| Human beta defensins (hBDs) are cationic antimicrobial peptides that can be produced in response to microbial challenge at mucosal sites. Acting as effector molecules of the innate immune system, hBDs can bind to and lyse microorganisms. Defensins have also been shown to bridge the innate and adaptive immune systems through the chemoattraction of immune cells. Here, we provide evidence that hBD-3 also activates antigen presenting cells. Following overnight exposure, hBD-3 can induce expression of the co-stimulatory molecules CD80, CD86, and CD40 on monocytes and myeloid dendritic cells. Activation of monocytes by hBD-3 is mediated by interaction with TLRs 1 and 2, resulting in signaling that requires MyD88 and results in IRAK-1 phosphorylation. Cell lines (HEK and CHO) that have been engineered to express Toll-like Receptors (TLRs) 1 and 2 could be activated by hBD-3, but cells that express other individual TLRs, or TLRs 1 and 6, could not. Antibodies to TLR 1 and TLR 2 were also able to inhibit the hBD-3 mediated induction of CD80 on monocytes.; HBD-3 can also induce expression of inflammatory cytokines (IL-8, IL-6, and IL-1beta) from purified monocytes. Exposure to hBD-3 induces the expression of the homing receptors CCR7, CD62L, and a key antigen-presenting molecule - HLA-DR on the surfaces of monocytes. Comparisons of the TLR1, 2 binding elements hBD-3 and Pam3CSK4, demonstrate that cell stimulation by these molecules results in differential outcomes. Exposure to Pam3CSK4 results in heightened production of IL-10 and decreased surface expression of CD86. HBD-3 does not stimulate IL-10 production and induces heightened levels of CD86 on monocytes. Also, induction of co-stimulatory molecule expression (CD80) on monocytes by hBD-3 and Pam3CSK4 is regulated differentially by the MAP kinase pathways. This work demonstrates for the first time, that a human antimicrobial peptide has the potential to stimulate cells in a TLR dependent manner; and that stimulation of TLRs 1 and 2 by unique ligands results in qualitative differences in protein expression. By activating antigen presenting cells through TLRs 1 and 2, HBD-3 may be playing a key role in bridging the innate and adaptive immune systems at mucosal sites. |
