Immunosuppressive properties of murine trophoblast

Microenvironment of the feto–maternal interface protects the semiallogenic fetus through its immunomodulatory activity on dendritic cells

OBJECTIVE

To investigate the immunomodulatory activity of decidual culture supernatant on dendritic cell (DC) functions.

DESIGN

In vivo and in vitro experimental study using mice.

SETTING

Academic research laboratory.

ANIMAL(S)

C57BL/6-mated female Balb/c mice.

INTERVENTION(S)

Culture supernatants of decidual cells obtained from the uteri of allogenic pregnant mice (Balb/c x C57BL/6) were collected. Dendritic cells were purified from Balb/c mice spleens and pulsed with antigen during overnight culture. In some cultures, decidual supernatant was added at 5%, 10%, or 20% final concentration. Endometrial culture supernatant-treated DCs served as a control. Antigen-pulsed DCs were injected into the front footpads of syngeneic mice.

MAIN OUTCOME MEASURE(S)

Lymph nodes of primed mice were removed 5 days after DC injection. Antigen-specific proliferation and interleukin-10 and interferon gamma production by lymphocytes were measured by (3)H-Thymidine incorporation and ELISA, respectively.

RESULT(S)

The results showed that decidual culture supernatant markedly blocked in vivo antigen presentation by DCs and inhibited their capacity to induce interferon gamma (but not interleukin-10) production by primed lymphocytes.

CONCLUSION(S)

It seems that soluble factors produced by decidual cells are important mediators of immunoregulation at the feto-maternal interface, which provide the two fundamental requirements for protection of the semiallogenic fetus, namely immunologic tolerance and predominance of T helper 2 immunity, through modulation of DCs function.

Removing the vertebrate-specific TBP N terminus disrupts placental beta2m-dependent interactions with the maternal immune system

Mammalian TBP consists of a 180 amino acid core that is common to all eukaryotes, fused to a vertebrate-specific N-terminal domain. We generated mice having a modified tbp allele, tbp(DeltaN), that produces a version of TBP lacking 111 of the 135 vertebrate-specific amino acids. Most tbp(DeltaN/DeltaN) fetuses (>90%) died in midgestation from an apparent defect in the placenta. tbp(DeltaN/DeltaN) fetuses could be rescued by supplying them with a wild-type tetraploid placenta. Mutants also could be rescued by rearing them in immunocompromised mothers. In immune-competent mothers, survival of tbp(DeltaN/DeltaN) fetuses increased when fetal/placental beta2m expression was genetically disrupted. These results suggest that the TBP N terminus functions in transcriptional regulation of a placental beta2m-dependent process that favors maternal immunotolerance of pregnancy.

Polytene chromosomes in mammalian cells

This article deals with the structural and functional organization of polytene chromosomes in mammals. Based on cytophotometric, autoradiographic, and electron microscopic data, the authors put forward a concept of nonclassic polytene chromosomes, with special reference to polytene chromosomes in the mammalian placenta. In cells with nonclassic polytene chromosomes, two phases of the polytene nucleus cycle are described, such as the endointerphase (S phase) and endoprophase (G phase). The authors generalize that the main feature of nonclassic polytene chromosomes is that forces binding the sister chromatids are much weaker than in the Diptera classic polytene chromosomes. This concept is confirmed by comparative studies of human, mink, and fox polytene chromosomes. The final step of the trophoblast giant cell differentiation is characterized by a transition from polyteny to polyploidy, with subsequent fragmentation of the highly polyploid nucleus into fragments of low ploidy. Similarities and dissimilarities of pathways of formation and rearrangement of nonclassic polytene chromosomes in mammals, insects, plants, and protozoans are compared. The authors discuss the significance of polyteny as one of the intrinsic conditions for performance of the fixed genetic program of trophoblast giant cell development, a program that provides for the possibility of a long coexistence between maternal and fetal allogenic organisms during pregnancy.

