Restriction of Cl hemolytic function by human proline-rich salivary proteins

Abstract

Acidic proline-rich salivary proteins, PRPI, PRPII, PRPIII, PRPIV, upper Db and Statherin, were isolated from parotid saliva and tested for interaction with complement. It was determined that each of the isolated proline-rich proteins blocked Cl hemolytic activity in assay systems where Cl was rate-limiting. Further studies comparing the specific activity of the proline-rich proteins to unfractionated parotid saliva indicated that other salivary substances (sensitive to urea treatment of parotid saliva) were also interacting with the first complement component but in a time-dependent manner. Based on a series of experiments examining the effect of the sequence of addition of the proline-rich proteins in the complement assay systems, it is postulated that these salivary proteins are able to block the proper interaction of Cl with EAC4 cells (sheep erythrocytes coated with antibody and C4 gp). The proline-rich salivary proteins had no effect on Cl once Cl was bound to the immune complexes on the EAC4 cells. The Cl macromolecular complex undergoes conformational changes upon interaction with immune complexes resulting in a more avid binding of the Clq-Clr-Cls subunits with one another. Thus it is speculated that the EAC4-bound Cl becomes resistant to disruption by the proline-rich salivary proteins. Although the urea-sensitive factors had the highest specific Cl-fixing activity, the activity of the acidic proline-rich proteins on Cl is important because of their relatively high concn in salivary secretions. Since complement-containing serous exudates and transudates are present on inflamed mucosal tissues, salivary ubstances which interact with Cl may play a role in regulating the initiation of the classical complement pathway, particularly at those mucosal sites where there is a high ratio of salivary secretion to serous exudate.