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J. Exp. Med., Volume 189, Number 8, April 19, 1999 1217-1228

Presentation of Antigen in Immune Complexes Is Boosted by Soluble Bacterial Immunoglobulin Binding Proteins

By Michel Léonetti,* Jérome Galon,Dagger Robert Thai,* Catherine Sautès-Fridman,Dagger Gervaise Moine,* and André Ménez*

From the * Commissariat à l'Energie Atomique, Département d'Ingénierie et d'Etudes des Protéines (DIEP) C.E. Saclay, Gif-Sur-Yvette cedex, France; and the Dagger  Institut National de la Santé et de la Recherche Médicale U255, Laboratoire d'Immunologie Cellulaire et Clinique, Institut Curie, Paris cedex 05, France

Using a snake toxin as a proteic antigen (Ag), two murine toxin-specific monoclonal antibodies (mAbs), splenocytes, and two murine Ag-specific T cell hybridomas, we showed that soluble protein A (SpA) from Staphylococcus aureus and protein G from Streptococcus subspecies, two Ig binding proteins (IBPs), not only abolish the capacity of the mAbs to decrease Ag presentation but also increase Ag presentation 20-100-fold. Five lines of evidence suggest that this phenomenon results from binding of an IBP-Ab-Ag complex to B cells possessing IBP receptors. First, we showed that SpA is likely to boost presentation of a free mAb, suggesting that the IBP-boosted presentation of an Ag in an immune complex results from the binding of IBP to the mAb. Second, FACS® analyses showed that an Ag-Ab complex is preferentially targeted by SpA to a subpopulation of splenocytes mainly composed of B cells. Third, SpA-dependent boosted presentation of an Ag-Ab complex is further enhanced when splenocytes are enriched in cells containing SpA receptors. Fourth, the boosting effect largely diminishes when splenocytes are depleted of cells containing SpA receptors. Fifth, the boosting effect occurs only when IBP simultaneously contains a Fab and an Fc binding site. Altogether, our data suggest that soluble IBPs can bridge immune complexes to APCs containing IBP receptors, raising the possibility that during an infection process by bacteria secreting these IBPs, Ag-specific T cells may activate IBP receptor-containing B cells by a mechanism of intermolecular help, thus leading to a nonspecific immune response.

Key words: Ag presentation;  protein A;  B cell superantigen


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