Differential tissue distribution and ontogeny of DC-1 and HLA-DR antigens

HLA class II antigen-processing pathway in tumors: Molecular defects and clinical relevance

The human leukocyte antigen (HLA) class II antigen-processing machinery (APM) presents to cognate CD4⁺ T-cells antigenic peptides mainly generated from exogeneous proteins in the endocytic compartment. These CD4⁺ T cells exert helper function, but may also act as effector cells, thereby recognizing HLA class II antigen-expressing tumor cells. Thus, HLA class II antigen expression by tumor cells influences the tumor antigen (TA)-specific immune responses and, depending on the cancer type, the clinical course of the disease. Many types of human cancers express HLA class II antigens, although with marked differences in their frequency. Some types of cancer lack HLA class II antigen expression, which could be due to structural defects or deregulation affecting different components of the complex HLA class II APM and/or from lack of cytokine(s) in the tumor microenvironment. In this review, we have summarized the information about HLA class II antigen distribution in normal tissues, the structural organization of the HLA class II APM, their expression and regulation in malignant cells, the defects, which have been identified in malignant cells, and their functional and clinical relevance.

HLA antigen and NK cell activating ligand expression in malignant cells: a story of loss or acquisition

Malignant transformation of cells is often associated with changes in classical and non-classical HLA class I antigen, HLA class II antigen as well as NK cell activating ligand (NKCAL) expression. These changes are believed to play a role in the clinical course of the disease since these molecules are critical to the interactions between tumor cells and components of both innate and adaptive immune system. For some time, it has been assumed that alterations in the expression profile of HLA antigens and NKCAL on malignant cells represented loss of classical HLA class I antigen and induction of HLA class II antigen, non-classical HLA class I antigen and/or NKCAL expression. In contrast to these assumptions, experimental evidence suggests that in some cases dysplastic and malignant cells can acquire classical HLA class I antigen expression and/or lose the ability to express HLA class II antigens. In light of the latter findings as well as of the revival of the cancer immune surveillance theory, a reevaluation of the interpretation of changes in HLA antigen and NKCAL expression in malignant lesions is warranted. In this article, we first briefly describe the conventional types of changes in HLA antigen and NKCAL expression that have been identified in malignant cells to date. Second, we discuss the evidence indicating that, in at least some cell types, classical HLA class I antigen expression can be acquired and/or the ability to express HLA class II antigens is lost. Third, we review the available evidence for the role of immune selective pressure in the generation of malignant lesions with changes in HLA antigen expression. This information contributes to our understanding of the role of the immune system in the control of tumor development and to the optimization of the design of immunotherapeutic strategies for the treatment of cancer.

Liver involvement in the histiocytic disorders of childhood

The liver can be involved directly, by infiltration, and indirectly--by remote effects--in the histiocytoses of childhood. Langerhans cell disease, the most well recognized of these, infiltrates the liver directly but has a remarkable selectivity for the bile ducts. Early involvement is by Langerhans cell histiocytosis (LCH) infiltration leading to a sclerosing cholangitis and, eventually, biliary cirrhosis. Gamma glutamyl transpeptidase is a sensitive indicator of liver infiltration in a child with LCH. The indirect effects on the liver of LCH elsewhere in the body are mediated through an accompanying macrophage activation syndrome that is most likely responsible for hepatomegaly and hypoalbuminemia but without direct infiltration. These indirect effects are completely reversible. Juvenile xanthogranuloma/xanthoma disseminatum, a related dendritic cell disorder that can have systemic manifestations, has a strikingly different pattern, with a predominantly portal infiltrate spilling over into the adjacent lobule but sparing the biliary tree. The biology of the liver lesions is not clear but regression has been documented. Myeloproliferative disorders and myeloid leukemias can express CD1a and/or S100 protein, mimicking LCH but distinguished by their sinusoidal pattern. The primary macrophage histiocytoses such as the familial hemophagocytic syndromes can lead to severe liver damage. Although a portal lymphohistiocytic infiltrate is most characteristic, it is probably cytokine-mediated hepatocellular damage that can cause substantial functional impairment or even hepatic failure as a presenting feature. Liver involvement in other, more unusual histiocytic disorders, is also illustrated.

Immunobiology of liver dendritic cells

Dendritic cells (DC) are rare, bone marrow-derived antigen-presenting cells that play a critical role in the induction and regulation of immune reactivity. In this article, we review the identification and characterization of liver DC, their ontogenic development, in vivo mobilization and population dynamics. In addition, we discuss the functions of DC isolated from liver tissue or celiac lymph, or propagated in vitro from liver-resident haemopoietic stem/progenitor cells. Evidence concerning the role of DC in viral hepatitis, liver tumours, autoimmune liver diseases, granulomatous inflammation and the outcome of liver transplantation is also discussed.

Hemopoiesis in human fetal and embryonic liver

In this review, we describe the topographic distribution of hemopoietic cells of the lymphoid, myeloid, and erythroid lineages in the human fetal and embryonic liver. The data are based on studies of frozen tissue, allowing the determination of a broad range of hemopoietic antigens, and studies on paraffin-embedded tissue, allowing the combination of optimal morphology and immunodetection of lineage-specific antigens. The different hemopoietic lineages each show their own immunophenotype and distribution; intercellular and microenvironmental relationships were easily determined. In a few cases, some scarce CD34-positive early progenitor cells were seen. The number of proliferating cells, identified by monoclonal antibody (MAb) Ki-67, varied from 350 to 2500 (median = 1,500) per square millimeter of tissue. Erythroid cells reacted with antisera to glycophorin A, CDw75, and CD43 and partly surround a central macrophage, whereas the myelomonocytic cells reacted with CD45, CD43, CD74, and antilysozyme serum, and with LN3 from 14 weeks onward. Myelopoietic (CD15 positive) cells were localized mainly around portal triad vessels and increased in number with gestational age. The lymphoid cells showed CD45, CD43, CD45RA/MT2, CD45RA/MB1, MB2, and CD74 reactivity. B cells and their precursors were scattered among the hepatocytes without any sign of focal development in the age range studied. We seldom found cells positive for delta-H chain or C3bR (CD35); C3dR (CD21)-positive cells were even more scarce. Cells reactive with MAb WT1 (CD7) were present in a scattered single pattern (< or = 20/mm2) among the parenchymal cells; cells expressing mature T-cell markers (CD2, CD3, CD5) were rare. Large (> 15 mu) CD43-positive hemopoietic cells in the fetal liver were distinguished that exclusively expressed CD43, probably representing early hemopoietic progenitor cells.

