Expression of HLA-DR, MB, MT and SB antigens on human mononuclear cells: identification of two phenotypically distinct monocyte populations

The expression of HLA-DR, SB, MB, and MT antigens in different populations of human mononuclear cells was investigated with the use of monoclonal antibodies that recognize distinct human Ia-like antigens. Our results indicate that in man, as previously reported in other species, two phenotypically distinct populations of monocytes or macrophages can be identified on the basis of expression of Class II MHC antigens. Virtually all circulating monocytes displayed determinants associated with HLA-DR, SB, and MT. In addition, a subpopulation of human monocytes expressed MB/DS-associated antigens, as detected with monoclonal antibodies specific for MB1, MB3, and DS-framework determinants. Most B lymphocytes expressed antigens associated with HLA-DR, and the specificities SB2, SB3, MB1, MB3, MT2, and MT3 were also present. Resting T lymphocytes were unreactive with antibodies that recognize all of the Class II MHC antigens tested. T lymphocytes activated by soluble antigen or alloantigens, and expanded in culture, expressed DR, SB, MB, and MT. The majority of the MB/DS+ cells present in the adherent population were monocytes, because they were phagocytic and had the monocyte-specific marker 63D3. The rest of the cells were not identified. They are likely to include mostly B lymphocytes. The presence of other cells, such as dendritic cells, in this subset needs to be determined.     

Accessory cell function of cells isolated from Mycobacterium leprae-induced granulomas

The large cells from Mycobacterium leprae-induced granulomas in guinea pig lymph nodes were separated by Percoll discontinuous density gradient centrifugation and on a fluorescence-activated cell sorter (FACS) using cross-reacting monoclonal antibody to human MHC Class II antigens. Large Percoll-separated cells (83% Class II antigen positive and 52% macrophage-specific antigen positive) and FACS-separated cells are able to act as antigen-presenting cells for T-cell proliferation to PPD. In previous studies, macrophage antigen-positive cells consistently failed to act as accessory cells. This indicates that there is a population of accessory cells which are macrophage antigen negative and MHC Class II antigen positive present in these M. leprae-induced granulomas.     

Cloned endothelium derived from autoimmune vascular disease retain structural and functional characteristics of normal endothelial cells.

MRL/1pr mice demonstrate anatomic specificity in their development of vasculitis including the small- and medium-sized muscular arteries of the mesentery. To define the functional role of endothelium in vasculitis, we have cloned endothelial cells derived from inflamed small- and medium-sized arteries. Primary cells were derived by enzymatic dispersement and endothelial cells were selected by utilizing a combination of specific culture conditions. Cloned endothelium were developed utilizing limiting dilution cultures supplemented by endothelial cell growth factor. The cloned endothelial cells express many structural features of mature endothelial cells including Factor VIII-RA, non-muscle-specific actin, and Weibel-Palade bodies. Functionally, the clones express functional receptors for the scavenger pathway for LDL metabolism. The cells do not express Class I MHC antigens; however, IFN-beta and IFN-gamma stimulate Class I MHC expression after 24 h, which induces lysis of virus-infected cloned endothelium by Class I-restricted virus-primed T cells. In direct contrast to site-identical vascular smooth muscle cells (VSMCs), endothelial cells do not spontaneously express Class II MHC antigens, nor do they secrete biologically relevant levels of IL-1 unless triggered by lipopolysaccharide. The availability of site-specific cloned endothelium along with cloned VSMCs from autoimmune mice should resolve major experimental controversies involving the pathophysiology of inflammatory vascular disease.     

