Structural and Functional Analysis of Porcine CR1-like Proteins in C4b-Mediated Immune Responses

The complement system is crucial for immune defense, linking innate and adaptive immunity. In the classical and lectin pathways, C4 is split into C4b, triggering opsonization, lysis, and the removal of pathogens and damaged cells. Dysregulated activation of C4 and other components of the classical pathway can lead to tissue damage and heightened inflammation, whereas appropriate regulation of C4b activity serves to mitigate excessive inflammation and prevent injury. ELISA analysis demonstrated C4 activation and cleavage during the co-incubation of PRRSV with fresh porcine serum. Immunoelectron microscopy revealed that porcine red blood cells could immunologically adhere to PRRSV, and C4b was involved in this adhesion process. BLAST (NCBI BLAST+ 2.14.1) analysis revealed that porcine CR1-like CCPs 1-3, CR1-like CCPs 12-14, and CR1-like CCPs 19-21 share high similarity with the CCP 1-3 region of human CR1, which mediates C4b binding. Yeast two-hybrid assays confirmed that all three CR1-like fragments bind C4b. To elucidate the interaction mechanism, homology models of C4b and CR1-like fragments were constructed, followed by molecular docking and dynamics simulations, identifying 18 key amino acids in porcine CR1-like involved in C4b binding. Surface plasmon resonance further validated the binding affinity of CR1-like CCPs 1-3, its mutant 118I, and C4b. These results enhance our understanding of complement regulation and provide a foundation for developing therapeutic strategies targeting complement-related diseases.