A virus originally isolated from cell cultures obtained by lysolecithin-induced fusion of human multiple sclerosis brain cells with CV-1 cells has been analyzed for its antigenic, RNA, and polypeptide compositions, and for selective biological properties. Our findings establish that this isolate, designated 6/94 virus, contains a 50 S RNA genome and is, as yet, indistinguishable from Sendai virus in its antigenic and total polypeptide compositions. Despite these similarities, the 6/94 and Sendai viruses differ in certain phenotypic properties. 6/94 virus is markedly less cytocidal for chick fibroblasts, especially at 37 C and, after β-propiolactone inactivation, it possesses a greater capacity for cell fusion and a lower toxicity than does comparably treated Sendai virus. In addition, 6/94 virus shows greater hemolytic activity.

The 6/94 virus, a type 1 parainfluenza virus recovered from multiple sclerosis brain cells after lysolecithin-induced fusion of these cells with African green monkey kidney cells (CV-1), has been found to grow in splenic and peritoneal macrophages obtained from outbred and different strains of inbred mice. Macrophages from C57BL animals were least susceptible to infection, a resistance apparently partially age related. The virus also has been found to replicate in IC21 cells, a line of simian virus 40 virus-transformed mouse macrophages. Viral growth was detected by development of hemadsorption in infected cells, followed by the appearance of infectious virus. The growth of 6/94 virus had different kinetics in mouse macrophages, in the standard continuous cell lines L, 3T3, and CV-1, and in primary mouse kidney and mouse embryo cells. The virus produced in macrophages could be passed in series to other macrophage cultures. In addition, once infected, the cultures continued to produce virus, and permanently infected cell lines were thus obtained. Macrophages from immunized mice with high titers of humoral neutralizing antibodies were found variably able to support virus growth.

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