Antigenic properties of California encephalitis viruses

The kinetics of inactivation by antiserum of several strains of California encephalitis virus were examined by the technique of plaque reduction. The antisera employed were obtained from rabbits following multiple injections of virus strains propagated in the brain tissues of suckling mice. The viruses used in the plaque reduction studies were propagated in cultures of baby hamster kidney cells. This was done to maximize the likelihood that the interaction between virus and antibody would involve virus-specific determinants and minimize the possibility of interference by antibodies directed against host cell components. An antiserum dilution was predetermined for each homologous virus-antiserum combination so that similar rates of inactivation resulted. That is, when the results of each of the homologous neutralization reactions of several viruses were compared graphically, the curves were found to coincide. When attempts were made to neutralize heterologous virus strains with each of these sera at their unique dilutions, three types of reaction were apparent. In some, the rates of inactivation of certain heterologous viruses were similar to that of the homologous virus, that is, inactivation was immediate and without a lag following admixture. In the second type of cross-reaction, a significant lag preceded neutralization, while in the third type of reaction no indication of cross-reaction could be detected. The demonstration of a kinetic lag preceding neutralization supported the concept of multihit inactivation kinetics rather than single hit kinetics. The differences between the two types of heterologous cross-reactivity suggested the significance of the configuration of the antigenic determinants in determining the ability of a cross-reacting antibody to recognize" a portion of the determinant. In the course of preliminary experiments it became apparent that phenotypic differences arose in genotypically identical viruses when different types of host cells were used to propagate the viruses. A significant persistent fraction of surviving virus was noted when neutralization experiments were conducted with viruses propagated in suckling mice. This fraction of non-neutralizable virus could not be reduced by any treatment of the virus suspension or the antiserum or by using different assay cells. Attempts to modify the surfaces of mouse brain-derived viruses by enzymatic digestion with trypsin, phospholipase C and neuraminidase did result in the detection of differences in the surfaces of viruses grown in these cells and in baby hamster kidney cells. However, treatment of mouse brain-propagated viruses with these enzymes did not enhance their neutralizability. As a part of this study, experiments were also conducted in which variables influencing plaque formation by California encephalitis viruses were investigated. The plaquability of the viruses was found to be extremely sensitive to the pH of the agarose overlay. In contrast to other viruses that exhibit pH sensitivities, California encephalitis viruses did not produce plaques when the initial pH of the overlay was greater than 7.5. Optimum conditions for plaque formation by most of the viruses were noted when the initial agarose overlay pH was between 7.0 and 7.2.