Chorionic gonadotropin-like proteins in the obplacental giant cells of the rabbit

Obplacental giant cells are enlarged cells, found following implantation, in the antimesometrial region of the rabbit uterus. They probably originate from trophoblastic knobs that traverse the uterine epithelium during early implantation. Little is known about their function. In this study, trophoblast, placental, paraplacental and obplacental tissues at days 7-15 post-coitum, and enzyme-isolated giant cells at day 15 were studied by two-dimensional gel electrophoresis, followed by immunoblotting and light-microscopic immunohistochemistry, for the presence of human chorionic gonadotropin-like proteins. Immunostaining was performed by using anti-human chorionic gonadotropin antibodies. In gel electrophoresis of obplacental tissue and isolated giant cells, two proteins of human chorionic gonadotropin-like antigenicity at 26 kDa with pIs equivalent to pH 6.4 and 6.6 were found; they were absent in the placenta, paraplacenta, day-7 blastocyst and day-8 trophoblast. The onset of synthesis of these proteins could be observed when day-8 trophoblastic tissue was cultured in vitro for 24 h. In immunohistochemistry, only the obplacental giant cells showed a positive reaction, indicating that the production of chorionic gonadotropin occurs in this cell type.

Immunologically mediated abortion (IMA)

Roughly 20% of all clinical pregnancies evolve into "spontaneous abortions". The causes of spontaneous abortion have been determined in under 60% of the total and comprise genetic, infectious, hormonal and immunological factors. In some cases the immune tolerance mechanism may be impaired and the foetus immunologically rejected (IMA, immunologically mediated abortion). The immunological mechanism implicated depends on the time in which pregnancy loss takes place. During preimplantation and up to the end of implantation (13th day) the cell-mediated immune mechanism (potential alloimmune etiologies) is responsible for early abortion. This mechanism involves immunocompetent decidual cells (eGL, endometrial granulated lymphocytes) already present during pre-decidualization (late luteal phase) and their production of soluble factors or cytokines. Once the implantation process is over, after blastocyst penetration of the stroma and the decidual reaction of uterine tissue, IMA could be caused by cell-mediated and humoral mechanism (anti-paternal cytotoxic antibodies or autoantibody etiology), by the production of paternal anti major histocompatibility complex antibodies, or even by an autoimmune disorder leading to the production of autoantibodies (antiphospholipid antibodies, antinuclear antibodies or polyclonal B cell activation). The diagnostic work-up adopted to select IMA patients is crucial and includes primary (karyotype of both partners, toxo-test, hysterosalpingography, endometrial biopsy, thyroid function tests, serum hprolactin, luteal phase dating) and secondary (full hemochromocytometric test, search for LE cells, lupus anticoagulant, anticardiolipin, antinuclear antibodies, Rheumatoid factor, blood complement VDRL) investigations. Therapeutical approaches vary. If autoimmune disorders are demonstrated therapies with different combinations of corticosteroids, aspirin and heparin or intravenous immunoglobulin are administered. Otherwise, therapy with paternal or donor peripheral blood mononuclear cells should be instituted.

Phenotypic characterization of normal human placental mononuclear cells

The placenta is a rich source of immunocompetent cells. We have studied the phenotype, number and origin of placental mononuclear blood cells isolated from 32 normal term placentae using 4 color flow cytometry. Respective maternal and cord blood leucocyte preparations were also compared. Placental tissue without extraembryonic membranes was cut into small pieces and divided. One portion was washed extensively with ice-cold PBS. Both tissue portions were disrupted in a blender and cells were dissociated by using a 180 mu sieve. Leucocytes were isolated by Ficoll-Hypaque density gradient centrifugation. Maternal and cord bloods were HLA typed and in cases of HLA-A2 or B7/40 disparity, monoclonal anti-HLA antibodies to these antigens showed that unwashed placental tissue contained 35% maternal and 65% fetal cells. This ratio, however, was not reflected for a given cell phenotype. In comparison, washed placental tissue contained cells of fetal origin only. Both unwashed and washed placental tissue contained fewer CD3 and CD4, but more CD8 cells than maternal and cord blood. Markers of NK cells such as, CD16, CD56, and CD57 showed this cellular phenotype to be 15 times more abundant in the placental preparations than in cord and maternal blood. The quantitative differences between peripheral blood and placental CD8 and NK cells were further explored with an antiprogesterone receptor antibody in combination with anti-CD8, anti-CD57 and anti-HLA-DR. The number of progesterone receptor (PGR) positive cells was three times higher in placental tissues than in cord or maternal blood. These data indicate that the phenotypic frequencies of certain placental leucocytes are significantly different from maternal and fetal peripheral blood. Progesterone and the presence of PGR may be important in the differential retention of placental leucocytes.

