CD1d-independent activation of mouse and human iNKT cells by bacterial superantigens

Invariant NKT (iNKT) cells are infrequent but important immunomodulatory lymphocytes that exhibit CD1d-restricted reactivity with glycolipid Ags. iNKT cells express a unique T-cell receptor (TCR) composed of an invariant alpha-chain, paired with a limited range of beta-chains. Superantigens (SAgs) are microbial toxins defined by their ability to activate conventional T cells in a TCR beta-chain variable domain (V beta)-specific manner. However, whether MKT cells are directly activated by bacterial SAgs remains an open question. Herein, we explored the responsiveness of mouse and human MKT cells to a panel of staphylococcal and streptococcal SAgs and examined the contribution of major histocompatibility complex (MHC) class II and CD1d to these responses. Bacterial SAgs that target mouse V beta 8, such as staphylococcal enterotoxin B (SEB), were able to activate mouse hybridoma and primary hepatic iNKT cells in the presence of mouse APCs expressing human leukocyte antigen (HLA)-DR4. iNKT cell-mediated cytokine secretion in SEB-challenged HLA-DR4-transgenic mice was CD1d-independent and accompanied by a high interferon-gamma:interleukin-4 ratio consistent with an in vivo Th1 bias. Furthermore, MKT cells from SEB-injected HLA-DR4-transgenic mice, and iNKT cells from SEB-treated human PBMCs, showed early activation by intracellular cytokine staining and CD69 expression. Unlike iNKT cell stimulation by alpha-galactosylceramide, stimulation by SEB did not induce TCR downregulation of either mouse or human iNKT cells. We conclude that V beta 8-targeting bacterial SAgs can activate MKT cells by utilizing a novel pathway that requires MHC class II interactions, but not CD1d. Therefore, iNKT cells fulfill important effector functions in response to bacterial SAgs and may provide attractive targets in the management of SAg-induced illnesses. Immunology and Cell Biology (2012) 90, 699-709; doi:10.1038/icb.2011.90; published online 1 November 2011