Streptococcal pyrogenic exotoxin B is a superantigen that induces murinesplenocyte proliferation and secretion of IL-2 and IFN-¿¿ ex vivo

Authors: Rasooly, Reuven; Do, Paula; He, Xiaohua; Hernlem, Bradley - Brad

Submitted to: FEMS Microbiology Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/27/2024
Publication Date: 5/28/2024
Citation: Rasooly, R., Do, P.M., He, X., Hernlem, B.J. 2024. Streptococcal pyrogenic exotoxin B is a superantigen that induces murinesplenocyte proliferation and secretion of IL-2 and IFN-¿¿ ex vivo. FEMS Microbiology Letters. 371.Article fnae036. https://doi.org/10.1093/femsle/fnae036.
DOI: https://doi.org/10.1093/femsle/fnae036

Interpretive Summary: SpeB (streptococcal pyrogenic exotoxin B) is made by Streptococcus pyogenes, a germ that causes strep throat, scarlet fever and other illnesses. They have also been found in foodborne outbreaks. SpeB is known to break down proteins but we studied whether it could also behave as a so-called “superantigen”. Indeed, using mouse spleen cells, we found that SpeB caused those cells to grow in number and also to make substances called cytokines, both typical of superantigens. We used a test for one cytokine, IL-2 (interleukin-2) to measure active SpeB and used that test to show that milk containing SpeB is not made safe by pasteurization, a process that otherwise kills the germ.

Technical Abstract: Streptococcus pyogenes is an important human pathogen, producing a range of virulence factors including streptococcal pyrogenic exotoxin B (SpeB) that is associated with foodborne outbreaks. This cysteine protease mediates cleavage of transmembrane proteins to permit bacterial penetration and is found in 25% of clinical isolates from STSS patients with extreme inflammation. Its interaction with host and streptococcal proteins has been well characterized but doubt remains about whether it constitutes a superantigen. In this study we show that SpeB does act as a superantigen by inducing proliferation of murine splenocytes and cytokine secretion, primarily of interleukin-2 (IL-2) as shown by cytometric bead array analysis. IL-2 secretion was confirmed by enzyme-linked immunoassay (ELISA) as well as secretion of interferon-y (IFN-y). The ELISA assay showed a dose-dependent relationship between SpeB concentration in splenocyte cells and IL-2 secretion levels, and it was shown that SpeB retains activity in milk pasteurized for 30 minutes at 63°C.