Progesterone and estradiol suppress human mononuclear cell cytotoxicity

Fetal trophoblast is generally resistant to lysis by cytotoxic cells. We hypothesized that progesterone and estrogens secreted by the trophoblast act at the choriodecidual interface where they are present in high concentrations to provide a local, paracrine immunosuppressive effect on cellular cytotoxicity. Using peripheral blood mononuclear cells as effector cells in a cytotoxicity assay, we evaluated the effects of progesterone, estrone, estradiol and estriol, either alone or in combination, on cellular cytotoxicity. Both progesterone and estradiol suppressed cytotoxicity in a dose-dependent manner. Estrone, estriol, pregnenolone and cholesterol had no effect. A synergistic suppression of cytotoxicity was observed when estrone, estradiol, estriol and progesterone were combined. We speculate that trophoblast production of progesterone and estradiol may be an important local immunosuppressive mechanism contributing to fetal survival.

In vitro and in vivo immunosuppressive effects of supernatants from human choriocarcinoma cell lines

Local immunosuppression mediated by placental suppressor factors may contribute to the absence of consistently demonstrable cellular immunity against the fetus. In this context, we have investigated the immunosuppressive capabilities of supernatants from human trophoblastic choriocarcinoma cell lines (HCS) by testing the effects of HCS on immune responses in vitro and in vivo in the human and murine systems. HCS suppresses mitogen-induced proliferation and mixed lymphocyte reactions in humans and in mice, as well as antigen-induced T cell proliferation in mice. HCS also suppresses the in vivo response of mice to allogeneic cells. Furthermore, HCS when injected intraperitoneally causes the induction of suppressor cells in mice which in turn prevent the mounting of an allogeneic response in other strains of mice. These results indicate that human choriocarcinoma cell lines secrete a suppressor factor(s) which induces suppression in vitro as well as in vivo through the generation of suppressor cells.

Differential modulation of the in vitro lymphocyte activation pathways by soluble and solubilized placental substances

Soluble and detergent-solubilized placental extracts were studied for their modulatory effects upon the proliferation of lymphocytes stimulated by various activating agents. It was shown that soluble placental extract (SPE) exerted an inhibitory effect on the lymphoproliferation triggered by alloantigen or LPS but not by Con A or the combined action of PMA + calcium ionophore A 23187. This effect was also observed with SPE precipitated by 30% of ammonium sulfate (SPE30). On the other hand, a solubilized placental extract (SzPE) that was obtained by using octyl-beta-D-glucopyranoside inhibited the stimulation triggered by alloantigen, LPS, and Con A but did not affect the protein kinase C pathway. The modulatory effects were observed not only when SPE (or SPE30) and SzPE were added at the time of culture initiation but also at 24 h before or after the activating agents. Preincubation with SPE30 or SzPE immobilized on plastic surface, however, transduced an enhanced lymphoproliferative response to alloantigen and mitogen Con A but not to LPS. The above results suggest that placental substances exerted their modulatory effects by interfering mainly with the antigen or mitogen lymphoproliferation pathways.

Obplacental giant cells of the domestic rabbit: development, morphology, and intermediate filament composition

Obplacental giant cells are large (less than or equal to 210 microns) polyploid cells that appear in the stroma of the pregnant uterus of the rabbit following ovoimplantation. Histological examination of a complete developmental series indicates that obplacental giant cells arise from trophoblastic knobs that have traversed the uterine epithelium during early implantation. During maturation, the cells undergo a massive (approximately 6,000%) increase in volume and penetrate deeply into the uterine stroma and myometrium, where they often become associated with blood vessels and smooth muscle cells. Giant cells at mid-gestation contain one or two large nuclei with prominent nucleoli and appear to be amitotic. They are rich in Golgi complexes, RER, SER, and cortically distributed cytoplasmic filaments, and contain intracellular canaliculi lined by microvilli. Giant cells vary with respect to the occurrence of lipid droplets, phagocytotic inclusions, lysosomal structures, and electron-dense granules. Immunocytochemistry demonstrates that the giant cells exhibit intermediate filaments related to cytokeratin and vimentin, but are negative for desmin and for an endothelial cell marker, Factor VIII-related antigen. The cells are positive for cytokeratin from their inception, but only become vimentin-positive between Days 12 and 15 of pregnancy, a change seemingly related to their detachment from epithelial tissue to take on an independent existence. Our findings indicate that the giant cells originate from obplacental trophoblast and, at maturity, exhibit cytoskeletal characteristics of isolated epithelial cells, as well as a complement of organelles suggestive of synthetic activity.

