Boosting regulatory T cell function for the treatment of autoimmune diseases - That's only half the battle!

Regulatory T cells (Treg) represent a subset of specialized T cells that are essential for the regulation of immune responses and maintenance of peripheral tolerance. Once activated, Treg exert powerful immunosuppressive properties, for example by inhibiting T cell-mediated immune responses against self-antigens, thereby protecting our body from autoimmunity. Autoimmune diseases such as multiple sclerosis, rheumatoid arthritis or systemic lupus erythematosus, exhibit an immunological imbalance mainly characterized by a reduced frequency and impaired function of Treg. In addition, there has been increasing evidence that - besides Treg dysfunction - immunoregulatory mechanisms fail to control autoreactive T cells due to a reduced responsiveness of T effector cells (Teff) for the suppressive properties of Treg, a process termed Treg resistance. In order to efficiently treat autoimmune diseases and thus fully induce immunological tolerance, a combined therapy aimed at both enhancing Treg function and restoring Teff responsiveness could most likely be beneficial. This review provides an overview of immunomodulating drugs that are currently used to treat various autoimmune diseases in the clinic and have been shown to increase Treg frequency as well as Teff sensitivity to Treg-mediated suppression. Furthermore, we discuss strategies on how to boost Treg activity and function, and their potential use in the treatment of autoimmunity. Finally, we present a humanized mouse model for the preclinical testing of Treg-activating substances in vivo.

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.

In vivo inhibition of epidermal growth factor receptor autophosphorylation prevents receptor internalization.

The question whether epidermal growth factor (EGF)-induced receptor endocytosis requires the prior autophosphorylation via the EGF receptor (EGFR) kinase domain has been a matter of long-standing debate. In the airway epithelial cell line NCI-H292, the EGFR kinase domain inhibitor BIBW 2948 BS was found to inhibit both autophosphorylation and subsequent internalization of the endogenous EGFR with similar IC50 values. Applying an ex vivo EGFR internalization assay in a clinical study, the in vivo effect of inhalatively administered BIBW 2948 BS was determined directly at the targeted receptor in airway tissues from COPD patients. In these experiments, the in vivo inhibition of the EGFR kinase domain prevented the EGF-induced internalization of EGFR.

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.

Generation of monoclonal antibodies against human regulatory T cells.

Natural CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs) control the activation of the immune system and therefore have become a major area of research in immunology. The generation of monoclonal antibodies against human Tregs offers the possibility to discover novel Treg-specific or Treg-associated surface markers and to identify targets for a therapeutic modulation of Tregs. Here we present a methodology optimized to efficiently induce and select mAb against human Tregs by repeated immunization of mice with Tregs from a single donor and a differential two-step flow cytometry-based hybridoma screening procedure.

Protection from graft-versus-host disease by HIV-1 envelope protein gp120-mediated activation of human CD4+CD25+ regulatory T cells.

Naturally occurring CD4(+)CD25(+) regulatory T cells (Tregs) represent a unique T-cell lineage that is endowed with the ability to actively suppress immune responses. Therefore, approaches to modulate Treg function in vivo could provide ways to enhance or reduce immune responses and lead to novel therapies. Here we show that the CD4 binding human immunodeficiency virus-1 envelope glycoprotein gp120 is a useful and potent tool for functional activation of human Tregs in vitro and in vivo. Gp120 activates human Tregs by binding and signaling through CD4. Upon stimulation with gp120, human Tregs accumulate cyclic adenosine monophosphate (cAMP) in their cytosol. Inhibition of endogeneous cAMP synthesis prevents gp120-mediated Treg activation. Employing a xenogeneic graft versus host disease model that has been shown to be applicable for the functional analysis of human Tregs in vivo, we further show that a single dose of gp120 is sufficient to prevent lethal graft versus host disease and that the tolerizing effect of gp120 is strictly dependent on the presence of human Tregs and their up-regulation of cAMP upon gp120-mediated activation. Our findings demonstrate that stimulation via the CD4 receptor represents a T-cell receptor-independent Treg activating pathway with potential to induce immunologic tolerance in vivo.

A convenient rapid culture assay for the detection of enteroviruses in clinical samples: comparison with conventional cell culture and RT-PCR.

A convenient rapid culture assay (RCA) for the detection of enteroviruses was evaluated against RT-PCR using 576 stool and 102 cerebrospinal fluid (CSF) samples. One hundred and ninety stool samples were also tested by conventional cell culture (CCC). The RCA used immunoperoxidase staining of cell culture plates with a blend of monoclonal antibodies (mAbs) against enterovirus VP1 on the second and sixth days after inoculation. This blend was composed of 5D8/1 (Dako) and four 'in-house' mAbs. CCC was performed using fluorescence staining with the Enterovirus Screening Set (Chemicon International) for culture confirmation. Detection of enteroviruses by the RCA was more successful in colonic carcinoma (CaCo-2) and rhabdomyosarcoma (RD) cells than in human embryonic lung fibroblasts, HEp2 and A549 cells. The performance of CCC in RD cells was hindered by rapid cell degeneration and non-specific staining of cells during culture confirmation. The sensitivity of the RCA compared to RT-PCR in stool samples was found to be 71 % (115/161) on the second day and 87 % (140/161) on the sixth day. The sensitivity of the RCA in CSF samples was 38 % (22/58) after 2 days and 59 % (34/58) after 6 days. The specificity of the RCA was 100 %. All CCC-positive samples were positive by the RCA. CCC required 3-14 days for virus recovery. In conclusion, the RCA has the same sensitivity as CCC, significantly shortens the time required for the detection of enteroviruses, and prevents pitfalls associated with using RD cells for CCC. For diagnosis of aseptic meningitis in CSF samples, RT-PCR should be performed.