Different patterns of HLA-DR antigen expression in normal epithelium, hyperplastic and neoplastic malignant lesions of the breast

Fifteen samples of non-tumoural breast tissue, 24 cases of benign lesions, four biopsies of inflammatory carcinomas and 94 tumour samples of primitive mammary carcinomas were analysed for HLA class II expression. We found, first, that HLA class II antigens were detectable in all cases of non-neoplastic breast tissue. Secondly, HLA class II antigen expression was notably increased in benign neoplasms and hyperplastic lesions. In contrast, only 32 out of 94 carcinomas showed expression of HLA-DR antigens, 17 tumours had HLA-DP antigens and 11 carcinomas were positive for the presence of DQ molecules. The expression of class II antigen was associated with the degree of histological differentiation (P < 0.05) but was independent of stromal leucocytic infiltration. Thirdly, HLA-DR was very strongly expressed in intravascular tumoural thrombi, especially in the 'inflammatory carcinomas'. The immunophenotype of inflammatory infiltrate was analysed in benign and malignant lesions. In malignant lesions the mean number of inflammatory cells was significantly higher than in benign lesions. Interestingly, we found no differences in the amount and composition of inflammatory infiltrate between HLA-DR positive and negative tumours.

Flow cytometric analysis of surface major histocompatibility complex class II expression on human epithelial cells prepared from small intestinal biopsies

A technique for preparing viable, single cell suspensions of the epithelial layer of small intestinal tissue obtained endoscopically is described. Constant agitation of four biopsies for 60 min in the presence of chelating and reducing agents gave yields of 1.2-6.7 x 10(6) cells, of which 11-30% were intraepithelial lymphocytes (IEL). Passage through a nylon wool column removed dead cells. This preparation was suitable for flow cytometric analysis. Using this technique, surface MHC class II molecule expression was studied in 14 patients with normal small intestinal mucosa. Fluorescence labelling of these cells showed strong HLA-DR expression by epithelial cells (EC), DP was expressed less strongly, while little DQ expression could be detected. This technique demonstrates that small intestinal biopsies taken during routine endoscopy can yield adequate numbers of viable epithelial cells to perform flow cytometric analysis.

Development of immune response markers in the trachea in the fetal period and the first year of life

Immunoglobulin (Ig)-producing cells, T cells (CD3) and epithelial expression of secretory component (SC) and HLA class II determinants (DR, -DP, -DQ) were studied by immunohistochemistry in 16 fetal and 15 postnatal specimens from the tracheal wall. Small amounts of secretory component (SC) was present in the tracheal surface and gland epithelium in the fetal period and increased towards term. A few IgM-, IgD- and IgG-producing cells were present in some fetal specimens but no IgA- and IgE-producing cells were found. Only very few CD3+ T-cells were present in fetal specimens and intraepithelial T-cells were virtually absent until after birth. Premature infants that lived for 1 week had less SC epithelial expression than mature infants of the same age. The density of CD3+ T-cells, IgA-, IgM-producing cells as well as the epithelial SC expression increased rapidly after birth. Epithelial MHC class II expression was absent in fetal specimens. HLA-DR was detected on the apical border of the surface epithelium one week after birth and was extensively expressed throughout the remaining postnatal period. Epithelial DP and PQ expression were virtually absent during this same period. These features probably reflect local activation of the immune system in response to environmental factors.

Expression of MHC class II antigens in human lung cancer cells

Surgical specimens of lung cancers were examined immunopathologically for the expression of major histocompatibility complex class II (MHC-II) antigens in the tumor cells and their relationship to the lymphocytic infiltration. A lymphocytic infiltrate was frequently observed in the tumor tissue, though its intensity differed among the various histological types. MHC-II antigens were often demonstrated in tumors with a lymphocytic infiltrate. They were detected predominantly in the cytoplasm of tumor cells and to a lesser extent on the cell membranes. The emergence of the MHC-II-positive tumor cells was closely related to a local infiltration by lymphocytes including interferon-gamma (IFN-gamma)-producing T-cells. On the basis of the histological findings, an in vitro experiment was carried out. Four types of lung cancer cells were incubated with recombinant IFN-gamma in order to induce MHC-II antigens. MHC-II antigens (HLA-DR as well as HLA-DQ and HLA-DP antigens) were elicited in three cancer cell lines depending on the concentration of IFN-gamma. Immunoelectron microscopic study revealed that they were expressed on the surface of the cell membrane, though to a lesser extent than in the cytoplasm. It was considered that MHC-II antigens could be induced in some tumor cells in the immunological environment where IFN-gamma was secreted from T-cells and concentrated locally.

Expression of HLA class I and II (DR, DP and DQ) determinants in fetal and postnatal salivary glands

Fetal (n = 20) and postnatal (n = 40) parotid glands were examined by two-colour immunohistochemistry combining monoclonal and polyclonal antibody reagents to study the expression of HLA class I and II (DR, DP, and DQ), CD45 and CD3. The epithelium lacked class II during fetal life, whereas class I determinants appeared in some acini and most major ducts. Fetal vessels were positive for both class I and class II (mainly DR), suggesting constitutive expression. Some class II-positive (DR greater than DP greater than DQ) histiocytic cells, scattered CD45+ leucocytes, and very few CD3+ T cells were present in the fetal stroma. The epithelium remained DR-negative the first few weeks after birth, but brisk expression was seen subsequently. DP and DQ remained virtually negative in the epithelium throughout the first year. A slight postnatal increase of class II expression (DR greater than DP greater than DQ), along with an apparent decrease in class I, was observed in the endothelium. The number of class II-positive histiocytic cells, CD45+ leucocytes and CD3+ T cells, as well as the proportion of presumably activated (DR+) T cells, increased a few weeks after birth. The local immune system hence seemed to be stimulated by extrinsic factors, but the overall number of T cells nevertheless remained small. Stimuli other than T cell-derived lymphokines, therefore, probably explained the brisk postnatal epithelial DR induction.

