A hybrid protein [Met-Ala-(His)6OprF190C342-OprI21C83] consisting of the mature outer membrane protein We (OprI) and amino acids 190 to 342 of OprF of was expressed in and purified by Ni2+ chelate-affinity chromatography. vaccine for use in humans. is definitely a leading cause of nosocomial infections and pneumonia in private hospitals (15, 21, 28). The pathogen affects primarily immunocompromised individuals, such as individuals with large burns up (36, 44, 45), or individuals undergoing immunosuppressive or cytostatic therapy for the prevention of rejection after organ transplantation (33) or for malignancy treatment (22, 51). Eradication of infections is definitely hampered, since strains isolated in private hospitals are highly resistant to antibiotics (23, 24, 31, 47, 49, 56). The effectiveness of vaccination against illness in burn individuals was demonstrated 20 years ago (1, 32, 37). However, the polyvalent vaccine, which was based on isolated lipopolysaccharides (LPS) of serotypes, was Velcade not approved for routine clinical use because of the toxicity associated with the lipid A portion of the LPS. Subunit vaccines based on oligosaccharides purified from LPS conjugated to exotoxin (5C7) or mucoid exopolysaccharide Velcade (alginate) of (40C43) were shown to be less toxic and have been used successfully to elicit antibodies in a number of volunteers and groups of individuals (6, 7, 40, 43). However, currently no medical vaccine against for which safety and effectiveness have been demonstrated in clinical tests with individuals from one of the major risk organizations for nosocomial illness is available for routine use. Our study during the last decade has been focused on the development of a vaccine against based on its outer membrane proteins (OPRs). A vaccine based on OPRs may have several advantages. OPRs, which induce cross-protective immunity among all 17 known serotypes (38), can be produced by recombinant DNA technology free of contaminating LPS. Additionally, cloned genes of OPRs would be relevant for naked DNA immunization (4, 8) or could be transfected into unique vectors such as nonpathogenic strains to induce a mucosal immune response (34, 50). The effectiveness of OPRs like a vaccine candidate was demonstrated by us and additional research organizations (12, 13, 18, 19, 35, 52, 53) in various animal models. We have cloned the major OPRs, outer membrane protein F (OprF) (9) and OprI (10). Recombinant OprI was indicated in and used to vaccinate human being volunteers (54). Vaccination was well tolerated. In addition, the elicited antibodies against advertised complement-dependent opsonization of infection of immunocompromised mice, the vaccine proved to be highly protective (53). The use of GST as a constituent of a clinical vaccine in humans, however, cannot be approved because of the induction of a high GST-specific, nonvaccine-related immune response, which may lead to cross-reacting autoantibodies. We therefore directed our attention toward the cloning of an OprF-OprI hybrid protein which can be expressed in without a fusion component. Because the expression of OprF190C342-OprI21C83 without a fusion protein in was not successful due to rapid degradation of the hybrid protein, modifications with various extensions of IP1 the hybrid protein were tested (14). Finally, two recombinant vaccine Velcade candidates could be expressed as histidine-tagged fusion proteins and tested in immunosuppressed mice (55). One of them, Met-Ala-(His)6OprF190C342-OprI21C83, was found to be partly soluble and was found in the pellet as well as in the supernatant of ruptured bacteria. Therefore, this protein could be purified under native conditions from the supernatant as well as from the inclusion bodies by solubilization under denaturing conditions with 6 M urea, followed by subsequent renaturation. The second candidate, OprF179C342-OprI21C83(His)6, remained totally soluble when expressed in infection in mice (55). We now present data demonstrating that Met-Ala-(His)6OprF190C342-OprI21C83 was isolated and purified from to yield a clinically applicable vaccine that was successfully used without any apparent side effects for the vaccination of human volunteers against were grown in phosphate-buffered Luria broth (LB) Velcade (1% tryptone, 0.5% yeast extract, 1% NaCl, 50 mM.