Immunosuppressive factor from Actinobacillus actinomycetemcomitans down regulates cytokine production

A cytoplasmic soluble fraction Actinobacillus actinomycetemcomitans Y4 was isolated and characterized as suppressing mitogen-stimulated proliferation of and cytokine production by C3H/HeN mouse splenic T cells, This factor, designated suppressive factor 1 (SP1), was isolated from the supernatant of sonicated whole bacteria and purified by Q-Sepharose Fast Flow column chromatography, DEAE-Sepharose Fast Flow column chromatography, hydroxyapatite high-pressure liquid chromatography (HPLC), and Protein Pack 300 & 125 gel filtration HPLC, Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis revealed that the purified SF1 migrated as a single band corresponding to a molecular mass of 14 kDa. This molecule was protease labile, heat resistant, and noncytotoxic, N'-terminal sequence analysis revealed no homology with any known peptides of periodontopathic bacteria or with any host-derived growth factors, Purified SF1 suppressed the proliferation of mouse splenic T cells which had been stimulated with concanavalin Al as well as suppressing the production of interleukin-2 (IL-2), gamma interferon, IL-4, and IL-5 from CD4(+) T cells at 0.1 mu g/ml or more, These data suggest that SF1 produced by the periodontal pathogen A. actinomycetemcomitans functions as a virulence factor by down regulating T-cell proliferation and cytokine production at local defense sites.