Human alpha-fetoprotein as a modulator of human lymphocyte transformation: correlation of biological potency with electrophoretic variants

Tumor-derived α-fetoprotein impairs the differentiation and T cell stimulatory activity of human dendritic cells

Several tumor-derived factors have been implicated in dendritic cell (DC) dysfunction in cancer patients. α-fetoprotein (AFP) is an oncofetal Ag that is highly expressed in abnormalities of prenatal development and several epithelial cancers, including hepatocellular carcinoma (HCC). In HCC patients exhibiting high levels of serum AFP, we observed a lower ratio of myeloid/plasmacytoid circulating DCs compared with patients with low serum AFP levels and healthy donors. To test the effect of AFP on DC differentiation in vitro, peripheral blood monocytes from healthy donors were cultured in the presence of cord blood-derived normal AFP (nAFP) or HCC tumor-derived AFP (tAFP), and DC phenotype and function were assessed. Although the nAFP and tAFP isoforms only differ at one carbohydrate group, low (physiological) levels of tAFP, but not nAFP, significantly inhibited DC differentiation. tAFP-conditioned DCs expressed diminished levels of DC maturation markers, retained a monocyte-like morphology, exhibited limited production of inflammatory mediators, and failed to induce robust T cell proliferative responses. Mechanistic studies revealed that the suppressive activity of tAFP is dependent on the presence of low molecular mass (LMM) species that copurify with tAFP and function equivalently to the LMM fractions of both tumor and nontumor cell lysates. These data reveal the unique ability of tAFP to serve as a chaperone protein for LMM molecules, both endogenous and ubiquitous in nature, which function cooperatively to impair DC differentiation and function. Therefore, novel therapeutic approaches that antagonize the regulatory properties of tAFP will be critical to enhance immunity and improve clinical outcomes.

Structure and function of alpha-fetoprotein: a biophysical overview

alpha-Fetoprotein (AFP) is a large serum glycoprotein belonging to the intriguing class of onco-developmental proteins. AFP has attracted considerable attention since it was shown that the change in its serum level during pregnancy is a hallmark of the development of numerous embryonic disorders, while the increase in its content in the plasma of adults correlates with the appearance of several pathological conditions. Over the past 30 years, some 11000 papers have been published concerning AFP, an average rate of over a publication a day since 1969. The majority of publications are about the application of the protein in diagnostics, or about other uses of AFP in biomedicine; though some of them describe the biochemical and functional properties of AFP, two aspects have been extensively reviewed. However, surprisingly little is currently known about structural properties of this protein as well as about the molecular mechanism of its function. The present review pursues the aim to describe the current state of the art in studies of structural properties and conformational stability of AFP. An attempt to establish the relationship between conformational transformations in AFP and its function is also made.

Evidence that immunosuppression is an intrinsic property of the alpha-fetoprotein molecule

Among the proteins that comprise the albumin family, alpha-fetoprotein (AFP) is the only member which exhibits immunoregulatory properties. However, some investigations have argued that AFP-mediated immunosuppression is not an inherent property of the molecule itself, but is instead, hypothesized to be either a function of a low molecular weight inhibitor bound to AFP or to a post-translational modification of the protein. AFP cannot be isolated from natural sources in quantities sufficient for the detailed biochemical and functional analyses required to resolve these issues. We have therefore produced recombinant forms of the protein (rAFP) by cloning the cDNA's for mouse and human AFP in both eukaryotic and prokaryotic expression systems. As described in this report, we were able to abundantly express rAFP's in bacterial, baculovirus and yeast expression systems. Recombinant proteins derived from each expression system were recognized by polyclonal and monoclonal anti-AFP antibodies as determined by immunoblot analysis. Pure recombinant protein samples, as characterized by polyacrylamide gel analyses, N-terminal sequencing and FPLC and HPLC chromatography, were evaluated for their immunoregulatory properties in murine and human in vitro immunological assays. The results of these studies establish that rAFP is functionally equivalent to natural fetal derived AFP molecules. Importantly, the data reported here demonstrate that AFP-mediated immunoregulation is an activity intrinsic to the molecule itself and cannot be attributed to either putative non-covalently bound moieties or to post-translational modifications such as glycosylation and sialylation. These studies provide a basis for initiating detailed investigations into the potential clinical usefulness of AFP as an immunotherapeutic agent.

