Mouse Mammary Tumor Viruses with Functional Superantigen Genes Are Selected During in Vivo Infection (Viral Recombination) T
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Proc. Natl. Acad. Sci. USA Vol. 92, pp. 4828-4832, May 1995 Immunology Mouse mammary tumor viruses with functional superantigen genes are selected during in vivo infection (viral recombination) T. V. GOLOVKINA*, J. P. DUDLEYt, A. B. JAFFE*, AND S. R. Ross*t *Department of Microbiology/Cancer Center, University of Pennsylvania, Philadelphia, PA 19104-6142; and tDepartment of Microbiology, University of Texas, Austin, TX 78712 Communicated by Philippa Marrack; Howard Hughes Medical Institute, Denver, CO, February 7, 1995 ABSTRACT Mouse mammary tumor virus (MMTV) en- viral genomic RNA. Surprisingly, offspring that nursed on codes a superantigen that is important for viral infectivity in these transgenic animals showed deletion of their cognate vivo. To determine whether superantigen function was re- Vp14+ T cells, with similar kinetics to that caused by wild-type quired for infection by milk-borne MMTV, we created HYB exogenous MMTV(C3H). Sequence analysis of the LTRs of PRO/Cla transgenic mice. These mice produced a full-length, the newly acquired viruses showed that recombination be- packaged viral RNA with a frameshift mutation that caused tween the transgene and endogenous Mtv-1 proviral RNA premature termination of the superantigen protein. Young caused reconstitution of the sag open reading frame. These HYB PRO/Cla mice showed no deletion of their cognate results indicate that there is selective pressure for viruses that V,14+ T cells, although they shed virus in their milLk The encode functional Sag proteins and suggest that Sag activity is nontransgenic offspring of the HYB PRO/Cla mice were required for virus transmission. infected with this virus, since transgene-specific viral tran- scripts were detected in their mammary glands. Surprisingly, these offspring demonstrated the progressive deletion of MATERIALS AND METHODS V114+ T cells characteristic of exogenous MMTV(C3H) in- Plasmids. The plasmid hybrid MMTV (HYB PRO in this fection. Sequence analysis demonstrated that these newly paper) was a gift from G. M. Shackleford, University of acquired viruses had reconstituted superantigen open reading Southern California (7) (Fig. 1). The HYB PRO/Cla trans- frames resulting from recombination between the HYB gene was constructed by partial Cla I digestion of HYB PRO, PRO/Cla and endogenous Mtv-l proviral RNAs. Thus, there filling in the 5' overhang with Klenow DNA polymerase, and is selection during the infection process for MMTVs with religation. The sequence of the mutation created in the 3' LTR functional superantigen genes. is shown in Fig. 1. Although the Cla I digest and fill-in should have generated a 2-bp insertion, only a 1-bp insertion was Mouse mammary tumor virus (MMTV) is a retrovirus that is created. acquired either through the germline as an endogenous virus Generation of Transgenic Mice. Female and male C3H/ or by milk-borne infection (1). Although the ultimate target for HeN MTV- and C3H/HeN MTV' inbred mice and SW MMTV is the mammary gland, cells of the immune system play outbred mice were purchased from the Frederick Cancer a role in milk-borne virus infection. MMTV encodes a viral Research Facility, Frederick, MD. Transgenic mice bearing protein [superantigen (Sag)] in its 3' long terminal repeat the HYB PRO construct have been described (8). The HYB (LTR) (2, 3) that functions in milk-borne transmission (4, 5). PRO/Cla transgene was introduced into C3H/HeN MTV- The virus-encoded Sag protein is presented by antigen- mice by our standard procedure. Transgenic animals were presenting cells (APCs), such as B cells, to T cells bearing identified by Southern blot analysis (8). specific 3-chain variable (Vp) regions of the T-cell antigen Cells and Flow Cytofluorometry. Peripheral blood lympho- receptor. This presentation of Sag causes the proliferation of cytes were stained with titrated amounts of antibodies [rat specific Vp-bearing T cells when it is recognized as foreign (2) anti-CD4 or anti-CD8 labeled with phycoerythrin (GIBCO/ and deletion of such T cells when it is recognized as self (3). BRL) and fluoresceinated rat anti-Vp14 (9) or rat anti-Vp6 Recent data indicate that the role of Sag is to stimulate division (PharMingen)] and analyzed on an electronically programma- and cytokine production by cognate T cells, thereby creating ble individual cell sorter (Coulter) (4). a reservoir of actively dividing T or bystander B cells suscep- RNase T1 Protection Analysis. Total RNA (40 ,ug) isolated tible to MMTV infection (4-6). These infected cells exist at from virgin and lactating mammary glands and a probe high enough levels for a long enough period of