Current understanding of the role of tyrosine kinase 2 signaling in immune responses.

Immune system is a complex network that clears pathogens, toxic substrates, and cancer cells. Distinguishing self-antigens from non-self-antigens is critical for the immune cell-mediated response against foreign antigens. The innate immune system elicits an early-phase response to various stimuli, whereas the adaptive immune response is tailored to previously encountered antigens. During immune responses, B cells differentiate into antibody-secreting cells, while naïve T cells differentiate into functionally specific effector cells [T helper 1 (Th1), Th2, Th17, and regulatory T cells]. However, enhanced or prolonged immune responses can result in autoimmune disorders, which are characterized by lymphocyte-mediated immune responses against self-antigens. Signal transduction of cytokines, which regulate the inflammatory cascades, is dependent on the members of the Janus family of protein kinases. Tyrosine kinase 2 (Tyk2) is associated with receptor subunits of immune-related cytokines, such as type I interferon, interleukin (IL)-6, IL-10, IL-12, and IL-23. Clinical studies on the therapeutic effects and the underlying mechanisms of Tyk2 inhibitors in autoimmune or chronic inflammatory diseases are currently ongoing. This review summarizes the findings of studies examining the role of Tyk2 in immune and/or inflammatory responses using Tyk2-deficient cells and mice.