A small percentage of invasive H. influenzae disease is (and probably has been) due to nontypable (i.e. unencapsulated) strains. With increasing immunization of children with H. influenzae conjugate capsular vaccines, the relative proportion of invasive disease due to nontypable organisms has increased: in certain locales it is the majority, in others only its prevalence has increased. Because of the concern that such cases will continue to increase in prevalence (because of the selective pressure exerted by vaccination against type b strains), it is important to understand the mechanism(s) used by unencapsulated H. influenzae to invade and cause bacteremia (and bacteremia-associated diseases) in immunocompetent immunized children. Our preliminary data indicate that the virulence determinant is carried by a novel bacteriophage (HP2) and the bacterium to replicate in blood by evading the complement system. Complement is recognized to be important in host defense against invasive H. influenzae infections in humans. The gene responsible for virulence has been designated the Human Serum Resistance 1 (hsr1) gene, and has no homolog in the current databases. Lysogenic conversion of only certain nontypeable H. influenzae results in conversion to virulence suggesting that "accessory" genes are necessary, for full expression of virulence. The proposed research seeks to define the additional gene(s) responsible for the unusual virulence properly, determine the prevalence of these virulence genes in a panel on invasive nontypeable H. influenzae, define the mechanisms by which HP2 mitigates complement activity, and identify the accessory genes needed for expression of this virulence trait using the avirulent laboratory strain H. influenzae Rd. KW20. Understanding this virulence mechanism, which is unusual for unencapsulated H. influenzae, will permit identification of new surface components, which are potential vaccinogens. Antigenic epitopes of the gene products operative in this mechanism can be used to produce a new H. influenzae conjugate vaccine active against invasive isolates.