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From the * Amgen Institute and Ontario Cancer Institute, Department of Medical Biophysics and
Immunology, University of Toronto, M5G 2C1 Toronto, Ontario, Canada; The dual specific kinase SAPK/ERK1 kinase (SEK1; mitogen-activated protein kinase kinase
4/Jun NH2 terminal kinase [ JNK] kinase) is a direct activator of stress-activated protein kinases ([SAPKs]/JNKs) in response to CD28 costimulation, CD40 signaling, or activation of the germinal center kinase. Here we show that SEK1
Ontario Cancer Institute,
Department of Medical Biophysics and Immunology, University of Toronto, Toronto, Ontario, Canada; § Institute for Radiation and Cell Research, University of Wuerzburg, D-97078 Wuerzburg,
Germany;
Samuel Lunenfeld Research Institute and Department of Medical Genetics, University of
Toronto, Mount Sinai Hospital, Toronto, Ontario, Canada; and ¶ Institute for Experimental
Immunology, University of Zürich, 8091, Zürich, Switzerland
/
recombination-activating gene (RAG)2
/
chimeric mice have a partial block in B cell maturation. However, peripheral B cells displayed
normal responses to IL-4, IgM, and CD40 cross-linking. SEK1
/
peripheral T cells showed
decreased proliferation and IL-2 production after CD28 costimulation and PMA/Ca2+ ionophore activation. Although CD28 expression was absolutely crucial to generate vesicular stomatitis virus (VSV)-specific germinal centers, SEK1
/
RAG2
/
chimeras mounted a protective
antiviral B cell response, exhibited normal IgG class switching, and made germinal centers in
response to VSV. Interestingly, PMA/Ca2+ ionophore stimulation, which mimics TCR-CD3
and CD28-mediated signal transduction, induced SAPK/JNK activation in peripheral T cells,
but not in thymocytes, from SEK1
/
mice. These results show that signaling pathways for
SAPK activation are developmentally regulated in T cells. Although SEK1
/
thymocytes
failed to induce SAPK/JNK in response to PMA/Ca2+ ionophore, SEK1
/
RAG2
/
thymocytes proliferated and made IL-2 after PMA/Ca2+ ionophore and CD3/CD28 stimulation,
albeit at significantly lower levels compared to SEK1+/+RAG2
/
thymocytes, implying that
CD28 costimulation and PMA/Ca2+ ionophore-triggered signaling pathways exist that can
mediate proliferation and IL-2 production independently of SAPK activation. Our data provide the first genetic evidence that SEK1 is an important effector molecule that relays CD28
signaling to IL-2 production and T cell proliferation.
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