The human blastocyst produces a soluble factor(s) that interferes with lymphocyte proliferation

Animal studies have reported that the blastocyst produces immunosuppressive factors that are critical to successful implantation. The production of such factor(s) by the human blastocyst has not yet been described. To test that hypothesis, the spent media of 67 human embryos developed in vitro was evaluated using either the allogeneic one-way mixed lymphocyte response (MLR) or the concanavalin A (Con-A) stimulated lymphocyte response (SLR). Conditioned embryo media was obtained at in vitro fertilization (IVF) in aliquots of (A) the first 24 hours in vitro, (B) the second 24 hours in vitro, and (C) from 24 hours until development of the blastocyst stage. Control media consisted of Ham's F-10 medium (Gibco, Grand Island, NY) with identical maternal serum concentration to test media. Conditioned media (either A, B, or C) or control media was added to 200-microL wells containing lymphocytes in either the MLR or SLR. A significant suppressive effect of 25% to 60% was observed in seven of nine samples of blastocyst-conditioned media (P less than 0.05). No significant suppressive effect was found in the earlier embryo media (A or B) in either the MLR or the SLR. These data suggest that the human blastocyst produces a factor(s) that interferes with lymphocyte proliferation.

Immunoregulatory effects of a suppressor factor from healthy pregnant women's lymphocytes after progesterone induction

Progesterone-treated pregnancy lymphocytes release an immunologic blocking factor. The mode of action of this substance was investigated. The supernatant of progesterone-treated pregnancy lymphocytes was highly suppressive of natural cytotoxicity toward human embryonic fibroblast target cells as well as of natural killer cell activity. The effect was not observed when progesterone induction was performed in the presence of RU 486, a progesterone receptor blocking agent. The factor was able to inhibit mixed lymphocyte reactions (MLRs), and transfer coculture experiments revealed that this effect was dependent on major histocompatibility complex nonspecific, nonrestricted suppressor T cells. The activation/expansion of suppressor inducer and suppressor effector T cells was further proved by fluorescence-activated cell sorter analysis of the populations from MLRs cultured in the presence of the inhibitory factor. These changes were not observed with MLRs performed in the presence of supernatants from progesterone + RU 486-treated peripheral blood lymphocytes. The inhibitory material, on the other hand, did not affect either production or function of IL-2. We conclude that in the presence of high local concentrations of progesterone, a suppressive pathway dependent on specific progesterone-CD8+ lymphocyte interaction might be established. This mechanism might play an important role in the maintenance of pregnancy.

Levels of interleukin-2 receptor in serum of patients during the menstrual cycle and following embryo/gamete transfer

In an attempt to establish whether nidation elicits a measureable systemic immunologic response in vivo, levels of interleukin-2 receptor (IL-2R) have been determined in serum drawn from 34 pregnant patients (IUP) immediately before embryo/gamete transfer, and 7 and 14 days later. For comparison, these same values were determined for 41 subjects who did not conceive, and for 18 who spontaneously aborted (SAB) during the first trimester of gestation. Serum IL-2R values were near 320 U/ml and did not differ between days within outcomes, or within days between outcomes. Furthermore, levels of IL-2R and beta-hCG in serum were not interrelated on days 7 or 14 of pregnancy (SAB or IUP). Serum IL-2R levels did not change during the menstrual cycle. The present results appear to traverse presumptive elements of existing hypotheses of "placental immunotropism."

Immunoactive products of human placenta. I. An immunoregulatory factor obtained from explant cultures of human placenta inhibits CTL generation and cytotoxic effector activity

Supernatants were prepared from short-duration explant cultures of term human placentas obtained after cesarean delivery. These supernatants inhibited murine and human mixed lymphocyte reactions, as well as CTL generation. The effects were reversed by an excess of IL-2-containing medium. Similarly, the material inhibited human natural killer cytotoxicity against K 562 targets. The material was subjected to gel-filtration chromatography on an ACA 44 or Bio-Gel A15m column. The apparent MW of the MLR-CML material was about 60-70 kDa, whereas the NK inhibiting activity was eluted in high-MW components (greater than 200 kDa) as well as in the 50-kDa range. The relevance of this material in local immunoregulation during human pregnancy is discussed.

