Suppression of antitumor immunity by macrophages in spleens of mice bearing a large MOPC-315 tumor

We had shown previously that progression of MOPC-315 plasmacytoma growth is associated with an increase in the percentage of macrophages in the spleen as well as a decrease in the ability of tumor-bearer spleen cells to mount an antitumor cytotoxic response upon in vitro immunization. Here we provide evidence that macrophages in the MOPC-315 tumor-bearer spleen are responsible at least in part for the suppression of the generation of antitumor cytotoxicity. Accordingly, removal of most macrophages by depletion of phagocytic cells or Sephadex G-10-adherent cells from spleens of mice bearing a large tumor resulted in augmented antitumor immune potential. Also, Sephadex G-10-adherent spleen cells from tumor-bearing (but not normal) mice drastically suppressed the in vitro generation of antitumor cytotoxicity by normal spleen cells. The suppressive activity of these adherent cells did not reside in contaminating suppressor T cells, since it was not reduced by treatment with monoclonal anti-Thy 1.2 antibody plus complement. The Sephadex G-10-adherent cell population from the tumor-bearer spleen suppressed the in vitro generation of antitumor cytotoxicity against autochthonous tumor cells but not against allogeneic EL4 tumor cells, and hence the suppression was apparently specific. The suppressive activity of the Sephadex G-10-adherent cell population from tumor-bearer spleens was overcome by treatment of the tumor-bearing mice with a low curative dose of cyclophosphamide. This immunomodulatory effect of a low dose of the drug in overcoming the suppression mediated by the Sephadex G-10-adherent cell population enables the effector arm of the immune system of tumor-bearing mice to cooperate effectively with the drug's tumoricidal activity in tumor eradication.     

Suppression of antitumor immunity by macrophages in spleens of mice bearing a large MOPC-315 tumor

Summary We had shown previously that progression of MOPC-315 plasmacytoma growth is associated with an increase in the percentage of macrophages in the spleen as well as a decrease in the ability of tumor-bearer spleen cells to mount an antitumor cytotoxic response upon in vitro immunization. Here we provide evidence that macrophages in the MOPC-315 tumor-bearer spleen are responsible at least in part for the suppression of the generation of antitumor cytotoxicity. Accordingly, removal of most macrophages by depletion of phagocytic cells or Sephadex G-10-adherent cells from spleens of mice bearing a large tumor resulted in augmented antitumor immune potential. Also, Sephadex G-10-adherent spleen cells from tumor-bearing (but not normal) mice drastically suppressed the in vitro generation of antitumor cytotoxicity by normal spleen cells. The suppressive activity of these adherent cells did not reside in contaminating suppressor T cells, since it was not reduced by treatment with monoclonal anti-Thy 1.2 antibody plus complement. The Sephadex G-10-adherent cell population from the tumor-bearer spleen suppressed the in vitro generation of antitumor cytotoxicity against autochthonous tumor cells but not against allogeneic EL4 tumor cells, and hence the suppression was apparently specific. The suppressive activity of the Sephadex G-10-adherent cell population from tumor-bearer spleens was overcome by treatment of the tumor-bearing mice with a low curative dose of cyclophosphamide. This immunomodulatory effect of a low dose of the drug in overcoming the suppression mediated by the Sephadex G-10-adherent cell population enables the effector arm of the immune system of tumor-bearing mice to cooperate effectively with the drug's tumoricidal activity in tumor eradication.This paper was presented in part at the annual meeting of the American Association of Immunologists, Chicago, Illinois, 10–15 April 1983     

Suppression of Con A mitogen-induced proliferation of normal spleen cells by macrophages from chickens with hereditary muscular dystrophy

Spleen cells from chickens with hereditary muscular dystrophy (MD) give low blastogenic responses to the T cell mitogen concanavalin A (Con A) while exhibiting normal mitogen stimulated blastogenic responses to the T cell mitogen phytohemagglutinin (PHA). The addition of MD spleen cells to normal spleen cells caused a marked suppression of the Con A response of the normal cells while not affecting the PHA response of the normal cells. The suppressive activity by the MD spleen cells requires viable cells and is contact mediated. The suppressive activity is attributed to the presence in MD spleens of a population of suppressor cells with characteristics typical of macrophages. The suppressor cell activity was not removable by complement-mediated lysis using anti-T or anti-B sera, but it was reversible by treatment with carrageenan or carbonyl iron magnet, by passage through a Sephadex G-10 column, and by adherence to plastic petri dishes or glass beads. MD spleen cells depleted of the suppressor cell population remained unable to respond to Con A.     

