ABSTRACT
All examined Haemophilus influenzae biogroup aegyptius isolates of the clone associated with Brazilian purpuric fever (the BPF clone) produced type 2 immunoglobulin A1 (IgA1) proteases encoded by identical iga genes that were distinct from the iga genes of other Brazilian H. influenzae biogroup aegyptius isolates. A partial nucleotide sequence analysis revealed close similarities to the iga genes of H. influenzae serotype c and one noncapsular H. influenzae biotype III strain isolated from a case of conjunctivitis in Tunisia, suggesting an evolutionary relationship. Epitopes recognized by neutralizing antibodies differed for the IgA1 proteases of the BPF clone and of other H. influenzae strains, including Brazilian H. influenzae biogroup aegyptius isolates from patients with noninvasive conjunctivitis. The low probability of developing cross-reacting neutralizing antibodies to the IgA1 protease of the BPF clone may contribute to the pathogenic potential of this virulent phenotype in Brazil.