Earlier studies implied a role for Mycoplasma arthritidis surface protein MAA2 in cytadherence and virulence and showed that it exhibited both size and phase variability. Here we report the further analysis of MAA2 and the cloning and sequencing of the maa2 gene from two M. arthritidis strains, 158p10p9 and H606, expressing two size variants of MAA2. Triton X-114 partitioning and metabolic labeling with [3H]palmitic acid suggested lipid modification of MAA2. Surface exposure of the C terminus was indicated by cleavage of monoclonal antibody-specific epitopes from intact cells by carboxypeptidase Y. The maa2genes from both strains were highly conserved, consisting largely of six (for 158p10p9) or five (for H606) nearly identical, 264-bp tandem direct repeats. The deduced amino acid sequence predicted a largely hydrophilic, highly basic protein with a 29-amino-acid lipoprotein signal peptide. The maa2 gene was expressed inEscherichia coli from the lacZ promoter of vector pGEM-T. The recombinant product was approximately 3 kDa larger than the native protein, suggesting that the signal peptide was not processed in E. coli. The maa2 gene and upstream DNA sequences were cloned from M. arthritidisclonal variants differing in MAA2 expression state. Expression state correlated with the length of a poly(T) tract just upstream of a putative −10 box. Full-sized recombinant MAA2 was expressed inE. coli from genes derived from both ON and OFF expression variants, indicating that control of expression did not include alterations within the coding region.
We previously identified two surface-exposed Mycoplasma arthritidis protein antigens, designated MAA1 and MAA2, that may be involved in cytadherence. Since adherence to host tissues is an important first step in most bacterial infections, we suggest that MAA1 and MAA2 may be virulence factors for M. arthritidis. In order to provide evidence for such a role, we conducted a series of experiments in which rats were actively immunized with each of these proteins purified from sodium dodecyl sulfate-polyacrylamide gels or passively immunized with poly-or monoclonal antibodies against MAA1 and MAA2. In each case, immunity against MAA1 and MAA2 conferred at least partial protection against M. arthritidis-induced disease. The greatest protection was achieved by passive immunization with monoclonal antibody A9a, directed against a surface-exposed epitope of putative adhesin MAA1. Because protective immunity in most bacterial infections is directed against major virulence factors, these results suggest that MAA1 and MAA2 may play a role in the pathogenesis of M. arthritidis-induced arthritis of rats, possibly by mediating initial colonization of joint tissues.
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