Effect of exogenous nitric oxide on murine immune response induced by Aggregatibacter actinomycetemcomitans lipopolysaccharide

Background and Objective: Elevated nitric oxide (NO) has been associated with destructive periodontal disease. The aim of the present study was to test the hypothesis that exogenous NO may inhibit a protective immune response to Aggregatibacter actinomycetemcomitans lipopolysaccharide (LPS) in a murine model. Material and Methods: Mice of the BALB/c strain were sham immunized, immunized with A. actinomycetemcomitans LPS, treated with S‐nitroso‐N‐acetyl penicillamine (SNAP; a NO donor) and immunized with A. actinomycetemcomitans LPS or treated with SNAP plus 2‐(4‐carboxyphenyl)‐4,4,5,5‐tetramethylimidazoline‐1‐oxyl‐3‐oxide (carboxy‐PTIO) and immunized with A. actinomycetemcomitans LPS. All animals were then challenged subcutaneously with viable A. actinomycetemcomitans. The serum‐specific immunoglobulin G (IgG) subclasses and both interferon‐γ (IFN‐γ) and interleukin‐4 (IL‐4) as well as splenic inducible nitric oxide synthase (iNOS) activity before and after bacterial challenge were assessed. The diameter of skin lesions was determined. Groups of mice were treated with l ‐N⁶‐(1 ‐iminoethyl)‐lysine (l ‐NIL), an iNOS inhibitor, or 1H‐(1,2,4)oxadiazolo(4,3‐a)quinoxalin‐1‐one (ODQ), a guanylyl cyclase inhibitor, prior to injections with SNAP and/or A. actinomycetemcomitans LPS, and the skin lesions were assessed. Results: Treatment with SNAP increased the iNOS activity, suppressed both serum‐specific IgG2a and IFN‐γ levels, and delayed the healing of the lesions. These SNAP‐induced immune alterations were restored by treatment with carboxy‐PTIO. Pretreatment with l ‐NIL resulted in partial healing, whereas pretreatment with ODQ induced a delayed healing of the lesions. Conclusion: The present study suggests that exogenous NO may suppress a protective T helper 1‐like murine immune response to A. actinomycetemcomitans LPS by an endogenous NO‐independent but a cyclic GMP‐dependent mechanism.