Anomalous expression of HLA class II molecules on keratinocytes and fibroblasts in hypertrophic scars consequent to thermal injury

Immunoperoxidase staining of skin sections obtained from 11 hypertrophic scars, six normotrophic scars and three samples of normal skin were performed using anti-HLA monoclonal antibodies (HLA-DR, -DQ, class I), anti-interleukin-2 receptor (IL-2R) and anti-CD1. Sections from all hypertrophic scars showed an anomalous expression of HLA-DR molecules on keratinocytes and fibroblasts. Moreover hypertrophic scars were characterized by dense infiltrates of IL-2R-positive cells and by the presence of abundant Langerhans (CD1+) cells in the epidermis and dermis. These results support the hypothesis that immunologic mechanisms play an important role in hypertrophic scarring and point to an involvement of cell-mediated immune phenomena.

Haemopoiesis in human fetal and embryonic liver. Immunohistochemical determination in B5-fixed paraffin-embedded tissues

Paraffin embedded tissue reactive monoclonal antibodies were used to study human embryonal and fetal haemopoiesis, combining optimal morphology with immunohistological determination of haemopoietic cell subtypes and their microenvironment. Seven embryonal and twelve fetal liver specimens were studied, having been fixed in B5-fixative and embedded in paraffin. The different haemopoietic lineages each showed their own immunophenotype and distribution; intercellular and microenvironmental relationships were easily determined. Erythroid cells are reactive with VIE-G4, LN1, and MT1, sometimes partly surrounding a central macrophage. Myelomonocytic cells react with LCA, MT1, MB3, LN2, and anti-lysozyme, and from 14 weeks onwards with LN3. Lymphoid cells show LCA, MT1, MT2, MB1, MB2, MB3, and LN2 reactivity. In a few cases some scarce My10+ early progenitor cells were seen. An important finding is the extensive MT1-reactivity distributed over all haemopoietic lineages, and the demonstration of immature haemopoietic blast cells exclusively expressing the MT1 antigen. Further studies employing MT1 are necessary to delineate the extent of the distribution and the possible function of the antigen. Use of the MT1 mAb may contribute to the elucidation of the exact nature of the haemopoietic blast cells and their place in haemopoietic development.

Histocompatibility antigen expression in pulmonary carcinomas as indication of differentiation and of special subtypes

In 70 human carcinomas of the lung the expression of histocompatibility antigens (HLA) was investigated by using monoclonal antibodies in frozen sections. The immunmoreactive tumor volume was determined morphometrically. The tumor types showed a different pattern of HLA expressions. In squamous cell carcinomas (SQC) and small cell carcinomas (SCC) the HLA-A,B,C expression varied in a great range from 0% to 100% of positive tumor volume. For the most part, HLA-DR was not demonstrable. In adenocarcinomas (AC) and large cell carcinomas two different populations were observed. One showed a pattern of HLA expression as seen in SQC. Electron microscopically these AC (type 1) (AC/1) were characterized by intracytoplasmic mucin granules indicating a phenotypical relationship to goblet cells. The other--nearly 50% of adenocarcinomas and one third of large cell carcinomas--expressed both HLA antigens homogeneously in the total tumor volume. Electron microscopically the characteristics of type II pneumocytes and Clara cells were seen in tumor cells of these AC (type II) (AC/II), consisting in lamellar bodies and apically located electron-dense granules. In SQC and AC/I a significant correlation was found between grades of differentiation and mitotic activity on the one side, and expression of HLA-A,B,C on the otrher. The loss of HLA-A,B,C expression seemed to be an indication of a lower grade of differentiation; the smalles expression of HLA-A,B,C occurred in SCC.

Altered expression of major histocompatibility complex (MHC) antigens by epidermal tumours

Alteration in the major histocompatibility complex (MHC) antigen expression by cutaneous tumours may enable them to escape host defence mechanisms and to invade surrounding tissue. Immunohistochemical studies in a wide range of epidermally derived tumours demonstrated expression by keratinocytes of the class II molecule HLA-DR in squamous cell carcinoma (SCC) (2 of 8 cases) and keratoacanthoma (KA) (2 of 7 cases). Additionally, HLA-DP and DQ were expressed by single cases of SCC and KA, although, unlike the widespread distribution of DR, DP and DQ, were only present on keratinocytes adjacent to the inflammatory infiltrate. Therefore, keratinocytes in cutaneous tumours, like carcinoma cells of the colon and breast, may express class II MHC antigens during tumour growth. Beta-2-microglobulin (B2M), an invariant MHC class I marker, was absent in all cases of basal cell carcinoma. Variable loss of B2M was observed in squamous cell carcinoma, Bowen's disease and actinic keratoses, suggesting reduced B2M expression by dysplastic cells. However, the variability in B2M staining both between and within diagnostic categories restricts it's immunodiagnostic usefulness.

Inappropriate major histocompatibility complex expression on cardiac tissue in dilated cardiomyopathy. Relevance for autoimmunity?

The inappropriate expression of major histocompatibility complex (MHC) molecules on epithelial and endothelial cells is a recognized marker of autoimmune disease. An autoimmune pathogenesis has been suspected in dilated cardiomyopathy (DCM). In the normal heart, MHC products are usually not detectable on myocytes using immunochemical techniques. MHC molecule expression has not, however, been assessed on cardiac endothelial cells. The aim of this study was to investigate possible autoimmune phenomena and MHC molecule expression in fresh endomyocardial biopsies from 29 patients with DCM. These were compared with those observed in surgical specimens from 63 patients with other acquired cardiac disease and from 22 with congenital heart disease (CHD) as normal controls. Conventional immunofluorescence (IFL) with monoclonal antibodies (MoAbs) to lymphocyte and macrophage markers and to MHC molecules was employed, and double IFL with antiserum to human Factor VIII was used for the identification of endothelial cells. Myocytes did not express MHC molecules in either DCM or controls. In normal hearts, Class II molecules were detected on endothelial and endocardial cells in only a few cases (3/22 and 2/22 respectively). By contrast, endothelial and endocardial cells inappropriately expressed Class II in a high proportion of DCM patients (28/29 and 22/29) but less frequently in other acquired cardiac diseases (19/63, P less than 0.001 and 11/63, P less than 0.001 respectively). In all the DCM biopsies examined there was a hierarchy of Class II subloci product expression (DR greater than DP greater than DQ); lymphocytic infiltration was a rare finding and macrophages/dendritic cells were not prominent. The finding of inappropriate MHC Class II molecule expression on cardiac endothelial and on endocardial cells suggests a possible pathogenic role for these cells in the initiation and/or perpetuation of DCM.

