Given the importance of Jak2 in cell signaling, a critical role for Jak2 in immune cells especially dendritic cells (DCs) has long been proposed. The exact function for Jak2 in DCs, however, remained poorly understood as Jak2 deficiency leads to embryonic lethality. Here we established Jak2 deficiency in adult Cre(+/+)Jak2(fl/fl) mice by tamoxifen induction. Loss of Jak2 significantly impaired DC development as manifested by reduced BMDC yield, smaller spleen size and reduced percentage of DCs in total splenocytes. Jak2 was also crucial for the capacity of DCs to mediate innate immune response. Jak2(-/-) DCs were less potent in response to inflammatory stimuli and showed reduced capacity to secrete proinflammatory cytokines such as TNFalpha and IL-12. As a result, Jak2(-/-) mice were defective for the early clearance of Listeria after infection. However, their potency to mediate adaptive immune response was not affected. Unlike DCs, Jak2(-/-) macrophages showed similar capacity secretion of proinflammatory cytokines, suggesting that Jak2 selectively modulates innate immune response in a DC-dependent manner. Consistent with these results, Jak2(-/-) mice were remarkably resistant to lethal dose of LPS-induced septic shock, a deadly sepsis characterized by the excessive innate immune response, and adoptive transfer of normal DCs restored their susceptibility to LPS-induced septic shock. Mechanistic studies revealed that Jak2/SATA5 signaling is pivotal for DC development and maturation, while the capacity for DCs secretion of proinflammatory cytokines is regulated by both Jak2/STAT5 and Jak2/STAT6 signaling.