Essential role of an ERV-derived Env38 protein in adaptive humoral immunity against an exogenous SVCV infection in a zebrafish model

Author summaryEndogenous retroviruses (ERVs) belong to an important subclass of transposon family that extensively exists in vertebrate and plant genomes. Understanding of the interplay between ERVs and host biological functions has long been a challenging frontier in life sciences. Fish is an indispensable integral part in this endeavor because it represents the most primitive host for retroviruses during vertebrate evolution. Here, by using a zebrafish model, we successfully identified the important role of an ERV-derived envelope protein (Env38) in adaptive humoral immunity against SVCV infection. We found that Env38 stimulates the initial activation of SVCV-induced CD4(+) T cells by cross-linking MHC-II and CD4 molecules during cell-cell interaction between MHC-II+ APCs and CD4(+) T cells, thereby uncovering a novel stimulatory molecule in the central supramolecular activating complex (c-SMAC) of an immunological synapse structure. This finding indicated that Env38 has been co-opted for beneficial adaptive antiviral response by zebrafish immune cells. This study will greatly enrich current knowledge on the molecular mechanism underlying the activation of adaptive immunity and deepen understanding on the correlation of ERVs with exogenous invading viruses and host immune reactions. Endogenous retroviruses (ERVs) are the relics of ancient retroviruses occupying a substantial fraction of vertebrate genomes. However, knowledge about the functional association of ERVs with cellular activities remains limited. Recently, we have identified approximately 3,315 ERVs from zebrafish at genome-wide level, among which 421 ERVs were actively expressed in response to the infection of Spring viraemia of carp virus (SVCV). These findings demonstrated the previously unrecognized activity of ERVs in zebrafish immunity, thereby making zebrafish an attractive model organism for deciphering the interplay among ERVs, exogenous invading viruses, and host immunity. In the present study, we investigated the functional role of an envelope protein (Env38) derived from an ERV-E5.1.38-DanRer element in zebrafish adaptive immunity against SVCV in view of its strong responsiveness to SVCV infection. This Env38 is a glycosylated membrane protein mainly distributed on MHC-II+ antigen-presenting cells (APCs). By performing blockade and knockdown/knockout assays, we found that the deficiency of Env38 markedly impaired the activation of SVCV-induced CD4(+) T cells and thereby led to the inhibition of IgM(+)/IgZ(+) B cell proliferation, IgM/IgZ Ab production, and zebrafish defense against SVCV challenge. Mechanistically, Env38 activates CD4(+) T cells by promoting the formation of pMHC-TCR-CD4 complex via cross-linking MHC-II and CD4 molecules between APCs and CD4(+) T cells, wherein the surface subunit (SU) of Env38 associates with the second immunoglobin domain of CD4 (CD4-D2) and the first alpha 1 domain of MHC-II alpha (MHC-II alpha 1). Notably, the expression and functionality of Env38 was strongly induced by zebrafish IFN phi 1, indicating that env38 acts as an IFN-stimulating gene (ISG) regulated by IFN signaling. To the best of our knowledge, this study is the first to identify the involvement of an Env protein in host immune defense against an exogenous invading virus by promoting the initial activation of adaptive humoral immunity. It improved the current understanding of the cooperation between ERVs and host adaptive immunity.