SUPERANTIGENIC PROPERTIES OF THE GROUP-A STREPTOCOCCAL EXOTOXIN SPEF (MF)

Streptococcal pyrogenic exotoxin F (SpeF), previously referred to as m itogenic factor, is a newly described potent mitogen produced by group A streptococci, To investigate whether this protein belongs to the fa mily of microbial superantigens, we analyzed the cellular and molecula r requirements for its presentation to T cells and compared it with th e 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 c ells (APCs) and an allogeneic B lymphoma cell line, Raji. In contrast, the monocytic cell line U937, which does not express major histocompa tibility complex (MHC) class II molecules, failed to present SpeF as w ell as SpeA but supported the response to PHA. Thus, the presentation of SpeF by APCs was class II dependent but not MHC restricted. The req uirement for HLA class II was further supported by the ability of anti -HLA-DQ monoclonal antibody to block the SpeF-induced proliferative re sponse by 75 to 100%. Paraformaldehyde (PFA) fixation of autologous AP Cs resulted in an impaired ability of SpeF and SpeA to induce optimal T cell proliferation. In contrast, fixation of Raji cells did not affe ct the induced proliferation. The stimulatory effect of PHA remained u naffected by both the use of PFA-fixed APCs and the addition of the HL A class II-specific monoclonal antibodies. The addition of a supernata nt enriched in interleukin 1 and interleukin 6 to fixed autologous APC s resulted in an increased SpeF-induced response; thus, the impairment was not due to a requirement for processing, but, rather, costimulato ry 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 th e speF gene in clinical isolates by Southern blot analyses, and the ge ne could be detected in 42 group A streptococcal strains, which repres ented 14 serotypes.