The 3' half of the mouse mammary tumor virus orf gene is not sufficient for its superantigen function in transgenic mice

Mutational and functional analysis of the C-terminal region of the C3H mouse mammary tumor virus superantigen

The mouse mammary tumor virus (MMTV) encodes within the U3 region of the long terminal repeat (LTR) a protein known as the superantigen (Sag). Sag is needed for the efficient transmission of milk-borne virus from the gut to target tissue in the mammary gland. MMTV-infected B cells in the gut express Sag as a type II transmembrane protein that is recognized by the variable region of particular beta chains (Vbeta) of the T-cell receptor (TCR) on the surface of T cells. Recognition of Sag by particular TCRs results in T-cell stimulation, release of cytokines, and amplification of MMTV infection in lymphoid cells that are needed for infection of adolescent mammary tissue. Because the C-terminal 30 to 40 amino acids of Sag are variable and correlate with recognition of particular TCR Vbeta chains, we prepared a series of C-terminal Sag mutations that were introduced into a cloned infectious MMTV provirus. Virus-producing XC rat cells were used for injection of susceptible BALB/c mice, and these mice were monitored for functional Sag activity by the deletion of C3H MMTV Sag-reactive (CD4+ Vbeta14+) T cells. Injected mice also were analyzed for mutant infection and tumor formation in mammary glands as well as milk-borne transmission of MMTV to offspring. Most mutations abrogated Sag function, although one mutation (HPA242) that changed the negative charge of the extreme C terminus to a positive charge created a weaker Sag that slowed the kinetics of Sag-mediated T-cell deletion. Despite the lack of Sag activity, many of the sag mutant viruses were capable of sporadic infections of the mammary glands of injected mice but not of offspring mice, indicating that functional Sag increases the probability of milk-borne MMTV infection. Furthermore, although most viruses encoding nonfunctional Sags were unable to cause mammary tumors, tumors were induced by such viruses carrying mutations in a negative regulatory element that overlaps the sag gene within the LTR, suggesting that loss of Sag function may be compensated, at least partially, by loss of transcriptional suppression in certain tissues. Together these results confirm the importance of Sag for efficient milk-borne transmission and indicate that the entire C-terminal region is needed for complete Sag function.

In vivo effects of a recombinant vaccinia virus expressing a mouse mammary tumor virus superantigen

Early after infection, the mouse mammary tumor virus (MMTV) expresses a superantigen (SAg) at the surface of B lymphocytes. Interaction with the T-cell receptor Vbeta domain induces a polyclonal proliferative response of the SAg-reactive T cells. Stimulated T cells become anergic and are deleted from the T-cell repertoire. We have used a recombinant vaccinia virus encoding the MMTV(GR) SAg to dissect the effects of the retroviral SAg during an unrelated viral infection. Subcutaneous infection with this recombinant vaccinia virus induces a very rapid increase of Vbeta14 T cells in the draining lymph node. This stimulation does not require a large Plumber of infectious particles and is not strictly dependent on the expression of the major histocompatibility complex class II I-E molecule, as it is required after MMTV(GR) infection. In contrast to MMTV infection during which B cells are infected, we do not observe any clonal deletion of the reactive T cells following the initial stimulation phase. Our data show that contrary to the case with MMTV, macrophages but not B cells are the targets of infection by vaccinia virus in the lymph node, indicating the ability of these cells to present a retroviral SAg. The altered SAg expression in a different target cell observed during recombinant vaccinia virus infection therefore results in significant changes in the SAg response.

Structure and biological activity of the subgenomic Mtv-6 endogenous provirus

The Mtv-6 provirus has an incomplete genome, but retains a functional superantigen gene (sag) which directs the thymic deletion of CD4+ T cells expressing T cell receptors containing the V beta 3 or V beta 5 chains. To better understand the Mtv-6 superantigen, the structure and biological activity of the Mtv-6 provirus was analyzed. First, the complete nucleotide sequence was determined, and the mutation producing the subgenomic provirus was identified. Second, the nucleotide sequence of the 5' end of the sag gene transcript (including the splice junction) was determined by sequence analysis of a cDNA clone. Third, the superantigen activity of Mtv-6 was analyzed in mice carrying the Mtv-6 provirus isolated by selective breeding on a genetic background free of endogenous and exogenous mouse mammary tumor virus (MMTV). These studies demonstrate that (i) the Mtv-6 provirus contains a 6.2-kb deletion between two 12-bp direct repeats encompassing the central portion of the provirus but not affecting sag gene splicing or translation, (ii) the sag gene transcript has the structure predicted from previous S1 nuclease mapping studies, and (iii) the Mtv-6 superantigen can direct thymic deletion of target V beta 3+ and V beta 5+ T cells in the absence of gene products from full-length MMTV proviruses.