Cross-Species Reactivity and Differential Anti-PRRSV Activities of CD163 SRCR4/SRCR5 Monoclonal Antibodies

CD163 not only acts as an essential receptor for porcine reproductive and respiratory syndrome virus (PRRSV) infection but also serves as a barrier to cross-species cellular infection by arteriviruses. Among its domains, the scavenger receptor cysteine-rich 5 (SRCR5) domain of CD163 is functionally indispensable. In this study, two monoclonal antibodies (mAbs), designated 5A and 11D, were developed against the SRCR4-6 region of porcine CD163. The linear epitopes recognized by mAbs 5A and 11D were identified as ⁵⁴⁶CEGHESHLSLCPVAP⁵⁶⁰ in SRCR5 and ⁴⁴⁹WDCKNW⁴⁵⁴ in SRCR4, respectively. Crystal structural analysis and sequence alignment showed that the ⁵⁴⁶CEGHESHLSLCPVAP⁵⁶⁰ epitope resides in the long loop 5-6 of SRCR5, which is a key structural region responsible for ligand binding. Meanwhile, the ⁴⁴⁹WDCKNW⁴⁵⁴ epitope is located within a highly conserved region of SRCR4, which confers broad cross-species reactivity. Notably, mAb 5A reduced PRRSV infection, while mAb 11D exhibited no anti-PRRSV activity in target cells. Collectively, this study provides essential molecular tools for exploring CD163-domain-dependent cross-species reactivity and developing anti-PRRSV strategies.