensitive assay for suppression, has found specific suppressor cells to be present in HGG tolerant mice at a time when the B cell compartment has recovered immunocompetence but when the T cell compartment is still tolerant. Numerous explanations can be put forth to explain these apparent paradoxes. The one I would like to offer for consideration (and hopefully for experimental verification) is that when T cells are removed from the environment in which the feedback induction of suppression is occurring, they rapidly lose suppressor characteristics. In this regard, it is worth emphasizing that in most studies in which dominant or 'infectious' tolerance has been shown by admixture of tolerant cells with normal cells in neutral recipients, the T dependence of the phenomenon has been shown by removal of T cells with the use of an anti-0 serum. Few, if any, of these studies have demonstrated the 'infectious' tolerance by iising isolated, purified, peripheral T cells. Although we have been able to show that purified T cells can act as suppressors on their own, they seem to be far less suppressive on a per cell basis than when they are mixed together with theta insensitive cells (Eardley & Gershon, in press and unpublished). Basten (personal commimication) has made similar findings. Interactions between suppressors and their inducers can also help explain the point I raised earlier; namely that suppression produced by adoptively transferred cells often becomes help when the transferred cells are sufficiently diluted. The dilution of the level of feedback products would remove the signal which is activating the suppressor T cells. This explanation is inherently more logical than arguments which state that suppression turns into help because there is competition between helper and suppressor T cells and the suppressor cells get diluted out first. No matter how much the cells were diluted, the ratio of suppressor to helper would remain the same. Without some ad hoc assumption, there is no reason to expect that one could dilute out suppression without diluting out help. On the other hand it is easy to see why reduction in helper cells would reduce feedback signals required for suppressor T cell activation. We have presented a large number of examples

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where this type of situation obtains (Gershon, 1974a, b, in press; Gershon et al. 1974). In addition to our published data, I would like to mention a recent result which is an exceptionally good example of the requirement for feedback signals for generation of suppressor cells. In these experiments we (H. Durkin & A. Schwartz) were trying to enhance the growth rate of the Lewis Limg tumor by the adoptive transfer of suppressor cells. We found that the adoptive transfer of spleen cells from mice in which we hoped to find suppressor T cell activity, had no effect on tumor growth rate in the recipients. However, when the adoptively transferred cells were prefractionated on discontinuous BSA gradients we found that a fraction in the middle density (c) band almost completely prevented tumor growth in the recipients. When the c band cells were recombined with the other fractions of the gradient, the adoptive immune response disappeared. Interestingly, none of the other fractions, when transferred by themselves, had any effect whatsoever on the tumor growth in the recipients. Thus, there clearly was a poptilation of cells present in the spleens which could suppress the activity of the c band cells, but which had no suppressor activity in the absence of c band cells, suggesting that suppressor activity was not inherently present but was only induced when the active c band cells were present. Another point in line with the notion that feedback signals are important in the induction of suppression is the observation that most forms of tolerance are preceded by a transient state of immimity (Dresser & Mitchison 1968). Many examples of 'infectious' suppression exhibit similsir kinetics in that the suppressed response proceeds at a normal rate for the first few days, and then abruptiy shuts off, while the non-suppressed response continues apace (Gershon et al. 1974, Rich & Pierce 1974, Baker et al. 1974). The start of the response may be the signal which activates suppressor activity. m . CAN SUPPRESSOR SIGNALS BE COUNTERMANDED?

In our original studies on the thymus dependence of the induction of immunological tolerance (Gershon & Kondo 1970), we found that thymus deprived mice which were reconstituted with a small number of thymocytes were rendered tolerant by the inoculation of a large dose of sheep RBC. The tolerant state of these mice could not be overcome by reinoculation of fresh thymocytes, although we were able to show in control animals that (a) the remaining antigen dose was not adequate to render the freshly inoculated cells tolerant, and (b) the freshly inoculated cells lodged and were able to ftmction in the host immune tissue. Further, transfer studies showed that the B cells of the tolerant mice were quite capable of responding to antigen when Transplant. Rev. (1975), Vol. 26

