Superantigens and retroviral infection: insights from mouse mammary tumor virus

Stimulation of basal transcription from the mouse mammary tumor virus promoter by Oct proteins

The steroid hormone-inducible promoter of mouse mammary tumor virus (MMTV) contains three overlapping sequences related to the consensus octamer motif ATGCAAAT. Basal promoter activity in the absence of hormone induction from a template in which all three octamer elements were mutated was decreased by two-to threefold in in vitro transcription assays. Oct-1 protein purified from HeLa cell nuclear extracts, as well as recombinant Oct-1 expressed in bacteria, recognized MMTV octamer-related sequences, as shown by DNase I footprinting. Furthermore, rabbit polyclonal antiserum directed against recombinant Oct-1 completely inhibited the formation of specific complexes between MMTV octamer-related sequences and proteins present in nuclear extracts of HeLa cells, indicating that Oct-1 is the major protein in HeLa nuclear extracts that recognizes octamer-related sequences in the MMTV promoter. In addition, depletion of Oct-1 from the nuclear extract by using Oct-1-specific antiserum or a sequence-specific DNA affinity resin decreased in vitro transcription from the wild-type MMTV promoter to a level identical to that obtained from a promoter in which all three octamer-related sequences were mutated. Addition of purified HeLa Oct-1 or recombinant Oct-1 to the depleted extract selectively increased transcription from the wild-type relative to the mutated promoter, demonstrating that Oct-1 transcription factor stimulates basal transcription from the MMTV promoter. A similar effect was observed when purified recombinant Oct-2 was added to the Oct-1-depleted extract, suggesting that Oct-2 may play an important role in MMTV transcription in B cells.

B-cell superantigens: implications for selection of the human antibody repertoire

For several decades, B-cell interactions with antigens were thought to occur only through a clonal activation mechanism, in which the hypervariable regions of the immunoglobulin receptor are exclusively involved in ligand binding. However, an additional mode of interaction can occur, whereby molecules, termed B-cell superantigens, can bind human B cells bearing immunoglobulin receptors of a given variable (V)-gene family. This mechanism requires contributions from regions outside the conventional hypervariable loops and results in a B-cell response of increased magnitude. Here, Moncef Zouali reviews recent in vitro and in vivo observations on human B-cell superantigens in the context of the current consensus of B-cell development, and discusses the implications of these novel concepts with respect to pathogenesis.

Direct physical interaction involving CD40 ligand on T cells and CD40 on B cells is required to propagate MMTV

The propagation of mouse mammary tumor virus (MMTV) has been analyzed in mice defective for expression of CD40 ligand (CD40L). Mice with endogenous viral superantigen (SAG) delete T cells with cognate V beta independent of CD40L expression. Nevertheless, CD40L-mice do not show deletion of cognate T cells after being exposed to infectious MMTV and have greatly diminished viral replication. The response of CD40L- T cells to SAG in vitro is also impaired, but can be reconstituted by adding B cells activated by recombinant CD40L to express costimulatory molecules. Thus, direct CD40L-dependent B cell activation appears to be a critical step in the life cycle of MMTV. The initial step in SAG-dependent T cell activation, and hence the MMTV life cycle, may be mediated by non-B cells, because splenocytes from B cell-deficient SAG-transgenic mice are able to activate cognate T cells.

Superantigens and pseudosuperantigens of gram-positive cocci

Superantigens use an elaborate and unique mechanism of T lymphocyte stimulation. Prototype superantigen are the pyrogenic exotoxins produced by Staphylococcus aureus and Streptococcus pyogenes. Many candidate proteins of bacterial, viral and protozoal origin have recently been reported to be superantigens. In most cases the evidence that these proteins are in fact superantigens is highly indirect. In this review the evidence that gram-positive cocci produce superantigens other than the pyrogenic exotoxins is critically discussed. Evidence in described demonstrating that the epidermolytic toxins of Staphylococcus aureus and the pyrogenic exotoxin B and M-proteins of Streptococcus pyrogenes are not superantigens. Criteria are described for acceptance of a candidate as a superantigen.

Biology of mouse mammary tumor virus (MMTV)

Mouse mammary tumor viruses (MMTV) replicate in the mammary gland, appear as infectious particles in mother's milk and invade the sucking pups from the intestinal tract. The immune system is essential for MMTV in the gut to reach the mammary gland. These properties make the life cycle of MMTV unique. We review the oncologically and immunologically intriguing events caused by MMTV in relation to the life cycle of the virus.

Superantigenic characteristics of mouse mammary tumor viruses play a critical role in susceptibility to infection in mice

Mouse mammary tumor viruses (MMTV) are retroviruses that induce mammary carcinomas. An interesting feature of these viruses is the superantigen (SAg) encoded in an open reading frame within the 3' long terminal repeat. The mechanism by which ingestion of milk-borne virus results in infection of the host mammary tissue remains incompletely understood. However, a working model has been proposed in which the interaction between viral SAg, T-cell receptor and MHC class II I-E facilitates viral replication and hence infectivity. In this review we summarize current studies demonstrating the role of SAg stimulation in susceptibility to MMTV infection.

AIDS as immune system activation. Key questions that remain

Immune system activation is gaining attention as a central part of HIV pathogenesis. Although there is no consensus yet as to the source of the signal or the result of the signalling, this line of thinking represents a significant shift in the paradigm away from considering HIV disease like any other cytopathic viral infection. Hopefully, completion of studies focussed on this approach will lead to more complete understanding of AIDS and more effective therapies, and will at least bring to the fore some of the central unanswered questions in modern cellular immunology.

From AIDS to parasite infection: pathogen-mediated subversion of programmed cell death as a mechanism for immune dysregulation

Premature cell death can result either from cell injury or degeneration, leading to necrosis, or from the activation of a physiological cell-suicide process, termed programmed cell death or apoptosis, that is regulated by intercellular signalling. This process plays an essential role in the selection of developing lymphocytes, and is also involved in the function of the mature adaptative immune system. A growing number of experimental findings during the last 4 years has provided support to our hypothesis that inappropriate HIV-mediated dysregulation of programmed T-cell death is relevant to AIDS pathogenesis. A series of recent experimental results also supports the general concept that the persistence and pathogenesis of several infectious pathogens, ranging from retroviruses to parasites, may be related to their capacity to dysregulate programmed cell death in various cell populations including lymphocytes. Subversion by pathogens of the physiological control of programmed cell death provides a paradigm for the pathogenesis of a wide range of infectious diseases that involve immune dysregulation and suggests therapeutic potential for the in vivo modulation of cell signalling.

New infectious mammary tumor virus superantigen with V beta-specificity identical to staphylococcal enterotoxin B (SEB)

Only few infectious mouse mammary tumor viruses (MMTV) have been characterized which induce a potent superantigen response in vivo. Here we describe the characterization of an MMTV which was isolated from milk of the highly mammary tumor-prone SHN mouse strain. Exposure of newborn mice to milk-borne MMTV (SHN) results in a very slow deletion of V beta 7, 8.1, 8.2 and 8.3 expressing peripheral T cells. Subcutaneous injection of adult mice with this virus induces a rapid and strong stimulation of all four affected V beta-subsets in vivo. Besides the strong T cell effect we observed an early proliferation and activation of the local B cell pool leading to the initial secretion of IgM followed by preferential secretion of IgG2a by day 6. Sequence comparison of the polymorphic C terminus with known open reading frames revealed high homology to the endogenous provirus Mtv-RCS. This is the first report of a virus having a complete overlap in V beta-specificity with a bacterial superantigen stimulating as many as 35% of the whole CD4+ T cell repertoire including V beta 8.2.