The USPA2 Protein and Serum Resistance of Moraxella Catarrhalis
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Most isolates of Moraxella (Branhamella) catarrhalis are resistant to the bactericidal activity of normal human serum. Several M. catarrhalis gene products have been linked to the serum-resistant phenotype but none of them was shown to be directly involved in this phenotype. This study provides the first evidence for the direct involvement of the UspA2 protein of several serum-resistant M. catarrhalis strains in the serum-resistant phenotype. This was achieved by using transformation and allelic exchange to introduce hybrid uspA2 genes into M. catarrhalis, together with cloning and expression of different UspA2 proteins in Haemophilus influenzae. Using different types of human sera, it was concluded that serum-sensitive M. catarrhalis strains are killed via the classical complement pathway. Analysis of complement deposition on four different serum-resistant M. catarrhalis strains and their serum-sensitive uspA2 mutants showed similar amounts of early complement components binding to these cells, but a significant reduction occurred in the amount of polymerized C9 on the wild-type strains relative to that on the uspA2 mutants. The binding of the UspA2 proteins of these strains to the complement regulator vitronectin was shown to be responsible for the protection of these strains against complementmediated killing. This represents the first example of vitronectin-mediated serum resistance on a microbe. In contrast, binding of the complement regulator C4BP by the M. catarrhalis strains used in this study did not correlate with serum resistance. Finally, analysis of the untranslated region upstream of the uspA2 open reading frame showed that the presence of a heteropolymeric nucleotide repeat (AGAT) in this region is necessary for both normal expression of the UspA2 protein and serum resistance. Also, it was shown that changes in the number of AGAT repeats affected transcription of the uspA2 gene, with 15-18 AGAT repeats yielding maximal levels of transcription. These results indicate that these AGAT repeats play a regulatory role in the expression of the uspA2 gene.