Primitive endothelial cell lines from the porcine embryonic yolk sac

Endothelial cell lines have been established from cells that were isolated from porcine yolk sacs from day 18 and day 22 embryos and propagated in vitro under various growth conditions. After expansion in vitro, the general properties of the cells proved similar for the different media used. The endothelial cells expressed cell surface receptors for acetylated low-density lipoprotein and also expressed cell surface-associated angiotensin-converting enzyme. The cells showed a characteristically high level of binding for Bandeiraea simplicifolia lectin I and Dolichos biflorus agglutinin but did not bind significant amounts of Ulex europaeus lectin I. The cells expressed low but serologically detectable levels of Class I major histocompatibility complex (MHC) antigens but failed to bind antibodies directed against Class II MHC antigens. Alpha5beta1 integrins were weakly expressed, whereas vascular cell adhesion molecule-1 (CD106) and alphavbeta3 integrins were not detected. Three-dimensional tube formation was readily observed in cultures grown on Matrigel and occurred even in uncoated plastic dishes in the absence of Matrigel. In contrast to most of the adult porcine endothelial cells, yolk sac-derived endothelial cells did not possess serologically detectable receptors for porcine growth hormone (GH), an observation consistent with the finding that GH did not increase the proliferative rate of these cells. Electron microscopic examination demonstrated the presence of Weibel-Palade bodies, tight endothelial cell junctions, and typical rough endoplasmic reticulum. Exposure of the cells to either concanavalin-A-stimulated porcine splenocyte culture supernatants or to human tumor necrosis factor alpha did not cause upregulation of VCAM-1 or Class II MHC antigens. Addition of porcine interferon-gamma led to an increase in the level of expression of Class I MHC. Yolk sac endothelial cells from day 22 embryos showed a low but detectable level of expression of Class II MHC antigens, whereas the endothelial cells from day 18 embryos showed no expression of Class II antigens after interferon-gamma stimulation. The cells maintained competence to develop vascular structures in vitro and could do so after coinjection with murine tumor cells into adult, immunocompromised mice.     

Ultrastructural immunocytochemistry of glia cells. Double labeling studies using LR White embedding and colloidal gold

The introduction of acrylate resins (Lowicryl K4M, LR White) into electronmicroscopic immunocytochemistry applied to embedded tissue (post-embedding method) has improved the localization of antigens because of a satisfactory preservation of both ultrastructure and antigenicity of tissues. Here we describe a method that allows double staining of intracellular and membranous determinants in ultrathin sections of nervous tissue and cultures of peripheral nervous system cells. Ultrathin sections of the rat central nervous system fixed on uncoated grids were stained first for MBP selectively on the one face, then the opposite face was stained for GFAP using monoclonal antibodies and indirect immunogold staining method (IGS). Cultured Schwann cells induced to express major histocompatibility complex (MHC) class II antigens were stained for class II antigens by pre-embedding method then followed by post-embedding IGS for the other intracytoplasmic antigens.     

Diabetes and tolerance in transgenic mice expressing class II MHC molecules in pancreatic beta cells

Insulin-dependent diabetes is caused by the loss of insulin-producing beta cells in pancreatic islets. It has been proposed that aberrant expression of Class II Major Histocompatibility Complex (MHC) molecules on beta cells stimulates an autoimmune attack against beta cell antigens. To test this hypothesis, we generated transgenic mice that express Class II MHC molecules (E alpha d/E beta b, or I-Eb) on beta cells. Diabetes was found in 100% of transgenic progeny from three expressing transgenic mouse lines, but without evidence for lymphocytic infiltrates. Furthermore, T lymphocytes appeared to be tolerant to the transgene I-Eb molecule, despite the absence of expression of I-Eb in the thymus or any other lymphoid tissue. The results suggest that novel expression of Class II MHC molecules on nonlymphoid cells is by itself insufficient to initiate autoimmune responses against tissue-specific antigens.     

Ultrastructural immunocytochemistry of glia cells

Summary The introduction of acrylate resins (Lowicryl K 4M, LR White) into electronmicroscopic immunocytochemistry applied to embedded tissue (post-embedding method) has improved the localization of antigens because of a satisfactory preservation of both ultrastructure and antigenicity of tissues. Here we describe a method that allows double staining of intracellular and membranous determinants in ultrathin sections of nervous tissue and cultures of peripheral nervous system cells. Ultrathin sections of the rat central nervous system fixed on uncoated grids were stained first for MBP selectively on the one face, then the opposite face was stained for GFAP using monoclonal antibodies and indirect immunogold staining method (IGS). Cultured Schwann cells induced to express major histocompatibility complex (MHC) class II antigens were stained for class. H antigens by pre-embedding method then followed by post-embedding IGS for the other intracytopasmic antigens.The Clinical Research Unit for Multiple Sclerosis is supported by Hermann and Lilly Schilling foundation     

Major-histocompatibility-complex-class-II-positive cells and interleukin-2-dependent proliferation of immune T cells are required to reject carcinoma cells in the guinea pig