Immunosuppression induced by staphylococcal enterotoxin B

Staphylococcal enterotoxin B (SEB) is a potent mitogen for both human and murine T lymphocytes. We report here studies which demonstrate that a suppressor cell population, capable of suppressing the primary immune response of normal syngeneic mouse splenocytes to heterologous sheep erythrocytes (SRBC), is activated by SEB. Enterotoxin concentrations ranging from 0.05 to 5.0 Mg ml^?1 are capable of activating this suppressor cell population, and significant suppression can be detected with relatively small numbers of SEB-primed spleen cells (SEB-PSC) in culture. Elimination of macrophages before or after priming splenocytes with SEB does not reduce the suppression of plaque-forming cell (PFC) responses when SEB-PSC are added to normal cells in Mishell-Dutton cultures. Treatment of cells with anti-Thy-1 serum plus complement, after priming with SEB, effectively eliminates the activity of the suppressor cell population. Enrichment for T cells before priming with SEB results in greater suppression of PFC responses than do SEB-PSC generated in cultures of nonfractionated splenocytes. Activation of suppressor cells with SEB in vitro appears to occur through the induction of the T-cell subpopulation expressing the Lyt-1^?,2⁺,3⁺ cell surface phenotype, since selective depletion of this T-cell subpopulation with monoclonal rat anti-mouse Lyt-2 antisera after priming cells with SEB virtually eliminates the suppressor activity.     

Stimulation of suppressor cells in the bone marrow and spleens of high dose cyclophosphamide-treated C57Bl/6 mice

Systemic administration of a single dose (300 mg/kg) of cyclophosphamide (Cy) induced the appearance of a population of suppressor cells in the bone marrow and spleens of mice. Suppressor cells were assayed by their capacity to inhibit the concanavalin A (Con A) blastogenesis or the mixed-lymphocyte response of normal C57Bl/6 spleen cells. Cy-induced bone marrow (Cy-BM) suppressor cells were present as early as 4 days following Cy therapy and their activity gradually decreased over the next 2 weeks. Cy-induced splenic (Cy-Sp) suppressor cells were maximally present on Days 6 through 10 following Cy therapy. Studies were performed to characterize the suppressor cells of bone marrow obtained 4 days after Cy treatment and of normal bone marrow (N-BM). Some suppressor activity was present in normal bone marrow. N-BM suppressor cells resembled cells of the monocyte/macrophage lineage in that they were slightly adherent to Sephadex G-10, sensitive to L-leucine methyl ester (LME), and insensitive to treatment either with anti-T-cell antibody and complement or with anti-immunoglobulin antibody and complement. Their suppressive activity was abrogated by incubation with either indomethacin or catalase. Cy-BM suppressor cells were also resistant to treatment with anti-T-cell and anti-immunoglobulin antibody and complement but were not adherent to Sephadex G-10 and not sensitive to LME. Their suppressive activity was partially eliminated by indomethacin alone or in combination with catalase. We conclude that Cy chemotherapy induces the appearance of a population of immune suppressive cells and that these cells appear first in the bone marrow and subsequently in the spleen.     

Inhibition of antibody production from the plasmacytoma cell line MOPC-315 by staphylococcal enterotoxin B-induced T-suppressor cells

Our previous results have shown that staphylococcal enterotoxin B (SEB) induces a population(s) of T cells which has the capacity to suppress the antibody response of splenocytes in vitro. In the present report we have attempted to investigate the effect of SEB-primed cells on the secretion of antibody by the plasmacytoma cell line MOPC-315. We have found that the secretion of antibody by MOPC-315 is significantly reduced in as little as 24 hr of coculture with the suppressor cells. The suppressive activity is not antigen- or isotype-specific, since the antibody secretion by both MPC-11 and HOPC1 plasmacytomas are also inhibited by the SEB-primed cells. In addition, we have found that the SEB-primed cell population which inhibits the antibody production by the MOPC-315 cell line expresses the Lyt-1+,2- and Thy-1+ cell surface markers. The apparent relationship between the SEB-primed suppressor cell population and the population which inhibits a conventional antibody response is discussed.     