The immunomodulatory activity of human amniotic fluid can be correlated with transforming growth factor-beta 1 (TGF-beta 1) and beta 2 activity

The role of alphafetoprotein (AFP) in the immunomodulatory activity of amniotic fluids (AF) from normally progressing human pregnancy (weeks 14-16) was investigated. A panel of 42 AF (25% v/v) reduced significantly phytohaemagglutinin (PHA)-induced peripheral blood mononuclear cell (PBMC) proliferation in serum-free cultures with a mean per cent inhibition of 68.4 +/- 5.5%. In contrast, AFP preparations, with one exception (U.AFP), failed to display inhibitory activity. Pretreatment of AF with anti-TGF-beta 1 and beta 2 antibodies used alone resulted in the mean per cent loss of inhibition of 33.1 +/- 3.9% and 52.3 +/- 7.5%, respectively. A summative loss of AF-mediated inhibition was detected when anti-TGF-beta 1 and beta 2 antibodies were used in combination, but immunomodulation was rarely abolished 100% by this treatment. Anti-TGF-beta 2 antibody treatment, unlike anti-TGF-beta 1 antibody treatment, reversed the inhibitory activity of U.AFP. The amount of TGF-beta 1 and beta 2 contained in human AF was studied by growth inhibition of Mv1 Lu cells. The mean levels of TGF-beta 1 and beta 2 in AF were 11 +/- 0.9 U/ml and 2.3 +/- 0.4 U/ml, respectively, which corresponds with a mean per cent inhibition of 49 +/- 4.7%. U.AFP also significantly inhibited Mv1 Lu cell growth. To investigate the mechanism of AF-mediated inhibition, the effect of AF and AFP on IL-2 production by concanavalin A (Con A)-stimulated PBMC blasts was determined by the CTLL-2 cell bioassay. IL-2 production was reduced 55.5% in AF-treated blasts and 61% in U.AFP-treated blasts compared with controls. Our findings indicate that the immunomodulatory activity of human AF can be correlated with TGF-beta 1 and beta 2 and not with AFP, the inhibitory activity of U.AFP preparation reflecting copurifying TGF-beta 2 activity.

Human alpha-fetoprotein (AFP) causes a selective down regulation of monocyte MHC class II molecules without altering other induced or noninduced monocyte markers or functions in monocytoid cell lines

Human alpha-fetoprotein (AFP) purified from human amniotic fluid was investigated for its effect on human monocytoid cell lines, including U 937 cells with established subclones. The impact of AFP on the expression of surface markers (MHC class I and II, CD4, CD18, CD45, Fc receptors for IgG) was analyzed using known inducers of monocyte-macrophage differentiation such as phorbol esters and IFN-gamma. Furthermore we investigated the effect of AFP on the induction of macrophage antibody-dependent cell-mediated cytolytic activity (ADCC). AFP did selectively induce a rapid down regulation of surface MHC class II expression. No evidence of alterations was found in the endogenous or differentiation-induced expression of other markers on the surface on monocytes, nor did AFP affect the functional maturation of surface Fc receptors or the ability to express ADCC.

