We have previously described two nucleolar proteins, named p14 and p21, in MMTV-induced T cell lymphomas. These proteins were identified by a monoclonal antibody (M-66) generated from a nontumorigenic, immunogenic variant of S49 T cell lymphoma. While p14 was common to several MMTV-derived T cell lymphomas, p21 was found only in highly tumorigenic variants of S49 cells. Here we report that p14 is the leader peptide of the MMTV env precursor. The epitope recognized by M-66 contains a putative nuclear localization signal. Actinomycin D was found to induce redistribution of p14/p21 from the nucleolus to the nucleoplasm. p14 coimmunoprecipitated and colocalized with the cellular protein, B23. Association with B23 has been previously reported for other auxiliary nucleolar retroviral proteins, such as Rev (HIV) and Rex (HTLV).

Currently there is no adequate experimental model available whereby the lethal infiltration of malignant lymphoma to the eye and CNS can be studied. Variant S49 mouse lymphoma cells that exhibit cell-cell adhesion properties (named Rev-2-T-6) were inoculated intraperitoneally into Balb/C mice at the ages of 6-60 days postnatal. Mice inoculated between days 6-11 postnatal developed signs of eye and CNS involvement with an apparent peak (58% of mice) at day 7. None of the mice inoculated beyond day 11 exhibited such signs. Histological analysis of these sites revealed tumorous infiltrates into a variety of structures in the orbit, intraocular tissues, along the optic nerve and in the brain. Additional analysis of the histopathological data, based on the structures demonstrating the highest frequency of lymphoma infiltration, suggests preferred routes of lymphoma entry to the brain and eye. Thus, entry to the brain can occur mainly through the choroid plexus and cranial nerves or cranial nerve ganglia. Entry to the eye may occur from the brain (along the optic nerve), and through hematogenous infiltration of orbital structures. No data were found that would support retrograde infiltration of the lymphoma from the eye to the brain. These findings present an experimental model for addressing the molecular mechanisms that govern homing of malignant lymphoma to the eye and brain, as well as the development of experimental therapeutic modalities for malignant lymphoma in these organs.

To characterize novel proteins expressed in lymphoma cells, monoclonal antibodies (MAbs) were raised against variant S49 mouse lymphoma cells. Immunoperoxidase analysis with a specific MAb, named M-66, revealed nuclear localization with prominent staining in the nucleoli of both tumorigenic (T-63) cells and nontumorigenic, immunogenic (T-25-Adh) cells. Weak signals were also observed in the cytoplasm and plasma membrane of both cells. Western blot analysis with M-66 antibody revealed a 14-kDa protein in nuclear extracts of both T-25-Adh and T-63 cells. An additional nuclear 21-kDa protein was evident only in T-63 cells. M-66 identified several clones from a T-25-Adh cDNA expression library. These clones demonstrated extensive homology (approximately 95% identity throughout their length) to the mouse mammary tumor virus (MMTV) env and LTR regions. Extensive amino acid sequence homology (approximately 90% identity) between the clones and the env protein was observed. M-66 identified the 14-kDa protein in another MMTV bearing T-cell lymphoma, EL-4. Immunoperoxidase analysis of EL-4 cells with M-66 also revealed prominent nucleolar staining. MMTV-negative cells and MMTV-positive cells of nonlymphocytic origin were devoid of both 14- and 21-kDa proteins. Moreover, an anti-MMTV gp52 (env) antibody precipitated the 21-kDa protein in T-63 cells. We thus suggest that MMTV bearing T-cell lymphomas express nucleolar proteins translated from the env region of MMTV.