The bacterial toxin protein A from Staphylococcus aureus (SpA) interacts with B cell antigen receptors encoded by variable region heavy chain (VH) clan III genes via a V region framework surface that has been highly conserved during the evolution of the adaptive immune system. We have investigated the consequences of exposure to this prototypic B cell superantigen, and found that treatment of neonates or adults induces a T cell–independent deletion of a large supraclonal set of susceptible B cells that includes clan III/VH S107 family–expressing lymphocytes. In studies of different SpA forms, the magnitude of the induced deletion directly correlated with the VH-specific binding affinity/avidity. Upon cessation of SpA exposure, the representation of conventional splenic (B-2 subset) lymphocytes normalized; however, we found that the VH family–restricted deficit of peritoneal B-1 cells persisted. SpA treatment also induced a persistent loss of splenic S107-μ transcripts, with a loss of certain natural antibodies and specific tolerance to phosphorylcholine immunogens that normally recruit protective antimicrobial responses dominated by the S107-expressing B-1 clone, T15. These studies illustrate how a B cell superantigen can exploit a primordial Achilles heel in the immune system, for which B-1 cells, an important source of natural antibodies and host immune responses, have special susceptibility.

The Tax protein of human T-cell leukemia virus type 1 (HTLV-1) upregulates the expression of several cellular genes by activating members of both the NF-kappaB and bZIP families of transcription factors. Recent studies demonstrate that the CD28 response element (CD28RE) of the interleukin 2 (IL-2) promoter is the site upregulated by Tax in stimulated T cells. Although some reports suggest that this site is transactivated by NF-kappaB family members, others disagree, leaving the identity of the transcription factor(s) binding the CD28RE unclear. The studies presented here further characterize the response of the IL-2 promoter and CD28RE to the HTLV-1 Tax protein and demonstrate that the TATA-proximal AP-1 binding site of the IL-2 promoter is also necessary for Tax transactivation in stimulated Jurkat cells. In contrast to its upregulation of the IL-2 promoter which requires T-cell stimulation, Tax transactivates the isolated CD28RE-AP-1 element without stimulation but is greatly synergized by calcium ionophore and phorbol ester. Additionally, transactivation of the IL-2 promoter requires the Tax activation domain involved in upregulation of bZIP-enhanced transcription while the NF-kappaB-activating domain of Tax is dispensable. Interestingly, both domains appear to be necessary for the activation of the isolated CD28RE-AP-1 sequence in the context of a heterologous promoter construct. This strongly suggests that activation of NF-kappaB is insufficient to activate transcription via the CD28RE-AP-1 element of the IL-2 promoter and that a different transcription factor, upregulated via the activation domain of the HTLV-1 Tax protein, may be involved.

The X region of human T-cell leukemia virus type I (HTLV-I) encodes two proteins that regulate viral gene expression. The tax protein is the product of the transactivator gene and has been shown to up-regulate the expression of some cellular genes controlling T-cell replication, including that of the interleukin-2 (IL-2) T-cell growth hormone and the a chain of its receptor (IL-2R). Several studies have shown that tax transactivation of the IL-2R a-chain promoter is mediated by binding sites for the transcriptional activator NF-KB, and this mechanism has also been implicated in the tax activation of IL-2 promoter activity. The rex gene product of HTLV-I regulates viral protein production by influencing mRNA expression and has been implicated in the stabilization of IL-2R a-chain mRNA. In the present studies, the ability of the tax and rex proteins to transactivate IL-2 gene expression has been reinvestigated. The ability of the tax protein to transactivate IL-2 promoter activity appears, at least in part, to be mediated by the recognition sequence for a DNA-binding complex known as CD28RC. Consistent with this hypothesis is the observation that tax-mediated activation of IL-2 gene expression is resistant to the immunosuppressive affects of cyclosporin A, a property postulated for the CD28RC binding complex. Unexpectedly, this tax-mediated up-regulation of IL-2 expression is synergized by the presence of the rex protein. These findings demonstrate that transactivation of IL-2 gene expression by tax is augmented by mechanisms distinct from NF-cB and raise the possibility that rex, as well as tax, contributes to the oncogenic capability of HTLV-I by altering the expression of the IL-2 gene in T cells infected with this retrovirus. * Corresponding author. pression of the IL-2 gene is tightly controlled, and activation of this gene product requires multiple signals. At the transcriptional level, the expression of the human IL-2 gene is activated by several transcription factors (for a review, see reference 46). One of these is the inducible NFAT-1 protein which is necessary but not sufficient for IL-2 gene activity through the T-cell receptor. The abrogation of IL-2 promoter activity by the immunosuppressant cyclosporin A has been shown to be due to a block in the activity of this transcription factor (12). Other proteins, such as NF-KB, AP-1 (from c-fos and c-jun), and octamer binding proteins, are also reported to be involved but not sufficient for IL-2 gene activation. The studies presented here show that transactivation of the IL-2 promoter by tax is mediated, at least in part, by the binding site for the CD28RC complex which is activated by CD28 stimulation of T cells (13). In addition, endogenous IL-2 production and IL-2 promoter activity are shown to be resistant to the immunosuppressive effects of cyclosporin A in the presence of tax. This response appears to be synergized by the presence of rex. These data support the hypothesis that IL-2 gene activation in the presence of HTLV-I proteins may be via a pathway...

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In vitro studies of several naturally occurring proteins have characterized VH family-specific B lymphocyte binding and stimulatory properties that appear analogous to those of T cell superantigens. To examine the in vivo consequences of exposure to a putative B cell superantigen, we treated neonatal BALB/c mice with a form of staphylococcal protein A (MS) devoid of Fcγ binding activity, which retains the clan VHIII Fab binding specificity. In naive adults, about 5% of peripheral B cells and >13% of splenic IgM-secreting cells display MS binding activity, in association with high IgM and low IgG circulating anti-MS Ab titers. Neonatal exposure to MS elicited two distinct temporal phases of immune responsiveness. The early phase, representing the first approximately 5 wk of life, was associated with MS-specific B cell and T cell tolerance. Microfluorometric assays revealed that exposure caused a dramatic MS-specific B cell clonal loss in bone marrow and spleen, but levels normalized by about 3 wk of life. The late phase (>6 wk of age) was associated with spontaneous priming for MS-specific T cell responses and production of MS-specific IgG1 Abs despite long term persistently depressed in vivo and in vitro MS-specific IgM responses. In vivo challenge during the late phase induced high frequencies of MS-specific IgG-secreting cells, indicating recruitment of highly focused Ab responses that were predominantly encoded by rearrangements of the S107 family, a member of the VHIII clan. These studies document the immunodominance of the VH-restricted Fab binding site on staphylococcal protein A and demonstrate the diverse effects of a B cell superantigen on the emerging peripheral B cell compartment.