Epstein–Barr Virus BALF0/1 Subverts the Caveolin and ERAD Pathways to Target B-cell Receptor Complexes for Degradation

Abstract

ABSTRACTEpstein–Barr virus (EBV) establishes persistent infection, causes infectious mononucleosis, is a major trigger for multiple sclerosis and contributes to multiple cancers. Yet, knowledge remains incomplete about how the virus remodels host B cells to support lytic replication. We previously identified that EBV lytic replication results in selective depletion of plasma membrane B-cell receptor (BCR) complexes, comprised of immunoglobulin and the CD79A and CD79B signaling chains. Here, we used proteomic and biochemical approaches to identify that the EBV early lytic protein BALF0/1 is responsible for EBV lytic cycle BCR degradation. Mechanistically, an immunoglobulin heavy chain cytoplasmic tail KVK motif was required for ubiquitin-mediated BCR degradation, while CD79A and CD79B were dispensable. BALF0/1 subverted caveolin-mediated endocytosis to internalize plasma membrane BCR complexes and to deliver them to the endoplasmic reticulum. BALF0/1 stimulated immunoglobulin heavy chain cytoplasmic tail ubiquitination, which together with the ATPase valosin-containing protein/p97 drove ER-associated degradation of BCR complexes by cytoplasmic proteasomes. BALF0/1 knockout reduced the viral load of secreted EBV particles from B-cells that expressed a monoclonal antibody against EBV glycoprotein 350 and increased viral particle immunoglobulin incorporation. Consistent with downmodulation of plasma membrane BCR, BALF0/1 overexpression reduced viability of a diffuse large B-cell lymphoma cell line dependent upon BCR signaling. Collectively, our results suggest that EBV BALF0/1 downmodulates immunoglobulin upon lytic reactivation to block BCR signaling and support virion release.SIGNIFICANCEEBV uses a biphasic lifecycle, in which it switches between a latent state that facilitates immune evasion and a lytic state, where virion are secreted. However, when EBV infects a B-cell that makes antibody against a virion protein, EBV must have a strategy to escape becoming trapped, since maturing virion and antibody each traffic through the secretory pathway. We identified that an EBV-encoded protein expressed, BALF0/1, associates with and targets immunoglobulin complexes for degradation. Intriguingly, BALF0/1 subverts the caveolin-1 and ERAD pathways to route antibody from the plasma membrane to cytoplasmic proteasomes for degradation. We present evidence that this enhances EBV secretion from cells that produce antibody against a viral glycoprotein, which could otherwise trap virus.