Increased immunogenicity of low-antigenic rat tumors after superinfection with endogenous murine C-type virus in nude mice

Abstract

Four chemical carcinogen‐induced and two polyoma‐virus‐induced rat tumors were repeatedly passaged through nude mice. A methylcholanthrene‐induced tumor in BDIX rats (MBDB) and a polyoma‐virus‐induced tumor in Wistar/Fu rats (PW41) became infected with endogenous mouse virus (EMV), as judged by the expression of murine C‐type virus‐associated gp71, p30 and p12 antigens on their cell surface. Two ethylnitrosourea‐induced tumors in BDIX rats (290T and GE3A) were exposed in vitro to the supernatant of EMV‐infected PW41. Subsequently, 290T but not GE3A converted to murine gp71, p30 and p12 positivity. All these successfully infected rat tumors (EMV‐MBDB, EMV‐PW41 and EMV‐290T) became less transplantable to and more rejectable in otherwise susceptible syngeneic rats. To compare the immunogenicity of the virus‐infected and non‐infected tumors, syngeneic rats were immunized three times with irradiated cells, and challenged with the non‐infected tumor. Wistar/Fu rats immunized with irradiated EMV‐PW41 showed no improvement in PW41 rejection, compared to rate immunized with irradiated, non‐infected cells. On the other hand, BDIX rats immunized with EMV‐MBDB or EMV‐290T rejected MBDB or 290T, respectively, with no cross‐immunity, while the rats immunized with irradiated but non‐infected tumors showed no significant rejection. These results indicate that EMV infection augmented the immunogenicity of non‐immunogenic or only low‐immunogenic rat tumors.