Chemistry and biology of alpha-fetoprotein

Alpha-Fetoprotein (AFP) is a product of specific fetal tissues and of neoplastic cells of hepatocyte or germ cell origin in adults. This protein belongs to a gene family that is phylogenetically most closely related to serum albumin. Its primary, secondary, and tertiary structural aspects appear similar to the three-domain concept proposed for the latter protein. The primary sequence of AFP departs most widely from serum albumin in the first 135 amino acid residues, with about 42% of the remaining 590 residues of the human proteins being identical. Some evidence exists that there are limited sequence differences in the AFP of a given animal species. AFP shows considerable charge heterogeneity that appears to relate mostly to its glycoid moiety. The proteins of some species such as the rat show more pronounced heterogeneities than that of humans. The variations in extent and type of glycosylations are evidenced by differences in the binding to various lectins. These interactions are being extensively explored in attempts to differentiate the sources of the protein produced by various normal and neoplastic cells and may provide valuable diagnostic methods. AFP, like serum albumin, shows relatively strong binding affinities for a variety of ligands. The most notable difference is the strong preferential binding of polyunsaturated fatty acids by AFP. This protein may play a role in transporting these substances to developing and to malignant cells. Various agents affect the synthesis of this protein both by specific fetal tissues and by neoplastic cells. Marked differences in the responses of cells, particularly those of neoplastic types, are indicative of variations in the genetic factors responsible for control of its synthesis. The subject of the genomic repression of the synthesis of AFP seen in fetal life upon maturation of the liver and the reoccurrence of synthesis upon malignant conversion of hepatocytes and of certain germ cells are of particular interest. The regulation of the closely related AFP and albumin genes is providing a powerful and attractive model to examine molecular events in the activation and inactivation of specific genes during development and in oncogenic processes. Extensive measurements of AFP during pregnancy and in the course of neoplasias, notably hepatoma, are being made to aid in following changes in such developments. Various specific physiological roles for this protein are also being proposed. One of these is its possible action in the regulation of immune processes.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of human alpha fetoprotein charge microheterogeneity during fetal development

Aliquotes of human amniotic fluid (AF), fetal serum (FS), and cord blood (CB) were obtained as by-products of routine clinical diagnostic procedures at term or in the second trimester of pregnancy. When samples of CB were applied to a pH 5.5-4 chromatofocusing gradient, three isoforms of AFP could be resolved; a pl 4.57 form (isoform IA, 52% AFP), a pl 4.27 form (isoform IB, 43% AFP), and one species that was bound to the column but could be eluted with 1.0 M NaCl (isoform II, pl less than 4.00, 5% AFP). Term AF displayed a profile similar to that observed in term CB. When samples of 15-20-week gestation AF were chromatofocused, the immunoreactive AFP recovered was distributed between isoform IA and IB (60%) and isoform II (40%). FS and AF obtained from same pregnancy (23-26 weeks) displayed an identical chromatofocusing profile. Aliquotes of AF subjected to conA revealed 83% reactive variants compared with greater than 95% reactive variants for CB. FS displayed a conA profile identical to CB. When individual CB charge isoforms were isolated and subjected to conA analysis, greater than 97% of the AFP bound to conA. In contrast, when AFP isoform IA and IB were isolated from midgestation AF, approximately 22% of the AFP did not bind to the lectin while 100% of isolated AFP isoform II eluted as the reactive variant. These data suggest that human AFP exists as at least three charge and two lectin variants and that the charge profile may change during fetal development.

Analytical- and preparative-scale separation of molecular variants of alpha-fetoprotein by anion-exchange chromatography on Monobead resins

A rapid and reliable purification procedure is described that is useful for both analytical detection and quantitative recovery of milligram amounts of individual molecular variants of mouse alpha-fetoprotein (AFP). The appropriate separation conditions were developed with an analytical-size Mono Q anion-exchange column linked to an automated Fast Protein Liquid Chromatography system. Effective separations of fetal-derived AFP variants was accomplished within 20 min under mild conditions with an L-histidine buffer. Employing the optimal separation conditions established on the Mono Q HR 5/5 column we upscaled the procedure by using a preparative Mono Q HR 16/10 column in order to obtain milligram quantities of each molecular variant of AFP. Seven distinct isomeric forms of AFP could be recovered on the preparative anion exchanger in a highly reproducible manner. Each of the seven protein peaks eluted from the Mono Q column were confirmed to be distinct isoforms of AFP by isoelectric focusing and Western blotting developed with monospecific anti-AFP antisera. This method in its scaled up version offers the benefit of providing milligram quantities of immunochemically pure AFP isomers for structure and function studies.

