Naturally occurring non-T suppressor cells in pregnant and neonatal mice: some functional and phenotypic characteristics

The identification of the neonatal NBXFO hybridoma cell and its mediator

The neonatal spleen:myeloma hybridoma cell, NBXFO, with immunosuppressive properties supported rodent hematopoietic colony formation. We identified this hybridoma to be an undifferentiated fibroblast that produced macrophage colony-stimulating factor (M-CSF). The bone marrow cells that grew in the presence of the NBXFO supernate were macrophages and were immunosuppressive towards lymphocytes. Neutralizing anti-M-CSF antibody partially inhibited the actions of the neonatal splenic suppressor cells. Neonatal splenocytes, but not the other parental cell line, FO, induced macrophage colony formation, possessed surface-associated M-CSF, and possessed M-CSF-specific transcripts. Therefore, we believe that the M-CSF-producing phenotype was contributed by a fibroblastic stromal cell and that these stromal cells could be responsible for the in situ generation of neonatal splenic suppressor cells.

Reactivity of monoclonal antibody 1E5.B5 with a novel phenotypic marker expressed on a murine natural suppressor cell subset

Natural suppressor (NS) cells are antigen-nonspecific, MHC-independent immunoregulatory cells that are typically found in murine bone marrow (BM), newborn (NB) mouse spleen, and in splenic tissue of adult mice during pregnancy and following cyclophosphamide (CY) treatment. There has been a pressing need for the development of NS cell-specific monoclonal antibodies (mAb) since NS cells are generally described as null cells which lack the usual phenotypic markers of mature T cells, B cells, and macrophages. Here we present evidence that mAb 1E5.B5, which was raised in rats against murine splenic pregnancy-associated NS (SPANS) cells, recognizes a unique antigenic marker expressed by some, but not all, murine NS cells. In the presence of complement, mAb 1E5.B5 effectively eliminates SPANS activity, and diminishes NS activity of CY-treated spleen cells in mixed lymphocyte reactions (MLR). However, cytotoxic pretreatment with mAb 1E5.B5 had minimal effects on NS activity of BM and NB spleen cells. We also show that pregnancy spleen cells and CY-spleen cells with moderate NS activity in MLR can be positively selected for by "panning" with mAb 1E5.B5. In contrast, only weakly inhibitory cells are isolated from BM and NB spleen by this procedure. Cellular ELISA and flow cytometry confirm that mAb 1E5.B5 has specificity for pregnancy spleen cells and CY-spleen cells, as well as for NB spleen and BM cell preparations. Western blot analysis reveals that mAb 1E5.B5 reacts with a novel 50 kDa NS cell-associated antigen which we have termed NS-1. The NS-1 antigen is not present on other null cells such as natural killer (NK) cells and natural cytotoxic (NC) cells since cytotoxic pretreatment of pregnancy spleen cells with mAb 1E5.B5 does not affect antibody-dependent cell-mediated cytotoxicity, NK or NC activity.

Altered immune response patterns in murine syngeneic pregnancy: presence of natural null suppressor cells in maternal spleen identifiable by monoclonal antibodies

Expression of certain autologous lymphocyte-activating antigenic determinants on the developing embryo is known to provide a stimulus for maternal anti-fetal autoproliferative responses. If left unregulated these responses could exert negative influences on the reproductive process by converting to autoaggressive forms of immune reactivity. In normal circumstances, immunological reactions of this nature are therefore likely to be under the control of pregnancy-associated immunoregulatory elements found within the maternal/fetal environment. In the present investigation we describe a naturally occurring splenic inhibitory cell type devoid of conventional T, B, and macrophage surface markers associated with syngeneic murine pregnancy that is capable of exerting potent immunosuppressive effects on an in vitro expression of fetal/newborn T cell autoreactivity, namely the autologous mixed lymphocyte reaction (AMLR). Maternal spleen cells inhibitory for AMLR were found to be highly resistant to cytotoxic pretreatment with a panel of conventional antisera directed against T cell-specific antigenic determinants. The non-T nature of the natural splenic suppressor cell was further indicated by experiments showing that purified spleen T cells had no inhibitory activity. Pregnancy spleen cell populations that were effectively depleted of macrophages retained full ability to inhibit AMLR. Maternal suppressor activity could be localized to the spleen cell population bearing receptors for the B cell-specific lectin, soybean agglutinin (SBA). A panel of monoclonal antibodies prepared against enriched populations of suppressor cells was screened and selected for specific reactivity using an ELISA against glutaraldehyde-fixed SBA+ spleen cell subpopulations from pregnant versus virgin animals. Several of the monoclonals developed against suppressor-enriched spleen cell populations from isopregnant as well as allopregnant animals were effective in reducing or eliminating suppressor cell activity following cytotoxic pretreatment in the presence of complement. The novel set of anti-suppressor monoclonal antibodies described here should prove useful in furthering the isolation and characterization of pregnancy-associated suppressor cells and in determining their relationship to natural suppressor cell populations described in other systems.