Summary Tumor immunity induced by bacillus Calmette-Guérin was studied in the line 10 hepatocellular carcinoma (line 10) in the strain-2 guinea pig. Line 10 immunity was investigatedin vitro with a lymphocyte proliferation assay using line 10 tumor protein extracted with 3 M KCl andin vivo by adoptive transfer of line-10-immune spleen cells. Monoclonal antibodies against guinea pig leucocyte markers were used to block functional properties of the immune cells in order to determine which cell types or cell markers are involved in the immune response to the line 10 tumor.In vitro cells from the spleen, peripheral blood and regional lymph node of immune animals reacted with a proliferative response to line 10 protein. This antigen-specific response was caused by T cells and was regulated by major histocompatibility complex (MHC) class II molecules. In blocking experiments it was found that CT5 (anti-PanT), or MSgp4 [anti-(MHC class I antigen)] monoclonal antibodies did not block but some-times stimulated the proliferative response. The effect of H159 (anti-PanT) was irregular, while H155 [anti-(T helper)], and 5C3 [anti-(IL-2 receptor)] monoclonal antibodies blocked the response almost completely. We studied the relevance of the resultsin vitro obtained and found that mAb 5C3 [anti-(IL-2 receptor)] inhibited the adoptive transfer of line 10 immunity, suggesting that the rejection of line 10 cells is caused by a mechanism that is interleukin-2 (IL-2)-dependent. Moreover, complement lysis of MHC-class-II-antigen-positive immune spleen cells inhibited completely the rejection of the line 10 tumor cell challenge in the adoptive-transfer experiments. In conclusion, our data show that MHC class II molecules or cells possessing these molecules are involved in immunity against line 10 tumor cells, as (a) monoclonal antibodies against MHC class II antigens inhibited thein vitro proliferative response of T cells to tumor antigens and (b) removal of MHC-class-II-positive immune spleen cells abrogated the antitumor effect in the adoptive-transfer experiments. Interleukin-2-dependent proliferation of immune T cells is required for the rejection of line 10 tumor cells.     

17β estradiol affects the expression of guinea pig blood leukocyte MHC antigens

Sex hormones have an effect on various immune responses but the mechanisms of action are unknown. One of these mechanisms might be a modification of expression of major histocompatibility complex (MHC) antigens in blood leucocytes. Estradiol-induced variations of the expression of guinea pig blood leukocytes MHC antigens (GPL-A) was studied. Class I and class II MHC antigens were detected by a sensitive rosetting method using specific alloimmune sera (AIS) and staphylococcal protein A-coated sheep red blood cells (SPA-SRBC) and evaluated by counting the number of bound SPA-SRBC per 100 cells. MHC antigens decreased after estrogen treatment. Estradiol modifies the expression of GPL-A antigens on the mononuclear cells including the Kurloff cells, which are involved in immunity or in a natural killer effect, but did not affect the expression of polymorphonuclear cells, ones which are not involved in immunity.     

Cross-linking of insulin receptors to MHC antigens in human B lymphocytes: evidence for selective molecular interactions

Molecular interactions between insulin receptors and MHC antigens were investigated in human B cells. Two B lymphoblastoid cell lines, IM-9 and 526, chosen for their high insulin binding capacity, were found to express 15,000 and 25,000 insulin receptors per cell, respectively. Insulin receptors were labeled with a 125I-photoreactive insulin analogue, and all other surface proteins by lactoperoxidase-catalyzed radioiodination. Neighbor proteins were cross-linked with a cleavable homobifunctional reagent dithio-bis-(succinimidyl propionate) (DSP) and solubilized before immunoprecipitation by anti-HLA monoclonal antibodies. Gel analysis of the precipitated proteins showed that 90% of insulin receptors precipitable by anti-insulin receptor antibodies were precipitated by anti-class I antibodies (anti-heavy chain and anti-beta 2-microglobulin) after cross-linking with 2 mM DSP. In neither IM-9- nor 526 cells could HLA antigens be precipitated by anti-insulin receptor antibodies, suggesting that the concentration of class I antigens largely exceeds the concentration of insulin receptors at the cell surface. In 526 lymphocytes, class I MHC antigens were also found to adjoin class II antigens, since both molecules could be coprecipitated with anti-HLA A, B, C and with anti-HLA-DR antibodies after chemical cross-linking. Down-regulation of insulin receptors by chronic exposure of IM-9 cells to insulin did not affect the amount of MHC molecules present on the cell surface, and conversely, class I MHC molecules were internalized in 526 cells irrespective of the presence of insulin. These results thus show that insulin receptors and MHC antigens form multimolecular complexes in the plasma membrane of cultured human B cells. These interactions, which do not appear to influence the regulation of these proteins on the cell surface, may be involved in the mechanism of hormone signaling.     

