Concomitant infection with exogenous mouse mammary tumor virus encoding I- E-dependent superantigen in I-E-negative mouse strain

We found that milk from II TES mice contained two species of exogenous mouse mammary tumor viruses (MMTV). Sequence analysis of the open reading frame (ORF) in the MMTV 3' long terminal repeat indicated that the two MMTV, MMTV (II TES2) and MMTV (II TES14), encode superantigens specific for Vβ2⁺ T cells and Vβ14⁺ T cells, respectively. In an experiment of subcutaneous injection of II TES milk, both T cells bearing TCR Vβ2 and Vβ14 proliferated vigorously in the draining lymph node from BALB/c mice (H-2(d) I-E⁺), whereas only Vβ14⁺ T cells showed significant proliferation in C57BL/6 mouse (B6 H-2(b) I-E^-) lymph nodes. These findings indicated that the superantigen encoded by MMTV (II TES2) required MHC class II I-E molecules exclusively for Ag presentation, but MMTV (II TES14) stimulated Vβ14⁺ T cells even in the absence of I-E molecules. Semiquantitative analysis of MMTV proviruses using PCR revealed that B6 mice were not infected with MMTV (II TES2) by injection of this MMTV alone. However, injection of II TES milk containing both MMTV (II TES14) and MMTV (II TES2) induced infection of B6 mice with MMTV (II TES2) besides MMTV (II TES14), in spite of no expansion of Vβ2⁺ T cells in this mouse strain. These results suggested that I-E-negative mice were concomitantly infected with MMTV (II TES2) with the help of I-E independent T cell activation mediated by MMTV (IITES14).