Isolation and characterization of a distinct immunoregulatory isoform of alpha-fetoprotein produced by the normal fetus

In this report, we examine the functional significance of the molecular microheterogeneity of alpha-fetoprotein (AFP). In doing so, we have taken the direct approach of purifying the naturally occurring isomeric forms of fetal-derived AFP using a preparative anion exchange column linked to an automated fast protein liquid chromatography (FPLC) system followed by parallel testing of each isolated molecular variant for in vitro immunoregulatory activity. The data obtained demonstrate the presence of seven distinct variants of AFP as defined by their retention volumes on FPLC elution profiles, by their pIs on analytical IEF gels, and by Western blot analysis. Molecular mass determination by SDS-PAGE showed each isomer to be equivalent in size to 69,000-dalton native unfractionated AFP molecules. All the immunosuppressive activity of AFP was localized to a single variant representing only 6% of the total composition of native AFP. The immunoregulating isomer termed AFP-1 was the least acidic of the seven isolated variants with a pI of 5.1 and displayed a sialic acid content of 1 mol/mol of protein. The inhibitory activity of AFP-1 could be readily measured on T cell-dependent antibody synthesis, Con A-induced stimulation of Lyt-1+23- thymocyte DNA synthesis, and lymphokine-activated NK cell activity. All other isomers were without effect in these test systems. The immunosuppressive AFP-1 isomer also displayed the strongest growth-promoting influence on cultured bone marrow lymphocytes. There was no correlation between functional activity and degree of expression of sialic acid residues on the AFP molecules. These findings demonstrate that the immunoregulating function of AFP is confined to a distinct and relatively small subpopulation of native AFP molecules and should therefore contribute to the resolution of outstanding questions regarding the structure/function relationship of this onco-fetal glycoprotein.

Studies on lectin affinities of chicken alpha-fetoprotein during ontogeny

Alpha-fetoprotein in the sera of chicken embryos (ch-AFP) of the 7th, 10th, 13th, 16th, and 19th day of incubation and of one-day-old chickens were examined in crossed-immunoaffino-electrophoresis (CIAE) using different lectins [concanavalin A (Con A), lentil, wheat germ lectin, and soybean lectin] as ligands. Differences in lectin affinities, suggesting different degrees or forms of glycosylation of ch-AFP during ontogeny, were demonstrated by this method. Sera of chicken embryos of the same stages were also subjected to Con A affinity chromatography and the elution patterns were evaluated qualitatively in CIAE and quantitatively in rocket immunoelectrophoresis. These experiments revealed a major Con A-nonreactive and a minor Con A-reactive fraction at the 7th day of incubation. In contrast, a minor Con A nonreactive fraction and a major Con A-reactive fraction were observed at later stages of embryonic life. Extracts from embryos of the 8th and 15th day of incubation did not exhibit heterogeneity when subjected to CIAE. The biological significance of these observations, as well as the biological role of AFP in general, remains to be elucidated. Examination of ch-AFP glycosylation patterns might be included into the spectrum of analyses when certain early teratological processes in the chicken embryo are to be monitored. In addition, these molecular changes have to be considered when in vitro or in vivo experiments on the biological, especially immunological, role of ch-AFP are envisaged.

Response of human peripheral blood lymphocytes in the presence of cord sera: relationship of lymphocyte transformation with number of pregnancies and levels of alpha-fetoprotein

Peripheral blood lymphocytes were stimulated with various mitogens (PHA, Con A and PWM) and allogenic cells in the presence of cord serum. The cord sera suppressed more effectively certain T cell populations. This immunoregulatory response of cord serum was compared with nonpregnant and pregnant sera from different gestations. Although the cord sera showed significantly higher inhibition compared to normal adult serum but the serum from gravidae in late gestation proved more inhibitory compared to cord serum. No correlation was found between the levels of alpha-fetoprotein (AFP) in the cord sera and the degree of inhibition observed in the cultures. As previously described for pregnant sera the cord serum obtained from women with different numbers of pregnancies did not correlate with the inhibition of cellular proliferation. Our results suggest a possible different suppressor factor in pregnant and cord sera and also excludes the possibility of AFP in cord sera as a suppressor agent of cell-mediated immunity.