Secretion of a suppressor cell inducing factor by an interleukin 3-dependent cell line with natural cytotoxic activity. III. Comparison with other interleukin 3-dependent cell lines

We previously described the interleukin 3 (IL-3)-dependent cell line, M1-A5, which has both natural cytotoxic (NC) and suppressor cell activities, the latter of which is mediated, in part, by the release of two cytokines which activate suppressor cells from unprimed lymphoid precursor cells. In this study we have compared the M1-A5 cell line with four other IL-3-dependent cell lines to determine whether these dual activities are universally associated with IL-3 dependence and to test the hypothesis that there is a direct relationship between the cytotoxic and the suppressive activities. The cell lines tested were a bone marrow derived Dexter culture derived line (FDC-P1), two Moloney leukemia virus induced leukemias (DA-1 and DA-3), and a mast cell line (PT18(A17]. All lines were dependent on IL-3 for survival but FDC-P1, DA-1, and DA-3 showed varying degrees of short-term proliferation in granulocyte-macrophage colony stimulating factor (GM-CSF). The cell lines all expressed asialo GM1 and Ly-5 surface markers but differed with respect to other markers. DA-1 expressed MAC-1, FDC-P1 and DA-3 expressed Thy-1, and PT18(A17) expressed receptors for the Fc portion of IgE. The cell lines varied greatly in their cytotoxic activity against WEHI-164. FDC-P1, DA-1, and PT18(A17) had low NC activity. DA-3 had consistently high activity, greater than that seen with M1-A5 cells. However, none of the cell lines secreted constitutively a suppressor cell inducing factor (SIF). In addition, it was demonstrated that recombinant murine TNF did not activate suppressor cells capable of inhibiting antibody synthesis and that anti-TNF did not block SIF activity, thus suggesting that TNF contamination of the M1-A5 derived SIF preparation is not responsible for the induction of suppressor cells. We conclude that suppressor cell inducing factors are not universally secreted by IL-3-dependent cell lines, that there is no correlation between NC and SIF activity, and that the dual activities of M1-A5 cells are not mediated by TNF.

Quantitative and functional change in T cells of primiparous mice following injection of benzo(a)pyrene at the second trimester of pregnancy

Progeny from benzo(a)pyrene (BP) exposed (150 micrograms/g body weight) primiparous mothers, injected during the second trimester of pregnancy, are severely compromised immunologically. In view of maternal-fetal associations, we studied, during pregnancy and postpartum, the quantitative and functional status of the maternal T cell population in the thymus and/or spleen. In the thymus, there is an exacerbated depression in the amounts of thymocytes, (theta +, Lyt 1+, Lyt 2+ cells) during pregnancy relative to the corn oil-injected controls, which is sustained postpartum. In the spleen, while there are inconsistencies in the level of theta + cells, The Lyt 1+ are depressed postpartum relative to virgins, but the Lyt 2+ are enhanced during pregnancy and postpartum, reaching levels greater than 700-fold of controls. In controls, while the number of Lyt 1+ cells was higher than BP-exposed mice or virgins during pregnancy, they virtually disappeared postpartum. A similar but reversed image is mirrored by the Lyt 2+ cells, i.e., they were virtually absent during pregnancy but increased postpartum. Splenic allogeneic and syngeneic mixed lymphocyte responses were subnormal. The data show that BP disrupts the maternal T cell repertoire, leading to an accumulation of Lyt 1-2+ cells, and suggest that splenic disruption may be due to changes in the differentiation potential of T precursor cells. These changes not only are most likely to affect maternal defenses, but also may have a direct bearing on the establishment of progeny immune status.

Murine pregnancy-associated modulations in lymphocyte reactivity to mitogens: identification of the cell populations affected

Lymphocytes from the thymus, spleen and inguinal lymph nodes of syngeneically pregnant and non-pregnant mice were compared in their responsiveness to polyclonal stimulation by mitogen. Pregnancy-associated changes in mitogen reactivity were detected, on a cell-per-cell basis, in thymocytes (increased) and spleen cells (decreased) but not in lymph node cells. The hyperreactivity of thymocytes during pregnancy correlated with physiological involution of the thymus occurring through the selective loss of relatively immature, non-mitogen-reactive, Lyt 1+2+ cells. The remaining cells were found largely to be mature Lyt 1+2- T cells with the capacity to respond to mitogenic stimulation. It is most likely the relative increase in the proportion of these Lyt 1+2- cells that causes the hyper-responsiveness of thymocytes to mitogens observed during pregnancy. On the other hand, while spleen cells from pregnant animals gave lower responses to mitogens than those from control virgin females, isolated splenic T cells from the two groups proved equally reactive to T cell mitogens. This supports the contention that at least some aspects of immunity during pregnancy are down-regulated by inhibitory cells within the non-T cell compartment. The results demonstrate the importance of identifying the reactive cell population in studies on changes in lymphocyte responsiveness in pregnancy.

