Immunity to antigens associated with primate C-type oncoviruses in pregnant women

Expression and functions of human endogenous retroviruses in the placenta: an update

The placenta is unique amongst normal tissues in transcribing many different human endogenous retrovirus (HERV) families at high levels and this has led to the suggestion that HERVs may fulfil important functions in reproduction. This review discusses our current knowledge of the placental expression of HERVs, in particular the envelope proteins of ERV3 and HERV-W which may have critical roles in placental function.

Human endogenous retroviruses: nature, occurrence, and clinical implications in human disease

Retroviral diagnostics have become standard in human laboratory medicine. While current emphasis is placed on the human exogenous viruses (human immunodeficiency virus and human T-cell leukemia virus), evidence implicating human endogenous retroviruses (HERVs) in various human disease entities continues to mount. Literature on the occurrence of HERVs in human tissues and cells was analyzed. Substantial evidence documents that retrovirus particles were clearly demonstrable in various tissues and cells in both health and disease and were abundant in the placenta and that their occurrence could be implicated in some of the reproductive diseases. The characteristics of HERVs are summarized, mechanisms of replication and regulation are outlined, and the consistent hormonal responsiveness of HERVs is noted. Clear evidence implicating HERV gene products as participants in glomerulonephritis in some cases of systemic lupus erythematosus is adduced. Data implicating HERVs as etiologic factors in reproductive diseases, in some of the autoimmune diseases, in some forms of rheumatoid arthritis and connective tissue disease, in psoriasis, and in some of the inflammatory neurologic diseases are reviewed. The current major needs are to improve methods for HERV detection, to identify the most appropriate HERV prototypes, and to develop diagnostic reagents so that the putative biologic and pathologic roles of HERVs can be better evaluated.

Increased antiretroviral antibody reactivity in sera from a defined population of patients with systemic lupus erythematosus. Correlation with autoantibodies and clinical manifestations

OBJECTIVE

The implied role of retroviruses in the pathogenesis of murine systemic lupus erythematosus (SLE) led us to study antiretroviral antibodies in a population-based SLE cohort.

METHODS

Immunoassays using whole virus and synthetic peptides were performed on sera from 72 patients with SLE and 88 control subjects.

RESULTS

Reactions with whole baboon endogenous virus occurred more frequently in patients with SLE, and correlated with the presence of anti-RNP and anti-Sm. Some retroviral env and gag peptides, several of which were similar to U1 small nuclear RNP, reacted more strongly in patients with SLE, and their presence was correlated with discoid rash, hematologic disorder, and other symptoms.

CONCLUSION

These results provide circumstantial evidence for involvement of retroviruses in the pathogenesis of human SLE; further studies should be carried out using other techniques for measurement of retroviral expression.

The human non-lineage antigen CD46 (HuLy-m5) and primate retroviral gp70 molecules share protein-defined antigenic determinants

The CD46 lymphocyte surface antigen of man (until recently called HuLy-m5), and defined by the E4.3 monoclonal antibody (MoAb), shares cross-reactive antigenic epitopes with the envelope gp70 glycoproteins of gibbon ape leukaemia virus (GaLV) and Mason Pfizer monkey virus (MPMV) primate retroviruses. It is now shown that the cross-reactive antigenic epitope shared by these three molecules is determined solely by the protein portion of these glycoproteins, and that the N-linked and O-linked carbohydrate moieties of these glycoproteins do not directly or sterically contribute to the antigenic cross-reactivity. When CD46 molecules (mol.wt = 66 and 56 kDa) from human thymocytes were stripped of sialic acid with neuraminidase, or stripped of N-linked carbohydrate with endoglycosidase F, the E4.3 MoAb was still able to bind and immunoprecipitate the protein core of CD46 (mol.wt = 56 and 44 kDa). Similarly, polyclonal antisera to GaLV and MPMV precipitated deglycosylated CD46, although at a reduced efficiency. The cross-reacting E4.3 MoAb, anti-GaLV and anti-MPMV antisera also immunoprecipitated HuLy-m5 primary translation protein lacking N- or O-linked carbohydrate from the in vitro translation products of human thymocyte mRNA. Thus, the antigenic cross-reactivity of CD46 molecules with GaLV gp70 and MPMV gp70 is both specific and due to protein structure rather than to carbohydrate; the findings suggest that retroviruses may have acquired a functional epitope from human CD46 or that an endogenous retroviral sequence of human may partially or completely encode the CD46 antigen.

Maternal immune responses to oncofetal antigens

The maternal host responds immunologically to antigens of the fetus. While the immune responses to paternally derived alloantigens and to placental antigens have been intensively studied, the immune responses to oncofetal antigens have been relatively unexplored. Oncofetal antigens are present on fetal and malignant tissues but absent from normal adult somatic tissues. These antigens elicit both cell-mediated and humoral immune responses in the parous female. Limited data suggest that these immune responses may influence reproductive processes. More investigation in this area is desirable.

