Abstract: Wang Shan, Zhao Zuotao, Wang Yuhan, Tu Ping, Liu Lingling Department of Dermatology, Peking University First Hospital, Beijing Key Laboratory of Molecular Diagnosis on Dermatoses, Beijing 100034, China（Current affiliation of the first author was Department of Dermatology, Beijing Children′s Hospital, Capital Medical University, National Center for Children′s Health, Beijing 100045, China） Corresponding author: Liu Lingling, Email: liulingling@medmail.com.cn 【Abstract】 Objective To investigate the role of Staphylococcus aureus enterotoxin B （SEB） in non-IgE mediated activation of mast cells （MCs） by in vitro co-culture of laboratory of allergic disease 2（LAD2） cells and SEB. Methods The LAD2 cells were incubated with SEB at different concentrations of 0.01, 0.1, 1, 10 and 100 μg/ml, A23187 positive control and negative control separately for 30 minutes. Then, effects of SEB on the morphology of MCs were observed by using a light microscope, and culture supernatants of the above incubation systems were collected. The concentration of tryptase released from MCs was analyzed by enzymatic activity assay, and the level of histamine was detected by enzyme-linked immunosorbent assay（ELISA）. Results After 30-minute co-culture of LAD2 cells and SEB, MCs showed larger size, obscure boundaries, increased number of protuberances on the cell surface and decreased refractivity, with a radial burr fin-like appearance. After 30-minute co-culture of LAD2 cells and SEB at different concentrations of 0.01, 0.1, 1, 10 and 100 μg/ml, the concentrations of tryptase in the culture supernatants were 4.116 ± 0.651, 5.344 ± 0.874, 3.806 ± 0.459, 1.309 ± 0.247, 0.310 ± 0.199 ng/ml respectively. Additionally, the tryptase levels were significantly higher in the 0.01-, 0.1-, 1-μg/ml SEB groups than in the negative control group（1.538 ± 0.490, all P < 0.05）, and gradually decreased along with the increase of SEB concentrations. The histamine levels in the 0.01-, 0.1-, 1-, 10- and 100-μg/ml SEB groups were 242.409 ± 63.915, 522.491 ± 73.466, 550.926 ± 84.466, 334.397 ± 33.640, 226.527 ± 5.678 ng/ml respectively. In the 0.01-, 0.1-, 1-μg/ml SEB groups, the levels of histamine released from MCs were gradually increased along with the increase of SEB concentrations, and were significantly higher than those in the negative control group （146.436 ± 3.100, all P < 0.05）. However, with the continued increase of SEB concentrations, the histamine levels gradually decreased. Conclusion SEB can directly activate MCs by a non-IgE mediated mechanism, followed by morphologic changes of MCs and release of tryptase and histamine.