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the theta sensitive cells were removed, and that the presence of the theta sensitive cells caused normal cells to become specifically unresponsive in neutral recipients (Gershon & Kondo 1971, Gershon 1974c). Thxis, the reason the normal thymocytes could not overcome the tolerance of the recipients was because they themselves were being rendered tolerant by a thymus dependent 'infectious' agent. However, when we transferred fresh thymocytes into our tolerant recipients and then immunized them with horse RBC (instead of sheep RBC), we found that the recipients made anti-sheep RBC responses which were similar to non-tolerant controls (Gershon & Kondo 1972, Gershon, 1974c). The anti-sheep RBC antibodies had no demonstrable affinity for horse RBC; five absorptions with horse RBC did not lower the anti-sheep RBC titer, although one absorption removed all the anti-horse RBC activity from the serum. We also found, in transfer experiments, that in order for the horse RBC to prevent the 'tolerance' from infecting the inoculated thymocytes, sheep RBC antigen to be present. In addition, fresh thymocytes had to be added for the horse RBC to yield an anti-sheep RBC response (i.e.: the horse RBC could not break an established state of tolerance but could only prevent the suppression of the newly added cells). These experiments can be interpreted as follows: normal thymocytes inoculated into sheep RBC tolerant mice are tolerized by the action of suppressor T cells. If, however, they are non-specifically activated, they make substances which render them resistant to the 'infectious tolerance' inducing agents. They then react with the residual sheep RBC in the tolerant mice and activate the non-tolerant B cells which are present, causing them to make anti-sheep RBC antibody. More recently we have repeated these findings using a diSerent system and have foimd that the signal which prevents the induction of 'infectious tolerance' in T cells inoculated into suppressed mice can be totally non-specific and does not require the close relationship that sheep and horse RBC have (Mitchell & Gershon, to be published). McCullagh (1970) has also found that non-specific activation of T cells inoculated into tolerant rats can prevent them from becoming suppressed and thus aUow them to break the tolerant state. (Besides the direct bearing these studies have on signals which countermand suppression, they add further evidence that space limitation is not the reason why normal cells cannot overcome tolerance, since it is quite clear that the normal cells can function as long as a signal is given which prevents the infectious tolerance from affecting them; this is because the non-specific signals fail to produce any specific activation unless normal cells are added to the tolerant or suppressed mice.) These findings may be extended, without too much mental gymnastics, to less experimentally contrived situations. In fact, they give a 'raison d'etre' for antigenic competition. If the immune response yields non-specific suppres-

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sion, there must be some non-specific augmentory signals which are being regxilated. We suggest that the non-specific augmentation overcomes the suppressor signals or renders them inoperative. The ability to produce this nonspecific augmentation, or 'contrasuppression,' is what Dresser (1962) has referred to as 'inherent adjuvanticity.' This notion is supported by the good correlation there is between the immimogenicity of substances and their ability to produce antigenic competition, if one considers antigenic competition to be the negative refiection of non-specific help. Of course external adjuvants should operate similarly. Thus, one key to producing tolerance is to avoid non-specific stimulation. A particularly good candidate for this type of tolerance inducer is deaggregated gamma globulin, which seems to lack 'inherent adjuvanticity' and is a poor inducer of the antigenic competition which is produced at rather high levels by the aggregated form of that antigen (Gershon et al. 1974). This subject and its ramifications have been discussed at further length elsewhere (Gershon 1974c). IV. CAN B CELLS BE MADE TOLERAMT WITHOUT T CELL HELP?

The answer to this question is a qualified 'yes,' although Kondo and I have often been quoted as having said 'this could not be done.' What we said was, 'those antibody-making cells that could produce antibody without assistance from T cells were made tolerant and the addition of T cells could not restore their activity' (Gershon & Kondo 1970). We also noted 'that thymus dependent B cells caimot be affected by antigen pretreatment in the absence of T cells.' Thus, we distinguished two subpopulations of B cells, one of which could make antibody without T cell help and could also be made tolerant in the absence of T cells. Another population of B cells made no response whatsoever, either to become tolerant or to make antibody, in the absence of T cells. We subsequently learned that the thymus dependent B cells did not become tolerant even when T cells were present, as we found the site of action of the suppressor T cells to be other T cells and not to be B cells (Gershon & Kondo 1972). Most other workers who have examined this question have made similar conclusions. Other workers who have produced B cell tolerance in the absence of T cell help have done so only imder special conditions. For example, tolerance induction in the B cells of nude mice with thymus dependent antigens is easily accomplished (Mitchell 1974, Schrader 1975a) while it is difficult or even impossible to accomplish this in splenic B cells of thymus deprived mice. There are several possible explanations for this discrepancy. One that has been championed by the Australian school is that thymus deprived mice contain some residual T cells (which they surely do), which act to prevent toler-

