The human cathelicidin peptide LL-37 is a multifunctional immunomodulatory and antimicrobial host defense peptide of the human immune system. LL-37 modulates host cell responses to microbial stimuli and also affects the action of other endogenous immune mediators such as IL-1beta and GM-CSF. This activity of LL-37 is known to be complex, with the functional outcomes being dependent on the cell type and activation status, timing of exposure, and other immune mediators present. It was demonstrated in this study that LL-37 inhibited cellular responses to IFN-gamma, the key cytokine of Th1-polarized immunity. The inhibitory activity of LL-37 on IFN-gamma responses was characterized in monocytes, macrophages, dendritic cells, and B lymphocytes, showing suppression of cell activation, proliferation, and production of proinflammatory and Th1-polarizing cytokines, and Abs. It was further shown that in monocytes the suppressive effects of LL-37 were mediated through inhibition of STAT1-independent signaling events, involving both the p65 subunit of NF-kappaB and p38 MAPK. This study suggests that LL-37 modulates IFN-gamma responses during both the innate and adaptive phases of immune responses, indicating a new immunomodulatory role for this endogenous peptide. These effects on IFN-gamma activity should be taken into consideration in the development of cathelicidin-based peptides for therapeutic applications as immunomodulatory or microbicidal agents.