Frequency and antigenicity of type C retrovirus-like particles in human placentas

Seventeen placentas from normal Japanese women, obtained from 15 full term gestations and at 2 earlier gestational periods, were observed by electron microscopy for the presence of type C retrovirus-like particles. Immature free and budding particles were found in 16 placentas including those obtained pre-term. Free virus-like particles were seen in the intercellular spaces and within the lysosomes of syncytiotrophoblasts of the chorionic villi. Forms budding from syncytiotrophoblasts were found in 3 cases. Coincident data with the electron microscopic observations were obtained by immunohistological methods. Specific positive staining with antisera against heterogeneic whole murine leukaemia virus were seen within the cytoplasm of trophoblastic cells both in and along the basal lamina.

Synthesis of retrovirus-like particles in testicular teratocarcinomas

Two lines of evidence led to the investigation of human teratocarcinoma cells in vitro for oncogenic retroviruses: the observation by electron microscopy of retrovirus-like particles budding from the syncytial trophoblasts of human placentas, and the demonstration that teratocarcinoma patients before treatment show a high serum antibody reactivity against envelope proteins of mammalian retroviruses. In all 5 teratocarcinoma cell lines studied so far, retrovirus-like particles have been detected by electron microscopy. The production of these human teratocarcinoma-derived (HTD)-particles is enhanced by induction procedures known to be effective in animal virus model systems. In parallel, virus induction also increases the level of chorionic gonadotropin in the culture supernatant, demonstrating syncytial trophoblast-like cells in the heterogeneous cell population of teratocarcinomas which could be responsible for HTD-particle production. The origin of these virus-like particles--endogenous or exogenous--as well as their role in the pathogenesis of teratocarcinomas is not known so far. The potential value of serum antibody reactivity in teratocarcinoma patients as a diagnostic marker is discussed.

Human antibodies recognizing the envelope glycoprotein of the baboon endogenous virus BaEV are of heterophil origin

Human sera were previously shown to possess antibodies capable of recognizing purified retrovirus envelope glycoproteins. In an extension of earlier studies we investigated sera from various groups of patients for an immune reaction against purified glycoprotein of the baboon endogenous virus BaEV. Reproducible demonstrations of oncovirus-like particles in human teratocarcinomas focused our main interest on sera from patients with testicular tumors. The specificity of the positive immune reaction of sera from these patients against BaEV gp 70 was analyzed in a competition RIAs with haptens and different cell lysates and experiments employing deglycosylated BaEV envelope antigen. From these experiments we conclude that the sera from teratocarcinoma patients contain naturally occurring, heterophil antibodies that react with the carbohydrate moieties of retrovirus envelope antigens.

Retrovirus p30-related antigen in human syncytiotrophoblasts and IgG antibodies in cord-blood sera

Sensitive immunological techniques were used to detect retrovirus markers in human pregnancy. A total of 1,540 human cord-blood sera were tested for retrovirus-reactive IgG antibodies using solid-phase enzyme immunoassay and purified RD 114 virus as antigen. Of these, 118 (7.7%) sera were positive. Blocking assays with specific animal anti-p30 sera, use of control antigen, and electrophoretic protein experiments combined with immunological detection indicated that the human antibodies reacted specifically with the p30 protein. The occurrence of antibodies in cord-blood serum had a highly significant correlation to complications during pregnancy and also correlated to the number of previous abortions and stillbirths. When goat anti-RD 114 p30 serum was used in the peroxidase-anti-peroxidase tissue staining procedure, p30-related antigen was detected in sections of all placental specimens (early and term pregnancies, blighted ova, hydatidiform moles, destructive moles and choriocarcinomas). However, in each case only syncytiotrophoblastic cells were positive. These findings, supplemented with different types of blocking tests, lead us to conclude that retrovirus p30-related antigen is selectively expressed in the highly differentiated syncytiotrophoblasts, which in the normal placenta are directly exposed to maternal blood. It is suggested that retrovirus-reactive antibodies may represent an autoimmune-like immune response to the p30-related syncytiotrophoblast antigen escaping during cellular damage.

Natural antibodies to the human T cell lymphoma virus in patients with cutaneous T cell lymphomas

Sera from patients with cutaneous T cell lymphoma and leukemia were screened for the presence of natural antibody to the human T cell lymphoma (leukemia) virus, HTLVCR, using a solid-phase radioimmunoassay. Sera from two patients, including patient CR, from whose cultured T lymphoblastic cell line (HUT102), the retrovirus HTLVCR was isolated, reacted specifically with proteins of HTLVCR. Serum from patient CR also reacted specifically with proteins of HTLVMB, an independent but highly related retroviral isolate from a patient with Sezary T cell leukemia. The specificity for HTLVCR proteins was demonstrated by solid-phase immunocompetition assays and competition radioimmunoprecipitation assays. Analysis of radioimmunoprecipitates indicated that the natural antibodies were directed against HTLVCR core proteins with molecular weights of 24,000 and 19,000 (p24 and p19). Whereas the serum reactivities for HTLVCR proteins were shown to be highly specific, additional reactivities seen against proteins of animal retroviruses including GaLV, SSV, FeLV, and BaEV were clearly shown not to be viral specific but rather were due to reactivity with cellular antigens contaminating the viral preparations or with related antigens present in fetal calf serum. These results demonstrating natural antibodies to HTLVCR provide the first evidence for a specific antibody response to a retrovirus in humans.

