Katja C. Beier

Recently, we have identified the inducible co-stimulator (ICOS), an activation-dependent, T cell-specific cell surface molecule related to CD28 and CTLA-4. Detailed analysis of human ICOS presented here shows that it is a 55-60-kDa homodimer with differently N-glycosylated subunits of 27 and 29 kDa. ICOS requires both phorbol 12-myristate 13-acetate and ionomycin for full induction, and is sensitive to Cyclosporin A. ICOS is up-regulated early on all T cells, including the CD28- subset, and continues to be expressed into later phases of T cell activation. On stimulation of T cells by antigen-presenting cells, the CD28/B7, but not the CD40 ligand/CD40 pathway is critically involved in the induction of ICOS. ICOS does not bind to B7-1 or B7-2, and CD28 does not bind to ICOS ligand; thus the CD28 and ICOS pathways do not cross-interact on the cell surface. In vivo, ICOS is expressed in the medulla of the fetal and newborn thymus, in the T cell zones of tonsils and lymph nodes, and in the apical light zones of germinal centers (predominant expression). Functionally, ICOS co-induces a variety of cytokines including IL-4, IL-5, IL-6, IFN-gamma, TNF-alpha, GM-CSF, but not IL-2, and superinduces IL-10. Furthermore, ICOS co-stimulation prevents the apoptosis of pre-activated T cells. The human ICOS gene maps to chromosome 2q33 - 34.

Autoimmune diseases and other inflammatory conditions are characterized by large lymphocytic tissue infiltrates in which T and B cells can be found in close contact. Here, using a murine airway inflammation model, we compare antigen-specific T and B cells in lung tissue versus lung-draining lymph node. In the lung we identify a B-cell population exhibiting a classical germinal centre phenotype without being organized into ectopic lymphoid tissue. By contrast, classical CXCR5(+) Bcl-6(+) T follicular helper cells are not present. Nevertheless, lung-infiltrating T cells exhibit follicular helper-like properties including the potential to provide help to naive B cells. The lung tissue is also a survival niche for memory T and B cells remaining in residual peribronchial infiltrates after resolution of inflammation. Collectively, this study shows the importance of T/B cooperation not only in lymph nodes but also in inflamed peripheral tissues for local antibody responses to infection and autoimmunity.

T-cells play a central role in allergic airway diseases such as bronchial asthma. The imbalance between allergen-specific pro-inflammatory and pro-allergic T-cell responses on one hand and regulatory or suppressive T-cell responses on the other may best explain the development of unwanted immune responses against environmental allergens, which lead to immunoglobulin E production and airway inflammation. A key role in the fine tuning of any T-cell response is provided by the engagement of so-called co-stimulatory molecules that are required for the full activation of T-cells and the recognition of antigens via the antigen-specific T-cell receptor. Many of these co-stimulatory molecules have been identified only recently, leading to a fundamental change in the overall understanding of T-cell regulation. Due to their pivotal impact on T-cell differentiation and control, co-stimulatory molecules are promising targets for therapeutic intervention in T-cell-regulated or -mediated immune disorders, including allergic diseases and asthma. In the present article, an attempt is made to summarise the current knowledge on the basic concept of co-stimulation, the presently known co-stimulatory molecules and their various functions on T-cell activation or suppression. The mini-series will be completed by two more articles describing the recent experimental studies and preliminary clinical findings regarding the role of co-stimulatory molecules in allergic disorders and bronchial asthma, and a discussion regarding the feasibility of co-stimulatory molecules as potential targets for the treatment of allergic airway disease. Although it is too early for any clinical implication or utilisation at this moment, the authors are convinced that a better understanding of co-stimulation in the context of allergic asthma will finally provide novel and promising approaches for treatment and prevention.