HLA-D-restricted antigen activation of sensitized T lymphocytes: studies on the ability of HLA-D/DR-expressing B lymphocytes to substitute for macrophages in antigen activation

Antigen processing and presentation to T cells

In this article Howard Grey and Robert Chesnut describe recent insights into the mechanism of antigen presentation and discuss the needfor antigen processing in the stimulation of T cells.

Inhibition of pokeweed mitogen-induced Ig secretion by IgG monoclonal antibodies to MHC class I and class II molecules requires binding of the intact antibody

Seven purified IgG monoclonal antibodies reactive with different epitopes on DQw1, DR, HLA-A3 or p85 glycoprotein of human lymphocytes have each been shown to inhibit pokeweed mitogen (PWM)-induced IgG and IgM secretion in a dose-dependent manner. Binding of these antibodies to their target antigens was required for the suppression. Antibodies of IgG1, IgG2 alpha, and IgG2b subclasses were able to inhibit both IgG and IgM secretion in the PWM system. The mechanisms by which two of the monoclonal antibodies (MoAbs)-77.34, specific for the class II antigen DQw1, and GAP A3, specific for the class I antigen HLA-A3-caused inhibition--were analyzed. The suppressive effects of 77.34 and GAP A3 were maximal when added at the initiation of the culture period. No inhibition of IL-2 production or cellular proliferation was detected. Supernatants obtained from inhibited cultures were not themselves suppressive. The F(ab')2 fragments of either 77.34 or GAP A3 failed to influence PWM-Ig secretion, indicating that intact IgG molecules were required. This suggests that the observed inhibition might be mediated via Fc receptors. Together F(ab')2 fragments of either 77.34 or GAP A3 and a control IgG2a protein did not reconstitute the inhibitory effects of intact 77.34 or GAP A3. Suppression, therefore, required intact Fc portions on the same IgG molecules as those that bound to DQw1 or HLA-A3. These studies suggest that populations of IgG molecules that crosslink sufficient numbers of Fc receptors with other cell surface antigens on peripheral blood mononuclear cells (PBMs) during the early stages of B-cell activation can inhibit Ig secretion. Crosslinking of B-cell Fc receptors with SIg has been proposed by others to act as a negative signal for Ig production; our data raise the possibility that crosslinking of FcR with B-cell plasma membrane components other than SIg can also suppress Ig secretion.

Epstein-Barr virus-transformed B cells process and present Mycobacterium tuberculosis particulate antigens to T-cell clones

We have analyzed the presentation of mycobacterial antigens by Epstein-Barr virus-transformed human B (EBV-B) cells to mycobacteria-specific T-cell clones and lines, and to purified resting T cells. EBV-B cells were able to process and present not only soluble forms of antigen, such as PPD and the expressate preparation of M. tuberculosis strain H37Rv, but also particulate forms of antigen, such as whole mycobacterial H37Rv or M. bovis organisms. Electron microscopy studies demonstrated the capacity of EBV-B cells to phagocytose mycobacterial cells in 18 hr and pulsing experiments confirmed that an 18-hr of incubation is required for an efficient processing and presentation of mycobacterial determinants to T cells. The processing of whole-H37Rv particulate antigen by EBV-B cells was inhibited by the lysosomotrophic compound chloroquine and by high doses of irradiation. Finally, the analysis of the presentation of soluble and particulate mycobacterial antigens by PPD-positive and PPD-negative EBV-B cell clones has shown a preferential presentation of both forms of antigen by PPD-positive EBV-B clones.

The role of HLA-DR antigens in PPD-stimulated lymphocyte-monocyte interactions

Autologous monocytes are required for an optimal lymphocyte proliferative response to purified protein derivate of tuberculin (PPD) in vitro and for a mixed lymphocyte culture induced by alloantigens. In the proliferative response to PPD we found that autologous monocytes could be replaced with HLA-DR-compatible monocytes and partly with HLA-DR semi-identical. In spite of a statistically significant difference between autologous and HLA-DR disparate monocytes in their cooperative capacity with PPD-stimulated lymphocytes, replacement in nearly one third of the cases was possible. These findings were supported by more detailed studies in which increasing numbers of allogenic and autologous monocytes were added to the isolated lymphocytes in the presence of PPD. It is concluded that the serologically defined HLA-DR antigens alone give insufficient information of the restriction elements controlling the PPD-stimulated lymphocyte-monocyte interactions.

