No abstract

The possible role of homodimerization events in intracellular signal transduction triggered by the bipartite human interleukin-4 receptor was addressed. We generated cell lines functionally expressing derivatives of the two receptor subunits alpha and gamma, which allow for a specific and background-free experimental induction of intracellular homo- and heterodimers. A heterodimer of alpha and gamma released an intracellular signal, whereas a gamma-gamma homodimer did not. Unexpectedly, we found the intracellular domain of interleukin-4 receptor alpha chain to evoke cell proliferation and activation of tyrosine kinase Jak1 as well as of transcription factor Stat6 upon homodimerization. Both recruitment of the common gamma chain and activation of kinase Jak3 were shown to be dispensible for these processes.

Interleukin-4 (IL-4) shows species-specific activity due to species-restricted interaction with the IL-4 receptor a (IL-4Ra) chain. The second subunit of a functional IL-4 receptor, the common 7 chain (yJ, is more promiscuous, since human IL-4 is able to activate IL-4 receptor complexes containing either human or murine common 7 receptor chain (yJ. We have stably transfected factor-dependent mouse cells of myeloid and lymphoid origin with combinations of human IL-4Ra and yc derivatives. In these cell lines, both human and murine yc receptors as well as IL-4Ra chains from both species are simultaneously expressed. Both versions of' yc readily form ternary complexes with either human IL-4 and human IL-4Ra or murine I t -4 and murine IL-4Ra. Due to distinct ligand-binding properties of human and murine yL, the two receptor complexes can be activated preferentially by different mutant variants of human IL-4. The contribution of murine common 7 chain to human TL-4-induced signal transduction is suppressed by an inhibitory antibody directed to the extracellular domain of the mouse yC. We present evidence that the two IL-4R complexes functionally interfere with each other and compete for responselimiting signalling components.

Haemoflagellates of the genus Trypanosoma are prevalent in freshwater fishes and are transmitted by leeches as vectors. As demonstrated by sequence comparisons of nuclear small subunit rRNA genes, trypanosomes isolated from several fish species at different localities can be divided into at least 2 closely related types, designated Type A and Type B. A clone derived from a Type A isolate from carp (Cyprinus carpio) was used to study the anti-parasite immune response in specified pathogen-free outbred carp. Infection leads to an initial rise in parasitaemia in the blood followed by a sharp decline in all fish (acute phase). Thereafter, in some carp, parasites become undetectable both in the blood and in internal organs while, in others, low numbers can be found in the blood for up to 1 year (chronic phase). Fish that have controlled an acute infection with the clone are not only protected against an homologous challenge infection, but also against the infection with parasite lines derived from carp in the chronic phase of infection. Passive immunization experiments with IgM purified from serum of recovered carp indicate that the infection is controlled by antibodies. The anti-parasite antibody level in recovered carp remains high for many months although the parasitaemia is controlled at very low levels and the half life of IgM, t1/2 = 22.5 days, is comparatively short. The effective control of trypanosomes in laboratory infections is in contrast to the high prevalence in natural and farmed freshwater fish populations.

Interleukin (IL)-4,1 a pleiotropic modulator of the immune system (1), exerts its activity on target cells through the interleukin-4 receptor. Different cell types, including lymphoid and myeloid blood cells, express a bipartite IL-4 receptor (IL-4R), which consists of the IL-4R ␣-chain (IL-4R␣) (2) and the common ␥ receptor chain (␥c) (3). IL-4R␣ may also form a functional IL-4 receptor in conjunction with the IL-13 receptor instead of ␥c, especially in nonimmune cells (4). In vitro IL-4R␣ has also been shown to trigger intracellular signal transduction as a homodimer without participation of a heterologous receptor subunit (5, 6); however, the possible physiological relevance of IL-4R␣ homodimers is not yet known.IL-4-induced dimerization of receptor subunits results in the rapid onset of various cytoplasmic events. Janus kinases JAK1 and JAK3, which are associated with IL-4R␣ and ␥c, respectively (7, 8), become activated, probably by transphosphorylation, and as a result several other constituents of the activated receptor complex are phosphorylated. Tyrosine phosphorylation of IL-4R␣, IRS-2 (insulin receptor substrate 2, originally termed 4PS/IL-4-induced phosphorylation substrate), phosphatidylinositol 3-kinase, STAT6, and probably other proteins generate a network of protein-protein contacts mediated by interactions between phosphotyrosines and Src homology 2 domains (9). Despite structure-function studies by several laboratories (10 -16), it is only partially understood how these IL-4-induced molecular events lead to long term cellular processes such as suppression of apoptosis, proliferation, and differentiation.The aspect of IL-4R-triggered intracellular signal transduction that has been best characterized is the JAK-STAT pathway. STAT (signal transducer and activator of transcription) factors are central components of signaling cascades triggered by cytokine receptors and are phosphorylated in response to ligand stimulation through receptor-associated Janus kinases (JAKs). STAT factors then dimerize and translocate to the cell nucleus, where they interact with cognate DNA sequences of ␥-interferon-responsive sites (GASs) and modulate transcription of target genes (17).Following signal induction by the ligand, both the IL-4 receptor and the IL-13 receptor specifically mediate the activation of STAT6 (15, 18 -21). Gene regulation by STAT6 appears to be of central importance for IL-4-governed immune regulation, since STAT6 knockout mice are unable to develop Th2 cells; are impaired in cell surface expression of CD23, IL-4R␣, and major histocompatibility complex class II; and show a drastic defect in immunoglobulin class switching (22)(23)(24).Results obtained in the course of investigations on related cytokine receptors raised the question of whether STAT6 is the only STAT protein involved in IL-4-induced intracellular signaling. The IL-4 receptor shares the common ␥-subunit with the receptors for IL-2, , and all cytokine receptors utilizing the ␥c subunit, with the exception of the IL-4R, activate STAT5 ...