Immunosuppressive activity of staphylococcal enterotoxin B. II. Activation of suppressor-effector cells by a staphylococcal enterotoxin B-induced suppressor factor

The capacity of staphylococcal enterotoxins to stimulate all T cells bearing certain T cell receptors has recently generated a great deal of interest. These toxins are believed to bind directly both to the TCR:CD4 complex via its V beta domains and to class II MHC molecules on accessory cells prior to T cell activation. Previous studies from this laboratory have demonstrated that staphylococcal enterotoxin B (SEB) is capable of inducing multiple T suppressor cell populations which can inhibit in vitro antibody responses. Additional studies have demonstrated that the suppressive activity of these cells is mediated, at least in part, by an I-J-restricted suppressor factor. Efforts to characterize the inhibitory activity of this factor have demonstrated that the suppressive element is capable of activating both early and late acting suppressor cell populations in vitro. Analysis by both positive and negative selection shows that cells bearing the Lyt1-2+ surface marker phenotype are active early, whereas Lyt1+2+ cells are active both early and late in the antibody response. Additional experiments using various strains of mice as sources of suppressor factor and of naive splenocyte populations have demonstrated that activation of suppressor-effector cells by this suppressor factor is restricted at the I-J, but not Igh, gene locus. These studies suggest that this SEB-induced suppressor factor alone provides the signals necessary for the induction and activation of suppressor-effector cell activity.     

Immunosuppressive factor(s) extracted from lymphoid cells of nonresponder mice primed with L-glutamic acid60-L-alanine30-L-tyrosine10 (GAT).

The synthetic terpolymer of L-glutamic acid60-L-alanine30-L-tyrosine10 (GAT) not only fails to elicit a GAT-specific antibody response in nonresponder mice, but also prior injection of GAT specifically decreases the ability of nonresponder mice to develop a GAT-specific antibody response to a subsequent challenge with GAT-MBSA. This inhibition is mediated by GAT-specific suppressor T cells. Further, a suppressive factor can be extracted from lymphoid cells of GAT-primed nonresponder mice that inhibits the development of primary GAT-specific antibody responses to GAT-MBSA and to GAT-PRBC- by normal syngeneic mice. The suppressive activity is dose-dependent and absorbed by GAT-Sepharose, but not by BSA-Sepharose. The suppressive activity elutes from a G-100 Sephadex column in the same fraction as ovalbumin, suggesting its m.w. is approximately 45,000 daltons.     

The role of adherent accessory cells in the generation of effector suppressor T cells

The role of accessory cell populations in the generation of effector suppressor (Ts3) cells was studied. By using an in vitro culture system, it was previously determined that the induction of NP-specific effector suppressor activity requires T cells, antigen, and an anti-idiotypic B cell population. We now demonstrate that the generation of Ts3 cells in this system also requires accessory cells. The accessory population appears to play a role in the processing and presentation of antigen. These antigen-presenting accessory cells are required early in the induction phase of Ts3 generation. These accessory cells can present NP coupled to immunogenic or non-immunogenic polypeptide carriers, including polymers of L-amino acids. However, NP coupled to polymers of poorly metabolized D-amino acids fail to induce suppressor T cell generation. Furthermore, the data demonstrate that an H-2 homology must exist between the Ts3 precursors and the antigen-presenting cell population if suppressor activity is to be generated. We also characterize the differential genetic restrictions that govern the induction of Ts3 cells that control suppression of either T cell or B cell responses. The data suggest that although I-J region encoded gene products control the induction and effector phases of suppressor cell activity as measured on T cell responses, the suppression of B cell responses appear to be controlled by I-A gene products. Possible cellular mechanisms that might explain these findings are discussed.     

