In the antigenically heterogeneous species a monoclonal antibody, mAb 26.7D, was previously found to recognize a 120 kDa polypeptide from 7488. This antibody did not react with the type strain PG21. The homologous gene from PG21 was cloned and sequenced and found to have a sequence identity of 91% with the gene of strain 7488. One hypervariable and two semivariable regions were detected. The epitope for mAb 26.7D was mapped to the hypervariable domain by expression of various parts of this domain in using expression vector systems. A polyclonal antiserum (pAb 121) generated against the hypervariable region of P120 from PG21 identified the P120 homologue in PG21. Fusion proteins of the hypervariable and constant parts of the proteins were constructed and tested for reactivity with 21 human sera. Twelve sera reacted with the 7488 hypervariable fusion protein, but only four reacted with the PG21 hypervariable fusion protein. No reactivity was seen with a fusion protein containing part of the constant region of P120. Gene fragments amplified from 18 isolates by PCR confirmed the heterogeneity of the hypervariable domain. Based on restriction endonuclease cleavage patterns of the hypervariable domain the 18 isolates could be divided into four classes. Reactivity with both mAb 26.7D and pAb 121 confirmed these classes. The hypervariable, but not the constant, part of P120 was recognized by the human humoral immune response. Such a variable domain may be important in evasion of the host's immune response, and thus aid survival of the micro-organism.


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