DEFINITION OF A DISCONTINUOUS IMMUNODOMINANT EPITOPE OF INTESTINAL ALKALINE-PHOSPHATASE, AN AUTOANTIGEN IN ACUTE BACTERIAL-INFECTIONS

In patients suffering hom acute bacterial infections specific autoanti bodies of the immunoglobulin class G (IgG), directed against intestina l alkaline phosphatase (IAP), are transiently expressed in high titer. The epitopes on IAP which are recognized by these IAP autoantibodies were investigated using chemical and enzymatic cleavage, followed by t wo dimensional electrophoresis and immunoblotting of the fragments. Im munoreactive and nonreactive cleavage fragments were isolated and N-te rminally sequenced. An epitope map was constructed by means of sequenc ing data, relative molecular mass of the fragments, and specific cleav age sites. To evaluate linear epitopes, the overlapping region of the immunoreactive fragments (amino acids 204-484 of the primary structure of IAP) was further analyzed by simultaneous synthesis of 95 peptides on an activated membrane. Four immunoreactive regions were revealed b y immunodetection with IAP autoantibody-positive sera. Nine peptides c omprising these regions were synthesized quantitatively and coupled to CH-Sepharose. However, specific LAP autoantibodies could not be isola ted. This result indicated the absence of continuous epitopes at least in the analyzed region of the molecule. Binding studies with, an IAP cleavage fragment suggested a discontinuous epitope involving amino ac ids 334-371. The results are indicative of an antigen-driven mechanism for the production of IAP autoantibodies initiated and maintained by self. (C) 1996 Academic Press, Inc.