The human amnion is a site of MHC class Ib expression: evidence for the expression of HLA-E and HLA-G.

The expression of HLA class I Ag by term human amnion epithelial cells was investigated. In immunostaining and FACS analysis, mAb to monomorphic class I Ag reacted extensively with amnion cells, whereas polymorphic mAb reactivity was more limited and variable. Further studies were conducted on amnion cell preparations containing negligible contaminants. Northern analysis with use of locus-specific probes demonstrated that amnion expresses two class Ib genes, HLA-E and HLA-G. Radio-immunoprecipitation with use of monomorphic mAb identified two fully glycosylated cell surface class I H chains of 44 and 41 kDa; polymorphic mAbs failed to immunoprecipitate the 41-kDa product, although 44-kDa products, typical of class Ia Ag, were identified in some preparations. Class I H chains were isolated from amnion by affinity chromatography. Microsequencing revealed that the first nine residues of the N-terminus of the 41-kDa product aligned perfectly only with HLA-E. Overall, amnion at term appears to express class Ib Ag with limited class Ia Ag. HLA-G is therefore expressed in two extrafetal epithelia: amnion and trophoblast. Identification of the class Ib protein HLA-E in amnion epithelium may have implications for preterm labor that can be associated with infection of the placental membranes.

The Lutheran blood group glycoprotein, another member of the immunoglobulin superfamily, is widely expressed in human tissues and is developmentally regulated in human liver.

Glycoproteins expressing the Lutheran blood group antigens were isolated from human erythrocyte membranes and from human fetal liver. Amino acid sequence analyses allowed the design of redundant oligonucleotides that were used to generate a 459-bp, sequence-specific probe by PCR. A cDNA clone of 2400 bp was isolated from a human placental lambda gt 11 library and sequenced, and the deduced amino acid sequence was studied. The predicted mature protein is a type I membrane protein of 597 amino acids with five potential N-glycosylation sites. There are five disulfide-bonded, extracellular, immunoglobulin superfamily domains (two variable-region set and three constant-region set), a single hydrophobic, membrane-spanning domain, and a cytoplasmic domain of 59 residues. The overall structure is similar to that of the human tumor marker MUC 18 and the chicken neural adhesion molecule SC1. The extracellular domains and cytoplasmic domain contain consensus motifs for the binding of integrin and Src homology 3 domains, respectively, suggesting possible receptor and signal-transduction function. Immunostaining of human tissues demonstrated a wide distribution and provided evidence that the glycoprotein is under developmental control in liver and may also be regulated during differentiation in other tissues.

Complement regulatory proteins in early human fetal life: CD59, membrane co-factor protein (MCP) and decay-accelerating factor (DAF) are differentially expressed in the developing liver.

The human fetus appears to be capable of protecting itself from maternal complement (C) from an early stage in development by expressing the C regulatory proteins decay-accelerating factor (DAF), membrane co-factor protein (MCP) and CD59 on fetally derived trophoblast at the feto-maternal interface. In this study we have examined the ontogeny of these proteins within the fetus itself and have focused on the liver which represents a major site of haemopoiesis during development. Immunostaining revealed that DAF, MCP and CD59 are all expressed from at least 6 weeks of gestation in the liver but that these proteins display distinct distribution patterns. CD59 was broadly distributed both within the epithelial and haemopoietic compartments, but expression of C3 convertase regulators was more restricted. DAF expression was limited to isolated cells within haemopoietic nests and the epithelium was DAF-negative. Although MCP expression on haemopoietic cells was also limited, by contrast with DAF the developing hepatic epithelium was strongly MCP-positive. Typical CD59 and MCP components were observed in fetal liver extracts by immunoblotting, although liver MCP components consistently migrated 4000-5000 MW ahead of those observed on placental trophoblast. Differences in the distribution of these proteins were also observed between the fetal and adult liver. In particular, by comparison with fetal hepatic epithelium, there was an apparent loss of MCP expression from adult hepatocytes. Thus, MCP appears to be developmentally regulated in the human liver and is expressed in the absence of DAF on the early hepatic epithelium. Overall, this study suggests that C regulatory proteins, and in particular CD59 and MCP, are required from the very early stages of gestation within the fetus itself.

Evidence for the expression of non-HLA-A,-B,-C class I genes in the human fetal liver.