Characterization and genetic restriction of suppressor-effector cells induced by staphylococcal enterotoxin B

The capacity of the staphylococcal enterotoxins to stimulate all T cells bearing certain (but not all) TCR has generated a great deal of interest. This stimulation appears to involve specific binding of the toxin to class II Ags and subsequent stimulation via the TCR. Previous studies from this laboratory have demonstrated that staphylococcal enterotoxin B (SEB) induces multiple T suppressor cell populations that inhibit both primary and secondary plaque-forming cell responses. Efforts to characterize these suppressor cell populations have demonstrated that the suppressor population active early in the antibody response expresses the Lyt-1-2+ cell surface phenotype, whereas depletion analysis suggests that the population active late in an ongoing response bears the Lyt-1+2+ cell-surface markers. In the present study, enrichment for this late acting effector population with the use of sequential panning with anti-Lyt mAb reveals significant suppressive activity at both the initiation and effector phases of a 5-day Mishell-Dutton coculture. Additional experiments using I-J disparate strains of mice have demonstrated a genetic restriction at the "I-J" gene locus between the cells mediating SEB-induced suppression and their target. Depletion of SEB-primed splenocytes with anti-I-J mAb suggests that both the early and late effector cells bear I-J molecules on their surface. Taken together, these results show that SEB induces suppressor cell populations with properties similar to those exhibited by Ag-specific cell circuits.     

Alloantigen-specific T suppressor-inducer and T suppressor-effector cells can be activated despite blocking the IL-2 receptor

To determine IL-2 requirement for activation of suppressor cells, PBMC were primed in one-way MLR in the presence of 10 micrograms/ml anti-IL-2R beta-chain antibody 2A3 (CD25) or control antibody, then irradiated and added as regulators in a fresh MLR. Cells primed in the presence of antibody 2A3 suppressed the proliferative response to fresh autologous lymphocytes to specific alloantigen but had no effect on the response to cells from third party donors. Priming in the presence of an antibody of irrelevant specificity induced only limited suppressor activity. Activated suppressor cells did not show cytolytic activity specific for the stimulators when tested at the time of the suppressor cell assay. To identify the subset(s) responsible for suppression, cells primed in the presence of antibody 2A3 were separated into CD4+/CD45RA+, CD4+/CD45RA-, and CD8+ subsets, which were irradiated and then tested. The suppressive activity was found predominantly in the CD4+/CD45RA+ subset, whereas CD8+ cells had some activity and CD4+/CD45RA- cells had none. No subset suppressed the response of autologous cells to third-party cells. When primed CD4+/CD45RA+ cells were cocultured with fresh autologous lymphocytes depleted of CD8+ cells, no suppression was observed, indicating that, although the CD4+/CD45RA+ cells can function as inducers of suppressors, they cannot function as suppressor-effectors. Conversely, CD8+ cells activated in MLR in the presence of 2A3 caused suppression, regardless of whether the fresh autologous responder population contained CD8+ cells. CD4+/CD45RA+ and CD8+ subsets isolated after priming in the presence of 2A3 also demonstrated Ag-specific suppression in the generation of cytotoxic T lymphocytes whereas CD4+/CD45RA- cells had no activity. Our data are consistent with the model that suppression of alloreactivity requires the cooperation of two types of cells, a CD4+/CD45RA+ suppressor-inducer and a CD8+ suppressor-effector population. Activated Tsi and fresh Tse or activated Tse alone can suppress lymphocyte proliferation and generation of CTL in response to specific Ag. Activation of Ag-specific T suppressor-inducer and T suppressor-effector cells appears to be relatively IL-2 independent and presumably require one or more other growth factors.     