Human placentas contain a specific inhibitor of RNA-directed DNA polymerase

Human placental extracts contain a specific inhibitor of mammalian retroviral RNA-directed DNA polymerase (deoxynucleosidetriphosphate:DNA deoxynucleotidyltransferase, EC 2.7.7.7) activity. This inhibitor copurifies with retrovirus-like particles in human placental extracts. The inhibitor can be removed from these particles by salt extraction, which leads to the recovery of the polymerase activity. Thus, the inhibitor does not irreversibly inactivate the particle-associated RNA-directed DNA polymerase activity. The inhibitory preparation contained no nuclease, protease, or phosphatase activity. Because its inhibitory action can be eliminated by the addition of more virus to the reaction, nonspecific inactivation of enzyme substrate has been ruled out. A partial characterization of the inhibitor indicates that it is (i) insensitive to ether, trypsin, and phospholipase C; (ii) stable to heat and pH 2-12; and (iii) nondialyzable.

Humans have antibodies capable of recognizing oncoviral glycoproteins: demonstration that these antibodies are formed in response to cellular modification of glycoproteins rather than as consequence of exposure to virus

There is controversy in the literature concerning the presence in humans of antibodies directed against the envelope glycoproteins of known oncoviruses. In the present report, we show that antibodies capable of precipitating a wide variety of oncoviral glycoproteins can be demonstrated under certain assay conditions. Substances as diverse as normal components of serum, extracts of bacteria, and even nonprotein molecules such as glycogen also shared the oncoviral glycoprotein determinants recognized by normal human sera. It was found that immunoprecipitation of a given viral glycoprotein by human sera was entirely dependent upon the cell in which the virus was grown. Human sera specifically did not recognize glycoproteins purified from oncoviruses grown in human or higher primate cells. These findings not only demonstrate that the antibodies were directed against cellular rather than the virus-coded antigenic determinants but also exclude the possibility that this immune response was elicited as a consequence of oncovirus exposure.

Specificity of human antibodies to oncovirus glycoproteins: recognition of antigen by natural antibodies directed against carbohydrate structures

Antibodies in human sera from healthy individuals were shown to be reactive with highly purified 70,000-dalton envelope glycoprotein (gp70) of the simian sarcoma virus-simian sarcoma-associated virus (SSV-SSAV) complex in radioimmunoprecipitation assays under certain conditions. The specificity of the reaction was analyzed in absorption tests with normal human serum proteins, assays of viral gp70 antigenicity after exposure to exo- and endoglycosidases or trypsin, and carbohydrate hapten inhibition studies. On the basis of the results obtained in these experiments we have concluded that immune recognition of SSV-SSAV gp70 can be mediated by naturally occurring heterophil antibodies in human sera that are reactive by virtue of binding to the carbohydrate moiety of the viral gp70 molecules. The results are consistent with the idea that the antibodies in question are elicited as a result of exposure to many natural substances possessing widely crossreacting antigens and are not a result of widespread infection of man with replication-competent oncoviruses.

Comparison of radioimmunoprecipitation assays for the detection of human anti-tumor virus antibodies

The demonstration of human antibodies reactive in radioimmunoprecipitation assays (RIAs) with primate tumor virus (oncornavirus) antigen has implications for a possible previously published negative findings and led to considerable scientific controversy. We feel much of the discrepancy may be of methodological origin. An attempt is therefore made in this communication to resolve these apparent discrepancies by comparing various published parameters of the RIAs used in the search for human antibodies reactive with oncornavirus antigens.

Placenta-bound immunoglobulins

Low pH eluates were prepared from trophoblasts derived from 8 term human placentas. A qualitative analysis for immunoglobulins revealed the presence of IgG, IgA, and IgM in these eluates. IgC-rich fractions were obtained by DEAE-cellulose chromatography of ammonium sulfate-concentrated eluates. These fractions were able to neutralize, in vitro, the catalytic activity of reverse transcriptases (RT) from several retroviruses. RT from baboon endogenous virus (BEV) seemed to be more susceptible to the neutralizing activity of some eluates. This was in contrast to RT from feline leukemia virus (FeLV) which were neutralized by eluates of leukocytes from chronic myelogenous leukemia. In contrast to previous and present results with purified IgG from leukemic leukocytes, the purified IgG from placenta eluates was incapable of RT neutralization. However, such purified IgG fractions inhibited mixed lymphocyte reactions.