Class II HLA antigen expression in familial polyposis coli is related to the degree of dysplasia

Class II HLA antigen expression was studied in 30 polyps from 3 patients who were diagnosed with familial polyposis coli. The highest levels of this expression were associated with the most severe grades of dysplasia (p less than 0.00001), the sequence of positivity being HLA-DR greater than DQ greater than DP. No association was observed between the expression of these antigens and the presence of a specific inflammatory leukocytic infiltrate. Our results imply that HLA class II molecule expression is somehow related to malignant transformation in familial polyposis coli in accordance with the adenoma-dysplastic adenoma-adenocarcinoma sequence. Thus these antigens may be useful markers to tumoral progression.

HLA DR, DP, DQ induction in human islet beta cells by the cytokine combination IFN-gamma + TNF-alpha

Human islet beta cells do not express HLA Class II normally, yet, in the diabetic pancreas, beta cells are selectively positive for Class II and this may facilitate their recognition by T cells. It has been demonstrated that human beta cells can be induced to express Class II when cultured with IFN-gamma + TNF-alpha or IFN-gamma + TNF-beta. To assess whether or not they can be induced to express the products of the Class II subregions, DR, DP and DQ, human islet cultures from 10 pancreas were supplemented with the combination of IFN-gamma + TNF-alpha using MoAbs specific for DR, DP and DQ products, and antibodies to insulin and glucagon. The combination IFN-gamma + TNF-alpha (100-1000 U/ml each) was able to induce the expression of the three subregions in both beta and alpha cells. The induction of subregion expression followed the hierarchy DR greater than DQ greater than or equal to DP. The capability of beta cells to express all three Class II subregions supports the possibility that these cells can present their self antigens to T cells.

Langerhans cells expressing HLA-DQ, HLA-DR and T6 antigens in normal oral mucosa and lichen planus

We compared Langerhans cells (LC) expressing HLA-DQ, HLA-DR and T6 antigens in biopsies from the same oral mucosal site in 12 patients with oral lichen planus and eight healthy volunteers. LC expressing each antigen were observed in all the specimens, but in lichen planus the cells were located in higher levels of the epithelium than in controls. Compared with controls, lichen planus contained significantly more HLA-DQ-positive LC (P = 0.04) and fewer HLA-DR-positive LC (P = 0.05), but there was no such difference in T6-positive LC. In lichen planus specimens, there were significantly more LC expressing HLA-DQ and T6 than HLA-DR (P = 0.0001 and 0.02 respectively); no such differences were found in normal mucosa. Epithelial cells in lichen planus expressed HLA-DR antigen, but not HLA-DQ or T6 antigens. We conclude that in lichen planus there is modulation of HLA-DR and HLA-DQ antigen expression by LC, or differences in the number of LC expressing those antigens.

Different staphylococcal enterotoxins bind preferentially to distinct major histocompatibility complex class II isotypes

The stimulation of T cells by staphylococcal enterotoxins (SE) is strictly dependent on major histocompatibility complex (MHC) class II-bearing cells. The interaction between SE and MHC class II molecules was studied on the human B cell lymphoma Raji and its MHC class II-negative variant RJ 2.2.5. Affinity purification with SEA and SEB matrix allowed the isolation of HLA-DR-like molecules from detergent lysates of 125I surface-labeled Raji cells, but not from RJ 2.2.5 cells. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis also revealed preferences in the binding of other SE such as SED, SEE and toxic shock syndrome toxin 1 to DR-like molecules, SEC2 to HLA-DQ-like molecules and SEC3 to DR- and DQ-like molecules. Preadsorption of the different MHC class II MHC isotypes confirmed the preferential binding of SEA to DR and of SEC2 to DQ. The implications of these findings for the understanding of SE-induced T cell activation and the potency of SE as a tool in the study of MHC class II antigens are discussed.

In situ study of haemopoiesis in human fetal liver

The anatomy of haemopoietic cells in human fetal liver was examined using immunohistological techniques on frozen sections of 31 fetuses (10-28 weeks gestational age). The immunohistological findings were consistent with reported cell suspension data. With regard to the location of haemopoietic activity no particular relationship existed between the various haemopoietic cell lineages. A large number of proliferating cells was present; only a few of these were reactive with haemopoietic progenitor cell monoclonal antibodies (MoAb) CD34. A population of haemopoietic cells expressed CD43 antigen (MoAb MT1) alone or together with anti-vimentin MoAb reactivity; this population needs further delineation. Erythropoiesis and myelopoiesis occurred in clusters around sinusoids and portal triad vessels respectively. Lack of MoAb reacting exclusively with early developmental stages of erythropoiesis and myelopoiesis precluded dissection of these lineages. Lymphopoiesis occurred in a loosely scattered pattern without any sign of focal development. Pre-B and B-cell numbers increased with gestational age. Cells expressing markers of more mature B cells (surface IgD, CD35, and CD21) were rare. Also, few cells reacted with mature T-cell markers, but CD7+ cells were obviously present. This expression of CD7 on haemopoietic fetal liver cells suggests that T-cell precursors develop in fetal liver as well as B cells.

The relationship between MHC antigen expression and metastasis

From the studies summarized here a complex picture of the role played by MHC products in determining tumorigenicity and metastasis is emerging. In order to be able to understand this relationship better, it is necessary to consider several factors. 1. Each tumor system or neoplastic tissue is unique, and its behavior reflects the influence of cell-specific characteristics, as well as its ability to modulate other cells and tissues--including cells belonging to the immune system--and also to be modulated by other cells and soluble factors. 2. Since metastasis formation is a multistep process in which only small subpopulations of tumor cells with complex and defined phenotypes are able to colonize secondary tissues, elimination of even one single phenotypic component of this structured process can easily reverse the metastatic capacity of the cells. Acquisition of metastatic ability, on the other hand, would be a more difficult task, since any new characteristic expressed by the cells or induced experimentally, such as gene transfection or results of IFN treatment, must be expressed in a temporal manner and in concert with other cellular characteristics. Therefore, an experimental protocol measuring a specific element in determining metastasis can easily produce conflicting results, depending on the type of cells and genetic background of the host studied. 3. The level of specific MHC products on tumor cells is one among many other cell characteristics that may determine the metastatic potential of cells. Moreover, each of the class 1 MHC products, and the relationship among them, including other than the classical K, L, or D products (Brickell et al., 1983), should be regarded as independent entities, with possible different regulatory roles in cell-cell recognition, in a general sense, which may be involved in determining invasiveness and homing as well as recognition by the immune system. 4. Both specific T-cell and nonspecific natural mediated immunity (which is much less understood) are involved in the selection of the metastatic cell population. 5. Immunogenicity of tumors is not necessarily determined by high levels of MHC antigen expression; it is also dependent on the level of TSA. Thus, immunoselection mediated by T lymphocytes during metastasis formation could be directed against both MHC and TSA antigens. Therefore, low expression of MHC antigens by metastatic cells as a result of immunoselection is not always observed.(ABSTRACT TRUNCATED AT 400 WORDS)

