Role of CD11a/CD18-CD54 interactions in suppression of human B cell responsiveness by CD4+ suppressor T cells

The role of CD40-CD40 ligand interactions in suppression of human B cell responsiveness by CD4+ T cells

Although human T cells have been shown to regulate humoral immune responses by directly inhibiting B cells, the precise sequelae for the mechanism of suppression have not yet been delineated. The present study was therefore examined to explore the nature of T cell-B cell collaboration to suppress B cell responses. Special attention was directed to the roles of Fas (CD95)-Fas ligand (FasL) interactions and CD40-CD40 ligand (CD40L) interactions. The suppressive activity was assessed by the effects of mitomycin C untreated CD4+ T cells (control CD4+ T cells) activated by immobilized anti-CD3 for 72 h on the production of IgM and IgG of B cells stimulated for 72 h with immobilized anti-CD3-activated mitomycin C treated CD4+ T cells. In this model system, B cells stimulated with anti-CD3-activated CD4+ T cells have been shown to express functional Fas receptor. Thus, anti-Fas mAb CH11 inhibited the production of IgM and IgG induced by anti-CD3-activated mitomycin C treated CD4+ T cells in a manner that was completely reversed by a neutralizing anti-Fas mAb ZB4. However, neither ZB4 nor anti-FasL mAb reversed the suppression of B cell responses by anti-CD3-activated control CD4+ T cells. Anti-CD40L mAb inhibited the production of IgM and IgG stimulated with anti-CD3-activated mitomycin C treated CD4+ T cells when it was added at the initiation of cultures. By contrast, anti-CD40L mAb markedly reversed the suppression of B cell production of IgM and IgG by anti-CD3-activated control CD4+ T cells when it was added after 72 h from the initiation of cultures. Consistently, the extent and intensity of CD40L on anti-CD3-stimulated CD4+ T cells declined upon treatment with mitomycin C in parallel with the loss of suppressive activities on B cell responses. These results indicate that signals achieved by direct interactions through CD40-CD40L exert bidirectional effects on the outcome of humoral immune responses depending on the state of activation of B cells and on the extent of CD40 ligation. Moreover, the data suggest that CD40-CD40L interactions rather than Fas-FasL interactions may play more critical roles in direct cellular collaboration between B cells and anti-CD3 stimulated CD4+ T cells to prevent the extension of responses of inappropriately activated B cells.

Suppression of human B cell responsiveness by CD4+ T cells does not involve CD95-CD95 ligand interactions

Although human CD4+ T cells have been shown to regulate humoral immune responses by directly inhibiting B cells, the precise sequelae for the mechanism of suppression have not yet been delineated. The present study was therefore designed to explore the nature of T cell-B cell collaboration to suppress B cell responses. Special attention was directed to the roles of Fas (CD95)-Fas ligand (FasL) interactions. The suppressive activity was assessed by the effects of mitomycin C-untreated CD4+ T cells activated by immobilized anti-CD3 for 72 h (CD4+ suppressors) on the production of IgM and IgG of B cells stimulated for 72 h with immobilized anti-CD3-activated mitomycin C-treated CD4+ T cells. In this model system, B cells stimulated with anti-CD3-activated mitomycin C-treated CD4+ T cells expressed functional Fas receptors. Accordingly, addition of anti-Fas mAb CH-11 inhibited the cluster formation and differentiation of activated B cells as a result of apoptotic cell death in a manner that was completely reversed by a neutralizing anti-Fas mAb ZB4. However, neither ZB4 nor anti-FasL mAb reversed the suppression of B cell responses by anti-CD3-induced CD4+ suppressors. Of interest, ZB4 significantly enhanced the production of IgM and IgG induced by anti-CD3-activated mitomycin C-treated CD4+ T cells in the absence of CD4+ suppressors. Consistently, mitomycin C-treated CD4+ T cells as well as mitomycin C-untreated CD4+ T cells expressed comparable levels of FasL upon activation with immobilized anti-CD3, although their intensities were very modest. These results indicate that B cells activated with anti-CD3-stimulated CD4+ T cells express functional Fas receptors and are sensitive to Fas-mediated apoptosis. However, the data also suggest that interactions other than Fas-FasL may play a critical role in direct cellular collaboration between activated B cells and anti-CD3-induced CD4+ suppressors to inhibit B cell responses.

Integrin alpha 4 beta 7 co-stimulation of human peripheral blood T cell proliferation

The integrin alpha 4 beta 7 mediates lymphocytes adhesion to VCAM-1 on activated endothelium, fibronectin in the extracellular matrix, and the mucosal vascular addressin MAdCAM-1. It is unclear whether alpha 4 beta 7 performs any function beyond directing specific adhesion reactions. We addressed the possibility that triggering of alpha 4 beta 7 with a specific monoclonal antibody was capable of delivering an accessory stimulus that would coactivate T cells and lead to proliferation. At submitogenic levels of anti-CD3 stimulation, triggering of alpha 4 beta 7 by immobilized mAb ACT-1 resulted in T cell blastogenesis, IL-2 production, expression of the IL-2 receptor alpha chain CD25, and ultimately DNA synthesis. These results indicate that the integrin alpha 4 beta 7 is involved in more than lymphocyte adhesion and homing but also plays a role in cell signaling.

MAb 18D3 triggering of integrin beta 1 will prevent but not terminate proliferation of human T cells

Triggering of integrins can deliver signals that will regulate T cell activation and proliferation when coupled with TCR/CD3 signaling. While co-activation stimuli can be achieved either with immobilized natural ligands or immobilized monoclonal antibodies specific for various integrin subunits, counterposing effects can be delivered by ligation of the integrin beta 1 chain (CD29) resulting in the downregulation of T cell proliferation. Thus, integrins may play a pivotal role in cell activation and are involved in both positive and negative regulatory pathways. In this report, anti-beta 1 mAb 18D3 was used to investigate the role of beta 1 in the negative regulation of T cell proliferation. T lymphocytes were stimulated to proliferate when activated with immobilized mAb to CD3 in conjunction with all of a panel of immobilized mAb to different alpha 4 (CD49d) and beta 1 epitopes, except the anti-beta 1.1 mAb 18D3. In soluble form, mAb 18D3 inhibited the induction of DNA synthesis dependent on costimulation of CD3 and the integrin alpha 4 subunit by a mechanism independent of anti-adhesive properties. In kinetic experiments, the addition of mAb 18D3 effectively inhibited the ultimate induction of DNA synthesis at all time points until the time coinciding with the onset of T cell proliferation, indicating that triggering the beta 1.1 epitope may only act to quench activation events prior to cellular commitment to synthesize DNA. MAb 18D3 did not induce cell death nor render cells incompetent for restimulation, but appeared to selectively inhibit IL-2 synthesis with little effect on the induction of IL-2 receptor expression.