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Molecular and Cellular Biology, May 2005, p. 4117-4128, Vol. 25, No. 10
0270-7306/05/$08.00+0     doi:10.1128/MCB.25.10.4117-4128.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Regulation of the Small GTPase Rap1 and Extracellular Signal-Regulated Kinases by the Costimulatory Molecule CTLA-4

Vollum Institute, Oregon Health & Science University, Portland, Oregon 97239,¹ Department of Molecular Microbiology and Immunology, Oregon Health & Science University, Portland, Oregon 97239²

Received 13 December 2004/ Returned for modification 19 January 2005/ Accepted 16 February 2005

The mitogen-activated protein kinase extracellular signal-regulated kinase (ERK) is activated following engagement of the T-cell receptor and is required for interleukin 2 (IL-2) production and T-cell proliferation. This activation is enhanced by stimulation of the coreceptor CD28 and inhibited by the coreceptor CTLA-4. We show that the small G protein Rap1 is regulated in the opposite manner; it is inhibited by CD28 and activated by CTLA-4. Together, CD3 and CTLA-4 activate Rap1 in a sustained manner. To delineate T-cell function in the absence of Rap1 activity, we generated transgenic mice expressing Rap1GAP1, a Rap1-specific GTPase-activating protein. Transgenic mice showed lymphadenopathy, and transgenic T cells displayed increased ERK activation, proliferation, and IL-2 production. More significantly, the inhibitory effect of CTLA-4 on T-cell function in Rap1GAP1-transgenic T cells was reduced. We demonstrate that CTLA-4 activates Rap1, and we propose that intracellular signals from CTLA-4 antagonize CD28, at least in part, at the level of Rap1.

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