ENHANCED INFECTIVITY OF MODIFIED BLUETONGUE VIRUS-PARTICLES FOR 2 INSECT-CELL LINES AND FOR 2 CULICOIDES VECTOR SPECIES

Previous studies (Mertens et al., Virology 157, 375-386, 1987) have sh own that removal of the outer capsid layer from bluetongue virus (BTV) significantly reduces (approximately X 10(-4)) the infectivity of the resultant core particle for mammalian cells (BHK 21 cells). In contra st, the studies reported here, using a cell line (KC cells) derived fr om a species of Culicoides that can act as a vector for BTV (Culicoide s variipennis), demonstrated a much higher infectivity of core particl es than that in mammalian cells (approximately X 10(3)). This increase resulted in a specific infectivity for cores that was only 20-fold le ss than that of purified disaggregated virus particles (stored in the presence of 0.1% sodium-N-lauroylsarcosine (NLS)). Removal of this det ergent caused intact virus particle aggregation and (as previously rep orted) resulted in an approximately 1 log(10) drop in the specific inf ectivity of those virus particles which remained in suspension. In con sequence the specific infectivity of core particles for the KC cells w as directly comparable to that of the intact but aggregated virus. The se data are compared with the results from oral infectivity studies us ing two vector species (C. variipennis and Culicoides nubeculosus), wh ich showed similar infection rates at comparable concentrations of pur ified cores, or of the intact but aggregated virus particles (NLS was toxic to adult flies). The role of the outer core proteins (VP7) in ce ll attachment and penetration, as an alternative route of initiation o f infection, is discussed. Previous studies (Mertens er at, Virology 1 57, 375-386, 1987) also showed that the outer capsid layer of BTV can be modified by proteases (including trypsin or chymotrypsin), thereby generating infectious subviral particles (ISVP). The specific infectiv ity of ISVP for mammalian cells (BHK21 cells) was shown to be similar to that of disaggregated virus particles. In contrast, we report a sig nificantly higher specific infectivity of ISVP but not of the intact v irus (approximately x 100) for two insect cell lines (KC cells and C6/ 36 mosquito cells (derived from Aedes albopictus)). In oral infection studies with adults of the two vector species, ISVP produced the same infection rate at approximately 100-fold lower concentrations than eit her core particles or the intact but aggregated virus particles. The i mportance of mammalian host serum proteases, or insect gut proteases, in modification of the intact virus particle to form ISVP and their ro le in initiation of infection and the vector status of the insect is d iscussed. (C) 1996 academic Press, Inc.