Spodoptera frugiperda (Sf) cell lines are widely used for baculovirus-mediated production of recombinant proteins, including human biologics. Thus, it was disconcerting when FDA investigators detected a rhabdovirus in some of these cell lines. This contaminant, now known as Sf-rhabdovirus, was initially detected by electron microscopy of Sf9 cell-conditioned media (3). In addition, massively parallel sequencing revealed rhabdovirus-related sequences in Sf9, Sf21, and in conditioned media from these cells. These results have been confirmed and extended (2, 4) and most investigators now recognize many, if not all Sf lines are contaminated with Sf-rhabdovirus.
Different Sf lines can harbor distinct Sf-rhabdoviral variants, including one with a 320 bp deletion in the X/L region (1-3). The X/L intergenic region includes a transcriptional control element for L gene expression. The position and, therefore, presence or absence of this element in the 320 bp deletion mutant has not been determined. Nevertheless, this has raised questions about the ability of this variant to replicate (1).
Our basic motivation for this study was to clarify the Sf-rhabdovirus contamination issue by directly examining Sf-rhabdoviral infectivity. We were particularly interested in comparing the infectivities of Sf-rhabdovirus variants with or without the 320 bp X/L deletion in Sf cells. These efforts were enabled by the isolation of an Sf-rhabdovirus-negative (Sf-RVN) Sf cell line, which has no detectable Sf-rhabdovirus sequences (4).
The “aha” moment came when we found Sf-rhabdoviruses with or without the 320 bp X/L deletion are both infectious for Sf-RVN cells. These results were the first to directly demonstrate Sf-rhabdovirus can establish persistent, productive infections in Sf cells. Importantly, they also showed Sf-rhabdovirus variants with the 320 bp X/L deletion are replicative. Thus, the bad news is Sf cell lines harboring this variant are contaminated with an infectious viral agent, whether we like it or not. The good news was we found no evidence of integrated Sf-rhabdovirus-specific sequences in Sf cell genomes, as has been reported in a study of one Sf cell line (1). We also found Sf-rhabdovirus was unable to establish persistent infections in heterologous insect or mammalian cell lines, supporting previous data indicating this virus has a limited host range.
Introducing the authors
Ajay Maghodia (left) is a Research Scientist with GlycoBac, LLC, which is a startup company housed at the University of Wyoming in Laramie, Wyoming. Don Jarvis (right) is a Professor in the Department of Molecular Biology at the Universtiy of Wyoming and the Founder of GlycoBac, LLC
About the research
Infectivity of Sf-rhabdovirus variants in insect and mammalian cell lines
Ajay B.Maghodia, Donald L.Jarvis
Virology, Volume 512, December 2017, Pages 234-245