Regulation of malignant B-cell proliferation by direct binding of anti-HLA class-II antibodies

Activated peripheral T lymphocytes undergo apoptosis when cultured with monocytes activated by HLA class II ligation

We treated PBMC with anti-MHC class II mAb known to inhibit T lymphocyte proliferation. Adherent cells from mAb-treated PBMC showed increased metabolic activity by the MTS assay that was not due to cell proliferation. PBMC cultured with solid-phase anti-class II mAb in chamber inserts inhibited, across a membrane, the proliferation of PBMC cultured with soluble anti-CD3 mAb. PBMC treated with both soluble mAb underwent apoptosis as shown by nucleosomal DNA fragmentation. The monocytes formed multinucleated giant cells as shown by fluorescent microscopy, and contained apoptotic bodies as shown by the TUNEL method and by electron microscopy. The apoptotic cells were identified as T cells by double-staining with anti-CD4/CD8-PE and annexin-V-FITC. Thus, MHC class II ligation stimulates monocytes to increase their metabolic activity, induce apoptosis of activated T lymphocytes, and phagocytize the apoptotic cells. TCR-mediated ligation of MHC class II may play a role in the downregulation of immune responses.

Requirements for induction of activation and proliferation of human B cells analysed with anti-idiotype monoclonal antibodies

We have produced five monoclonal antibodies (MoAb) directed against idiotypic determinants of surface immunoglobulins (sIg) expressed on malignant B cells from a patient suffering from prolymphatic leukaemia (B-PLL). The anti-idiotype MoAb were characterized by immunofluorescence-binding studies, allotype reactivity, and immunoprecipitation studies and were found to recognize at least two distinct epitopes on the sIg of the neoplastic B cells. Differential effects of the soluble anti-idiotype MoAb on phorbol myristate acetate (PMA)-induced B-PLL-cell proliferation were found; three types of anti-idiotype MoAb could be distinguished: (1) MoAb that enhanced the PMA-induced B-PLL-cell proliferation, (2) MoAb without effect, and (3) MoAb that inhibited the PMA-induced B-PLL-cell proliferation. Addition of the calcium ionophore A23187 could abolish the differential effects of the anti-idiotype MoAb on PMA-induced B-PLL-cell proliferation. The negative effect of the type 3 MoAb on PMA-induced B-PLL-cell proliferation was associated with a more than 10-fold stronger binding to sIg expressed on the B-PLL cells, compared with the other anti-idiotype MoAb. The differential effects of the types 1 and 2 MoAb cannot be explained solely by differences in the Fc portion of the MoAb and binding characteristics. Neither did differences in epitope specificity necessarily lead to differential effects on PMA-induced B-PLL-cell proliferation. In contrast to the clear differences found in the proliferation induction experiments, all MoAb were equally able to induce an early rise in free intracellular Ca2+ concentration. Our data suggest that for B-cell proliferation to occur, apart from early rises of intracellular Ca2+, more prolonged [Ca2+]i elevations or additional intracellular activation signals are required. The differences in proliferative responses of B-PLL cells to anti-idiotype MoAb may be relevant for immunotherapy of B-cell tumours with anti-idiotype MoAb.

Signal transduction via MHC class II antigens on B lymphocytes

The role of the MHC class II antigens in the activation of resting human B lymphocytes (B-Go) was examined with respect to both early and late events in the activation process. The (Ca2+)i induced by anti-IgM was enhanced in the presence of, or following pre-incubation with, an anti-MHC class II DR antibody (D1.12). Pre-incubation with a sepharose conjugated antibody (Seph.-D1.12) augmented the proliferation of B-Go in response to a sub-optimal concentration of anti-IgM. The 2D PAGE profile of B-Go differed from that of in vivo activated B lymphocytes. The 2D PAGE profile of B-Go activated by Seph.-D1.12 was not identical to the profile of B-Go activated by either anti-IgM or PMA. These data suggest that the activation of B-Go via the class II antigens shares part of the pathway of anti-IgM induced activation but does not follow an identical pathway.

A role for MHC class II antigens in B-cell activation

Class II antigens of the major histocompatibility complex (MHC) have a well-defined role in restricting cellular interactions and presenting processed antigen to T cells. In addition, a fundamental role for Class II antigens in cellular activation has been suggested, following studies demonstrating that Class II antigen binding alters the proliferation of various cell types. This is further supported by biochemical evidence of signal transduction by second messengers after ligation of the Class II antigens. We have investigated the role of HLA Class II antigens in the activation of B cells. Both activated and resting B cells proliferate in the presence of Sepharose--conjugated anti-Class II antibodies. This proliferation was not epitope-restricted and was unaffected by low m.w. BCGF. Intracellular free calcium elevation was also examined as a marker of cellular activation. (Ca2+)i was increased after the binding and cross-linking of an anti-DR antibody. The above results further support the role of Class II antigens as signal-transducing molecules.