Monoclonal antibodies against murine neonatal and pregnancy-associated natural suppressor cells induce resorption of the fetus

Pregnant and neonatal/fetal mice have been shown to harbour naturally occurring inhibitory cells of both T and non-T type. Non-T suppressor cells present in the spleen of primiparous pregnant and newborn animals inhibit proliferative responses in autologous and allogeneic mixed lymphocyte reactions. Such cells can be positively selected for by agglutination with the B cell-specific lectin soybean agglutinin (SBA). We generated rat IgG monoclonal antibodies against unique cell surface structures on the non-T inhibitory cells, and cytotoxic pretreatment of spleen cells from pregnant or neonatal/fetal mice largely abrogates their suppressive activity on proliferative responses. Furthermore, in vivo administration of such antibodies to pregnant inbred and outbred mice results in termination of the pregnancy or decreased litter size. Since injection of anti-T cell IgG monoclonal antibodies does not interfere with the delivery of normal sized litters it is evident from these studies that the non-T immunoregulatory cells, in contrast to T-inhibitory cells, are of great importance in ensuring immunological homeostasis in the fetal-placental environment during pregnancy in mice.

Neonatal cells in the immunoregulation of parental alloreactivity

The ability of neonatal murine F1 cells to regulate parental graft vs. host (GvH) reactions was investigated. Neonatal F1 splenocytes were able to decrease significantly the deleterious effects of systemic GvH reactions induced either with maternal or paternal splenocytes in a third party strain. Both neonatal F1 splenocytes or thymocytes were able to decrease local GvH reactions induced with maternal splenocytes towards paternal histocompatibility antigens. In the same experimental conditions, however, neonatal F1 cells were unable to decrease local GvH reactions induced with paternal splenocytes towards maternal histocompatibility antigens; using different numbers of neonatal F1 cells not only was no suppressive effect detected but even, a significant increase in GvH was registered. Similar results were obtained when mortality assays were carried out. It can be concluded that neonatal F1 mice differ in their capacity for regulating parental alloreactive T reactions against self histocompatibility antigens either of maternal or paternal origin.

Secretion of a suppressor cell-inducing factor by an interleukin 3-dependent cell line with natural cytotoxic activity. II. Range, potency, and kinetics of suppressive activity

We have shown previously that the interleukin 3 (IL 3)-dependent, M1-A5 cell line has natural cytotoxic (NC) activity and secretes a small molecular weight factor which activates suppressor cells capable of inhibiting the in vitro generation of antibody forming cells (PFC). In this paper, we show that the M1-A5 cells are capable of suppressing the generation of both PFC and cytotoxic T cells (CTL) as well as the proliferative response in a mixed lymphocyte reaction (MLR). Very little suppression of Concanavalin A (Con A)-stimulated proliferation is seen. Suppression is seen with as few as 10(2) M1-A5 cells, a 10(-4) dilution of a cell-free supernatant from the M1-A5 cells, or as few as 10(4) suppressor cells activated by the suppressor cell inducing factor (SIF) secreted by the M1-A5 cell line. Kinetic studies show that, in all cases, the cells or suppressive factors must be present in the initial 24 to 48 h of the culture for significant suppression. In contrast, a second IL 3-dependent cell line FDC-P1 is directly suppressive, but this suppression is clearly due to a different mechanism, since cell-free supernatants are not inhibitory and do not activate suppressor cells.

Suppression by alpha-fetoprotein of murine natural killer cell activity stimulated in vitro and in vivo by interferon and interleukin 2

