The expression of type 1 ribbriae (pili) of Escherichia coli is turned on and off at the transcriptional level at a high frequency (10-3 per cell per generation) in a process termed phase variation. Using Southern blot and DNA sequence analysis, we have detected a genomic rearrangement in the switch region immediately upstream of the fimbrial structural gene. This rearrangement involves an invertible 314-basepair segment of DNA whose alternating orientation apparently results in the on-and-off activation of a promoter that determines the state of fimbrial expression.Type 1 fimbriae (pili) are major surface appendages that mediate binding of Escherichia coli to eukaryotic cells by a ligand-receptor mechanism that is sensitive to the presence of mannose. The fimbrial protein adhesin is thought to be a virulence factor during the initial colonization stage. During the invasive stage it would seem to be to the pathogen's advantage not to express adhesins that could mediate binding to phagocytic cells. A genetic regulatory system that would result in an on-and-off expression of an adhesin would itself be a virulence factor (1).The phase variation between fimbriate (Fim+) and nonfimbriate (Fim-) E. coli cells occurs at the transcriptional level (2). In addition, it has been shown that a cis-dominant DNA switch is turned on and off by means of a trans-active factor (3) different from that reported by Orndorff and Falkow (4). Here we report that the molecular basis of this switch is the inversion of a relatively small segment of DNA that results in the alternating activation of the fimbrial promoter in correlation with phase variation. This system resembles in general that which controls flagellar phase variation in Salmonella (5) but differs both in its recombinase specificity and in its small size, so that the element does not encode its own recombinase. MATERIALS AND METHODS Bacterial Strains and Media. E. coli K-12 strain CSH50[ara A(lac-pro) rpsL thi] was the parental strain used. The genetic construction of strain VL412 and bacteriophage X412 has been presented in detail (3). 5-Bromo-4-chloro-3-indolyl ,B-D-galactopyranoside (X-Gal) RESULTSThe Lac-E. coli K-12 strain CSH50, used in the genetic constructions outlined below, undergoes fimbrial phase variation. Eisenstein (2) described the construction of afim-lac operon fusion in this strain and used its oscillating Lac';±Lac-phenotype to show that phase variation is under transcriptional control (2). The fim', fim-lac merodiploid strain VL412 (Fig. 1) was constructed through integration of a specialized X phage, carrying the fim-lac operon fusion, by homologous recombination with the fimD gene (3). This merodiploid strain exhibits independent, reversible, and noncoordinated phase variation of both the Lac and Fim phenotypes, which indicates the presence of two cis-acting switches, each adjacent to its respective operon (3). Ultraviolet induction of VL412 yields the phage X412, which alternates between a very dark blue (Dk) and light blue (Lt) plaque phe...

While studying the alpha beta T cell receptor repertoire in rheumatoid arthritis (RA) patients, we found that the frequency of V beta 14+ T cells was significantly higher in the synovial fluid of affected joints than in the peripheral blood. In fact, V beta 14+ T cells were virtually undetectable in the peripheral blood of a majority of these RA patients. beta-chain sequences indicated that one or a few clones dominated the V beta 14+ population in the synovial fluid of individual RA patients, whereas oligoclonality was less marked for other V beta's and for V beta 14 in other types of inflammatory arthritis. These results implicate V beta 14-bearing T cells in the pathology of RA. They also suggest that the etiology of RA may involve initial activation of V beta 14+ T cells by a V beta 14-specific superantigen with subsequent recruitment of a few activated autoreactive v beta 14+ T cell clones to the joints while the majority of other V beta 14+ T cells disappear.

The alpha beta T-cell receptors (TCRs) react with complex ligands composed of peptides bound to major histocompatibility complex (MHC) proteins. In the absence of foreign antigens the peptides bound to MHC molecules come from the proteins of the host itself. Interactions between TCRs and these self-peptide-MHC ligands work positively to drive T-cell development in the thymus and negatively to delete or inactivate T cells with potential self-reactivity. On the cell surface, MHC proteins are associated with many different self peptides, making it impossible to know which self peptide was involved in positive or negative interactions with a particular T cell. These studies as well as in vitro studies on TCR-peptide-MHC interactions would be aided by a means of producing MHC molecules containing a single peptide. We have tackled this problem for MHC class II proteins by genetically attaching the peptide by a flexible peptide linker to the amino terminus of the class II beta-chain. Here we report that a secreted, soluble form of this covalent peptide-MHC complex can be expressed in insect cells. The peptide is engaged by the peptide-binding groove of the secreted MHC molecule and this complex is recognized by T cells bearing receptors specific for that combination.

Sunllnal'yStaphylococcal enterotoxin B (SEB) is both a superantigen and toxin. As a superantigen, SEB can bind to major histocompatibility complex (MHC) class II molecules to form a ligand for ot/B T cell receptors bearing particular VB elements. As a toxin, SEB causes rapid weight loss in mice sometimes leading to death. We show here that both of these functions map to the NH2-terminal portion of the toxin. Three regions were identified: one important in MHC class II binding, one in T cell recognition, and one in both functions. These results support the conclusion that the toxicity of SEB is related to massive T cell stimulation and release of cytokine mediators and show that the residues interacting with MHC and the T cell receptor are intertwined:S taphylococcus aureus produces a set of exotoxins harmful to humans and other species (1-4). These include a group of enterotoxins (Staphylococcal enterotoxins [SEs]I), associated with food poisoning; toxic shock syndrome toxin 1 (TSST-1), associated with toxic shock syndrome; and exfoliating toxins (ExF), associated with scalded skin syndrome. Many of these toxins were purified to homogeneity some years ago and most have been cloned and sequenced (5-12). Nevertheless, there still is no consensus on their mechanisms of action in human diseases.These toxins were shown some time ago to be powerful T cell stimulators (13-15), a capacity that we and others have recently attributed to their properties as microbial superantigens (16-24). As superantigens, each toxin has the ability to bind to class II molecules of the MHC and create a ligand for virtually all T cell ot/~ antigen receptors containing particular variable dements on the B chain (VB). Amino acid differences in the other receptor variable components (D~, JB, Vot, Jot) do not play a major role in this interaction. Since there are a limited number of V~s (at least 57 in humans [25] and 22 in mice), the frequency ofT cells reactive to each toxin is very high. A recent study (26) has indicated that T cell stimulation occurs by interaction of the MHC-bound toxin with specific residues in the V/3 element lying on the side of the receptor rather than the variable face thought to contact conventional antigen bound to MHC. We have sug-1 Abbreviations used in this paper: SE, Staphylococcal enterotoxin; TSST-1, toxic shock syndrome toxin 1.gested that the pathology of these toxins involves the massive release of bioactive cytokines accompanying this high frequency T cell response in vivo (22,27).Although binding of the toxin to class II MHC is a prerequisite for T cell recognition (16,(28)(29)(30), the process is much more permissive than that seen with the binding of conventional peptide antigens. Whereas peptide antigens are very dependent on allelic MHC residues for binding, toxin superantigens bind to a wide variety of allelic and isotypic forms of class II MHC in mouse and humans (31-34). Furthermore, T cells rarely recognize peptide antigens bound to allo-MHC molecules, while individual T cell clones can respond t...