time to allow containing a region of the 3' LTR of exogenous MMTV(C3H) the mammary gland, in which cells begin to divide under the were used for RNase T1 protection analysis (8). hormonal stimulation of puberty (at 3-4 weeks of age), to cDNA Library Construction and Screening. RNA was iso- become infected. lated from the virus fraction of milk (10) and a cDNA library Although the importance of the immune system in the was made with the SuperScript plasmid system for cDNA MMTV life cycle is well known, an absolute requirement for and the sag gene in this cycle has not been established. It is possible synthesis plasmid cloning (GIBCO/BRL). The plasmid that there are alternative pathways of MMTV infection that DNAs ofclones that hybridized with a MMTV LTR probe (11) bypass the need for the Sag protein. To test this, we created were sequenced. HYB PRO/Cla transgenic mice carrying a new MMTV pro- Sequence Analysis. The plasmids were sequenced with a virus with a mutation that caused premature termination ofthe Sequenase kit (United States Biochemical). The primers used MMTV(C3H) Sag. The HYB PRO/Cla transgenic females (arrows in Fig. 1) were no. 1, 5'-AATTCGGAGAACTCGAC- produced milk-borne virus containing the transgene as the Abbreviations: LTR, long terminal repeat; MMTV, mouse mammary tumor virus; RT, reverse transcription. The publication costs of this article were defrayed in part by page charge tTo whom reprint requests should be addressed at: Department of payment. This article must therefore be hereby marked "advertisement" in Microbiology/Cancer Center, University of Pennsylvania, 526 CRB, accordance with 18 U.S.C. §1734 solely to indicate this fact. 415 Curie Boulevard, Philadelphia, PA 19104-6142. 4828 Downloaded by guest on September 26, 2021 Immunology: Golovkina et at Proc. Natl. Acad ScL USA 92 (1995) 4829 Mtv-1 R C3H exo I L HYB PRO gag Pol w~~~~~ - 1 2 3 4 "107 cim TCC TCT TAC M CCO CAT CMmTTOT CCT TCA GMATA AGA... S Y K P H R F C PT E I R CUT WITH Cbl FILL4N FIG. 2. Pedigree of the HYB PRO/Cla transgenic mice. 0, Non- LIGATE transgenic females; o, nontransgenic male; o, transgenic female; o, transgenic male. TCC TCT TAC CCO CAT CeC ATT TTO TCC TTC AGAM T AGA MT MT OC TOA S 8 Y K P H R I V F R N R NKN 8 W was used for RNase T1 protection assays (Fig. 3). Expression FIG. 1. Map of the HYB PRO construct with the three viral of the transgene was detected in the mammary glands of both transcripts. The numbered arrows denote the approximate location of strains of transgenic HYB PRO/Cla females (shown for strain the primers used for sequencing of the recombinant viruses. Also t of 340 nt shown is the Cla I frameshift mutation (underlined) introduced into 28; band in Fig. 3, lane 6). The transgene in both the 3' LTR of the HYB PRO/Cla transgene. The boxed sequence 1 2 3 4 5 6 7 8 9 TAA is the premature termination codon. R, EcoRI; C, Cla I. - 404 CTTCC-3' [nt 268-289 in the MMTV LTR (12)]; no. 2, -327 5'-TTGTAAGAGGAAGTTGGCTG-3' (nt 326-307); no. 3, 5'-CTCCTTGGTATGGAAAATCTTTCC-3' (nt 894-871); - 231 and no. 4, 5'-GGACTGTTGCAAGTTTACTC-3' (nt 1203- 1184). Reverse Transcription (RT)-PCR Analysis. Total RNA (20 jig) isolated from lactating mammary gland was reverse tran- - 141 scribed with SuperScript II reverse transcriptase in the buffer supplied by the manufacture (GIBCO/BRL) and a (dT)15 Mtv-1 < primer (Promega). One-fourth of each cDNA preparation was used for PCR with primers specific for exogenous MMTV(C3H) [no. 1 and no. 5 (5'-TAATGTTCTATTAGTC- CAGCCACTGT-3; nt 923-898)] or endogenous and exoge- nous MMTVs [no. 6 (5'-GAGACGCTCAACCTCAATTGA- 3'; nt 507-527) and no. 4], as described (6). After PCR amplification, 20 ,lI of each reaction mixture was digested with Mun I (GIBCO/BRL). The PCR products were analyzed in 1.8% agarose gels. 118 -15 107 440 -> Probe RESULTS HYB PRO/Cla Transgenic Females Produce Virus. We 340 Full-length protection previously created a strain of mice, HYB PRO, with a genet- ically engineered MMTV provirus as a transgene in the FIG. 3. Mice nursed on HYB PRO/Cla transgenic females get C3H/HeN MMTV- inbred background (8). This transgene infected with MMTV. RNase protection analysis was performed on encodes a Sag protein with the same Vp specificity as exoge- RNA isolated from the mammary glands of various lactating and nous MMTV(C3H) virus (Vp14). C3H/HeN MMTV- mice virgin females (see Fig. 2), using the MMTV(C3H)-specific probe (8, were also used to create transgenic mice carrying the HYB 10). Lane 1, tRNA; lanes 2 and 3, lactating mammary glands of PRO construct with a frameshift mutation at aa 113 that C3H/HeN MMTV+ mice; lane 4, virgin mammary gland from the resulted in premature termination of the Sag protein encoded transgenic offspring of a HYB PRO/Cla 28 female (mouse B in Fig. by the 3' LTR (HYB PRO/Cla) (Fig.