Jak3, a member of the Janus kinase (JAK) family, is a non-receptor tyrosine kinase. It is mainly expressed in hematopoietic tissue cells and is uniquely associated with cytokines shared in the common gamma-chain receptor subunit. Jak3 plays a crucial role in immune regulation by activating the signal transducer and activator of transcription (STAT) proteins, thereby participating in multiple cytokine-mediated signaling pathways [4].

JAK3 inhibitors have shown potential in treating various inflammatory and autoimmune diseases. For example, PF-06651600, an irreversible JAK3 inhibitor, can also inhibit five TEC kinase family members due to a unique cysteine residue (Cys-909) in JAK3's catalytic domain. This dual inhibition may provide a beneficial inhibitory profile for diseases like rheumatoid arthritis, inflammatory bowel disease, alopecia areata, and vitiligo [1]. A JAK3 inhibitor can suppress multipotent ILC2s and attenuate steroid-resistant asthma, as the JAK3/STAT pathways contribute to the acquisition of steroid-resistant ILC2s [2]. Ritlecitinib, an oral JAK3/TEC family kinase inhibitor, downregulated immune biomarkers and upregulated melanocyte-related markers in vitiligo patients [3].

In conclusion, Jak3 is essential for immune-related functions through its role in cytokine-mediated signaling. The use of JAK3 inhibitors in pre-clinical and clinical studies has demonstrated their potential in treating inflammatory and autoimmune diseases such as rheumatoid arthritis, steroid-resistant asthma, and vitiligo, highlighting the importance of Jak3 as a therapeutic target in these disease areas.