FcγRIIA signalling as a host-associated determinant of endosomal trafficking and antibody-dependent enhancement in flavivirus infection.

Background: Antibody-dependent enhancement (ADE) is a major determinant of disease severity in flavivirus infections, yet the host-associated factors that regulate the transition from antibody-mediated uptake to productive infection remain incompletely defined. Fc gamma receptor IIA (FcγRIIA/CD32A), an activating Fc receptor expressed on myeloid cells, represents a key host determinant linking pre-existing immunity to viral entry pathways. Here, we investigated how FcγRIIA signalling influences intracellular trafficking and ADE efficiency across dengue virus (DENV), Zika virus (ZIKV), and Japanese encephalitis virus (JEV). Method: Using BHK-21 cells expressing either wild-type FcγRIIA (WT) or a cytoplasmic-truncated variant lacking the immunoreceptor tyrosine-based activation motif (ITAM), we show that immune-complex internalization occurs independently of the cytoplasmic domain, but that signalling competence significantly modulates downstream infection efficiency. WT cells generally showed higher ADE efficiency, whereas cytoplasmic-truncated receptors preserved ADE activity but showed virus-dependent reductions in enhancement magnitude. Result: FcγRIIA signalling altered the intracellular routing of virus-antibody complexes. WT receptors promoted rapid trafficking from EEA1-positive early endosomes to Rab7-positive late endosomes, whereas signalling-deficient receptors displayed delayed progression and increased association with IRAP- and Rab14-positive compartments. These trafficking differences correlated with virus-specific patterns of enhancement, with DENV showing the strongest dependence on late endosomal maturation, while ZIKV and JEV utilized more flexible or alternative pathways. Transcriptomic analysis revealed minimal differences between WT and truncated receptors at later time points, indicating that FcγRIIA primarily modulates early post-entry events rather than sustained transcriptional responses. Conclusion: Collectively, these findings identify FcγRIIA signalling as a host-associated determinant that regulates the intracellular fate of virus-antibody complexes and influences ADE efficiency through modulation of endosomal trafficking dynamics. The work provides a mechanistic framework linking host receptor signalling to variability in infection outcomes under conditions of pre-existing immunity, with implications for understanding disease heterogeneity and optimizing vaccine and antibody-based interventions.