Novel mechanisms of specific suppression of anti-hapten antibody response mediated by monoclonal anti-carrier antibody

The cellular mechanisms of the antibody-induced suppression of immune responses were analyzed in the keyhole limpet hemocyanin (KLH) system. Some of the monoclonal anti-KLH antibodies, like KLH-specific suppressor T cell factor (KLH-TsF), were demonstrated to suppress the anti-2,4-dinitrophenyl IgG but not IgM plaque-forming cell responses in a KLH-specific and H-2-restricted manner. The anti-KLH antibodies with suppressive activity reacted with, and in turn, stimulated the suppressor hybridoma (34S-281) with the anti-idiotypic receptor complementary to the idiotypic KLH-TsF of the inducer type. Moreover, because the suppressive activity of the anti-KLH antibody was completely abolished by the treatment of responding spleen cells with anti-Lyt-2 and complement, it was apparent that the suppressive antibody activated suppressor T cell pathways. The isotype or affinity of antibodies is not related to the suppressive activity, because suppressive and nonsuppressive antibodies possess a similar affinity belonging to the same Ig isotypes. It also has been demonstrated that the Fc portion is not the functional site, because the F(ab')2 fragment still has the activity. The antibody specificity is found to be important for determining whether the antibody is suppressive or not. In fact, anti-KLH 26, but not other antibodies without activity, recognizes the particular KLH epitope seen by KLH-TsF, and exclusively interacts with the anti-idiotypic suppressor T cells. Thus, the anti-idiotypic suppressor T cell receives signals both from the suppressive anti-KLH antibody and from KLH-TsF, and transmits the antibody-induced suppressor signals to the effector-suppressor pathway. The size of the repertoire of anti-idiotypic suppressor T cells involved in the suppression seems to be very limited, because only four out of 120 monoclonal anti-KLH antibodies were found to have suppressor activity. The possible mechanisms of the cell interaction mediated by the suppressive antibody are discussed.     

Accessory cells reduce lipoprotein suppression of lymphocyte activation

Plasma lipoproteins of d less than or equal to 1.063 g/ml suppress lymphocyte activation triggered in vitro by polyclonal T cell mitogens. The extent of suppression decreases as the number of accessory cells per culture increases. Accessory cells isolated by glass adherence and by counter-flow centrifugation reduce lipoprotein suppression to the same extent. Modulation of lipoprotein suppression by accessory cells is independent of the amount and type of polyclonal activator. Reduction of lipoprotein suppression requires viable accessory cells and that they be present with lymphocytes, mitogen and lipoproteins during the initial 24-h culture period. It is within this same time period that lipoproteins exert their suppressive effect. Accessory cells isolated from a patient with the homozygous form (receptor-defective) of familial hypercholesterolemia also reduce the extent of lipoprotein suppression, suggesting that modulation is not mediated by the classic low density lipoprotein receptor. There appear to be at least two mechanisms by which accessory cells may alter lipoprotein suppression of T lymphocyte activation: by secretion of a soluble factor, probably not interleukin 1, that decreases the extent of suppression and by direct modification of the population of suppressive lipoproteins. Neither mechanism accounts for the lipoprotein-enhanced activation that occurs when cultures contain approximately equal numbers of T lymphocytes and accessory cells.     

In vitro growth of murine T cells. IV. Use of T-cell growth factor to clone lymphoid cells

Murine bone marrow cells can suppress the in vitro primary antibody response of normal spleen cells without apparent cytotoxicity. The bone marrow cells suppress the response to both T-dependent (SRBC) and T-independent (DNP-Ficoll) antigens. When bone marrow cells are fractionated on a sucrose density gradient, the suppressive activity is found in the residue rather than the lymphocyte fraction. The suppressive activity is either unaffected or enhanced by treatment with anti-T- and anti-B-cell serums. Pretreatment of mice with phenylhydrazine which reduces the number of pre-B cells did not reduce the suppressive activity of their bone marrow cells. Suppressive activity is abolished by irradiation of the marrow cells in vitro with 1000 R prior to assay. The activity is present in the marrow of thymus deficient (nude) mice, infant mice, and mice which have been made polycythemic by transfusion. Furthermore, the suppressor cell can phagocytize iron carbonyl particles, is slightly adherent to plastic and Sephadex G-10, and can bind to EA monolayers. We conclude that the suppressor cell is not a mature lymphocyte or granulocyte nor a member of the erythrocytic series, but is likely to be an immature cell possibly of the myeloid series. We speculate on the physiologic role of this cell.     

