Fine-tuning of MEK signaling is pivotal for limiting B and T cell activation

MEK1 and MEK2, the only known activators of ERK, are attractive therapeutic candidates for both cancer and autoimmune diseases. However, how MEK signaling finely regulates immune cell activation is only partially understood. To address this question, we specifically delete Mek1 in hematopoietic cells in the Mek2 null background. Characterization of an allelic series of Mek mutants reveals the presence of distinct degrees of spontaneous B cell activation, which are inversely proportional to the levels of MEK proteins and ERK activation. While Mek1 and Mek2 null mutants have a normal lifespan, 1Mek1 and 1Mek2 mutants retaining only one functional Mek1 or Mek2 allele in hematopoietic cell lineages die from glomerulonephritis and lymphoproliferative disorders, respectively. This establishes that the fine-tuning of the ERK/MAPK pathway is critical to regulate B and T cell activation and function and that each MEK isoform plays distinct roles during lymphocyte activation and disease development. [Display omitted] •ERK activation in B and T cells is proportional to MEK protein levels•Mek1 and Mek2 play non-redundant roles during lymphocyte activation•1Mek1 mutant mice develop a lupus-like disease•1Mek2 mutant mice die from lymphoproliferative disorders Houde et al. report that the kinases MEK1 and MEK2 play distinct roles in lymphocyte activation. Reduced Mek1/Mek2 expression led to reduced levels of ERK activation and to spontaneous B and T cell activation. Moreover, 1Mek1 mutant mice develop lupus-like disease, establishing Mek expression as a key gatekeeper of B cell tolerance.