Distribution of the major histocompatibility complex antigens on different cellular components of human liver

Monoclonal mouse antibodies to the "framework" determinants of the class I and II molecules of the major histocompatibility complex (MHC) were used to demonstrate the presence of the MHC antigens in human liver. First, the localization of these antigens was demonstrated from frozen section histology with indirect FITC immunofluorescence and the cell component(s) binding the mouse antibody were identified by rabbit marker antisera and indirect TRITC immunofluorescence. Second, the antigen expression on the cell surface was analyzed by the Staphylococcus aureus rosette method from cytological cell smears. All antibodies reacted with cells in the liver sinusoids, both with the Kupffer cells and at least partially with the sinusoidal endothelial cells. The same antisera reacted also with the bile duct cells, though weaker, and with some stromal cells in close proximity of the blood vessels. The vascular endothelial cells of hepatic artery, hepatic vein, and portal vein displayed no reaction. Thus human liver differs strikingly from, e.g., human kidney, where the vascular endothelial cells contain large amounts of MHC antigens on the cell surface. This difference may be one explanation to why liver allografts are less promptly rejected than renal allografts in man.     

Detailed analysis of early immunopathologic events during lesion formation in acute experimental autoimmune encephalomyelitis

To investigate early immunopathologic events, SJL/J mice were challenged for acute experimental autoimmune encephalomyelitis (EAE) and sampled between 12 hr and 14 days postinoculation (PI). Complete Freund's adjuvant (CFA)-inoculated mice served as controls. T cells, T cell subsets, Class II major histocompatibility (MHC) antigen (Ia)-positive and immunoglobulin (Ig)-positive cells, albumin and Ig deposits, and myelin antigens were localized in frozen sections of central nervous system (CNS) and non-CNS tissue (heart, liver, kidney) by immunocytochemical techniques. In both experimental groups, a few Ia-positive endothelial cells and low-grade diffuse infiltration by T cells, T cell subsets, and Ia+ and Ig+ cells were seen from 12 hr PI onward in CNS and non-CNS tissue. Only in acute EAE but not in CFA-challenged mice were these early changes followed at 10 days PI by extensive inflammation which was restricted to the CNS and was accompanied by Ia-positive astrocytes. Thus, in acute EAE, immunopathologic changes appear to develop in two stages. During the early low-grade generalized phase, recirculation of lymphocytes is moderately enhanced while during the late phase, extensive immunopathology is focused upon the target organ, the CNS.     

Antibodies reactive with class II antigens encoded for by the major histocompatibility complex inhibit human B cell activation

Although class II antigens encoded by genes in the major histocompatibility complex (MHC) are important as recognition structures for immunoregulatory cell interactions, the precise functional role of these molecules in the biological responses of B lymphocytes is unknown. In the studies described here, we have examined the effects of six monoclonal antibodies reactive with human class II MHC antigens on B cell activation and proliferation. Peripheral blood IgM+ B cells purified by fluorescence-activated cell sorter (FACS) techniques were stimulated with anti-mu antibodies, protein A-bearing Staphylococcus aureus (SAC), or in T cell-dependent activation cultures. The B cell proliferative responses induced by these stimuli were inhibited 68 to 90% by low concentrations (1 to 5 micrograms/ml) of antibodies reactive with class II MHC antigens. Antibodies specific for DR and DQ antigens were both effective inhibitors of B cell proliferation. This inhibition was not due to the binding of antibody to B cell Fc-IgG receptors, because IgM and IgG anti-class II antibodies were equally potent as inhibitors. When responses of B cells fractionated on the basis of cell size by forward angle light scatter were analyzed, anti-DR and anti-DQ antibodies inhibited the proliferation of small, resting IgM+ cells induced by T-independent as well as T-dependent stimuli. Activation-dependent increases in B cell size and RNA synthesis were similarly inhibited. In contrast, the responses of large B cells (that had been preactivated in vivo) to T cell-derived B cell growth factors were not affected by anti-class II antibodies. These data suggest that class II MHC molecules do not serve merely as cellular interaction structures but also directly participate in early events of the B cell activation cascade that precede cell enlargement or increased RNA synthesis. After activation and expression of receptors for growth factors, however, B cell class II MHC antigens no longer mediate signals required for mitogenesis.     