Lymphocyte function in human bone marrow. I. Characterization of two T cell populations regulating immunoglobulin secretion

We analyzed the regulation of immunoglobulin (Ig) production in short-term cultures of human (rib) bone marrow cells. In contrast to blood or tonsil cell cultures, large quantities of IgG and IgA, but not IgM, were secreted by unstimulated marrow cells. The addition of pokeweed mitogen or phytohemagglutinin resulted in the suppression of this Ig secretion. Both mitogens induced the production of high levels of interleukin 2 (IL 2) in marrow cultures, and the addition of IL 2 alone mimicked the suppressive effect of mitogens. Incubation of marrow cells with Epstein Barr virus resulted in enhanced Ig secretion, primarily of the IgM isotype. The addition of mitogen or IL 2 suppressed Ig production in these cultures as well. The mitogen-induced suppression of Ig secretion in stimulated or unstimulated marrow cultures was inhibited by the monoclonal anti-TAC (IL 2 receptor) antibody. Cell separation experiments indicated that the induction of suppressor activity in marrow cultures involved two distinct populations of marrow-resident T lineage cells. The first population responds to activation by mitogens with the production of IL 2. This population has a surface phenotype appropriate for helper T cells. The second T cell population expresses T8 and TAC determinants. These cells acquire suppressor cell activity after exposure to IL 2. The expression of suppressor function does not require additional (e.g., mitogenic) activation signals. The IL 2-dependent marrow suppressor T cells represent a newly recognized T lymphocyte subset. The regulatory pathway delineated may be important for the regulation of antibody formation in bone marrow, the major site of Ig production in man.     

Induction of CD4 suppressor T cells with anti-Leu-8 antibody

To characterize the conditions under which CD4 T cells suppress polyclonal immunoglobulin synthesis, we investigated the capacity of CD4 T cells that coexpress the surface antigen recognized by the monoclonal antibody anti-Leu-8 to mediate suppression. In an in vitro system devoid of CD8 T cells, CD4, Leu-8+ T cells suppressed pokeweed mitogen-induced immunoglobulin synthesis. Similarly, suppressor function was induced in unfractionated CD4 T cell populations after incubation with anti-Leu-8 antibody under cross-linking conditions. This induction of suppressor function by anti-Leu-8 antibody was not due to expansion of the CD4, Leu-8+ T cell population because CD4 T cells did not proliferate in response to anti-Leu-8 antibody. However, CD4, Leu-8+ T cell-mediated suppression was radiosensitive. Finally, CD4, Leu-8+ T cells do not inhibit immunoglobulin synthesis when T cell lymphokines were used in place of helper CD4 T cells (CD4, Leu-8- T cells), suggesting that CD4 T cell-mediated suppression occurs at the T cell level. We conclude that CD4 T cells can be induced to suppress immunoglobulin synthesis by modulation of the membrane antigen recognized by anti-Leu-8 antibody.     

Immunosuppressive activity of staphylococcal enterotoxin B. I. Characterization of staphylococcal enterotoxin-B-induced suppressor cells.

Staphylococcal enterotoxin B (SEB) binds specifically to major histocompatibility complex class II molecules on the surface of accessory cells and stimulates virtually all T cells bearing certain, but not all, T cell-receptor V beta alleles. We have previously shown that this superantigen is a potent inducer of multiple regulatory T cell populations. In the present report we show that SEB induces a population of suppressor T cells which inhibits the generation of alloantigen-induced cytotoxic T cell activity. Using both negative- and positive-selection analysis, we found that this suppressor population is a CD4- CD8- CD5+ IL-2R+ T cell. This cell population inhibited both syngeneic and allogeneic cytotoxic T lymphocyte activity, but the cell population which inhibited allogeneic CTL activity was radiation sensitive. In addition, allogeneic SEB-primed cells appeared to develop cytolytic activity as a result of the additional stimulation in the mixed-lymphocyte reaction culture. The relationship of the SEB-primed CD4- CD8- CD5+ T cells to related regulatory T cell populations is discussed.     

