Salivary agglutinin is the major component in human saliva that modulates the lectin pathway of the complement system

Saliva interacts with blood after mucosal damage or leakage of gingival crevicular fluid. Surface-adsorbed salivary agglutinin (SAG) activates the lectin pathway (LP) of the complement system via mannose-binding lectin, while SAG in solution inhibits complement activation. In the present study we investigated if, next to SAG, whole and glandular saliva itself and other salivary glycoproteins activate or inhibit the LP. Complement activation was measured by detecting C4 deposition on microtiter plates coated with saliva or purified proteins. Complement inhibition was measured after incubating serum with saliva or proteins in microtiter plates coated with mannan, an LP activator. Adsorbed whole, sublingual and submandibular saliva showed LP-dependent complement activation. Blood group secretors, but not non-secretors, activated the LP. Saliva of both secretors and non-secretors inhibited C4 deposition on mannan. After depletion of SAG, saliva no longer inhibited the LP. Other salivary proteins, including amylase, MUC5B and histatin 2, did not activate or inhibit the LP. Surface-adsorbed whole saliva and glandular saliva samples activate the LP of complement, depending on the presence of SAG and the secretor status of the donor. In solution, saliva inhibits the LP, depending on the presence of SAG, but independent of the secretor status.