Hypothesis: blocking factors and human pregnancy: an alternative explanation for the success of lymphocyte transfusion therapy in abortion-prone women

Fetal-maternal immune interaction: blocking antibody and survival of the fetus

In the late 1940s it became clear that the homograft reaction was essentially the result of an immune response. Subsequently, Medawar commented on the apparent paradox of the survival of the mammalian fetus in the face of such a potential (cell-mediated) immune response. In an outbred population the fetal-placental unit will be antigenically different to the mother by virtue of its complement of paternal genes and additionally there may be developmental or stage-specific gene products that are immunogenic. Many mechanisms have been proposed to account for the survival of the fetus in the face of a potential immune attack and, while many of these have been investigated in considerable detail, there has been no clear-cut indication that any one plays a predominant role. Either control of immune rejection of the fetus is exercised by an as yet undiscovered mechanism or, more probably, by a combination of some or all of the mechanisms that have been proposed by many workers over the last three decades. Potential controlling processes, which will be reviewed briefly, include: systemic and local modification of maternal responsiveness; altered expression of MHC antigens on extra-embryonic tissues; the placenta as a barrier; and blocking antibody responses. We discuss some of our recent studies in which we have started to look for potential blocking antibodies in a mouse model system. Cells secreting immunoglobulins M and G, characterized in hemolytic plaque assays, have been mapped to areas close to the midgestation mouse embryo, using an immunocryohistological technique. A scaled-down version of hybridoma technology has been used as an analytical probe of the specificity and isotype of immunoglobulin secreted by cells originating either from close to the embryo/fetus or from the para-aortic lymph nodes (PALN). So far monoclonal (IgG1) antibodies with specificity for embryonic cells have been derived together with some monoclonal immunoglobulins with as yet uncharacterized antibody specificity.

The normal fetomaternal immune relationship

The antigenic status of the preimplantation embryo is ill-defined and there are no clearly recognized maternal immune reactions against this early stage of development. Following implantation, the pregnant female shows evidence of immune recognition of her intrauterine allogeneic conceptus. In a proportion of pregnancies, particularly in multiparous women, there are maternal cytotoxic antibodies exhibiting specificity for the paternally inherited HLA antigens of the fetus. When these are undetectable there may be other antibodies that are non-complement fixing and non-cytotoxic or antibodies that are not present as free molecules and incapable of identification in conventional assays. Anti-HLA antibodies pose no threat to the fetus, principally owing to their absorption by the placenta and, very likely, the harmless binding of any that do reach the fetal circulation. No potentially deleterious cytotoxic T lymphocyte generation occurs in most pregnancies. The extent to which this is due to maternal immunoregulatory control processes is not yet established. The fetal trophoblast is able to act as a protective barrier by virtue of unique properties, including a lack of conventional class I and class II HLA molecules, that render it insusceptible to immune attack. The nature and significance of any maternal recognition of non-HLA antigens on trophoblast await elucidation. Maternal immune cell traffic across the placenta occurs only at a very low level, if at all, in normal pregnancy. This may take place to a greater degree in some of the rare instances of fetal graft-versus-host disease, but this is complicated by the associated fetal immunodeficiency. Maternal IgG antibodies are transmitted across the placental trophoblast by receptor-dependent mechanisms to provide immediate protection for the neonate against environmental pathogens. Leakage of fetal erythrocytes, leukocytes and platelets into the maternal circulation can elicit IgG isoantibodies that take advantage of the same mechanisms to gain access to the fetus, with pathological consequences. Autoantibodies in women with various disease states may similarly pass into the fetus but these normally produce only mild and transient effects. The development of the fetal immune system begins at an early stage of gestation. It is competent to respond to intrauterine infections from as early as 12 weeks and has full functional potential at birth. Maternally acquired IgG is available for up to 9 months of life until the infant's own immune system has been adequately primed and activated following first exposure to specific antigens. The normal fetomaternal immune relationship represents a remarkable harmonious association between two genetically disparate individuals.(ABSTRACT TRUNCATED AT 400 WORDS)

Reduction in popliteal lymph node graft-versus-host reactivity by homologous and heterologous pregnancy serum

The popliteal lymph node assay was used to investigate the effect of pregnancy on graft-versus-host reactivity (GvHR) of mouse spleen cells. After local injection of splenocytes from primiparous syngeneically pregnant (by BALB/cJ males) or allogeneically pregnant (by CBA/Ca males) mice no differences in lymph node weight gain were observed in F1 recipients (CBA/Ca x BALB/cJ) when compared to injections of cells from age-matched non-pregnant BALB/cJ mice. However, lymphocytes of pregnant BALB/cJ females which had previously been pregnant between 4 and 6 times by CBA/Ca males induced a significantly lower GvHR compared to cells of matched non-pregnant multiparous mice. These results suggested an inhibitory effect of gestation on cells possibly primed towards paternal antigens by multiple pregnancies. To test this hypothesis, virgin BALB/cJ mice were actively immunized with lymphocytes of male CBA/Ca mice. Before injection into F1 recipients, spleen cells of immunized animals were incubated for 1 h at 37 degrees C in heat-inactivated serum of primiparous pregnant or virgin non-pregnant mice. Pre-incubation in pregnancy serum had no effect on unprimed cells, but GvHR of cells derived from immunized donors was significantly depressed in female recipients. In male animals this effect was only irregularly observed. Inhibition of GvHR was also observed with serum from pregnant but not non-pregnant pigs. Depression of cellular immune response was observed as early as days 4-9 post-coitum (p.c.) with mouse serum and days 16-19 p.c. with pig serum. These results indicate that pregnancy serum contains factor(s) which modulates the GvHR of primed lymphocytes in both a species- and an antigen-non-specific manner while reactivity of naive spleen cells is not changed.