Toxic Shock Syndrome (TSS) is an acute, multisystem disease that is caused by an infection with a strain of Staphylococcus aureus bacterium that secretes an exotoxin known as toxic shock syndrome toxin-1 (TSST-1). TSS was first described by Dr. Jim Todd, an epidemiologist at Children's Hospital in Denver, CO in 1978, and is "characterized by high fever, hypotension, generalized erythroderma, desquamation of the skin, and dysfunction of multiple organ systems" (Chatila et al. 1992). Other symptoms that are associated with this disease include chills, headache, vomiting, diarrhea, muscle pain, hallucinations, confusion, sore throat, dryness in the mouth, vagina, or eyes, elevated white blood cell count, and low blood pressure. Although TSS was once believed to be caused by the use of tampons, this notion has been dismissed as the body of knowledge surrounding this formerly elusive disease continues to expand.
What type of molecule is TSST-1?
TSST-1 belongs to a family of proteins known as pyrogenic toxin superantigens (PTSAs). This family of proteins also includes the staphylococcal enterotoxins (SE) A through I (except F) and streptococcal pyrogenic exotoxins (SPE) A-C and F. (Li et al. 1999) TSST-1, SEs, and SPEs are all "extremely resistant to proteases, are generally stable to temperatures of 60° C or higher, and survive a range of pH from 2.5 to 11" (Earhart et al. 1998). Superantigens like TSST-1 are capable of cross-linking major histocompatibility class II (MHCII) molecules and T-cell receptors (TCRs), leading to the activation of a substantial number of T cells. However, unlike antibodies, which stimulate T cells according to the epitope specified by the variable region of the TCR, superantigens like TSST-1 interact with the TCR depending on the haplotype of its (the TCR's) variable region's beta chain (Vß ). This activation often leads to the release of several cytokines, which we will see play an important role in the pathogenesis of TSS.
What is the structure of TSST-1?
TSST-1 is a
"single chain polypeptide [with] a molecular mass of approximately 22 kDa
and an isoelctric point of 7.2" (Deresiewicz et al. 1994). TSST-1
has been compared to other superantigens in its family, and its been found
that it shares only 20-30% of its primary nucleotide sequence with other
PTSAs. Yet despite this incongruence, there are some structural features
TSST-1 shares with its fellow superantigens. For instance, like the
SEs and SPEs, the three dimensional structure of TSST-1 consists of two
domains, a large and a small (domains A and B, respectively). According
to the 1999 Annual Review of Immunology, the "small domain is a ß-barrel
made up of two ß-sheets, whereas the large domain contains a ß-grasp
motif, an alpha-helix packed against a mixed ß-sheet that connects
the peripheral strands" (Li et al. 1999). However, another study
I came across concluded that the bilobal structure of all PTSAs contains
one domain (B), that is "composed of a five-strand mixed ß-barrel
and a second domain (A) built around a long, central helix lying against
a five-strand ß-sheet" (Earhart et al. 1998). Thus, although the
general structure of superantigens is fairly well documented, it is clear
that there remain some discrepensies as to the details.
According to a study published in Biochemistry,"there are a large number of charged amino acids spread over the surface of TSST-1, [and]... this protein has two pronounced grooves located in the middle of the molecule" (Prasad et al. 1993). The larger groove is located on the back of the molecule and the smaller is located on the front side of TSST-1. Some unique structural characteristics of TSST-1 include "the absence of a metal binding site and an alpha helix at the bottom of the ß barrel domains between strands ß3 and ß4, the lack of a disulfide loop at the top of the ß domain, and the extent to which the long central alpha helix is covered on the front and rear of the molecule" (Earhart et al. 1998).
How does the structure of TSST-1 relate to its function?
is interesting to note that much of what we now know about the relationship
between structure and function in TSST-1 we have obtained through studies
which focused on mutant forms of TSST-1 in which the toxin's biological
activity is reduced. In particular, TSST-1's toxicity to the host
(lethality) and its ability to stimulate T cell proliferation (mitogenicity)
have been major focal points of these experiments using mutant toxins.
