Activation of THP-1 cells by needle proteins results in cytokine secretion. THP-1 cells were treated with PBS, 1 μg/ml of HKLM, or needle proteins (1 μg/ml). After 5 h (TNF-α) and 24 h (IL-6 and IL-8), supernatants were collected and tested by ELISA for production of TNF-α (A), IL-6 (B), and IL-8 (C). Error bars represent SEM (n = 3). Data shown are representative of at least three experiments. ***, P < 0.001; *, P value of 0.01 to 0.05.