Studies on the Biological and Antigenic Properties of ESP-1 Type C Virus Particles1

ESP-1 virus provides a helper function to defective Friend spleen focus-forming virus (SFFV). This helper activity is demonstrable only in B-type mice (Fv-lbb) under conditions in which the N-tropic lymphatic leukemia-inducing virus contained in the Friend virus complex is inhibited. ESP-1 virus therefore expresses a B-tropic host range in vivo with respect to its ability to provide a helper function to defective SFFV. Further, this helper activity is directly associated with the presence of the ESP-1 virus since activity was recovered only from gradient samples ranging from 1.15 to 1.17 g/ml. All samples contained ESP-1 virus and Mycoplasma. However, Mycoplasma derived from the ESP-1 cell line and maintained in broth culture only enhanced SFFV titer estimates and failed to provide a helper function to SFFV. ESP-1 virus helper activity is dose dependent and heat labile and is neutralized by rat antiserum directed against both whole and Tween:etherdisrupted ESP-1 virus. Further, ESP-1 virus is not neutralized by type-specific murine antiserum directed against both the Rauscher virus complex and the lymphatic leukemia-inducing helper virus component of the Rauscher virus complex and Moloney leukemia virus. These findings, together with the observations that these same murine antisera neutralized human-embryo-kidney-cell-grown Rauscher leukemia virus, suggest that the type C virus designated ESP-1 has some of the biological properties of the murine leukemia viruses (i.e., helper activity for the induction of leukemia in mice) but does not contain type-specific envelope antigen(s) characteristic of Moloney leukemia virus, N-tropic Friend lymphatic leukemiainducing virus, or human-embryo-kidney-cell-grown Rauscher leukemia virus.