Chromatographic separations of alphavirus strains by hydroxylapatite.

Hydroxylapatite column chromatography methods were developed to characterize selected alphavirus populations. Different conditions of pH and phosphate molarity were required to obtain satisfactory elution profiles and separations for Western equine encephalomyelitis virus strains, compared with Eastern equine encephalomyelitis virus and Semliki Forest virus strains. Raising the pH of the buffers effected earlier elutions of all viruses. Selection of phosphate gradients with more gentle slopes and adjustment to the proper pH effected better separations of virus subpopulations. Elution profiles were not affected by 0.85% NaCl, 10% fetal calf serum, or 1% bovine serum albumin, which are common constituents of virus-stabilizing diluents. Passage of Western equine encephalomyelitis and Semliki Forest viruses in BHK-21, Vero, or duck embryo cell cultures or in suckling mouse brains did not usually affect elution profiles, unless passage also resulted in a shift in the plaque size marker. Essentially all infectious virus applied to the column was recoverable in appropriate fractions. This permitted accurate determinations of heterogeneity within alphavirus populations. For Western equine encephalomyelitis large-plaque (LP) and small-plaque (SP) virus populations, it was possible to detect ratios of 1 LP in a population of 10(6) SP, and 1 SP in 10(3) LP by using linear phosphate gradients. When stepwise elution procedures were used, it was possible to detect ratios of 1 SP in a population of 10(5) LP. Hydroxylapatite column chromatography therefore appears to be a useful tool for characterizing alphaviruses and for isolating minority subpopulations of viruses of biological or epidemiological importance from apparently homogeneous virus stocks.