Interferon-gamma induction of major histocompatibility complex antigens on cultured bovine luteal cells

To investigate immunological mechanisms that may be involved in luteal function, the presence of Class I and Class II major histocompatibility complex (MHC) antigens on cultured bovine luteal cells was examined. After 72 h in serum-free culture, Class I antigens were markedly expressed on luteal cells, as determined by indirect immunofluorescence, whereas expression of Class II antigens was limited. The expression of MHC antigens on luteal cells was increased by treatment with the T-lymphocyte factor, interferon-gamma (IFN-gamma). Class I and II antigens were elevated 25% and 370% above controls, respectively, after IFN-gamma exposure. Since the corpus luteum is regulated by luteinizing hormone (LH), luteal cells were treated with either hormone alone or hormone in addition to IFN-gamma, and antigen expression was determined. LH treatment attenuated IFN-gamma-induction of Class II antigens on bovine luteal cells. These observations are the first to demonstrate the presence of MHC antigens on bovine luteal cells and the modulation of antigen expression by the lymphokine IFN-gamma and by LH.     

Expression of class II antigens by subsets of activated T cells

Gene products coded for within the HLA complex play an important role in the control of immune responses. Class I antigens, coded for by the HLA-A, B, and C loci, are expressed by virtually all mononuclear blood cells. Class II antigens, coded for by the DR, DQ, and DP loci, have a more limited tissue distribution. They are expressed by B cells, monocytes, and by activated, but not by resting, T cells. The class II molecules of B cells and antigen-presenting cells have long been of interest to immunologists, since they are involved in the presentation of antigen, in communication between T cells and B cells and between T cells and adherent cells, and in susceptibility to certain diseases. The class II antigens expressed by activated T cells, however, remain largely uncharacterized in terms of their specificity, functional significance, and molecular nature. We have studied the expression of DR and DQ antigens by activated T cells and then examined the expression of DR versus DQ antigens by Leu 2a and Leu 3a subsets of mitogen-activated populations. Our results demonstrated that, as for class II-positive macrophages, the intensity of staining with monoclonal antibodies directed against DR antigens was much greater than that obtained with those directed against DQ antigens. Interestingly, the percentages of Leu 2a- and Leu 3a-positive cells which expressed DR antigens were quite similar, as were the percentages of Leu 2a and Leu 3a cells which expressed DQ. Thus, there does not seem to be preferential expression of DR versus DQ antigens by mitogen-activated T-cell subsets. Finally, though both DR-positive-DQ-positive and DR-positive-DQ-negative populations were detected, few or no DR-negative-DQ-positive cells were observed in these populations.     

Mouse monoclonal antibodies specific for endothelial cells

Summary Balb/c mice were immunized with a human endothelial cell pool. Spleen cells were then fused with a NS-0 hybridoma cell line. A number of hybridomas secreted antibodies that reacted with the immunizing endothelial cell pool as well as with every other tested umbilical cord vein~derived human endothelial cell. These monoclonal antibodies also stained pig, rabbit and ox aortic endothelial cells indicating their specificity for this cell type. Five of 16 monoclonal antibodies additionally reacted with human fibroblasts (HFIB). The produced monoclonal antibodies did not recognize FVIIIRAG or MHC determinants. They can therefore be regarded as additional and reliable markers for endothelial cells in vitro.     

Unilateral T cell maturation arrest in the thymus of CBA/H mice as a long-term effect after neutron irradiation

Thymuses of CBA/H mice were investigated up to 570 days after whole-body irradiation with 2.5 Gy fast fission neutrons or 6.0 Gy X rays. A number of these thymuses, observed 220-270 days after neutron irradiation, have two equal sized lobes, one of which has an abnormal T cell distribution. The present paper reports on the distribution of lymphoid and stromal cell types in these thymuses. For this purpose, we employed immunohistology using the indirect immunoperoxidase method. We incubated frozen sections of these aberrant thymuses with monoclonal antibodies directed to cell surface differentiation antigens on lymphoid cells, such as Thy-1, T-200, MT-4, Lyt-1, Lyt-2, and MEL-14; monoclonal antibodies directed to major histocompatibility complex (MHC) antigens, such as I-A and H-2K; and monoclonal antibodies directed to determinants in various thymic stromal cell types. The results of this study show a T cell differentiation arrest in only one of the two thymic lobes. T cells in the aberrant lobe express Thy-1, T-200, and MEL-14 antigens but are MT-4- and Lyt-1-. In some lobes, a weak Lyt-2 expression was observed. The observed T cell maturation arrest is mainly restricted to the cortex since in the medulla, in addition to cells with an aberrant cortical phenotype, normal T cell phenotypes are observed. This indicates that cortex and medulla have independent generation kinetics in T cell maturation. The stromal cell composition in these abnormal lobes is not different from that in the normal lobe, but the size of the medulla tends to be smaller. Furthermore, the I-A expression on the cortical epithelial cells does not reveal the characteristic reticular staining pattern that is observed in the normal lobe, since the I-A determinants are not strictly confined to the epithelial cells. In addition, cortical lymphoid and stromal cells in these lobes are slightly H-2K+. These alterations in MHC expression in the cortex are discussed in relation to the observed T cell maturation arrest.     