The results from these experiments suggest that "the superantigenicity
of [TSST-1] involves the region surrounding the carboxyl end of the central
helix, . . . [where as] the amino terminal half of the central helix is
important in controlling the lethality of TSST-1" (Prasad et al. 1993).
Furthermore, "the top surface of domain B is not likely to be involved
with either lethality or mitogenicity" (Prasad et al. 1993).
Similarly, it has been hypothesized by several researchers that the lack of emetic activity associated with TSS can be attributed to the lack of cysteineyl residues, which ultimately leads to TSST-1's inability to form a disulfide loop. Also, TSST-1's inability to bind zinc may contribute to the nature of this toxin's interaction with MHCII molecules, for "the binding of zinc to [other superantigens] are believed to have direct effects [on] the recognition of MHCII molecules" (Papageorgiou et al. 1996). Regardless of the role this metal binding site might play, it is now clear that "a number of residues from the N-terminal ß barrel are involved in MHCII binding, . . . but TSST-1 possess[es] only one binding site for MHCII molecules involving [these N-terminal residues]" (Papageorgiou et al. 1996). X-ray crystallography studies of the TSST-1:DR1 complex (toxin:MHCII complex) have shown that when binding to MHCII, TSST-1 "covers most of the peptide binding site, contacting all the polymorphic residues on the alpha chain alpha helix, residues on the bound peptide, and part of the ß chain alpha helix, across the peptide binding site" (Kim et al. 1994). This seems somewhat counter-intuitive, for it suggests that "TSST-1 binding to DR1 may be in part peptide-dependent" (Kim et al. 1994). This idea is further supported by the fact that "a mutation in the peptide-binding groove [of MHCII] molecules disrupted TSST-1 binding sites" (Thibodeau et al. 1994).
Other research has focused on the localization of the T-cell epitope of TSST-1. Using an improved method that involved co-stimulation of the toxin:TCR complex using CD28 and synthetic peptides, Hu et al. concluded that the T-cell epitope of TSST-1 lies somewhere between residues 125-158. (1998) In summary, TSST-1 acts as a wedge between the TCR ß chain and the MHCII alpha chain, allowing it to circumvent the normal mechanism for T cell activation by specific peptide:MHCII complexes. This leads to polyclonal activation of large numbers of T cells that express certain Vß haplotypes. Unlike other superantigens, the mode of binding of TSST-1 may represent a special example that demonstrates its ability to regulate the degree of TCR-MHCII interactions within the TCR-TSST-1-peptide:MHC complex. This will be explored in greater depth in the next section.
What mechanisms does TSST-1 employ to elicit an immune response?
Even at extremely high concentrations, "TSST-1
does not exert direct toxic effects on the vast majority of tissues, but
rahter, induction of TSS by TSST-1 may result from massive and unregulated
stimulation of the immune system" (Chatila et al. 1992). To further
explore this idea that immune cell activation plays a vital role in the
pathogenesis of this disease, it is useful to follow the course of a typical
After TSST-1 manages to invade the body of a susceptible, it is bound by Ia molecules that serve as receptors for the superantigen. The Ia-bound TSST-1 elicits an immune response using two major mechanisms. The first mechanism "accounts for the activation of T lymphocytes by TSST-1 and involves the engagement of the Vß component of the T-cell receptor by toxin/Ia complexes resulting in T cell activation an proliferation" (Chatila et al. 1992). In other words, only those T lymphocytes expressing Vß2 proliferate in response to TSST-1. The second mechanism "involves the transduction via Ia molecules of signals that result in the activation of Ia+ immune cells including B lymphocytes, monocytes, activated natural killer (NK) cells and activated T lymphocytes" (Chatila et al. 1992).
