Crustacean Mab21 proteins drive tissue-specific antiviral immunity by activating IKKε outside the canonical nucleic-acid sensing paradigm.

The Mab21/cGAS protein family has diversified across metazoans to regulate development and innate immunity. In vertebrates, cGAS detects cytosolic DNA and synthesizes 2'3'-cGAMP to activate STING-TBK1-IRF signaling, while invertebrate cGAS-like receptors (cGLRs) recognize RNA or DNA and generate non-canonical cyclic dinucleotides. However, whether shrimp Mab21 proteins function as canonical nucleic acid sensors remains unresolved. Here, we identified three Mab21 proteins from Litopenaeus vannamei-LvMab21-1, LvMab21-2, and LvMab21-3. Although they are phylogenetically related to cGAS-like proteins, none bound dsDNA or dsRNA or synthesized cGAMP in response to ISD or poly(I:C). Instead, all three interacted directly with the TBK1 homolog LvIKKε, promoted its phosphorylation at serine 175, and thereby activated the downstream IRF-Vago4 signaling axis. This mechanism defines a non-canonical nucleic acid sensing paradigm, whereby Mab21 proteins act as protein-based enhancers of kinase activation rather than as nucleic acid-dependent CDN synthases. We further show that these proteins display tissue-specific antiviral functions: all three act in hemocytes, LvMab21-1 predominates in hepatopancreas, LvMab21-2 and LvMab21-3 are most critical in gills, and LvMab21-1 and LvMab21-3 cooperate in intestine. Silencing any Mab21 paralog reduced survival and increased white spot syndrome virus (WSSV) burden, underscoring their physiological relevance. Together, our findings expand the known repertoire of innate immune strategies within the Mab21 family, highlight a previously unrecognized non-canonical mechanism of interferon-like activation, and reveal tissue-specific specialization that tailors antiviral responses across shrimp organs. These insights provide both evolutionary context and candidate targets for breeding disease-resistant shrimp.