Antibody-induced association between discrete regions of HLA class I and II antigens

The anti-HLA-DR + DP monoclonal antibody (MoAb) CR11-462 was unexpectedly found to cross-inhibit the binding to B lymphoid cells of the anti-HLA Class I MoAb CR10-215 and CR11-115. The latter two antibodies recognized the same or spatially close antigenic determinant. The cross-blocking of anti-HLA Class I MoAb CR10-215 and CR11-115 by MoAb CR11-462 reflects neither its contamination by anti-HLA Class I antibodies nor its cross-reactivity with HLA Class I antigens. On the other hand, the cross-blocking appears to reflect redistribution of HLA Class II antigens by the MoAb CR11-462, since the MoAb CR10-215 and CR11-115 are not susceptible to blocking when lymphoid cells are treated with 0.025% glutaraldehyde or are coated with Fab' fragments of the MoAb CR11-462. Furthermore, immunoprecipitates from B lymphoid cells preincubated with the MoAb CR11-462 before solubilization contain HLA Class I antigens. Therefore, these results have shown for the first time an antibody-induced association between discrete regions of HLA Class I and Class II antigens on the membrane of B lymphoid cells.     

Role of class II histocompatibility antigens in Staphylococcus aureus protein A-induced activation of human T lymphocytes

The capacity of peripheral blood monocytes and B lymphocytes to support staphylococcal protein A (SpA)-induced proliferation of autologous and allogeneic T cells, as well as the role of major histocompatibility complex (MHC) class I and II molecules in this activation process, were investigated. Highly purified peripheral T lymphocytes did not proliferate in response to SpA, but their response was reconstituted by both irradiated (or mitomycin C-treated) monocytes and B lymphocytes. The effect of B cells on the SpA-induced T-cell response could not be explained by a contamination of residual accessory cells because long-term continuous B-cell lines restored SpA-induced T-cell DNA synthesis as effectively as did monocytes. Support of SpA responsiveness by B cells could not be accounted for by polyclonal binding of SpA to cell surface immunoglobulins, since the ability of SpA-unreactive and SpA-reactive B cells was comparable. The cells from two human leukemic lines--K562 and Raji--showed the same ability in supporting the pokeweed mitogen-induced T-cell response, but the class II-positive Raji cells were much more effective than class II-negative K562 cells in restoring the T-cell responsiveness to SpA. Monoclonal antibodies specific for monomorphic determinants of MHC class II antigens, as well as their F(ab')2 fragments, consistently inhibited the SpA-induced proliferative response, whereas antibodies specific for MHC class I antigens were without effect. The antibodies specific for class II antigens appeared to act at the level of accessory cell, since pretreatment with these antibodies inhibited the ability of SpA-pulsed monocytes or Raji cells to present SpA to autologous or allogeneic T lymphocytes, respectively. These data indicate that either monocytes or normal and lymphoblastoid B cells can act as accessory cells for the proliferative response of human T cells to soluble SpA and that monomorphic determinants of MHC class II molecules play an important role in this activation process.     

Accessory cell function of human endothelial cells. I. A subpopulation of Ia positive cells is required for antigen presentation

The expression of class I and class II HLA antigens on preparations of human endothelial cells, isolated from umbilical cord veins, was investigated by immunofluorescence. While virtually all endothelial cells expressed class I antigens, less than 1% were positive for class II antigens, as detected with a panel of 10 different monoclonal antibodies. Antigen specific T cell lines proliferated in response to mumps antigen in the presence of endothelial cells or blood monocytes from HLA-DR matched donors. However, these T cell lines failed to respond in the absence of accessory cells or when accessory cells from HLA-D-region mismatched cord donors were used. The ability of both monocytes and endothelial cells to present antigen was abolished by treatment of the cells with monoclonal antibodies specific for either class I or class II HLA antigens plus complement. Similar treatment with monoclonal antibodies specific for monocytes greatly reduced antigen presentation by endothelial cells. These results indicate that preparations of endothelial cells contain a subpopulation of Ia positive cells, distinct from monocytes, which are required for antigen presentation.