Background: Extracts of the plant Andrographis paniculata have been used to treat inflammatory diseases in Asian countries. A recent double-blind, placebo-controlled trial of HMPL-004 (A. paniculata extract) has demonstrated its safety and effectiveness for induction of clinical response, remission, and mucosal healing in patients with mild to moderate ulcerative colitis (UC). We aimed to determine if HMPL-004 could prevent the development of T-cell-dependent murine colitis and to define its in vivo mechanism(s) of action. Methods: CD4+CD45RBhigh T cells were transferred into Rag1 -/- mice and gavaged daily with HMPL-004 or methyl cellulose (MC). Severity of colitis was evaluated by weight loss, histology, and cytokine expression. Results: Mice treated with MC developed colitis within 4-7 weeks, as evaluated by weight loss, and severe intestinal inflammation. HMPL-004-treated mice did not lose weight and displayed only very mild intestinal inflammation. Tumor necrosis factor alpha (TNF-α), interleukin (IL)-1β, interferongamma (IFN-γ), and IL-22 expression were significantly decreased in HMPL-004-treated mice. We observed higher percentages of naïve CD4 + T cells in the lamina propria of HMPL-004-treated mice. At early timepoints HMPL-004-treated mice have significantly reduced splenic cell counts, reduced CD4+, and IL-17+, and IFN-γ+ T cells. Furthermore, HMPL-004 inhibited the proliferation of CD4+ T cells and differentiation into TH1/TH17 cells in vitro. Conclusions: HMPL-004 inhibits the development of chronic colitis by affecting early T-cell proliferation, differentiation, and TH1/TH17 responses in a T-cell-driven model of colitis, presenting a unique mechanism of action. Our data suggest that HMPL-004 could be an attractive herbal therapeutic for inflammatory bowel disease. Copyright © 2013 Crohn's & Colitis Foundation of America, Inc.