Purification and physicochemical and immunological analysis of chicken alpha-fetoprotein

Chicken alpha-fetoprotein was isolated from 12 to 13-day-old embryonic chicken serum by column chromatography on CM-Sephadex C-50. Hydroxyapatite and DEAE-Sephadex A-25. The purified protein was homogeneous based on polyacrylamide gel electrophoresis, immunoelectrophoresis and isoelectric focusing. The purified protein had the following physicochemical and immunological properties. (1) It was a glycoprotein with a single polypeptide chain. (2) The molecular weight of the protein was estimated at 71,000 by SDS-polyacrylamide gel electrophoresis. (3) The isoelectric point of the protein was 4.90. (4) The amino acid composition of the protein was similar to those of mammalian alpha-fetoproteins. (5) The protein showed no steroid-binding capacity. (6) It was immunologically distinct from mammalian alpha-fetoprotein. (7) No immunological cross-reaction was observed between the protein and chicken albumin.

Characterization of the suppressor cell activity in human cord blood lymphocytes

Newborn mononuclear cells are known to have increased suppressor activity when compared with adult cells. However, the precise phenotypic description of the suppressor cell subpopulation has not yet been reported. This study was designed to examine the surface markers on human cord blood cells as defined by monoclonal antibodies and to characterize cord blood mononuclear cells that suppress in vitro pokeweed mitogen-driven immunoglobulin production. The pattern of fluorescence with the 9,6-monoclonal antibody suggested either a decreased density or a partial blocking of E-receptors on cord blood lymphocytes. Otherwise, human cord and adult cells had similar proportions of T cell subpopulations. Different subsets of newborn cells isolated by a monoclonal antibody rosetting technique were tested for suppressor activity. Cord blood lymphocytes recognized by the OKT8 monoclonal antibody constituted the major functional suppressor cell subpopulation. These cells were present in both E-rosetting and non-E-rosetting populations. Removal of OKT8+ mononuclear cord blood cells abrogated most of the suppressor activity. Thus the major suppressor cell present in cord blood is an OKT8 positive lymphocyte.

Effects of polycationic compounds on mitogen stimulation

The effects of polycations added to phytomitogen stimulated human lymphocyte cultures have been studied. Within certain dose ranges all polycations tested gave rise to augmented thymidine uptake in mitogen stimulated cultures. The optimum enhancing concentrations of polycations was depending on the serum concentration in the culture medium. This was found to be due to two types of interactions: (a) Interference with mitogen binding serum factors, (b) Reaction with immunosuppressive serum proteins. Suggestive evidence for an affect of polycations also directly on the cells was found by pretreatment and cell density experiments. The direct effect on cells was found not to be due to monocyte bypass or to activation of non-T cells by the mitogen. It is pointed out that effect of chemicals on in vitro immunresponses have to be considered in relation to the charge properties of the compound.

Inhibition and stimulation of rat thymocytes in vitro by partial hepatectomy

Male Sprague Dawley rats were partially hepatectomized. At different times after operation, their thymocytes were isolated and incubated for 5 hours at 37 degrees C in an RPMI medium with 10% rat control serum and their incorporation of 3HTdR was then determined. 1 and 2 days after partial hepatectomy, the thymocytes incorporate much less 3H TdR; from day 3 to day 10 a stimulation of proliferation was observed. Thymocytes of control animals incorporate much less 3HTdR when incubated with sera from animals 1 day to 4 days post partial hepatectomy. Corticosterone, cortisol, testosterone, alpha-fetoglobulin, putrescine, sperimidine, spermine and cadaverine do not influence the incorporation of labelled thymidine into control thymocytes during an incubation of 5 hours.

Culture medium components modify the effects exerted by alpha-fetoprotein (AFP) on lymphocyte response

Incorporation of [methyl-3H]thymidine to DNA of lymphocytes cultured in the presence of various concentrations of alpha-fetoprotein (AFP) from human cord blood serum was investigated. It was found that AFP by itself does not exert mitogenic effects. In phytohemagglutinin (PHA) stimulated cultures it may inhibit DNA biosynthesis or exert augmentative effects on this process dependent on the composition of the culture medium.

