Staphylococcal superantigens and T cell expansions in Wegener's granulomatosis

Abstract

In Wegener's granulomatosis (WG), a form of autoimmune systemic vasculitis, chronic carriage of Staphylococcus aureus constitutes a risk factor for the development of exacerbations. Circulating T cells in this disease are persistently activated, suggesting the presence of a chronic stimulus. A causal link between chronic carriage of S. aureus and chronic T cell activation in WG is conceivable, because S. aureus produces superantigens (SAg), which are potent T cell stimulators. Superantigenic stimulation of T cells results in expansion of T cell subsets expressing SAg-binding T cell receptor V-beta (Vbeta) chains. In the present study we hypothesized that in WG the presence of staphylococcal SAg is accompanied by expansion of SAg-reacting T cell subsets. We tested our hypothesis in a cross-sectional and a longitudinal study in which the association between seven staphylococcal SAg genes [typed by poplymerase chain reaction (PCR)], eight SAg-binding Vbeta chains and four SAg-non-binding Vbeta chains (assessed by flow-cytometry) was assessed. Both studies showed that T cell expansions were present at a significantly higher rate in WG patients than in healthy individuals, but were not associated with the presence of either S. aureus or its SAg. Moreover, T cell expansions were generally of small extent, and did not appear simultaneously in both CD4 and CD8 subsets. We conclude that in WG S. aureus effects its supposed pathogenic function by a mechanism other than superantigenic T cell activation.

Fig. 1

Typing of staphylococcal SAg by multiplex PCR. Seven staphylococcal SAg genes (SEA-SEE, TSST-1 and ETA) can be typed in three separate reactions using this multiplex PCR protocol. Typing of the genes makes use of the fact that the SAg-specific primers yield different amplicon sizes. Thus, SEA, SEC and TSST-1 (lane 2), SED and SEE (lane 3) and SEB and ETA (lane 4) can be detected in the same reaction. Additionally, the S. aureus specific nuclease-a gene was co-amplified in order to confirm the strain species. To illustrate the method, total DNA from S. aureus strains carrying one of the genes encoding for the SAg SEA-SEE, TSST-1 or ETA, respectively, were mixed, thus creating an artificial positive control. The100 base pair ladder was used as a size marker.

Fig. 2

Expansions of T cell subsets expressing SAg-binding Vβ chains. Percentages of T cells expressing SAg-binding TCR Vβ chains were determined by flow cytometry. Closed symbols denote measurements in the CD4 T cell subset, open symbols denote measurements in the CD8 T cell subset. Horizontal lines represent the threshold for expansions, determined for the group of healthy controls.