HLA class I Ag expression was investigated in the human fetal liver. By immunostaining, mAb to monomorphic class I Ag showed widespread reactivity with both epithelial and hemopoietic cells. By contrast, mAb to polymorphic determinants showed more restricted reactivity that was confined to a proportion of hemopoietic cells: the hepatic epithelium was essentially unreactive. This suggested that the developing liver might express nonclassical HLA class I. Class I Ag were examined in membrane and cytosol fractions of mid-trimester fetal liver. Because of its broad reactivity with HLA-A,-B, mAb Q1/28 was selected to identify classical class I Ag in these studies. Immunoprecipitations were carried out against radiolabeled glycoprotein extracts of fetal liver membranes. W6/32 detected a 40-kDa product characteristic of nonclassical class I proteins, as well as a 43-kDa product, in lysates immunodepleted with Q1/28. By immunoblotting, an anti-H chain antiserum (HC) identified a Q1/28- 40-kDa component and a 43-kDa Q1/28+ component in fetal liver membrane glycoproteins. The fetal liver cytosol fraction was found to contain a 42- to 43-kDa product by W6/32-chromatography. This component partitioned to the aqueous phase upon condensation in TritonX-114 detergent and by immunoblotting was reactive with monomorphic mAb HC10 but not with Q1/28. Total RNA and polymerase chain reaction amplified class I transcripts of fetal liver were probed using oligonucleotides specific for HLA-E, -F, and -G. HLA-F, was readily detected in total RNAs by Northern analysis. HLA-E, HLA-F, and HLA-G were all detected in fetal liver by polymerase chain reaction. Differential expression of these genes may occur between the first and second trimester of liver development. Overall therefore, the human fetal liver expresses multiple class I protein products and contains transcripts for non-classical class I genes; particularly HLA-F.

Complement regulatory proteins at the feto-maternal interface during human placental development: distribution of CD59 by comparison with membrane cofactor protein (CD46) and decay accelerating factor (CD55).

The complement (C) regulatory proteins decay-accelerating factor (DAF, CD55) and membrane cofactor protein (MCP, CD46), which control C3 convertases, together with CD59, an inhibitor of the membrane attack complex (MAC), were found to be present in the developing human placenta from at least 6 weeks of gestation until term. Immunostaining revealed differences in the distribution of these proteins on the fetally derived trophoblast epithelium, especially in early placentae which contain trophoblast populations of diverse proliferative potential and differentiation status. Expression of all three proteins occurred on the terminally differentiated syncytiotrophoblast epithelium covering chorionic villi and which is in direct contact with maternal blood. CD59 was also expressed on the underlying villous cytotrophoblast cells and on their extra-villous derivatives. These two populations showed differential expression of the C3 convertase regulators. Villous cytotrophoblast cells expressed MCP but were largely devoid of DAF. Proliferation of this population to generate extra-villous cytotrophoblast cell columns was associated with both an increase in DAF expression and a decrease in MCP expression. Throughout placental development, expression of DAF appeared to be lower than that of MCP and CD59 as assessed by solid-phase binding assays on isolated trophoblast membranes. Early placentae were also found to contain both DAF+ and DAF- chorionic villi. Conversely, expression of CD59 appeared comparatively high and transcripts for CD59 were found to be much more abundant than those for DAF in purified trophoblast cells. C regulatory proteins appear to play an important role throughout gestation in protecting the fetally derived human conceptus from maternal C. The differential expression patterns of the proteins on trophoblast may reflect differences in requirement for specific functional activities at different locations within the placenta.

Preferential expression of the complement regulatory protein decay accelerating factor at the fetomaternal interface during human pregnancy.

The expression of decay accelerating factor (DAF) was investigated in human fetal and extra-fetal tissues using a panel of mAb directed against different epitopes on the DAF molecule. By immunostaining, extensive reactivity was observed on the placental trophoblast epithelium and this was confined exclusively to sites of direct contact with maternal blood and tissues at the fetomaternal interface. Within the fetus, by contrast, very little staining was observed especially on hemopoietic cell populations in the developing liver. The antibodies identified a component of 70,000 m.w., comigrating on SDS-PAGE with red cell DAF, on isolated trophoblast membranes and an apparently quantitative increase in the expression of DAF was observed during placental development. Northern analysis using a DAF cDNA clone revealed 1.5-, 1.6-, and 2.2-kb mRNA transcripts typical of DAF in mRNA prepared from whole term placentae and from purified trophoblast cells. DAF appears to be preferentially expressed at the fetomaternal interface during development and may function specifically to inhibit amplification convertases formed at this site either directly or indirectly as a result of maternal complement activation. This molecule may play an important role in protecting the semiallogeneic human conceptus from maternal C-mediated attack.