Atopic allergic disorders such as allergic asthma and/or atopic dermatitis are immunologically characterized by elevated serum IgE levels and eosinophilia. Based on the analysis of allergen-specific T helper (Th) cell clones of allergic and non-allergic individuals, we have previously shown that these aberrant immunological features in atopic patients are related to the occurrence and activation of memory Th cells (Th2 cells) that do not produce the type 1 cytokine interferon-g (IFN-gamma), but high quantities of the type 2 cytokines interleukin (IL)-4 and IL-5.
The cytokine secretion profile of Th cells is highly sensitive to the modulatory impact of various micro-environmental factors during antigen-specific Th cell activation. An important factor in this respect is the antigen-presenting cell-derived cytokine IL-12, which selectively upregulates IFN-gamma production in activated T cells. In vivo experiments with Th1- and Th2-proned inbred mouse strains showed that the cytokine profile of memory Th cells is strongly determined by their IL-12 responsiveness during maturation from naive Th cells and thus by their expression levels of IL-12 receptors (IL-12R). Functional, high-affinity IL-12R consist of a beta1 and a beta2 chain, of which the latter is mainly involved in signal transduction. Mouse experiments indicated that Th2 cell development from naive Th cells is associated with suppression of IL-12Rbeta2 chain expression, leading to loss of IL-12 responsiveness and, consequently, the inability to promote IFN-gamma production.
This project includes a more detailed and comparative study of the levels of expression of IL-12Rbeta2 on T helper cells of allergic asthma and atopic dermatitis patients. Specific fundamental research aims are (i) to investigate the physiological induction and regulation mechanisms underlying normal expression of the IL-12Rbeta2 chain, (ii) to investigate how this proces of regulation is altered in atopic patients, and (iii) to explore possibilities for pharmacological modulation of the expression of this molecule.
We have previously reported that allergen-specific Th2 clones from atopic patients completely lack signalling through the IL-12R, as evident from the lack of IFN-gamma induction and STAT4 phosphorylation upon exposure to IL-12. Because FACS analysis showed normal surface expression of the IL-12Rbeta1 chain, these findings strongly suggest the absence of functional IL-12Rbeta2 chains on human Th2 cells, similar to the situation in mouse Th2 cells. Indeed, RNAse-protection assays with a human IL-12Rbeta2-specific DNA probe (recognizing a sequence encoding the intracellular domain) indicated the IL-4-dependent downregulation of IL-12Rbeta2 mRNA in human TCR (anti-CD3)-stimulated naive T cells. So far, we have not been able to confirm these findings using a quantitative RT-PCR based on the sequence encoding the extracellular part of the IL-12Rbeta2 chain. Moreover, preliminary data with a newly generated antibody against the beta2-chain suggest normal expression of the IL-12Rbeta2 protein on the membrane of Th2 cell clones and activated naive T cells, unaffected by exposure to IL-4. In contrast to TCR-induced IL-12Rbeta2 expression, our RT-PCR detecting mRNA encoding for the extracellular part of the ß2-chain clearly indicated suppression by IL-4 of the IL-12-mediated superinduction of TCR-induced IL-12Rbeta2 mRNA expression. We are currently exploring the possibility that the expression of the IL-12Rbeta2 chain is subject to at least two levels of regulation, including a TCR-mediated mechanism and a cytokine (JAK/STAT)-mediated pathway. The present data further suggest differential regulation of the intracellular and extracellular domains of the IL-12Rbeta2 chain in the presence of IL-4. |