When the TSST-1/Ia complex engages the Vß2, a set of events similar to those observed in nominal antigen:TCR interaction is set into motion. These similar activation events include: the activation of TCR-coupled phospholipase C, which leads to the generation of second messengers such as diacylglycerol, which in turn activates protein kinase C as well as inositol phosphates. It is thought that these early activation events "mediate the induction of lymphokine and lymphokine receptor gene expression as well as the progression of T cells through the cell cycle" (Chatila et al. 1992). Also, it has been shown that TSST-1 can "induce the expression of the CD40 ligand (CD40L) on the TSST-1 responsive Vß2 bearing subset of T cells" (Jabara et al. 1996). This induced expression of CD40L can cause isotype switching in B cells.
Since B lymphocytes express Ia molecules throughout much of their development, they are prime targets for TSST-1 binding and activation, but this activation is dependent on the presence of T lymphocytes. Indeed, it has been demonstrated that a ratio of 1:20 (T:B lymphocytes) that is required for intense proliferation of and differentiation of B cells into Ig secreting cells "corresponds to the ration of T to B lymphocytes normally found in the lymph node follicles" (Chatila et al. 1992).
Upon activation by TSST-1, a T lymphocyte is induced to "begin produc[ing] elevated levels of the cytokines interleukin-2 (IL-2), interleukin-4 (IL-4), interleukin-6 (IL-6), and interferon-gamma (IFN-g)" (Papageorgiou et al. 1998). These cytokines play an important role in the activation of NK cells, which express IL-2 receptors and can be activated by IFN-g directly. Similarly, "TSST-1 is a potent inducer of interleukin-1 (IL-1) production by monocytes/macrophage" (Jupin et al. 1988). The same study also reported that TSST-1 is capable of inducing the production of tumor necrosis factor (TNF), a molecule that plays a role in transcriptional activation. These cytokines help recruit other effector cells to the area of infection, and in doing so, stimulate an even larger immune response, leading to a viscious cycle.
Thus, following the engagement of MHCII molecules by a superantigen such as TSST-1 (see previous section), a cascade of cytoplasmic and nuclear ctivation events" occur, for "TSST-1 transduces activation signals via Ia molecules that result in the induction of monokine gene transcription" (Jupin et al. 1988). It seems as though the body's response to superantigens is the ultimate cause of a majority of symptoms associated with TSS, and the more we can learn about the pathogenicity of this disease, the better equipped we are to combat this rare ailment.
Who is at risk to develop TSS?
Although TSS was thought to be a female only
disorder for many years, we now know that this is not accurate, for TSS
cases have been found in men,women, and children. However, according
to a recent article in Emerging Infectious Diseases, "93% of all TSS cases
(5296 total) that were reported to the Center for Disease Control (CDC)
were among women" (Hajjeh et al. 1999). Furthermore, women who use
superabsorbant tampons are more likely to develop TSS, for a tampon has
been shown to provide an opportunistic environment for the colonization
by S. aureus. Thus, everyone is potentially at risk
to develop TSS, for just about any laceration or minor abration has the
potential to become infected with S. aureus, but menstruation can increase
susceptibility to this rare disease.
Since 1980, when TSS made its biggest headlines and was linked to the overuse of non-regulated, super(duper) absorbant tampons, there has been a dramatic decrease in the number of reported cases. "The incidence rates decreased from 6 to 12 per 100,000 among women 12 to 49 years of age in 1980 to 1 per 100,000 among women 15 to 44 years of age in 1986" (Hajjeh et al. 1999). However, this newfound awareness of women has led to a general shift in the epidemiology of TSS, and the number of nonmenstral TSS cases now appears to be increasing. I would suggest that this apparent increase may be linked to the growing number of Staphylococcal strains that have developed resistance to many commonly used antibiotics, but this is merely speculation, for I was unable to find any specific causitive evidence to support this theory.
How is TSS treated?
Shock Syndrome is most commonly treated with antibiotics, which help boost
the adaptive immune response and decrease the rate of infection by S.
aureus. In addition, intravenous fluids are admistered and hospitalization
is often needed to help control the manifestations of shock. Although
more research is needed, I was able to find one other potential treatment
option for the future. A group of researchers have found that "[g]lycerol
monolaurate (GML), a naturally occurring surfactant that is used widely
as an emulsifier in the food and cosmetics industries...inhibits the synthesis
of most staphylococal toxins and other exoproteins" (Projan et al. 1994).