Gamma interferon induces different keratinocyte cellular patterns of expression of HLA-DR and DQ and intercellular adhesion molecule-I (ICAM-I) antigens

With indirect immunofluorescence techniques we demonstrated that recombinant gamma-interferon induced the expression of the class II antigens HLA-DR and HLA-DQ as well as intercellular adhesion molecule-I (ICAM-I) on normal, cultured human keratinocytes grown in low-calcium, serum-free medium. Each antigen displayed a distinctive cellular staining pattern. HLA-DR was strongly localized to perinuclear zones with intense cell surface expression; HLA-DQ displayed a perinuclear accentuation, but with minimal cell surface staining, and ICAM-I was strongly expressed in a diffuse cytoplasmic pattern with intense cell surface expression. Keratinocytes grown in medium supplemented with 10% fetal calf serum underwent differentiation, with a diminished expression of all three antigens as compared to those grown in low-calcium, serum-free medium. These results confirm that gamma interferon can differentially regulate HLA-DR and HLA-DQ expression; that there are probably different biochemical metabolic pathways by which these three molecules are expressed on keratinocytes, and that the expression is also a function of the degree of keratinocyte differentiation. The strong cell surface expression of ICAM-I is suggested to be of major importance as the recognition molecule, by which T cells bind to gamma interferon exposed keratinocytes, and suggests an integral role for this molecule in epidermal lymphocyte trafficking.

Products of class II major histocompatibility complex gene subregions are differentially expressed on keratinocytes in cutaneous diseases

Aberrant expression of class II major histocompatibility complex (MHC) subregion antigens by keratinocytes was examined immunohistochemically in a range of cutaneous disorders. Although cell surface expression of human lymphocyte antigen (HLA)-DR was observed, HLA-DQ and HLA-DP were not expressed in any disorder investigated except for allergic contact dermatitis. Epidermal Langerhans cells expressed antigens of all three subregions on the cell surface. Differential expression of class II MHC subregion antigens may be related to tissue levels of gamma-interferon or to different functions of each subregion.

HLA class II sublocus expression in benign and malignant breast epithelium

Using immunohistochemistry and a panel of five monoclonal antibodies, the epithelial expression of HLA class II sublocus products by benign and malignant breast has been studied. The magnitude of the stromal mononuclear inflammatory cell infiltrate was assessed. There was expression of HLA class II by 75 per cent of epithelial cells in the benign tissues, with little variation in intensity and between antibodies. There was coordinate expression of DR and DQW1. Epithelial expression by carcinomas was more complex and variable. Most (61 per cent) carcinomas exhibited variable loss of epithelial expression of class II products, as detected by three antibodies recognizing epitopes on DP, DQ, and DR together. Thirteen (28 per cent) carcinomas were completely negative or had very occasional positive cells. The extent of this loss was unrelated to the magnitude of the inflammatory infiltrate and axillary lymph node status. No well-differentiated carcinomas exhibited complete loss. Furthermore, non-coordinate expression of DR and DQW1 was present in 8 out of 40 carcinomas, with the proportion of DQW1 positive epithelium always being less than that of DR. Carcinomas exhibiting non-coordinate expression were never well differentiated; there was no relationship with the extent of the inflammatory infiltrate. This is the first study to detail HLA class II expression in breast, and our results suggest that alterations in expression of these products may modify or reflect tumour behaviour.

Immunohistochemical analysis of the stage-specific expression of Ia antigens in the rat mammary gland during pregnancy and lactation

The expression of Class II MHC antigens in the rat mammary gland was assessed by immunoperoxidase using monoclonal antibodies against rat homologues of the murine I-A and I-E antigens. Our results showed that, while I-A and I-E were inconspicuous in undifferentiated mammary epithelium in virgin rats, during early pregnancy, the ductal epithelium becomes intensely positive for both antigens. A monoclonal antibody specific for macrophages showed that they were abundant in the stroma but did not correspond to the epithelial staining. During late pregnancy and lactation, the ductal epithelium expressed varying levels of Ia, while the alveolar epithelial cells were consistently Ia-negative. Ia-positive mononuclear cells of round or dendritic morphology present in the alveolar areas were enumerated as cells per high power field (HPF). Maximal cell counts for I-A and I-E were seen in late pregnancy and early lactation, with a significant decline in mid-lactation. Counts were higher in intraepithelial locations than in the subjacent connective tissue at all stages. The possible role of these stage-specific alterations in regulating local immune responses and transfer of immune components into milk is discussed.

Major histocompatibility complex (MHC) class II antigen (HLA-DR, DQ, and DP) expression in human fetal endocrine organs and gut

Monoclonal antibodies were used to analyse adrenal, pancreas, thyroid, and gut samples from human fetuses (14-19 weeks estimated gestational age; EGA) for the presence of class II major histocompatibility complex (MHC) antigens (HLA-DR, DQ, DP) by immunohistochemistry. In the adrenal definitive and fetal cortex, HLA-DR+, DP-, DQ- cells were clearly demonstrated. These DR+ cells were identified, phentotypically, as predominantly tissue macrophages and a small population of CD45R+, IgM+ lymphoid cells. Within the pancreas, numerous cells throughout the tissue were strongly DR+ but DQ-; DP+ cells were not observed until 17 weeks EGA. Using a double-labelling procedure, minor proportions of these DR+ cells were identified as macrophages or as (CD19+) B cells, while endocrine and endothelial cells were negative. Throughout the thyroid, small numbers of DR+ macrophages and small lymphoid cells were detected, although the thyroid epithelial cells were DR-, DP-, and DQ-. Large numbers of DR+, DP+, DQ- cells were observed in the stomach wall and mucosa. In the intestine, DR+, DP+, DQ+ cells positive for all MHC class II loci products were abundant throughout the lamina propria and lymphoid aggregates. The class II antigens appeared in the proportion DR greater than DP greater than DQ and expression was most prevalent in the mid-gut region. A small proportion of epithelial cells of the villi along the gut were weakly DR+ but DQ-, DP-. These results show that DR+ cells in fetal endocrine tissue are mainly 'passenger leucocytes' and that, in contrast to recent reports concerning normal adult tissue, the adrenal and pancreatic endothelial cells are DR-.