T regulatory and primed uncommitted CD4 T cells express CD73, which suppresses effector CD4 T cells by converting 5'-adenosine monophosphate to adenosine

CD73 (5'-ectonucleotidase) is expressed by two distinct mouse CD4 T cell populations: CD25+ (FoxP3+) T regulatory (Treg) cells that suppress T cell proliferation but do not secrete IL-2, and CD25- uncommitted primed precursor Th (Thpp) cells that secrete IL-2 but do not suppress in standard Treg suppressor assays. CD73 on both Treg and Thpp cells converted extracellular 5'-AMP to adenosine. Adenosine suppressed proliferation and cytokine secretion of Th1 and Th2 effector cells, even when target cells were activated by anti-CD3 and anti-CD28. This represents an additional suppressive mechanism of Treg cells and a previously unrecognized suppressive activity of Thpp cells. Infiltration of either Treg or Thpp cells at inflammatory sites could potentially convert 5'-AMP generated by neutrophils or dying cells into the anti-inflammatory mediator adenosine, thus dampening excessive immune reactions.     

Hormone-induced preimplantation Lyt 2+ murine uterine suppressor cells persist after implantation and may reduce the spontaneous abortion rate in CBA/J mice

Immunoregulatory cells in the maternal uterine endometrium and decidua are thought to play an important role in ensuring the success of the semiallogeneic conceptus. Two phases of suppression have been described in pregnant mice. Prior to implantation, the hormonal changes triggered by mating activate or recruit a population of nonspecific Lyt 2+ suppressor cells that inhibit cytotoxic T lymphocyte generation: this suppression appears to wane at the time or implantation and 4-5 days after implantation, a non-T suppressor cell population activated or recruited by fetal trophoblast cells develops. In this paper we confirm the non-major histocompatibility complex specificity of the hormone-regulated preimplantation suppressor cell. We show that this activity persists in the uterus during the early postimplantation period where its suppressive activity is masked by an Fc-receptor-positive cell population recruited by the implanting embryo. The potential importance of the persisting suppressor cells is suggested by an increase in the rate of spontaneous abortion of DBA2-mated CBA/J mice following injection of monoclonal anti-Lyt 2+ antibody in the early postimplantation period.     

Role of CD11a/CD18-CD54 interactions in suppression of human B cell responsiveness by CD4+ suppressor T cells.

The role of leukocyte function-associated Ag-1 (LFA-1, CD11a/CD18) and intercellular adhesion molecule 1 (ICAM-1, CD54) interactions in the suppression of human B cell function by immobilized anti-CD3-activated CD4+ T cells was examined by studying the effects of mAb to these determinants. The suppressive activity was assessed by the effects of CD4+ T cells without mitomycin C treatment activated by immobilized anti-CD3 for 72 hr on the differentiation into Ig-secreting cells of B cells activated for 72 hr with immobilized anti-CD3-stimulated CD4+ T cells that had been treated with mitomycin C (T4 mito). Suppression was not observed when activated CD4+ T cells and B cells were separated by filter membranes, indicating that the suppression requires the direct interactions between anti-CD3-activated CD4+ T cells and activated B cells. In this model system, mAb to either the alpha (CD11a) or beta (CD18) chain of LFA-1 or ICAM-1 (CD54) reversed the suppression of B cell function by suppressor CD4+ T cells significantly. Reversal of suppression of B cell function was most marked when activated B cells were treated with mAb to ICAM-1 and suppressor CD4+ T cells were treated with mAb to LFA-1, but not vice versa. Studies using fluorescence-activated cell sorter revealed marked increase of expression of ICAM-1 on B cells after 72 hr of activation with immobilized anti-CD3-stimulated T4 mito. These results indicate that the interactions between LFA-1 and ICAM-1 play an important role in mediating the suppressive activity of anti-CD3-activated CD4+ T cells to B cells. Moreover, the data are consistent with a model of T-cell-mediated B cell suppression in which interactions between LFA-1 on suppressor T cells and ICAM-1 on activated B cells play a central role in the suppression of B cell function.