Some of the most frequently studied HDPs are the cathelicidins, including human LL-37 and its rodent ortholog mouse cathelin-related antimicrobial peptide (mCRAMP). Cathelicidins are characterized by a conserved cathelin pro-domain located near the N-terminus that is removed as the peptide is secreted, leaving the active HDP 1, 5. It is well known that cathelicidins and other HDPs influence adaptive immunity by acting on APCs (Fig. 1). Cathelicidins are secreted and taken up by macrophages, B cells, and DCs and their effects on these cells lead to selective
immune activation 1, 2, 6. Immature monocyte-derived DCs (MDDCs) transport LL-37 into the cytoplasm and nucleus, click here where LL-37 acts to upregulate CD86 and HLA-DR expression 7. MDDCs derived in the presence of LL-37 also show various changes in surface expression including increased CD86 and CD11b in immature MDDCs 8. These markers are associated with activation of the adaptive response; however, in response to Toll-like receptor (TLR) ligands, including lipopolysaccharide (LPS), cathelicidins can limit DC activation. For example, a model of allergic contact dermatitis found that wild-type mice had significantly decreased DC maturation and inflammation
in response to LPS sensitization as compared with mice lacking mCRAMP 9. Kandler et al. 10 found that LPS and other TLR ligands BMN673 in combination with LL-37 led to a decrease in expression of HLA-DR, CD86, and other markers when applied to DCs. Protein kinase N1 When such DCs were co-cultured with CD4+ T cells, this reduced T-cell proliferation and their production of the T-cell activators IL-2 and IFN-γ 10. Conversely, MDDCs derived with LL-37 in the culture medium showed normal maturation and increased CD11b
and CD86 expression in response to LPS, and co-cultured T cells exposed to LPS and LL-37 had increased IFN-γ production but no significant change in cell proliferation 8, consistent with the concept that HDPs modulate rather than suppress or stimulate immune responses. Other APCs include the M1 and M2 macrophages, polarized to a pro- and anti-inflammatory response, respectively. M1 macrophages promote the maturation of naïve CD4+ T cells into Th1 cells, leading to activation of cell-mediated immunity, whereas M2 macrophages promote the development of Th2 cells and the humoral response. Both M1 and M2 macrophages show decreased TNF-α production in response to LL-37 11, but LL-37 has also been demonstrated to make M2 macrophages more pro-inflammatory 12. Together, these studies show that immune responses to cathelicidins depend on when the cathelicidin is applied and the presence of other signaling molecules such as TLR ligands. While cathelicidins clearly influence APCs and their interactions with adaptive immune cells, evidence is emerging that cathelicidins have a more direct influence on the adaptive response.