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ance induction (Mitchell 1974, Schrader 1975b). This explanation is xmlikely for the following reasons: we gave thymus deprived mice up to 13 injections of 2 X 10' sheep RBC before reconstituting them with T cells and produced no tolerance whatsoever in the thymus dependent B cells (Gershon & Kondo 1970). On the other hand, Mitchell produced B cell tolerance with one injection in nude mice. Stirely the first twelve injections of sheep RBC wotild have made the residual T cells of our thymus deprived mice tolerant and thus the basis for the difference in the results would appear to reside in differences in the B cell populations. It wotild seem that tolerance induction is easily produced in immature B cells £ind becomes more difficult as the B cells mature (Nossal & Pike 1975). We suggest that the B cells of nude mice are less mattire than those of mice with a thymus and have recently simmiarized the evidence which supports this notion (Gershon, in press). Other workers have shown that some special antigens like TNP conjugated to a polymer of dextrorotary amino acids (DNP-DGL) (Katz 1974) or conjugated to motise immunoglobulin (Borel & Aldo-Benson 1974) or mouse RBC (Hamilton et al. 1974) and when injected into mice, seem to be able to induce direct B cell tolerance qtiite easily; these antigens are extremely weak immtmogens. The mechanism by which these antigens produce B cell tolerance is obscure, but a recent study has shown that B cell tolerance carmot be induced in a motise strain (CBA/N) which is incapable of making an antibody response to thymtis independent antigens (Cohen et al., in press). The authors suggested that B cell tolerance requires cells capable of actively responding to the tolerogen, which is the same conclusion we arrived at in our original studies (Gershon & Kondo 1970). It is clear that further evidence is required before a clear picture can emerge on how B cells are made tolerant. I think it fair to say, in light of available evidence, that there are subpopulations of thymus dependent B cells which caimot be rendered directly tolerant by antigens without some help from either T cells or other B cells. Qearly there are other subpopulations which are less mattire and are able to be rendered directly tolerant by antigen. V. WHAT IS THE RELATIONSHIP BETWEEN THE SUPPRESSION INDUCED BY EXOGENOUSLY ADMINISTERED ANTIGENS TO THE SUPPRESSION OF INTRINSIC IMMUNE RESPONSIVENESS SUCH AS CHRONIC ALLOTYPE UPPRESSION AND INFECTIOUS' AGAMMAGLOBULINENEMIA?

There are no data that I know of that bear directly on this question, but since I feel it is of some importance I offer my inttiitive feelings on this subject which (and this is the usual rationalization for over-exercizing one's imagina-

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tion) may help in designing experiments to test the hypothesis. The answer to the question is 'not much.' It seems to me that this type of suppression has the same basis as does the failure of histoincompatible T and B cells to cooperate. That is, when T cells recognize certain antigens on B cells, they may suppress the function of those B cells. This suppression, just like its mirror image the allogeneic helper effect (Katz 1972), probably has very little bystander effect. Therefore I think that chronic allotype suppression (Herzenberg & Herzenberg 1974) is caused by the removal of the allotype from the circulation by the acute (antibody) suppression, which results in a loss of T cell tolerance to that antigen, and when the acute suppression wanes the T cells recognize the allotype as foreign. They then react against cells bearing that allotype just as they would in a GVH reaction, resulting in suppression of the B cells which they are reacting against. The histology of the lymphoid tissue in mice with chronic allotype suppression is highly reminiscent of a chronic GVH reaction (Gershon, unpublished observations). I would guess that infectious agammaglobulinenemia (Blaese 1974) has the same basis; that is there is a temporary condition which removes B cell differentiation antigens causing loss of T cell self-tolerance. When the B cells try to return and express their differentiation antigens the T cells recognize them as foreign and suppress them. It should be remembered that a chronic GVH is an excellent way to produce hypo or even agammaglobulinenemia (Byfield et al. 1973). I would suspect the same mechanism would be responsible for T cell dependent suppression of idiotypes (Eichmann, 1974; Bangasser & Nisonoff in press). VI. I've saved the easiest question to answer for last. That is, what is the mechanism by which the suppressor T cell effects its suppression? The answer, of course, is the exact obverse of the one by which the helper T cell produces help. SUMMARY

