CHARACTERIZATION OF A VARIANT ERYTHROCYTE SURFACE-ANTIGEN (VESA1) EXPRESSED BY BABESIA-BOVIS DURING ANTIGENIC VARIATION

Babesiu bovis, an intraerythrocytic, protozoal parasite of cattle, und ergoes clonal antigenic variation (Allred DR, Cinque RM, Lane TJ, Ahre ns KP. Infect Immun 1994;62:91-98). This ability could provide a mecha nism by which the parasite escapes host immune defenses to establish c hronic infection. Previous work identified two parasite-derived antige ns of M-r 128 000 and 113 000 that were present on the surface of the infected erythrocyte and appeared to be associated with clonal antigen ic variation (Allred DR, Cinque RM, Lane TJ, Ahrens KP. Infect Immun 1 994;62:91-98). Two monoclonal antibodies (mAbs), 3F7.1H11 and 4D9.1G1, which recognize the variant erythrocyte surface antigen (VESA1) have been identified. These mAbs react only with the surface of erythrocyte s infected with the B. bovis C9.1 clone in live-cell immunofluorescenc e assays. In both conventional and surface immunoprecipitations, the m Abs precipitate a variant antigen doublet that matches in mass the inf ected red blood cell (IRBC) surface antigens precipitated with bovine serum. In contrast, Western blot analysis revealed that only the M-r 1 28 000 polypeptide is recognized by the mAbs. Neither mAb recognizes a ntigenically variant progenitor or progeny parasite clones in any of t he immunoassays, confirming the involvement of this antigen in rapid c lonal antigenic variation. Failure to label this antigen with [9,10(n) -H-3]myristic acid, [9,10(n)-H-3]palmitic acid or D-[6-H-3]glucosamine indicates that these polypeptides are neither N-glycosylated nor fatt y acylated. Identity of the variant antigen recognized by the mAbs wit h that putatively identified with immune serum was confirmed by compar ison of partial proteolytic digestion products. Unambiguous identifica tion of the VESA1 antigen as a component of antigenic variation will f acilitate characterization of the events leading to antigenic variatio n on the B. bovis-infected erythrocyte surface and its significance to parasite survival during chronic infection. (C) 1997 Elsevier Science B.V.