Suppression of lymphocyte activation by a soluble factor released from the human placental chorionic membrane: chemical analysis and functional characterization

A heat-resistant factor that markedly and reversibly inhibits human lymphocyte activation was discovered in culture medium of human placental chorion. The chorionic factor inhibits lymphocyte proliferation in response to polyclonal mitogens and in the mixed leukocyte response. The inhibitory action is most effective if the factor is added during the first 24 h of lymphocyte culture and is reversible. The chorionic factor is sensitive to proteinase K, pepsin and bovine pancreatic protease; its activity is completely lost after papain digestion and following treatment with trichloracetic acid (TCA). The factor prevents the expression of IL-2 receptors and class II MHC glycoproteins (HLA-DR) on phytohemagglutinin-stimulated PBMC but does not affect the expression of MHC class I molecules. It inhibits the replication of IL-2-dependent CTLD cells but is without effect on the growth of various human or murine cell lines or acute leukemic cells. Human placental chorion is thus capable of releasing in vitro a nontoxic heat-resistant factor with protein characteristics that reversibly inhibits processes associated with the early stages of lymphoid cell triggering. This factor may play a role in an immunoregulatory mechanism that prevents maternally mediated immune rejection of the conceptus.

Assessment of immunoregulation by cultured, pre-attachment bovine embryos

The possibility that the viability of bovine embryos might be predicted by measuring their release of immunoregulatory substances during culture has been investigated. Bovine embryos between days 2 and 19 of gestation were cultured for 24-48 h, the embryo-conditioned medium was harvested and studied for suppression of PHA-stimulated bovine leukocyte cultures. Medium incubated in the absence of any conditioning tissues served as control. Artefactual immunosuppression was detected in incubated control material that could be attributed, in part, to the mixing of different tissue culture media, the type of plastic-ware employed for incubation and supplementation of media with additional L-glutamine. It was observed that day-2 to day-9 bovine embryos, cultured in medium able to support the lymphocyte proliferation assay, did not release immunosuppressive substances. Medium conditioned by day-10 to day-12 embryos produced variable immunosuppression while that conditioned by trophoblastic vesicles derived from day-14 to day-19 embryos was consistently highly suppressive. Since bovine embryo transfer is normally conducted at 6-8 days of gestation, it is unlikely that measuring the immunosuppressive products released from bovine embryos will be of value for predicting their viability.

In vitro immunomodulatory effects of placental substances on preparatory MLR and resulting modifications of lymphocyte reactivities

The immunomodulatory effects of murine placental extracts (PE) were studied in vitro using mixed lymphocyte culture (MLC) and resulting cell-mediated lympholysis (CML). The results showed that preparative cultures in the presence of PE syngeneic to the responding cells led to a low secondary MLR response with a concomitant generation of suppressor cells. At the efferent phase, cells from the same preparative culture showed a weaker cytotoxic activity than controls cultured in the absence of extract. Furthermore, the induction of regulatory cells able to inhibit CTL in vitro activity was also observed. The active substances can be found in the 30% ammonium sulphate precipitate as well as in some gel filtration fractions showing several main bands from 115 to 43 kDa in SDS-PAGE.

Placental immunoregulatory factors

Local immune suppression appears to be a key feature in the success of the fetal allograft. Various placental factors have been described endowed with immunoregulatory activity. Much of the available evidence concerning the biological function and biochemical characteristics of these molecules is reviewed here, as well as the controversial issues surrounding them. In the final section, some outstanding questions relating to these immunoregulators are considered.