Maternal and fetal cellular relationships in the human placental basal plate

Maternal and fetal cellular relationships in the normal human term placental basal plate were investigated by single and double immunohistochemical labelling techniques. Extravillous fetal trophoblast in the basal plate was uniformly reactive with markers of low-molecular-weight cytokeratins. The predominant maternal leucocyte population in the basal plate consisted of leucocyte-common-antigen-positive, class II MHC-positive macrophages, which exhibited acid phosphatase activity. Double-labelling methods highlighted the close association of these macrophages with extravillous trophoblast: they often extended processes around the fetal cells and were also observed within islands of cytotrophoblast. Other leucocytes were uncommon, although aggregates of T cells were apparent in some tissues.

Differential expression of HLA-DR, HLA-DP, HLA-DQ and associated invariant chain (Ii) in normal colorectal mucosa, adenoma and carcinoma

The expression of MHC class II antigens (HLA-DR, HLA-DP and HLA-DQ) and the associated invariant chain (Ii) was studied in epithelial cells of normal colorectal mucosae, colorectal adenomas and carcinomas, using a sensitive immunoperoxidase technique with monoclonal antibodies on frozen sections. In contrast to class II antigens, Ii was detected in some normal mucosae distant from the tumour. In residual non-neoplastic mucosa adjacent to carcinomas, Ii and class II antigens were induced in the order Ii greater than or equal to HLA-DR greater than or equal to HLA-DP greater than or equal to HLA-DQ, the reactions being most pronounced in cases with inflammatory alteration of the crypts. In 22/37 adenomas and 77/123 carcinomas, Ii expression clearly exceeded class II antigen expression. Class II antigens were found in 20/37 adenomas and 62/123 carcinomas, mostly in a non-coordinate manner, following the above order. A detailed analysis of the expression patterns in normal and neoplastic colon epithelial cells revealed a closer association of HLA-DP with HLA-DQ than of HLA-DR with HLA-DP, or HLA-DQ.

B-cell lymphomas of high-grade malignancy frequently lack HLA-DR, -DP and -DQ antigens and associated invariant chain

The expression of HLA-DR, HLA-DP and HLA-DQ antigens was studied in an unselected series of 66 B-cell lymphomas by means of immunohistology using monoclonal antibodies against non-polymorphic determinants in a sensitive immunoperoxidase technique. In addition, the expression of the MHC class-II antigen-associated invariant chain (li) was examined. The tumors were classified according to the Kiel classification, 30 being of high-grade and 36 being of low-grade malignancy. Only 6 lymphomas of high-grade and 16 lymphomas of low-grade malignancy showed coordinate strong expression of all HLA class-II antigens and invariant chain as observed in the normal peripheral B cell. Six further tumors of high-grade and 8 tumors of low-grade malignancy contained tumor-cell subsets with reduced expression of one or several of the antigens. Eighteen lymphomas of high-grade and 12 lymphomas of low-grade malignancy contained varying tumor-cell subsets that were negative for HLA-DR, -DP, -DQ and li in a selective or combined manner. Three highly malignant tumors were devoid of all class-II antigens and li; 2 highly malignant tumors expressed invariant chain only. The presence of high-grade malignancy was significantly correlated with the occurrence of tumor cells lacking HLA-DR (p = 0.004), HLA-DP (p = 0.013), HLA-DQ (p = 0.007) or li (p = 0.024).

Expression of HLA-D-locus (DP, DQ, DR)-coded antigens, beta 2-microglobulin, and the interleukin 2 receptor in Sjögren's syndrome

The expression of beta 2-microglobulin (beta 2m), class II major histocompatibility complex (MHC) antigens (HLA-DP, -DQ, -DR) and the interleukin 2 receptor (IL-2R) on resident and infiltrating cells in labial salivary glands of patients with Sjögren's syndrome (SS) was studied using an immunoperoxidase technique based on staining with monoclonal antibodies. The progression of the inflammatory process was accompanied by increasing numbers of immunocompetent cells as well as glandular epithelial cells expressing beta 2m and class II MHC antigens. Up to 60% of the infiltrating lymphocytes were DQ- and DR-positive while fewer cells stained for DP antigen. Glandular epithelium (acinar and ductal cells) stained for products of all three HLA-D subregions but with varying degrees of expression following the pattern DR greater than DP greater than DQ related to the severity of inflammation. A minor portion of the infiltrating lymphocytes expressed IL-2R. These findings emphasize the potential importance of epithelial expression of class II antigens in the local activation of T lymphocytes in salivary gland lesions of SS patients. They also indicate that differences may exist in this respect between T-cell reactions restricted by different class II MHC-encoded gene products.

Differential expression of class II alloantigens by keratinocytes in disease

The purpose of this study was to determine whether keratinocytes in certain disease states such as cutaneous T-cell lymphoma and lichen planus, express HLA-DR antigens (corresponding to the murine I-E antigens) only or whether they are also capable of expressing HLA-DQ antigens (analogues of the murine I-A antigens). Cryostat sections from 11 biopsies from cutaneous T-cell lymphoma and from 11 lichen planus biopsy specimens were submitted to indirect immunofluorescence and a 4-step immunoperoxidase method. This consists of applying monoclonal antibodies recognizing HLA-DR and HLA-DQ molecules and the intracytoplasmic invariant chain of the class II molecules. In 8 of the 11 cutaneous T-cell lymphoma specimens and in 3 of the 11 lichen planus biopsies concomitant expression of HLA-DR and HLA-DQ molecules by keratinocytes was detectable with the immunoperoxidase method. However, with the indirect immunofluorescence technique HLA-DQ antigens on keratinocytes could not be detected. The simultaneous expression of surface-bound HLA-DR antigens and intracytoplasmic gamma-chains was demonstrable in all cases investigated and with both the immunohistologic methods applied.

Epidermal Langerhans' cells and dermal dendritic cells in human fetal and neonatal skin: an immunohistochemical study

The dendritic cells in skin biopsies from 36 fetuses (from 16 weeks' gestational age on) and neonatal infants of different ages were investigated using a number of markers for various cells of the immune system and immunohistochemistry. Epidermal Langerhans' cells were found in all cases in a predominantly basal localization. The number of OKT6-positive Langerhans' cells and the development of their dendritic processes gradually increased with age. Dermal dendritic cells were present as well. Both HLA-DR-positive and Leu-10-positive dermal dendritic cells were found in the superficial and deep dermis, often in a perivascular distribution. The OKT6-positive dermal dendritic cells were few in number and occurred only in the upper dermis.