This study found that GML acts at the level of transcription, for it "blocks
the induction, but not the constitutive synthesis of ß-lactamase,
suggesting that it acts by interfering with signal transduction" (Projan
et al. 1994). Although the authors were unable to ascertain how exactly
GML is able to limit the transcription of S. aureus genes that are necessary
for the synthesis of exotoxin, these results provide a possible molecular
mechanism by which we can attack and possible treat TSS. Likewise,
if researchers were somehow able to develop an antagonist to Ia molecules,
it may be possible to block the initial steps involved in TSS pathogenesis,
but once again, this type of treatment is still in the developmental stages.
Chatila, T.; Scholl, P.; Ramesh, N.; Trede, N.; Fuleihan, R.; Morio, T.; Geha, R.S. 1992. Cellular and Molecular Mechanisms of Immune Activation by Microbial Superantigens: Studies Using Toxic Shock Syndrome Toxin-1. Chemical Immunology 55:146-71.
Deresiewicz, R.L.; Woo, J.; Chan, M.; Fingerg, R.; Kasper, D.L. 1994. Mutations Affecting the Activity of Toxic Shock Syndrome Toxin-1. Biochemistry 33(43):12844-51.
Earhart, C.A; Mitchell, D.T.; Murray, D.L.; Pinheiro, D.M.; Matsumura, M.; Schlievert, P.M.; Ohlendorf, D.H. 1998. Structures of Five Mutants of Toxic Shock Syndrome Toxin-1 with Reduced Biological Activity. Biochemistry 37(20):7194-7202.
Hajjeh, R.A.; Reingold, A. 1999. Toxic Shock Syndrome in the United States: Surveillance Udate, 1979-1996. Emerging Infectious Diseases 5(6):807.
Hu, W.; Zhu, X.; Wu, Y.; Jia, Z. 1998. Localization of a T-Cell Epitope of Superantigen Toxic Shock Syndrome Toxin-1 to Residues 125-158. Infection and Immunity 66(10): 4971-5.
Jupin, C.; Anderson, S.; Damais, C.; Alouf, J.E.; Parant, M. 1988. Toxic Shock Syndrome Toxin 1 as an Inducer of Human Tumor Necrosis Factors and Interferon-gamma. Journal of Experimental Medicine 167(3):752-61.
Kim, J.; Urban, R.G.; Strominger, J.L.; Wiley, D.C. 1994. Toxic Shock Syndrome Toxin-1 Complexed with a Class II Major Histocompatibility Molecule HLA-DR1. Science 266(5192):1870-4.
Papageorgiou, A.C.; Brehm, R.D.; Leonidas, D.D.; Tranter, H.S.; Acharya, K.R. 1998. The Refined Crystal Structure of Toxic Shock Syndrome Toxin-1 at 2.07 Å Resolution. Journal of Molecular Biology 260(4): 553-69.
Prasad, G.S.; Earhart, C.A.; Murray, D.L.; Novick, R.P.; Schlievert, P.M.; Ohlendorf, D.H. 1993. Structure of Toxic Shock Syndrome Toxin 1. Biochemistry 32(50):13761-6.
Projan, S.J.; Brown-Skrobot, S.; Schlievert, P.M.; Vandenesch, F.; Novick, R.P. 1994. Glycerol Monolaurate Inhibits the Production of ß-lactamase, Toxic Shock Syndrome Toxin-1, and Other Staphylococcal Exoproteins by Interfering with Signal Transduction. Journal of Bacteriology 176(14):4204-9.
Thibodeau, J.; Cloutier, I.; Lavoie, P.M.; Labrecque, N.; Mourad, W.;
Jardetzky, T.; Sekaly, R. 1994. Subsets of HLA-DR1
Molecules Defined by SEB and TSST-1 Binding. Science 266(5192):1874-7.
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