CLL, together with C1qR, suppresses WSSV infection by regulating the activation of Dorsal.

There is no evidence that arthropods possess a complement system. In arthropods, only gC1qR has been reported. The functional convergence of gC1qR in arthropods and vertebrates during evolution remains to be further elucidated. In the present study, a C-type lectin named PcCLL, which contains a C1q-like sequence (a segment of the C1q globular head) and might be involved in antiviral immunity, was identified in Procambarus clarkii. When crayfish are infected with white spot syndrome virus (WSSV), some C1qRs (gC1qR homologs), which are located at the cell membrane, recognize WSSV by interacting with the envelope protein VP28. Subsequently, C1qR recruits PcCLL in the cell, and the C1qR-PcCLL complexes synergistically promote the nuclear translocation of Dorsal. Serine-arginine protein kinase (SRPK) is an essential molecule in this process. The C1qR-PcCLL-SRPK axis regulates the activation and translocation of Dorsal. Finally, the C1qR-PcCLL-Dorsal axis can promote resistance to WSSV infection by regulating the expression of the lectin-like protein (PcLT). In both vertebrates and arthropods, C1qR appears to participate in antiviral immunity, but the mechanisms were different. This may highlight a fascinating evolutionary trajectory across animals.

Importance: The body's recognition of pathogenic microorganisms plays an important role in host resistance to pathogen infection. Here, we identified a lectin, PcCLL, in the red swamp crayfish Procambarus clarkii, which contains a C1q-like sequence and might be involved in antiviral immunity. C1qR located on the cell membrane can recognize white spot syndrome virus by interacting with VP28. Then, it recruits PcCLL and promotes the nuclear translocation of Dorsal. This process cannot be achieved without SRPK, which is a splicing regulator that can phosphorylate proteins. CLL (C1q-like motif that is part of the globular head of C1q) and C1qR (gC1qR homolog) interact with each other, and they are involved in the antiviral immune response by activating Dorsal. This may be a new mechanism for the host to recognize pathogens.