Defensins are small positively charged peptides (29–47 amino acids) that are highly conserved among species and play a role in host defense against bacteria and viruses. There are two categories of defensins: A-defensins are secreted from human neutrophils, and B-defensins are produced from the epithelial cells. Each peptide contains six cystein amino acid residues, connected with disulfide bonds. The physicochemical properties of defensins are similar with the properties of another molecule, B2-glycoprotein I (B2-GPI), the major target of the autoantibodies found in the sera of patients with antiphospholipid syndrome (APS).
We investigated whether sera from patients with APS as well as primary Sjogren Syndrome (pSS), systemic lupus erythematosus (SLE), and rheumatoid arthritis (RA) contain autoantibodies that react with defensins.
We synthesized the linear peptides NH2-ACYCRIPACIAGERRYGTCIYQGRLWAFCC-OH and NH2-GIGDPVTCLKSGAICHPVFCPRRYKQIGTCGLPGTKCCKKP-OH, corresponding to the amino acid sequences of the mature forms of A1 (defA1) and B2 (defB2) defensins, respectively. These peptides were oxidized, in order for the disulfide bonds of the native molecules to be created, and subsequently were lyophilized. The peptides were dissolved in water and they were tested in ELISA experiments against sera from patients with APS (n = 24), pSS (n = 24), SLE (n = 16), and RA (n = 8). Additionally, sera from normal individuals were tested. Homologous inhibition experiments were performed in order to examine the specificity of the immune response against defensins.
None from the tested sera reacted against the defA1. Sera from patients with systemic autoimmune diseases contained autoantibodies to defB2 as follows: 21% of patients with APS and 25%, 31%, and 12% of the sera from the patients with pSS, SLE, and RA, respectively, gave a positive reaction against the same peptide. None of the normal sera reacted with the peptides at all. In the inhibition experiments the defB2 peptide, when it was used as soluble inhibitor, inhibited the binding of the antibodies at the plate-bound defB2 by 64%.
Defensins are components of the innate immunity and share common physicochemical properties with the B2-GPI molecule. A rather small proportion of sera from patients with systemic autoimmune diseases contain antibodies that react specifically with the defB2 peptide. The presence of these autoantibodies is not disease specific and their pathogenic significance, if any, remains to be elucidated.