Endothelial cell presentation of antigen to human T cells

Activation of human T cells requires presentation of antigen by Ia (HLA-DR in man) bearing cells of the mononuclear phagocytic series (macrophages, MO, and more recently Langerhans cells, dendritic cells, and vascular endothelial cells. Since T cells must cross endothelial barriers to enter extravascular tissues during immune reactions, we investigated the role of endothelial cells in antigen presentation. Endothelial cells were cultured from human umbilical veins and identified by classic morphology and specific markers (factor VIII related antigen, and so on). Antigen-pulsed endothelial cells were used to present antigen to MO-depleted human T cells; activation was assessed by 3H-thymidine uptake. The HLA-DR compatible endothelial cells were as effective as MO in reconstituting MO-depleted T-cell responses. The endothelial cell reconstituted responses were antigen specific, HLA-DR restricted, and blocked by monoclonal antibodies to HLA-DR framework structures. Moreover, the T-cell responses were clonal with respect to HLA-DR. A monoclonal antibody completely eliminated MO reconstitution of the MO-depleted response without diminution of endothelial cell reconstitution of the same response. Fibroblasts and smooth muscle cells cultured from the same umbilical veins could not reconstitute the MO-depleted T-cell response. These data indicate that endothelial cells play an important and distinctive role in lymphocyte triggering.

Antigen presentation to human T lymphocytes. I. Different requirements for stimulation by hapten-modified cells vs. cell sonicates

We have investigated the cellular and antigenic requirements for incubation of secondary proliferative responses by human T lymphocytes. Two distinct properties of antigen-presenting peripheral blood mononuclear cells were studied: (a) the ability for appropriate cell surface constituents to construct an immunogenic moiety, and (b) the ability to present similar antigenic determinants when they are not covalently bound. Only Ia+ hapten-modified cells were effective stimulators. In contrast, both Ia+ and Ia- cell sonicates could stimulate secondary proliferative responses, but only in the presence of an accessory cell. This accessory cell was present in Ia+ macrophage, but not in Ia+ non-T lymphocyte, preparations. In contrast, macrophages or soluble factors produced by macrophages were not required for primed T cells to undergo hapten-specific proliferation in response to hapten-modified Ia+ stimulator cells. Thus, although all Ia+ cells tested can stimulate primed cells to proliferate, not all Ia+ cells can function as accessory cells for responses to sonicates. This may reflect the unique ability of a subpopulation(s) of Ia+ cells to bind or process sonicates or soluble antigens for appropriate recognition by primed T cells.

Antigen presentation and regulatory functions of human monocytes in the in vitro response of lymphocytes against purified protein derivative of tuberculin (PPD)

Monocyte functions in the primary and secondary proliferative responses of human lymphocytes against purified protein derivative of tuberculin (PPD) have been studied. By applying the antigens either directly to the responding cells or on antigen-treated autologous monocytes, it was possible to distinguish between two different monocyte functions: antigen presentation and regulation. Their helper function in lymphocyte proliferation was found in cell-free supernatants of autologous and allogeneic monocytes and was expressed better in the secondary rather than in the primary responses. The antigen-presenting function of monocytes, however, was found to be more substantial in the primary response of lymphocytes. These results suggest that macrophage dependency of certain immune functions should be considered in regard to the various functions of macrophages, since at least quantitative differences for each of the macrophage functions were found in the primary and secondary proliferative responses.

Studies on human epidermal Langerhans cells. I. Allo-activating and antigen-presenting capacity

Human epidermis was separated from dermis by means of a suction blister device and dissociated with trypsin. The epidermal cell suspensions obtained contained 3--5% Langerhans cells as judged by immunofluorescence staining ot the cells with a rabbit anti-DR antiserum. The epidermal cells were co-cultured with purified allogeneic T cells and with autologous T cells with or without PPD of tuberculin. A strong T-cell response to allogeneic epidermal cells was obtained, as was a strong T-cell response to PPD, provided autologous epidermal cells were also present. Pre-treatment of the epidermal cells with anti-DR antiserum plus complement abolished both these responses. These data indicate that epidermal cells are able to substitute for macrophages both in the allo-activating and in the antigen-presenting function. Since the responsible cells were DR-positive, it is highly probable that the cells responsible for these functions are the Langerhans cells.