Natural killer (NK) cells are 'spontaneously' cytotoxic cells thought to be involved in surveillance against tumour cells, rejection of virally infected cells, and regulation of haematopoietic stem cell differentiation and antibody synthesis. Fetus-derived alpha-fetoprotein (AFP) has been shown to regulate certain T cell-mediated immune reactions in vitro and in vivo. The lack of NK activity in newborn mice with high endogenous levels of AFP, together with the presence of cells expressing NK surface markers, also suggests that AFP may regulate NK activity. In this study we compared the effects of AFP on spontaneous versus activated murine NK activity. The lytic ability of both freshly prepared splenic NK cells and those arising after incubation for 24 h with interferon, Poly I:C, or T-cell growth factor (TCGF) was not affected by AFP if the latter was present only during the killing phase. However, if AFP was added at the beginning and retained for the duration of the 24-h in vitro lymphokine stimulation, the subsequent NK activity induced by interferon, Poly I:C, and TCGF was found to be significantly suppressed. This inhibition is both dose- and time-dependent. Delayed addition experiments showed that when AFP is present during the first 6 h of in vitro stimulation it will suppress interferon and TCGF-boosted NK activity by 50-80%. The AFP-mediated inhibitory effect on lymphokine-stimulated NK activity is not the result of increased death of effector cells nor, in the case of interferon and polyribonucleotides, of non-specific binding of AFP to the enhancing agents. In vivo injections of Poly I:C or TCGF failed to increase neonatal NK function, while administration of interferon did cause slightly higher levels of NK activity. However, spleen cells from newborn animals cultured for 24 h in the presence of lymphokines resulted in markedly elevated NK function and this in vitro activation could be suppressed by purified fetus-derived AFP. Thus, the in vivo pattern of NK activation in newborns with high endogenous levels of AFP was very similar to that of adult NK stimulation in vitro when exogenous AFP was added.

Naturally occurring, spleen-associated suppressor activity of the newborn mouse. Biochemical and functional identification of three monokines secreted by newborn suppressor-inducer monocytes

Immature monocytes residing in the spleens of newborn mice less than 6-7 days of age are known to suppress T-dependent and T-independent immune responses in vitro and in vivo. Suppression is mediated in part through products secreted by the newborn monocytes. In the present study, we have identified 3 monokines secreted by newborn, but not adult, monocytes correlating with materials possessing significant suppressor activity in vitro. These monokines have molecular weights of 58, 10.8, and 10 kilodaltons, appear to be proteins, and are insensitive to heat inactivation. The 58 kd material is antigenically distinct from alpha-fetoprotein, while the lower molecular weight materials are too large to be prostaglandins. Although these monokines can be shown to have activity in vitro, they are not as efficient as newborn monocytes in preventing alloreactivity in vivo.

Increased maternal T cell autoreactivity associated with syngeneic murine pregnancy

In this study T cells isolated from isopregnant and virgin CBA/J mice were examined for reactivity to self antigen(s) in vitro and in vivo. The autoproliferative capacity of maternal versus virgin T cells was tested in vitro using autologous mixed lymphocyte reactions (AMLR). The popliteal lymph node (PLN) assay was used to compare the ability of maternal versus virgin T lymphocytes to mediate syngeneic graft-versus-host (SGvH) reactions in vivo. Splenic T cells obtained from pregnant animals near term were found to be approximately 10-fold more reactive towards syngeneic virgin non-T stimulator cells in AMLR than splenic T lymphocytes from age-matched virgin animals. In addition, T cells isolated from the spleens of gravid CBA/J mice displayed a significantly enhanced capacity to mediate SGvH reactions in virgin CBA/J females as measured by regional lymph node enlargement. These findings indicate that syngeneic murine pregnancy is accompanied by an increase in autoreactive T cell activity.

In vitro activation of bone marrow-derived T-and non-T-cell subsets by alpha-fetoprotein

Alpha-Fetoprotein (AFP) is a major serum glycoprotein during embryonic and early postnatal life. A number of diverse biologic functions have been attributed to AFP, including osmotic and carrier function and immunosuppressive activity. In this study we demonstrate that AFP selectively stimulates in vitro proliferation of two distinct subsets of adult murine bone marrow cells. One population of AFP-reactive bone marrow cells expresses surface receptors for soybean agglutinin (SBA) lectin. SBA+ bone marrow cells are resistant to cytotoxic pretreatment with T-cell-specific antisera and are not retained on Ig-anti-Ig affinity columns. The absence of conventional T- and B-cell markers, coupled with the presence of SBA receptors, suggests that AFP-activated non-T bone marrow cells may belong to an immature set of B lymphocytes. A second population of AFP-responsive bone marrow cells expresses the Thy-1+ Lyt 1+2- phenotype characteristic of conventional mature adult T helper cells. The potential physiological relevance of the mitogenic effects of AFP on bone marrow cells with respect to immunoregulatory processes in the fetal/newborn environments is discussed.

Altered mitogen- and alloantigen-induced lymphoproliferative responses in primary lymphoid organs of pregnant mice

The effect of syngeneic murine pregnancy on the response of lymphocytes from the primary lymphoid organs to mitogenic and alloantigenic stimulation was investigated. Thymocytes and bone marrow cells from gravid animals were found to exhibit elevated reactivity to T cell mitogens and allogeneic stimulator cells. In contrast, the LPS response of bone marrow cells from pregnant mice did not differ significantly from that of virgin animals. These findings indicate that the immune reactivity of T cells derived from the primary lymphoid organs is altered during syngeneic murine pregnancy. Pregnancy-induced modifications in T lymphocyte reactivity may contribute to the ability of the maternal immune system to recognize and react against fetal antigens.