The main points that I have put forth are that: (1) suppressor T cell activity cannot be explained as simply being too much help; (2) feedback signals from target cells are of crucial importance in determining and maintaining the acti'vdty of suppressor T cells; (3) whenever T cells are triggered by antigen, suppression occurs. Immune responses only occur when countermanding signals are also generated. Both intrinsic and extrinsic adjuvanticity is the operational production of countermanding signals; (4) memory T cells are qualitatively different from normal T cells in their sensitivity to feedback signals and also in their susceptibility to suppression; (5) mature thymus dependent B cells cannot be rendered tolerant by the direct action of antigen, while immature and thymus independent B cells can; (6) the mechanism of suppres-

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sion induced by exogenously administered antigens and that by normal differentiation products (i.e.: GVH; allotypes), is different; (7) generation of suppressor cells requires or results from complex interactions between subpopulations of cells, making it impossible under present conditions to determine which cell is doing what and to which; (8) further work is required before a full understanding of the importance, mechanism of action and other aspects of suppressor T cell function can be fully understood. ACKNOWLEDGMENTS My research was supported by U.S.P.H.S. Grants CA-08593 and AI-10497 and a contract, CB-43994, from the National Cancer Institute. I am deeply indebted to my colleagues, students and technical assistants who participated in these studies and particularly to Kazunari Kondo, who stands behind the accuracy of the data but who absolves himself from all blame and contumely which may result from the interpretations thereof. REFERENCES Askenase, P. W., Hayden, B. J. & Gershon, R. K. (1975) Augmentation of delayed-type hypersensitivity by doses of cyclophosphamide which do not a&ect antibody responses. / . exp. Med. 141, 697. Asofsky, R., Cantor, H. & Tigelaar, R. (1971) Cell interactions in the graft- versus hostresponse. In Progress in Immunology, ed. Amos, B., p. 369. Academic Press, New York. Baker, P. K., Prescott,B., Stashak,P.W. & Amsbaugh, D. F. (1974) Regulation of the antibody response to type m pneumococcal polysaccharide by thymic-derived cells. In: The Immune System: Genes, Receptors, Signals, ed. Sercarz, E., Williamson, A. & Fox, C. F., p. 415. Academic Press, New York. Baker, P. J., Stashak, P. W., Amsbaugh, D. F., Prescott, B. & Barth, R. F. (1970) Evidence for the existence of two functionally distinct types of cells which regulate the antibody response to type m pneumococcal polysaccharide. / . Immunol. 105, 1581. Bangasser, S. B. & NisonofE, A. (in press) Immunologic suppression of idiotypic spedfcities. Transplant. Rev. 27. Barrymore, D. & Frank, J. (1957) Too much too soon. Holt, Rheinhart & Winston, New York, New York. Basten, A., Miller, J. F. A. P., Sprent, J. & Cheers, C. (1974) Cell-to-cell interaction in the immune response. X. T-cell dependent suppression in tolerant mice. / . exp. Med. 140, 199. Bell, E. B. c& Shand, F. L. (1975) Changes in lymphocyte redrculation and liberation of the adoptive memory response from cellular regulation in irradiated recipients. Europ. J. Immunol. S, 1. Benjamin, D. C. (1975) Evidence for specific suppression in the maintenance of immunological tolerance. /. exp. Med. 141, 635. Billingham, R. E., Brent, L. & Medawar, P. B. (1956) Quantitative studies on tissue trans-

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