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Superantigenic properties of the group A streptococcal exotoxin SpeF (MF)
A Norrby-Teglund1, D Newton, M Kotb
1Department of Clinical Bacteriology, Umeå University, Sweden.
Abstract:
Streptococcal pyrogenic exotoxin F (SpeF), previously referred to as mitogenic factor, is a newly described potent mitogen produced by group A streptococci. To investigate whether this protein belongs to the family of microbial superantigens, we analyzed the cellular and molecular requirements for its presentation to T cells and compared it with the known streptococcal superantigen pyrogenic exotoxin A (SpeA) and the nonspecific polyclonal T-cell mitogen phytohemagglutinin (PHA). SpeF and SpeA were efficiently presented by autologous antigen-presenting cells (APCs) and an allogeneic B lymphoma cell line, Raji. In contrast, the monocytic cell line U937, which does not express major histocompatibility complex (MHC) class II molecules, failed to present SpeF as well as SpeA but supported the response to PHA. Thus, the presentation of SpeF by APCs was class II dependent but not MHC restricted. The requirement for HLA class II was further supported by the ability of anti-HLA-DQ monoclonal antibody to block the SpeF-induced proliferative response by 75 to 100%. Paraformaldehyde (PFA) fixation of autologous APCs resulted in an impaired ability of SpeF and SpeA to induce optimal T-cell proliferation. In contrast, fixation of Raji cells did not affect the induced proliferation. The stimulatory effect of PHA remained unaffected by both the use of PFA-fixed APCs and the addition of the HLA class II-specific monoclonal antibodies. The addition of a supernatant enriched in interleukin 1 and interleukin 6 to fixed autologous APCs resulted in an increased SpeF-induced response; thus, the impairment was not due to a requirement for processing, but, rather, costimulatory factors produced by metabolically active APCs were needed. SpeF was found to preferentially activate T cells bearing V beta 2, 4, 8, 15, and 19, as determined by quantitative PCR. The data presented clearly show that SpeF is a superantigen. We also studied the prevalence of the speF gene in clinical isolates by Southern blot analyses, and the gene could be detected in 42 group A streptococcal strains, which represented 14 serotypes.
Insights
Streptococcal pyrogenic exotoxin F (SpeF) acts as a superantigen, requiring antigen-presenting cells (APCs) for T-cell activation. This potent mitogen is prevalent in Group A Streptococcus strains.
Area of Science:
- Immunology
- Microbiology
- Molecular Biology
Background:
- Streptococcal pyrogenic exotoxin F (SpeF), a potent mitogen from Group A Streptococcus, was previously known as mitogenic factor.
- Microbial superantigens are known to potently activate T cells, bypassing conventional antigen processing pathways.
Purpose of the Study:
- To determine if SpeF functions as a microbial superantigen.
- To elucidate the cellular and molecular requirements for SpeF presentation to T cells.
- To compare SpeF's presentation with SpeA and phytohemagglutinin (PHA).
Main Methods:
- T-cell proliferation assays using autologous antigen-presenting cells (APCs), allogeneic B lymphoma cell line (Raji), and monocytic cell line (U937).
- Major histocompatibility complex (MHC) class II dependency assessed using anti-HLA-DQ monoclonal antibodies.
- Impact of paraformaldehyde (PFA) fixation on APCs and T-cell response.
- Quantitative PCR to identify T-cell receptor Vβ usage.
- Southern blot analysis to detect the prevalence of the speF gene in clinical isolates.
Main Results:
- SpeF presentation by APCs is MHC class II dependent but not MHC restricted, similar to SpeA.
- T-cell proliferation induced by SpeF and SpeA requires metabolically active APCs, suggesting a need for costimulatory factors rather than processing.
- SpeF preferentially activates T cells expressing specific Vβ elements (Vβ 2, 4, 8, 15, and 19).
- The speF gene was detected in 42 out of 14 serotypes of clinical Group A Streptococcus isolates.
Conclusions:
- The data strongly support SpeF's classification as a superantigen.
- SpeF's mechanism of T-cell activation involves MHC class II presentation and requires costimulatory factors from active APCs.
- The widespread presence of the speF gene indicates its potential significance in Group A Streptococcus pathogenesis.