A novel sequence discovered in a computational screen appears distantly related to the p35 subunit of IL-12. This factor, which we term p19, shows no biological activity by itself; instead, it combines with the p40 subunit of IL-12 to form a novel, biologically active, composite cytokine, which we term IL-23. Activated dendritic cells secrete detectable levels of this complex. IL-23 binds to IL-12R beta 1 but fails to engage IL-12R beta 2; nonetheless, IL-23 activates Stat4 in PHA blast T cells. IL-23 induces strong proliferation of mouse memory (CD4(+)CD45Rb(low)) T cells, a unique activity of IL-23 as IL-12 has no effect on this cell population. Similar to IL-12, human IL-23 stimulates IFN-gamma production and proliferation in PHA blast T cells, as well as in CD45RO (memory) T cells.

In spite of thymic involution early in life, the numbers of naive CD4 + T cells only slowly decline in ageing humans implying peripheral post-thymic naive CD4 + T cell expansion. This proliferation may compensate for continuous activation and death of naive CD4 + T cells but may also have negative consequences for protective immunity. Here we show that naive CD4 + T cells that have proliferated in the periphery are characterized by a highly restricted oligoclonal TCR repertoire. Additionally these cells, which constitute the majority of naive CD4 + T cells in the elderly, display signatures of recent TCR engagement. Our results demonstrate for the first time that peripheral post-thymic proliferation of naive CD4 + T cells in healthy human individuals causes a significant contraction of the peripheral TCR repertoire. This age-dependent deterioration of CD4 + T cell immunity could entail ageing-associated autoimmunity, increased susceptibility to infection or cancer and decreased efficiency of vaccination.

Interleukin (IL)-6, IL-11 and cililary neurotrophic factor (CNTF) belong to the same family of hematopoietic and neurotrophic cytokines. Their receptor complexes contain a cytokine-binding a receptor and the common glycoprotein (gp)130 subunit for signal transduction. The extracellular parts of the a-receptor subunits consist of a membrane-proximal cytokine-binding domain and an N-terminal immunoglobulin (Ig)-like domain with unknown function. We examined the role of the Ig-like domain of IL-6R by constructing deletion mutants lacking the Ig domain (IL-6RDIg and soluble IL-6RDIg). IL-6RDIg was shed as effectively as wild-type IL-6R from transfected COS-7 cells upon 4b-phorbol 12-myristate 13-acetate (PMA) treatment, whereas nonstimulated shedding of IL-6RDIg was not observed. The shed sIL-6RDIg from PMA-treated cells, as well as the transmembrane IL-6RDIg, had the same biological activity as wild-type sIL-6R, as measured by the induction of haptoglobin secretion in HepG2-IL-6 cells and IL-6-dependent proliferation of IL-6RDIg transfected BAF/gp130 cells. In COS-7 cells transfected with IL-6RDIg or soluble IL-6RDIg cDNA, transport of the deletion mutants through the secretory pathway appeared to be delayed because a sizeable proportion of the mutants was detected as an endo-b-N-acetylglucosaminidase-sensitive intermediate, suggesting that transport and processing of the DIg mutants on the secretory pathway were impaired. These experiments suggest that the Ig-like domain of the IL-6R is important for intracellular transport of IL-6R through the secretory pathway. Furthermore, the Ig-like domain is necessary for noninduced shedding of the IL-6R, whereas it has no function in PKC-dependent shedding of the IL-6R.

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