MAJOR MEMBRANE-PROTEINS AND LIPOPROTEINS AS HIGHLY VARIABLE IMMUNOGENIC SURFACE COMPONENTS AND STRAIN-SPECIFIC ANTIGENIC MARKERS OF MYCOPLASMA-ARTHRITIDIS

Surface antigenic variation was investigated in Mycoplasma arthritidis , an agent that produces chronic arthritis in rats which shares severa l features with many mycoplasma-induced diseases and thus defines a we ll-characterized model system. Hyperimmune rabbit antisera (anti-ISR1, anti-PG6, anti-H606 and anti-158p10) to whole M. arthritidis organism s were used as immunological probes in Western immunoblots of four M. arthritidis prototype strains (ISR1, PG6, H606 and D263) and five rat- passaged substrains (ISR1p1, ISR1p7, ISR1p8 158p10 and D263p1). Severa l prominent antigens were identified that varied in expression. By Tri ton X-114 phase fractionation and treatment of whole cells with trypsi n and carboxypeptidase Y, these strain-variant antigens were shown to be integral membrane proteins with C-termini and portions of the polyp eptide chains oriented outside the membrane. Western blot immunoscreen ing of a large number of randomly selected clonal isolates and well-es tablished clonal lineages from stock cultures of M. arthritidis ISR1p7 , 158p10, PG6 and H606 revealed an expanded repertoire of variant memb rane proteins whose expression was subject to independent, reversible phase variation. Colony immunoblots of these clonal populations with a hyperimmune rabbit antiserum to a gel-purified variant membrane prote in (P36) showed that this phase switching occurred at a high frequency (10(-4) to 10(-2) per generation). Detailed immunological and biochem ical characterization of the phase-variant membrane proteins demonstra ted that they are: (i) antigenically related or distinct; (ii) apparen tly specific to particular strain populations; (iii) proteins or lipop roteins; (iv) major immunogens of M. arthritidis, recognized by serum antibodies from convalescent rat; and (v) able to undergo variation in expression during in vivo passage. Thus, M. arthritidis possesses a c omplex system capable of creating large repertoires of cell surface ph enotypes which may affect the multiple interactions of this organism w ith its host and dictate its potential as a successful infectious agen t and pathogen.