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Studies seem to suggest that Vitamin D3 plays an important role in production of  Defensins, just is it apparently does with the more widely discussed antimicrobial peptides, Cathelicidins (LL-37):

• Computational promotor analysis of mouse, rat and human antimicrobial peptide-coding genes

• Cutting Edge: 1,25-Dihydroxyvitamin D₃ Is a Direct Inducer of Antimicrobial Peptide Gene Expression

• Convergence of IL-1beta and VDR activation pathways in human TLR2/1-induced antimicrobial responses

While less discussed than their antimicrobial "cousins", cathelicidins, studies suggest that Alpha and Beta Defensins also play an important role in host defense.   Here are a few such studies:

Alpha Defensins (also knowns as Neutrophil Defensins):

• Interactions of {alpha}-, {beta}-, and {theta}-Defensins with Influenza A Virus and Surfactant Protein D

• Neutrophil primary granule proteins HBP and HNP1-3 boost bacterial phagocytosis by human and murine macrophages

• Human alpha-defensins inhibit BK virus infection by aggregating virions and blocking binding to host cells

• The capsule sensitizes Streptococcus pneumoniae to alpha-defensins human neutrophil proteins 1 to 3

• Susceptibility of Chlamydia trachomatis to protegrins and defensins

   

Beta Defensins:

• The Human -Defensins (-1, -2, -3, -4) and Cathelicidin LL-37 Induce IL-18 Secretion through p38 and ERK MAPK Activation in Primary Human Keratinocytes

• Influences of human beta-defensin 1 on the replication and expression of HPV18 in Hela cell

• Antifungal activities of human beta-defensins HBD-1 to HBD-3 and their C-terminal analogs Phd1 to Phd3

• Interaction of antibacterial peptides spanning the carboxy-terminal region of human beta-defensins 1-3 with phospholipids at the air-water interface and inner membrane of E. coli

• Role of human beta-defensin-2 during tumor necrosis factor-alpha/NF-kappaB-mediated innate antiviral response against human respiratory syncytial virus

• Human beta-defensins suppress human immunodeficiency virus infection: potential role in mucosal protection

• Isolation and Characterization of Human -Defensin-3, a Novel Human Inducible Peptide Antibiotic

• Human beta-defensin-3 promotes wound healing in infected diabetic wounds

• Human beta-defensin 4: a novel inducible peptide with a specific salt-sensitive spectrum of antimicrobial activity

• Expression of human beta defensin 4 in genetically modified keratinocytes enhances antimicrobial activity

Here's an excellent article on the various currently identified defensins and their molecular mechanisms of action:

• Through the Looking Glass: Mechanistic Insights from Enantiomeric Human Defensins

Studies suggest that in addition to their antimicrobial effects, defensins may also play an important role in regulating immuneⁱ responses and inflammationⁱ, as well as in wound repair:

• Defensins and cathelicidins in inflammatory lung disease: beyond antimicrobial activity