IMPAIRED CD28-MEDIATED INTERLEUKIN-2 PRODUCTION AND PROLIFERATION IN STRESS KINASE SAPK ERK1 KINASE (SEK1) MITOGEN-ACTIVATED PROTEIN-KINASEKINASE-4 (MKK4)-DEFICIENT T-LYMPHOCYTES/

The dual specific kinase SAPK/ERK1 kinase (SEK1; mitogen-activated pro tein kinase kinase 4/Jun NH2 terminal kinase [JNK] kinase) is a direct activator of stress-activated protein kinases ([SAPKs]/JNKs) in respo nse to CD28 costimulation, CD40 signaling, or activation of the germin al center kinase. Here we show that SEK1(-/-) recombination-activating gene (RAG)2(-/-) chimeric mice have a partial block in B cell maturat ion. However, peripheral B cells displayed normal responses to IL-4, I gM, and CD40 cross-linking. SEK1(-/-) peripheral T cells showed decrea sed proliferation and IL-2 production after CD28 costimulation and PMA /Ca2+ ionophore activation. Although CD28 expression was absolutely cr ucial to generate vesicular stomatitis virus (VSV)-specific germinal c enters, SEK1(-/-)RAG2(-/-) chimeras mounted a protective antiviral B c ell response, exhibited normal IgG class switching, and made germinal centers in response to VSV. Interestingly, PMA/Ca2+ ionophore stimulat ion, which mimics TCR-CDS and CD28-mediated signal transduction, induc ed SAPK/JNK activation in peripheral T cells, but not in thymocytes, f rom SEK1(-/-) mice. These results show that signaling pathways for SAP K activation are developmentally regulated in T cells. Although SEK1(- /-) thymocytes failed to induce SAPK/JNK in response to PMA/Ca2+ ionop hore, SEK1(-/-)RAG2(-/-) thymocytes proliferated and made IL-2 after P MA/Ca2+ ionophore and CD3/CD28 stimulation, albeit at significantly lo wer levels compared to SEK1(+/+)RAG2(-/-) thymocytes, implying that CD 28 costimulation and PMA/Ca2+ ionophore-triggered signaling pathways e xist that can mediate proliferation and IL-2 production independently of SAPK activation. Our data provide the first genetic evidence that S EK1 is an important effector molecule that relays CD28 signaling to IL -2 production and T cell proliferation.