Differences in the susceptibility of MHC and non-MHC mixed lymphocyte reactions to suppression by murine amnionic fluid and its components

The ability of mouse amniotic fluid (MAF), alpha-foetoprotein (AFP) and MAF depleted of AFP (MAF - AFP) to suppress primary one-way MLR's was investigated. It was found that MAF, AFP and MAF - AFP were all suppressive of MLR's specific for MHC, K, D or I + S determinants. Suppression was observed when either lymph node or spleen cells were used as the responder cells. Nylon wool column passage of these cells did not significantly affect the immunosuppressive action of these substances. In contrast, MLR's specific for non-MHC/M-locus determinants demonstrated either diminished suppression or augmentation of the response, compared with the MHC stimulated MLR's. Our results show a differential effect of whole MAF and its fractions on the proliferative responses induced by various allogeneic stimuli and suggest that suppression is not due to a non-specific effect on proliferation regardless of the stimulus or cell type involved.

Inhibition of blastic transformation and immunoglobulin release in lymphocytes cultured in the presence of alpha-fetoprotein (AFP)

Blastic transformation of phytohemagglutinin-stimulated lymphocytes in cultures containing various concentrations of homogenous alpha-fetoprotein (AFP) was studied. It was found that AFP in concentrations of 4-12 microgram/ml inhibits both blastic transformation and immunoglobulin release (IgA, IgG) in lymphocyte cultures. It is suggested that AFP plays an important role in immunoregulation during the fetal life.

Developmental changes in carbohydrate moiety of human alpha-fetoprotein

Human AFP purified from fetal serum and amniotic fluid was separated into three different variants by chromatography on concanavalin A insolubilized on Sepharose (Con A--Sepharose). The three variants were indistinguishable in immunodiffusion and radioimmunoassay. Sera from patients with yolk-sac tumor and amniotic fluid from early pregnancy were found to contain a high proportion (15-45%) of AFP which does not bind to Con A, while AFP in fetal and newborn sera, and in amniotic fluid from late pregnancy, contained less (2-6%) of this variant. The use of a large excess of Con A--Sepharose and the fact that the non-bound AFP consistently eluted as non-bound in rechromatography showed that this AFP is non-reactive with Con A. Fractionation of radiolabelled AFP from cord serum in a mixture with amniotic fluid verified the difference in the amount of the Con-A nonreactive variant in AFP from these two sources. These results suggest that AFP synthesized by the yolk-sac tissue and by the liver are glycosylated differently. The variant may prove to be diagnostically useful.

Decrease of human serum fucosyltransferase as an indicator of successful tumor therapy

Surgical removal of colon carcinomas leads to a decrease in the rate of incorporation of [14C]fucose into its endogenous acceptor in human serum; normal incorporation rates are attained within 14 days. A similar time course has been determined for alpha2- and alpha3-fucosyltransferase when either desialo- or desialodegalactofetuin are employed as exogenous acceptors. A correlation has also been seen between transferase activity and the therapeutic response of patients with breast cancer. These results indicate that the determination of fucosyltransferase activity can facilitate the diagnosis of neoplasia, and the success of surgery, chemotherapy, or radiation.

Effects of human alpha-foetoprotein on human B and T lymphocyte proliferation in vitro

Purified human alpha-foetoprotein (AFP) isolated from extracts of foetal and hepatoma tissues, and from cord serum was evaluated as to its suppressive effects on in vitro lymphocyte responses to stimuli which selectively trigger human B or T cells. The effects of equivalent concentrations of individual AFP preparations were compared on lymphocyte cultures stimulated with the human B cell mitogen Staphylococcus aureus strain Cowan I organisms, with the T cell mitogen phytohaemagglutinin (PHA), and with irradiated allogeneic lymphocytes in the one-way mixed lymphocyte reaction (MLR). PHA responses were significantly inhibited by most purified preparations of AFP in a dose-dependent manner, within the concentration range of 300 to 18 μg/ml. However, individual foetal-derived AFP preparations did vary in suppressive potency on PHA responses, and attempts to reactivate an inactive AFP were unsuccessful. In parallel cultures the mitogenic response to protein A expressing Staph. aureus bacteria was normal or even slightly enhanced by AFP. The one-way MLR was effectively suppressed at higher concentrations of AFP (300–600 μg/ml) than were required for inhibition of PHA responses. The inhibitory effect of AFP on PHA-induced lymphocyte proliferation was not altered by increases in the mitogen dose. No evidence was found that AFP merely inhibits PHA responses by direct interference with mitogen or by competition for cell surface receptors with the mitogen. The results reported here indicate that human AFP effectively suppresses certain T cell-mediated reactions, but not B cell responses in vitro, and these are in line with previously reported findings in the murine AFP system.