Class II antigen (HLA-DR) expression by intestinal epithelial cells in inflammatory diseases of colon

Eighty four colonic biopsy specimens were obtained from patients with ulcerative colitis, Crohn's disease, radiation colitis, infectious colitis, and from normal controls. Paired specimens were examined by histological and immunohistochemical methods using monoclonal antibodies to the beta chain of HLA-DR antigen. The expression of HLA-DR antigen in mucosal epithelial cells was strongly related to whether the specimens were actively inflamed: epithelial cells from 34 of 37 inflamed specimens (nu three of 42 non-inflamed specimens) were HLA-DR positive (p less than 0.0001). Epithelial cells were uniformly HLA-DR negative in specimens from normal control patients despite the presence of HLA-DR positive lymphoid cells and macrophages in the lamina propria. Epithelial cells in specimens from patients with ulcerative colitis, Crohn's disease, and radiation colitis were HLA-DR positive in 30 of 33 inflamed biopsy specimens and in only three of 25 non-inflamed specimens (p less than 0.0001). Epithelial cells were HLA-DR positive in nine of 10 biopsy specimens from patients with acute infectious colitis (p less than 0.01).

Induction of various HLA class II molecules in a human colonic adenocarcinoma cell line

The colonic carcinoma cell line HT-29 had no constitutive expression of HLA class II molecules. Gamma interferon (IFN-gamma) induced expression of HLA class II molecules in a dose-dependent manner with 100 U/ml as an optimal dose. The expression of HLA-DR, HLA-DP, and HLA-DQ molecules seemed to follow different kinetics. While DR and DP molecules were maximally induced after 2 days, DQ molecules appeared later with maximum percentage positive cells after 8 days. Treatment with a prostaglandin synthesis inhibitor (indomethacin) neither induced class II expression nor altered the dose-response curve for IFN-gamma; this indicated that possible endogenous production of prostaglandins in this cell line did not interfere with its class II expression. The lectins phytohaemagglutinin (PHA), concanavalin A (Con A), pokeweed mitogen (PWM), and wheat germ agglutinin (WGA) did not induce class II expression.

HLA-D region antigen expression by human epidermal Langerhans cells

We have systematically examined HLA-D region antigen expression by normal human epidermal Langerhans cells with an improved double label indirect immunofluorescence technique in order to better understand the immunologic potential of this human interstitial dendritic cell type. The results of this study which differ somewhat from earlier observations strongly suggest that 100% of OKT-6-positive epidermal Langerhans cells constitutively express HLA-DQ and HLA-DP as well as HLA-DR molecules. Since HLA-DQ expression has been positively correlated with the ability to present nominal antigen, our findings would suggest the possibility that epidermal Langerhans cells, like other members of the human dendritic cell series, might be more efficient than monocytes at nominal antigen presentation since only about 50% of peripheral blood monocytes express HLA-DQ molecules.

Class II antigens in Hashimoto thyroiditis. I. Synthesis and expression of HLA-DR and HLA-DQ by thyroid epithelial cells

Aberrant expression of HLA-DR antigens on epithelial cells is seen in various organ-specific autoimmune disorders including Hashimoto thyroiditis (HT). Expression of HLA-DQ has so far not been demonstrated on these cells. We report here that thyroid epithelial cells (TEC) in HT, in addition to the known aberrant expression of HLA-DR, coexpress HLA-DQ antigens. Furthermore we provide evidence that class II antigens are synthesized by TEC themselves by demonstration of intracellular HLA-DR gamma-chain. These findings support the theory that TEC may be able to present (auto)antigens in vivo thus perhaps contributing to the perpetuation of thyroid destruction. As expression of class II antigens on TEC was never observed in non- or weakly infiltrated areas, we propose that infiltration by T cells is necessary to induce this aberrant expression of class II antigens.

Functional roles of human class II antigens in T cell proliferative response. Comparison of the DR and DQ molecules using mouse monoclonal antibodies

The monoclonal antibody (mAb) NC1 recognizing a monomorphic DR determinant and the anti-DQ mAbs PLM2, PLM9, and PLM12 recognizing polymorphic DQ determinants were produced. NC1 reacted with the H1L1 (DR) and H1L2(DRw52, DRw53 and allelic products) complexes of a DR family, while the specificity of PLM12 was DQw3 (previously called TB21). Both PLM2 and PLM9 reacted with the allodeterminant of TA10 (a subtype of DQw3). These three mAbs immunoprecipitated the same H2L3 complex of a DQ family. With the aid of these mAbs, DR molecules but not DQw3 molecules were detected on monocytes in significant quantities. The functional roles of these class II molecules in the T cell proliferative responses to three soluble antigens (PPD, mite, Candida) and alloantigens were examined by blocking assays in vitro. NC1 almost completely inhibited T cell proliferative responses against both soluble antigens and allogeneic antigens. In contrast, PLM2, PLM9 and PLM12 showed no significant inhibitory effects at all. These results indicate that DR and DQ antigens are different in their functional roles as well as their serological and immunochemical characteristics and tissue distributions; these three soluble antigens and the alloantigen were presented in association with determinants residing predominantly on two forms of NC1-reactive molecule, namely the H1L1 and/or H1L2 complex of a DR family but not in association with the DQw3 allodeterminant on the H2L3 complex of a DQ family.

Ontogenic and functional implications of the differential expression of HLA-DQ antigens on leukemic cells

We have examined the HLA class II antigenic profiles on different types of leukemic cells and have attempted to relate these findings to the normal differentiation pathways of the cells from which they have arisen. Monoclonal antibodies reacting with the different HLA class II determinants, HLA-DR, DRw52(MT), and DQ, were used to study the expression of these antigens on Epstein-Barr virus transformed cell lines, chronic lymphocytic leukemic cells, acute lymphoblastic leukemic blasts, acute myeloblastic leukemic blasts, and established leukemic cell lines by indirect immunofluorescence binding and immunoprecipitations. The results showed that whereas the HLA-DR and HLA-DRw52(MT2) antigens are normally expressed on the majority of the cells tested, there is a different expression of the HLA-DQ antigens on acute leukemic blasts, chronic lymphocytic leukemic cells, and leukemic cell lines indicating that the DQ molecules may be differentiation antigens preferentially expressed on mature cells. Furthermore, when the pre-B cell leukemic line NALM 6 was induced to differentiate with phorbol ester (TPA), normal expression of the HLA-DQ antigen was obtained after 5 days of culture. The absence of HLA-DQ antigens from the acute leukemic blasts suggests that these immature cells "froze" in the early stages of cell differentiation. We discuss these findings in relation to the role of these HLA class II antigens in cell differentiation and the immune response.