In vitro inhibitory effect of human syncytiotrophoblast plasma membranes on the cytolytic activities of CTL and NK cells

We have previously described the purification procedure for syncytiotrophoblast plasma membranes (STPM) and have now studied their immunomodulatory properties on the in vitro cytotoxic assays of generated cytotoxic T-lymphocytes (CTL) and natural killer cells (NK). STPM inhibited the cytolytic activities of CTL and NK against their target cells, whereas RBC ghosts, even at the highest protein concentration used, were ineffective. This inhibitory effect was dose-dependent upon the STPM-protein concentration and appeared to be particularly distinct at low effector/target ratios. It is hypothesized that the inhibitory activity of STPM may be exerted by blocking the effector cells or by masking their targets. Regardless of the mode of action, since cytotoxic cell activities are known to play an important role in the allograft rejection process, this suppressive inhibitory effect of STPM might be a crucial mechanism in the tolerance of the semiallogenic fetal graft.

Production of immunosuppressor factor(s) by preimplantation human embryos

Embryonic loss prior to implantation appears to be a significantly frequent phenomenon and this is further reinforced by the very low pregnancy rates reported by in vitro fertilization and embryo transfer programs. Implantation failure may possibly be the result of rejection of the antigenic embryo by the hostile maternal immune system. The mechanism by which embryos in successful pregnancies escape these rejection responses may depend upon their ability to produce factor(s) that suppress in vitro mitogen-induced lymphocyte proliferation. Only 43% of cleaved embryos demonstrated this ability. We postulate that successful pregnancies are dependent upon the production by embryos of immunosuppressor factor(s) that has a direct suppressive effect on the maternal immune response.

Inhibition of lymphocyte aggregation by progesterone

Investigations were carried out on the influence of progesterone and murine trophoblast culture supernatants on the capacity of lymphocytes to form aggregates (clusters), an important feature in immunologic recognition and response. Both progesterone and trophoblast supernatants inhibited this cluster formation in a dose-dependent way. The effect of trophoblast supernatants appeared to be mediated mainly by progesterone since they lost their inhibitory effect on the cluster formation after treatment with anti-progesterone serum (APS). Preparations with I1-2 activity of rat and mouse origin could either prevent or restore the suppressive effect of both progesterone and trophoblast supernatants on lymphocyte aggregation. The interference with lymphocyte interaction by trophoblast may represent one of the mechanisms by which the fetal allograft is protected against maternal recognition.

Immunobiology of complete molar pregnancy and gestational trophoblastic tumor

The unique curability of gestational trophoblastic tumors may in part be attributable to a host immunologic response. The occurrence of rapidly progressive and fatal choriocarcinoma may be favored by histocompatibility between patients and their partners. However, histocompatibility is not a prerequisite for the development and persistence of gestational choriocarcinoma. The expression of HLA by choriocarcinoma cells in culture is enhanced following incubation with gamma-interferon and this may be of both biologic and clinical significance. Complete molar pregnancy is a complete allograft because all molar chromosomes are of paternal origin. Patients with complete mole are sensitized to paternal HLA antigen which is expressed in molar tissue. Other polymorphic antigen systems including trophoblast-leukocyte common antigens and placental-type alkaline phosphatase are also expressed in molar tissue. We have studied the immunopathology of the molar implantation site to investigate possible humoral and cellular immune responses. The relationships among normal placenta, complete mole and choriocarcinoma are not clearly understood. The pattern of expression of oncofetal antigens in these three gestational tissues may be used to assess trophoblastic differentiation. In studies to date, molar trophoblast has the same pattern of expression of oncofetal antigens as normal placental trophoblast. We will review recent advances in our understanding of the immunobiology of gestational trophoblastic disease and suggest new directions for further research.

Immunoregulatory activities of human trophoblasts mediated by polyamine complexes

In a previous publication we described the presence in human placenta (HP) of immunosuppressive factors inhibiting the lymphoproliferative responses to mitogen. The results of further study reported herein indicate that the substance involved is of a syncytiotrophoblastic origin, that it is thermostable to 100 degrees C for 1 hr, and of low molecular weight, i.e. 3,500. It was defined as a polyamine conjugate with nucleic acids. Trophoblast cell extracts lost their immunosuppressive ability after heating in cultures of human lymphocytes supplemented with 5% autologous serum. These effects were, however, preserved both in cultures assayed in 5% fetal calf serum and in those to which purified polyamine oxidase (PAO) was added to autologous serum. Trophoblast cell extract was found to contain polyamine oxidases. Placental PAO can be inhibited by quinacrine a typical inhibitor of flavoprotein enzymes but not by isoniazid, an inhibitor of pyridoxal enzymes; this would suggest that the enzymes in human placenta are of a tissular rather than seric origin. The implication of these observations is that immunosuppression is mediated by oxidative products issued from an interaction between polyamine and polyamine oxidase in the syncytiotrophoblast cytosol. This interaction may constitute the basis for a local immunological barrier and may be involved in the protection of the fetus against maternal immune rejection.