Human alpha-fetoprotein in the epidermal cells of Bowen's disease

Human alpha-fetoprotein (HAFP) was demonstrated in clusters of atypical cells of an extensive Bowen's disease involving the pubic area. Frozen sections were studied by indirect immunofluorescence using monospecific rabbit anti-HAFP antiserum or purified rabbit anti-HAFP antibody followed by staining with fluorescein isothiocyanate (FITC)-labeled goat anti-rabbit gamma-globulin and by a direct technique using FITC-labeled rabbit anti-HAFP Fab2 fragments. Sections of fetal liver served as positive controls and sections of adult human skin and liver were used as negative controls. Preabsorption of the anti-HAFP antibodies with purified HAFP blocked the fluorescence. This finding adds Bowen's disease to the list of premalignant disorders which possess HAFP as an oncofetal marker of their malignant potential. HAFP might possibly enhance malignant behavior through interference with cellular immunosurveillance mechanisms.

Inhibition of human lymphocyte transformation by human alpha-foetoprotein (HAFP): studies on the mode of HAFP action and the role of HAFP polymorphism

In an effort to explain the mechanism of inhibition of human lymphocyte transformation by human alpha-foetoprotein (HAFP), we sought for, and failed to find, evidence of physical association between HAFP and phytomitogens or antihuman thymocyte antiserum. In addition, 20–40 fold increases in mitogen dose do not reverse the inhibition of lymphocyte transformation by a constant dose of HAFP. The presence of HAFP does not interfere with the attachment of ¹²⁵I-labelled phytohaemagglutinin to the lymphocyte surface.

When analyzed by 2-dimensional crossed immunoelectrophoresis, HAFP isolated from the body fluids of hepatoma patients displays electrophoretic heterogeneity, and demonstrates three charged species of HAFP designated as HAFP-1 (the most cathodal), HAFP-2, and HAFP-3 (the most anodal). The potency of hepatoma-HAFP isolates in inhibiting lymphocyte transformation can be positively correlated with the ratio HAFP-3:HAFP-1 in each preparation. Passage of hepatoma HAFP isolates over CM-cellulose allows the isolation of two distinct HAFP fractions, one at pH 4.95, and the other at pH 5.24. The pH 4.95 CM-cellulose isolate is enriched in the more electronegative HAFP species (HAFP-3) and is, on the average, two times more potent than the pH 5.24 CM-cellulose HAFP isolate. The latter, by comparison with native HAFP, is enriched in the more electropositive HAFP species (HAFP-1).

The structural basis for the charge differences which govern the biological potency of HAFP as a modulator of lymphocyte responses is unknown, but it is independent of HAFP sialic acid content. Inhibition of lymphocyte transformation by HAFP cannot be explained by simple competition between the lymphocyte membrane and HAFP for the mitogen combining sites.

A population of human cord blood mononuclear cells with surface alpha fetoprotein