Imbalanced MHC class II molecule expression at surface of murine B cell lymphomas

To study the role of class II MHC expression in mouse lymphomagenesis, we examined the cell surface expression of I-A/E antigens on 24 spontaneous or murine leukemia virus (MuLV)-induced mouse B10.A (I-Ak, I-Ek) B cell lymphomas. Two primary B10.A B cell lymphomas were observed with strong I-Ek expression but with only minimal cell surface I-Ak expression. Both tumors are readily transplantable in syngeneic mice, with maintenance of their I-A-, I-E+ phenotype. Strikingly, one I-A-, I-E+ B cell lymphoma contains a (11; 17) translocation with a breakpoint on chromosome 17 that is localized within or very close to the H-2 complex. DNA of both tumors contains normal restriction enzyme fragments of the A alpha and A beta genes. Northern blot analyses indicated that one I-A-, I-E+ tumor strongly expressed A alpha, E alpha, and E beta mRNAs but possessed only a weak expression of A beta mRNA. The other B cell lymphoma showed A beta, E alpha, and E beta mRNA expression but only minimal A alpha mRNA expression. In 11 primary B10.A B cell lymphomas with a normal I-A+, I-E+ phenotype, no imbalances in A alpha/A beta mRNA levels were observed. The implications of these findings for the role of class II MHC expression in mouse B cell lymphoma-genesis are discussed.

Expression of MHC class II antigens in human B-cell leukaemia and non-Hodgkin's lymphoma

In this review we have summarized our experiences of serological analysis of MHC class II antigen expression in human B cell malignant disease. Cells from a large number of cases of B-cell chronic lymphocytic leukaemia (CLL) and non-Hodgkin's lymphoma (NHL) have been examined for expression of class II antigens. Using a number of monoclonal antibodies which in some cases are specific for class II subregion products (DP, DQ and DR), MHC class II antigens were detected by indirect immunofluorescence and fluorescent activated cell sorter analysis in CLL and by immunohistochemical staining in NHL. At the cell surface in many cases of B cell malignant disease, products of the different class II subregion genes are non-coordinately expressed. The most commonly occurring pattern of non-coordinate expression of class II molecules is of expression of DP and DR antigens in the absence of detectable DQ expression. These findings are in contrast to normal B lymphocytes where DP, DQ and DR antigens are expressed together at the cell surface. There is considerable heterogeneity among cases comprising individual histopathological categories of B cell malignancy, and in many instances heterogeneous class II phenotypes are also found on cells from the same tumour. In chronic lymphocytic leukaemia, class II antigen expression is inducible in vitro by treating the cells with the phorbol ester TPA. CLL cells treated with TPA have much increased levels of class II antigen expression at the cell surface and much increased steady state levels of class II specific mRNA transcripts detectable with complementary DNA probes. Aberrant class II antigen expression may be involved in the pathogenesis of B cell malignant disease.

Gene products of the HLA-D region in normal and malignant tissues of nonlymphoid origin

Indirect immunofluorescence staining with monoclonal antibodies has shown a differential distribution of HLA-DR, DQ, and DP antigens in normal tissues of nonlymphoid origin. The distribution of HLA-DP antigens is similar to that of HLA-DR antigens, while that of HLA-DQ antigens is more restricted. Malignant transformation of cells of nonlymphoid origin may be associated with the appearance of the gene products of the HLA-D region. HLA-DR antigens appear more frequently than the other two types of HLA class II antigens and HLA-DP antigens more frequently than HLA-DQ antigens. Differential expression of the gene products of the HLA-D region was also found in autologous metastases removed from different anatomic sites from patients with melanoma. The HLA class II phenotype of surgically removed malignant lesions did not correlate with the degree of differentiation of tumor cells and/or with the expression and/or cellular distribution of HLA class I antigens. Furthermore, in melanoma lesions, no relationship was found between the HLA class II phenotype and the expression of 3 membrane bound and 1 cytoplasmic melanoma associated antigen recognized by monoclonal antibodies. The functional significance and the practical implications of the differential expression of the gene products of the HLA-D region by tumor cells are discussed.

Differential susceptibility to modulation by recombinant immune interferon of HLA-DR and -DQ antigens synthesized by melanoma COLO 38 cells

Recombinant immune interferon (IFN-gamma) induced a dose-related increase in the synthesis and expression of HLA class II antigens by the cultured melanoma cells COLO 38. Although IFN-gamma-treated melanoma cells COLO 38 continue to express higher levels of HLA-DR antigens than of HLA-DQ antigens, the effect of IFN-gamma was more marked on HLA-DQ antigens than on HLA-DR antigens, as indicated by the dose and incubation time required to induce the changes and by the extent and duration of the increase. The effect of IFN-gamma on HLA class II antigens is significantly higher than that of leukocyte and fibroblast interferons. Analysis by one- and two-dimensional gel electrophoresis of antigens synthesized by melanoma cells COLO 38 in the presence of IFN-gamma did not detect any significant change in the structural profile of the subunits of HLA-DR and -DQ antigens.

Thymoma epithelial cells secrete thymic hormone but do not express class II antigens of the major histocompatibility complex

17 thymomas were studied by indirect immunofluorescence for the presence of thymic hormones and antigens of the major histocompatibility complex (MHC). The thymoma epithelial cells (specifically identified by their keratin content) contained thymic hormones (thymulin and thymosin alpha 1), a finding corroborated by the observation of elevated thymulin serum levels. In contrast with normal or hyperplastic thymuses, thymoma epithelial cells did not express HLA-DR and HLA-DC antigens as assessed by immunofluorescence as well as immunoblot analyses. Conversely, MHC class I antigens (HLA-ABC) were normally expressed. Thus, we conclude that thymoma epithelial cells are endocrinologically active but are defective for the expression of some MHC products (class II molecules) known to play an essential role in intrathymic T cell differentiation.