Design and Screening of the Peptide SAMP-12aa Derived from LL-37, Which Exhibits Anti-H. Pylori Activity and Immunomodulatory Effects

The appearance of antibiotic-resistant strains of <i>Helicobacter pylori</i> (<i>H. pylori</i>) is leading to a decreased eradication rate of <i>H. pylori</i> infection. There is an urgent need to find new agents with antimicrobial mechanisms different from those of antibiotics, with therapeutic potential to clear colonization of <i>H. pylori</i> in the stomach. Some antimicrobial peptides (AMPs) possess bactericidal activity by enhancing the permeability of the outer membrane and damaging the integrity of the cell membrane. Bacteria are not susceptible to drug resistance through this antimicrobial mechanism. In this study, 28 short peptides containing 12 amino acid residues were designed based on nine amino acid fragments (KRIVQRIKD) from human cathelicidin LL-37, which is stable in gastric juice, and 3 amino acids were added at the C-terminus of the peptide. These designed peptides were not digested and degraded by pepsin at low pH values. The peptides were predicted using the online tool platform. Then, the strongest antimicrobial peptide, named SAMP-12aa (KRIVQRIKDVIR), was screened from 28 short peptides. Further studies found that SAMP-12aa retained anti-<i>H. pylori</i> activity after incubation in simulated gastric juice. The MIC and MBC of SAMP-12aa were 8 μg/mL and 32 μg/mL, respectively. SAMP-12aa showed good bactericidal kinetics. SAMP-12aa was found to have cell selectivity, penetrating and damaging bacterial cell membranes and exhibiting almost no toxicity to human cells at a relatively high concentration (128 μg/mL). Regulatory T (Treg) cells express CD25^High with immunosuppressive activity that induces immune tolerance in response to <i>H. pylori</i>. Molecular docking prediction revealed that SAMP-12aa could target the active center of Foxp3. Flow cytometry analysis revealed that SAMP-12aa can inhibit Foxp3 activity and downregulate CD25 protein expression on CD4⁺ T cells, thereby reducing the development and differentiation of CD4⁺Foxp3⁺CD25^High Treg cells with immunosuppressive effects. Further research revealed that the levels of the cytokine interferon-γ (IFN-γ), which activates CD8⁺ T-cell activity, were significantly elevated, and the levels of transforming growth factor-β (TGF-β), which inhibits CD8⁺ T-cell activity, were significantly reduced. The results of this study reveal that SAMP-12aa not only possesses antibacterial activity but also has immunomodulatory effects.