Defective C-Type Retrovirus Particles Secreted by L1210 Leukemia Cells¹

Cultures of 1210 leukemia cells (L1210) have been found to release C-type retrovirus. Since L1210 is used as a common model system in cancer research, the potential influence of these virus particles in the course of tumor cell growth was studied. The virus particle was found to be nononcogenic and noninfectious in standard assays for xenotropic and ecotropic murine retroviruses. The virus, designated as L1210 virus, had a normal complement of viral proteins, except that it has no detectable gp70 (a glycoprotein with a molecular weight of 70,000). Quantitative radioimmunoassay for the major structural protein, p30 (a protein with a molecular weight of 30,000) indicated that the virus is at least 1000-fold more defective than are ecotropic viruses such as AKR and Moloney leukemia virus.

L1210 cells grown in vitro released about 20-fold more particle-associated p30 than did in vivo-cultured cells, but cell-associated p30 and gp70 were found at similar levels. Both in vitro- and in vivo-cultured cells displayed gp70 on their surfaces that appeared to be of larger molecular weight than gp70's from most ecotropic viruses. Cells grown in vitro released a high level of non-virion-associated gp70 into the growth medium, but the high concentration of gp70 in the serum of normal DBA/2 mice precluded such measurements on in vivo-cultured cells. These results indicate that L1210 virus does not act as a classical infectious leukemia virus during growth of L1210 cells, but the presence of relatively large amounts of viral antigens in these cells may affect the immune response to them.