Molecular mechanisms of thymic stromal lymphopoietin signaling

Thymic stromal lymphopoietin (TSLP) is an IL-7-like cytokine that requires a heterodimeric receptor complex composed of the interleukin 7 receptor a chain and the TSLP receptor, which is related to the common gamma chain. TSLP has been shown to play an important role in the development of allergic inflammation and acute lymphoblastic leukemia. Chimeric receptors composed of the cytoplasmic region of the TSLP receptor fused to the extracellular regions of homodimeric receptors have been used to dissect signaling events induced by the TSLP receptor. Intriguingly, studies using such chimeric TSLP receptors revealed that the human, but not mouse, TSLP receptor cytoplasmic domain can support proliferation of growth factor dependent cells after homodimerization. Here, we used a systematic approach to investigate the mechanistic basis of this difference. Our studies revealed that induced homodimerization of receptor chimeras containing the transmembrane and cytoplasmic domains of both human and mouse TSLP receptors is not sufficient for driving cell proliferation. Our data suggests that in the chimeric receptor context, the transmembrane and juxtamembrane domain of mouse Epo receptor are essential for the cytoplasmic domain of human TSLPR to drive proliferation. It has previously been suggested that the lone tyrosine residue in the mouse TSLPR cytoplasmic domain is required for cell proliferation using chimeric receptor systems. Also the role of tyrosine residues in the IL-7Ralpha cytoplasmic domain in TSLP signaling has not yet been investigated. Therefore, I undertook a systematic analysis to test the role of tyrosine residues of both the IL-7Ralpha and the TSLPR in TSLP-induced cell proliferation. Our site-directed mutagenesis experiments revealed that the single tyrosine residue in human TSLPR was not required for TSLP-dependent cell proliferation. Interestingly, in contrast to IL-7 signaling, none of tyrosine residues in the human IL-7Ralpha cytoplasmic domain were required for TSLP-dependent cell proliferation in the presence of a wild type TSLPR. However, the mutation of all cytoplasmic four tyrosine residues of human IL-7Ralpha and human TSLPR to phenylalanine residues abolished the proliferative ability of the TSLP receptor complex in response to TSLP. These results suggest that TSLP requires at least one cytoplasmic tyrosine residue to transmit proliferative signals. However, unlike other members of IL-2 cytokine family, tyrosine residues in IL-7Ralpha and TSLPR cytoplasmic domains play a redundant role in TSLP-mediated cell growth.