Suspensions of mononuclear cells from adult peripheral blood (PBL) and mononuclear cells from cord blood (CBL) were examined for the presence of surface alpha-fetoprotein (AFP) using a fluoresceinated F(ab')2 fragment of rabbit IgG anti-human AFP. The mean proportion of CBL with AFP was increased (10%) when compared with PBL (1%) although some CBL specimens did not demonstrate such an increase (range 0--15%). The presence of AFP on CBL could be either due to cytophilic AFP attached to a unique surface receptor or intrinsic AFP synthesis. The following observations could not distinguish between these two possibilities: (1) After treatment with trypsin, only minor reappearance of surface AFP could be observed in AFP-free medium in contrast to the larger numbers observed in medium containing AFP. Such selective reappearance depending on the media could be related to either cytophilic attachment of heterologous or homologous AFP or preferential stimulation of intrinsic AFP synthesis. (2) The reappearance of AFP positive CBL following trypsin treatment and incubation in media with or without AFP containing sera was inhibited by cyclohexamide. Such inhibition could be due to inhibition of synthesis of an AFP surface receptor or intrinsic AFP. (3) The shedding of surface AFP observed at 2--4 degrees C could be due to release of exogenous cytophilic AFP or the continued "turnover" of intrinsic AFP without concomitant AFP synthesis due to the cold temperature. Finally, the removal of AFP positive cells via selective depletion of B cells using bead columns coated with IgG-anti-IgG and the absence of depletion of AFP positive cells after successive gradient centrifugation of E-rosettes and cells with IgG-Fc receptors are consistent with the identity of AFP positive CBL as cells without IgG-Fc receptors or lymphocytes without conventional T-cell markers as defined by E-rosettes.

Human alpha-fetoprotein: immunosuppressive activity and microheterogeneity

Purified human alpha-fetoprotein (HAFP) from five patients with hepatoma, one with gastric carcinoma, one with an embryonal cell tumor, and from fetal liver has demonstrated immunosuppressive potencies in vitro which vary over three orders of magnitude. A reversible association of HAFP with the cell surface and a predominant effect on T cells are suggested. No evidence of complex formation between HAFP and mitogen has been found. The microheterogeneity of HAFP has been detailed with crossed immunoelectrophoresis and isoelectric focusing in polyacrylamide gels containing 8M urea, and the immunosuppressive potency of HAFP isolated from a given source can be correlated with the proportion of certain HAFP species contained in it.

Immunosuppressive activity of isolated rat and human fetoproteins

Alpha-fetoprotein (AFP) was purified from rat hepatoma sera and pooled human cord sera. Both AFP-rich fractions prepared by consecutive ion exchange and gel filtration chromatography and pure rat AFP prepared by liquid phase immunoabsorption lacked in vitro suppressive activity. Human AFP purified by affinity chromatography was suppressive, but so was similarly purified human cord albumin. Alteration of ionic conditions was shown to affect the activity of both human AFP and cord albumin. The primary mixed lymphocyte response, the generation of the secondary response, and the memory cell, per se, were all found to be sensitive to active human AFP under the appropriate culture conditions.

A postsynthetic modification of human alpha-fetoprotein controls its immunosuppressive potency

In a previous study, we demonstrated three variants of human alphafetoprotein by crossed immunoelectrophoresis. In addition, we correlated the capacity of alpha-fetoprotein isolates from various hepatoma and fetal sources to suppress human lymphocyte transformation in vitro with the relative proportion of the electronegative variant, HAFP-3, present in each isolate. We have now isolated alpha-fetoprotein from the serum, ascitic fluid, and saline extract of tumor from a single hepatoma patient and from a homogenate of fetal livers. When tested for their capacity to inhibit human lymphocyte transformation in vitro, tumor and fetal liver alphafetoprotein were found to be extremely potent, serum alphafetoprotein had intermediate potency, and ascitic fluid alpha-fetoprotein was the least potent. Analysis of these isolates by crossed immunoelectrophoresis confirmed the correlation between the proportion of HAFP-3 and the immunosuppressive potency of each isolate. In addition, analysis of these isolates by isoelectric focusing in polyacrylamide gels containing 8 M urea revealed further evidence of microheterogeneity; at least six molecular variants were apparent. The proportion of one of these variants, termed HAFP-3a, in each isolate was correlated with the immunosupressive potency of the isolate. The sialic acid content of the various alpha-fetoprotein isolates did not vary significantly. Our data suggest that a postsynthetic modification of alphafetoprotein occurs, probably after secretion, which reduces immunosuppressive potency by converting the active electronegative species to an inactive electropositive form. This modification probably involves a charged moiety other than sialic acid on the molecule.