Opsonophagocytosis of a prototype type III strain, M781, and six type III clinical isolates by rabbit rAlp3 antiserum (diluted 1:100), complement, and human peripheral blood leukocytes was measured (bottom). GBS, MI-136 compared MI-136 TEL1 with 43% survival among those born to dams that received the uncoupled mixture ( 0.0001). A tricomponent conjugate of type III CPS, rAlp3, and a GBS recombinant beta C protein lacking its IgA binding site (III-rAlp3-rBCPIgA) provided protection against a serotype III strain and a serotype Ia strain bearing beta C protein. High-titered anti-rAlp3 rabbit serum opsonized Alp3-containing strains of two GBS serotypes (types V and VIII) and invasive type III strains bearing the cross-reactive Rib protein for in vitro killing by human peripheral blood leukocytes. Thus, the potential exists for the inclusion of rAlp3 in a GBS vaccine formulated to provide multiserotype coverage. (GBS), is an important cause of invasive infection in newborns, pregnant women, the elderly, and immunocompromised adults. The most prevalent GBS serotypes causing neonatal GBS diseases in the world (16, 18, 19), including the United States, Germany, and Malawi, are type Ia (14% to 30%) and type III (50% to 68%). Serotype V has emerged as the most common serotype (30%) isolated from nonpregnant adults (14, 19), and serotypes VI and VIII are prevalent colonizers of pregnant women in Japan (25). Early vaccine trials with purified, uncoupled type Ia, II, and III capsule polysaccharides (CPSs) showed that these antigens evoked low and variable type-specific antibody responses in healthy adults (2, 4). Later vaccine trials of conjugate vaccines created by the covalent attachment of the CPS to an immunogenic carrier protein, such as tetanus toxoid (TT), enhanced the immune response to the CPS and resulted in high levels of type-specific, functionally active antibody predominantly of the immunoglobulin G (IgG) class (1, 40). Thus far, only a few protein carriers have been used for the preparation of licensed conjugate vaccines (15). Three protein carriers, including diphtheria mutant protein cross-reactive material (CRM197), TT, and diphtheria toxoid have been used in type b conjugate vaccines, whereas CRM197, TT, and diphtheria toxoid have been used in conjugate vaccines (15, 39). Seven pneumococcal CPSs have each been coupled to CRM197 MI-136 to create the multivalent conjugate vaccine against infections (13). While all of these carrier proteins improved the immune response to the CPS antigens, there was concern regarding their overuse. Indeed, studies have shown that healthy adults and children vaccinated with the protein carrier subsequently have suppressed antibody responses to the CPS epitopes (6, 10), and apparent epitopic suppression was observed when multiple vaccines sharing common protein epitopes were administered simultaneously (12). The use of alternative carrier proteins may reduce the potential for immune suppression in primed individuals as more conjugate vaccines are added to the routine vaccination program. Therefore, the development of alternative protein carriers that are immunogenic, protective, and easy to produce and ideally expand coverage is desired. To date, several GBS protein carriers that confer protection in animals have been described, including the alpha C protein (ACP) (17, 29), beta C protein (BCP) (32), Rib (29, 45), Sip (9), C5a peptidase (11), and surface-exposed antigens GBS67, GBS80, and GBS104 (36). The majority of GBS isolates express one or more of a family of surface-anchored proteins that form ladder-like patterns on Western blot due to a large number of repeat units. These proteins include ACP, Rib, alpha-like protein 2 (Alp2), and alpha-like protein 3 (Alp3) (31). The Rib protein is present in most type III strains, and Alp3 is present on most type V and type VIII strains (22, 31). The Alp3 protein has N-terminal regions that are highly similar to those of the ACP and BCP, and its tandem MI-136 repeats are essentially identical to the repeat region of the Rib protein (24). This tandem repeat region in Alp3 might constitute the antigenic site that was also present in the Rib protein (34, 33). Except for the number of repeats, the sequence of Alp3 is 98% identical to the sequence of R28 from disease in.