Soluble GARP has potent antiinflammatory and immunomodulatory impact on human CD4⁺ T cells.

Glycoprotein A repetitions predominant (GARP) is expressed on the surface of activated human regulatory T cells (Treg) and regulates the bioavailability of transforming growth factor-ß (TGF-ß). GARP has been assumed to require membrane anchoring. To investigate the function of GARP in more detail, we generated a soluble GARP protein (sGARP) and analyzed its impact on differentiation and activation of human CD4⁺ T cells. We demonstrate that sGARP efficiently represses proliferation and differentiation of naïve CD4⁺ T cells into T effector cells. Exposure to sGARP induces Foxp3, decreases proliferation and represses interleukin (IL)-2 and interferon-γ production, resulting in differentiation of naïve T cells into induced Treg. This is associated with Smad2/3 phosphorylation and partially inhibited by blockade of TGF-ß signaling. Furthermore, in the presence of the proinflammatory cytokines IL-6 and IL-23, sGARP facilitates the differentiation of naïve T cells into Th17 cells. More important, in a preclinical humanized mouse model of xenogeneic graft-versus-host disease (GVHD), sGARP prevents T cell-mediated destructive inflammation by enhancing Treg and inhibiting T effector cell activity. These results demonstrate a crucial role of sGARP in modulation of peripheral tolerance and T effector cell function, opening the possibility to use sGARP as a potent immunomodulator of inflammatory diseases including transplant rejection, autoimmunity, and allergy.

Immune regulation by dendritic cells and T cells--basic science, diagnostic, and clinical application.

Dendritic cells (DC) are professional antigen-presenting cells defined by their ability to transport incoming infectious signals from the periphery to T cell areas in lymphoid organs and by their unique properties to induce primary T cell activation. As sentinels of immunity DC play a critical role in the initiation of immune responses. Thus, they are key targets in antigen-specific immunotherapeutic strategies for cancer. However, beside this essential immunostimulatory function in the immune system, DC also play an important role in the maintenance of peripheral tolerance. Dependent on subtype and mode of activation, tissue resident immature DC differentiate into immunostimulatory or immunosuppressive antigen-presenting cells with a strong capacity to activate or to inhibit T cell responses, respectively. This review summarizes our current knowledge about the complex interaction between DC and T cells considering both--immunity and tolerance--as well as the possibilities to use this knowledge for development of novel diagnostic and immunotherapeutic strategies to treat immune-imbalanced human diseases such as cancer, allergy, and autoimmunity.

Increased regulatory T-cell frequencies in patients with advanced melanoma correlate with a generally impaired T-cell responsiveness and are restored after dendritic cell-based vaccination.

Naturally occurring CD4(+) CD25(+) regulatory T-cell (Treg) activity is assumed to facilitate tumor development and progression. To elucidate the possible role of Tregs in the course of melanoma progression, we analysed the frequency of Tregs in the peripheral blood of patients at melanoma stages I-IV and in patients at melanoma stage IV that underwent dendritic cell (DC)-based immunotherapy. Using CD25, Foxp3, CD127 and HLA-DR as Treg associated markers, we observed increased Treg frequencies in patients at the late melanoma stage (stage IV) when compared to healthy donors. Accumulation of Tregs in patients with progressed melanoma correlated with a general reduction of T-cell responsiveness to the recall antigens tetanus toxoid and tuberculin-GT. However, DC-based immunotherapy not only restored antigen-specific immunity, but also decreased the frequency of Tregs in peripheral blood of patients with melanoma. These findings indicate that tumor progression in patients with melanoma result in general immunosuppression that is associated with Treg expansion in the periphery and can be overcome by DC-based vaccination.

Large scale preparation of human MHC class II+ integrin beta(1)+ Tregs.

The human CD4+CD25+FoxP3+ regulatory T cell population (Tregs) contains both MHC class II+ and MHC class II(-) cells. MHC class II+ Tregs belong to the integrin alpha(4)beta(1)+ subpopulation and exclusively execute contact-dependent suppressive activity. Here we present a method optimized for isolation of these MHC class II expressing Tregs from large leukaphereses products using magnetic microbeads that achieves a reproducible purity of more than 90% and enables the use of this small-sized Treg population in pre-clinical application and basic research.