Negative-feedback regulation of CD28 costimulation by a novel mitogen-activated protein kinase phosphatase, MKP6

TCR and CD28 costimulatory receptor-cooperative induction of T cell IL-2 se cretion is dependent upon activation of mitogen-activated protein (MAP) kin ases, Using yeast-hybrid technology, we cloned a novel CD28 cytoplasmic tai l (CD28 CYT) interacting protein, MAP kinase phosphatase-6 (MKP6), which we demonstrate inactivates MAP kinases. Several lines of evidence indicate th at MKP6 plays an important functional role in CD28 costimulatory signaling. First, in human peripheral blood T cells (PBT), expression of MKP6 is stro ngly up-regulated by CD28 costimulation. Second, transfer of dominant-negat ive MKP6 to PET with the use of retroviruses primes PET for the secretion o f substantially larger quantities of IL-2, specifically in response to CD28 costimulation. A similar enhancement of IL-2 secretion is observed neither in response to TCR plus CD28 costimulatory receptor engagement nor in resp onse to other mitogenic stimuli such as phorbol ester and ionomycin. Furthe rmore, this hypersensitivity to CD28 costimulation is associated with CD28- mediated hyperactivation of MAP kinases, Third, a retroviral transduced chi meric receptor with a CD28 CYT that is specifically unable to bind MKP6 cos timulates considerably larger quantities of IL-2 from PET than a similar tr ansduced chimeric receptor that contains a wild-type CD28 CYT. Taken togeth er, these results suggest that MKP6 functions as a novel negative-feedback regulator of CD28 costimulatory signaling that controls the activation of M AP kinases.