Deviation of humoral and cellular alloimmune reactions by placental extracts

Modifications of the alloimmune response at both the humoral and the cellular levels by placental extracts (PE) syngeneic to the recipient were studied in the mouse using two different H-2 strain combinations. CBA (H-2k) or C57BL/Ks (H-2d), immunized with A/J (H-2a) spleen cells. The tests included in vivo tumor allograft evolution (accelerated rejection or enhancement reactions), and in vitro analysis of the involved immune agents, both cellular and humoral, using mixed lymphocyte reactions (MLR) and biological activity studies of serum samples. Animals from the recipient strains exhibited a delayed rejection of A/J tumor Sa 1 allografts if preimmunization was carried out with 10(6) A/J spleen cells combined with PE syngeneic to the recipients, as compared to controls immunized with A/J cells only or supplemented with isogeneic liver extracts (LE). The serological analysis revealed that PE treatment did not modify the overall hemagglutinating antibody production but resulted simultaneously in both a decreased production of cytotoxic complement fixing antibodies and an increase of specific anaphylactic mast cell degranulating antibodies, as compared to controls. The sera from PE-treated donors also demonstrated enhancing activity following passive transfer to isogeneic recipients. MLR regulatory activity was exhibited by spleen cells from PE- and immunogen-treated mice although the same or stronger activity was obtained from mice immunized without the addition of PE. However, in vivo transfer of these cells to syngeneic recipients showed that PE treatment erased the accelerated rejection caused by allogeneic immunization in the absence of PE and could even cause some degree of allografted tumor enhancement. The cells responsible for this inhibitory effect were mainly IJ+ lymphocytes, since their elimination with a relevant anti-IJ serum and complement restored a secondary type rejection pattern. These results show that PE present during the onset of immunization can promote the activation of regulatory agents such as enhancing antibodies and suppressor cells favoring allograft survival.

Modulation of mouse anti-SRBC antibody response by placental extracts

Mouse placental extracts (PE) and corresponding Sephadex G-200 fractions were administered to isogeneic CBA mice along with an optimal immunizing dose of SRBC. Spleen cells were harvested 8 days later and transferred to CBA recipients, subsequently immunized with SRBC. The immunoregulatory activity of spleen cells from PE-treated donors was compared to cells from liver extract (LE)-treated controls or from mice immunized with SRBC only, using Cunningham's PFC direct and indirect tests. Within the dose range used, selective modulatory activities were obtained with cells from PE, but not from LE, treated mice, the latter being comparable to cell transfer effects from donors immunized with SRBC only. Spleen cells from animals injected with low doses of PE (0.25 to 4 mg per mouse) added to immunizing SRBC had a suppressive effect on the primary IgM response of recipients immunized against SRBC. In contrast, when SRBC were given to donor animals with higher doses of PE (8 to 13 mg), transferred spleen cells potentiated the IgM response of the recipients. These opposite suppressive and potentiating activities were found in distinct Sephadex G-200 fractions of 40 and 60 kDa, respectively. When the effect of PE treatment was tested within the same animal, the indirect secondary PFC response following a challenge with SRBC was significantly modified. We observed an overall suppression of the different isotypes after treatment with lower doses of PE or with its 40-kDa fraction. PE doses of 0.5 to 2 mg resulted in a stronger inhibition of IgM than IgG1 production. This phenomenon was also obtained with the 40 KDa fraction. IgG2 responses were significantly reduced by all doses of this fraction. In contrast, all doses of the 60-kDa fraction gave a strong stimulation of IgG2 and IgM responses and a constant suppression of the IgG1 response. This shows a clear dissociation between IgG1 and C'-fixing (IgM, IgG2) antibody classes as far as the influence of placental substances is concerned in their regulation. These data emphasize the relevance of isogeneic placental products as a useful physiological material capable of modulating xenogeneic immune responses (as well as allogeneic systems).