Haemophilus influenzae type b (Hib) is an important pathogen for young children, and children can be protected with antibodies (Abs) to Hib polysaccharide (PS) capsule, a linear polymer of ribosyl ribitol phosphate. The structure of anti-Hib-PS Abs has been well characterized at the molecular level; about two-thirds of anti-Hib-PS Abs use a V kappa gene named A2, and the remaining anti-Hib-PS Abs use one of many other VL genes. In order to understand the structural basis for the variability in the function of these Abs, we prepared 18 clonally pure Abs from adults and studied their affinity, avidity, bactericidal potency in vitro, and ability to reduce bacteremia in newborn rats. Affinities and avidities were determined as the inverse of the concentrations of short (3 repeating units) and long (20 repeating units) ligands which could bind 50% of anti-Hib-PS Ab in solution, respectively. No significant correlations between the protection of newborn rats and affinity (r = 0.02) or avidity (r = 0.16) were observed. The amount of Ab required to kill 50% of bacteria in vitro decreased with avidity (r = -0.32), as expected. However, Abs with high affinity were unexpectedly found to have less bactericidal activity (r = 0.38). This suggests that avidity may be a better predictor of Ab function than affinity. Affinity and avidity results were negatively correlated (r = 0.76, P = 0.0022), and Abs that had A2 V kappa gene products had higher avidity (P < 0.05) and lower affinity (P = 0.06) than Abs that had other VL genes. A possible explanation of these observations is that the epitope for Abs with the A2 gene is within the Hib-PS chain itself, whereas the epitope for Abs with a non-A2 gene is the terminus of Hib-PS.

Haemophilus influenzae type b (Hib) is a significant pathogen for young children, and three Hib vaccines (named PRP-OMPC, HbOC, and PRP-T) are currently available for young children. Extensive studies of anti-Hib polysaccharide (PS) antibodies (Abs) have shown that the V regions of Abs against the Hib PS comprise a VH gene in the VH3 gene family and a VL gene from various K kappa and V lambda subgroups. To study immunogenic properties of the three vaccines in young children, we determined the VL subgroups and avidities of anti-Hib-PS Abs induced by the three clinically available conjugate vaccines. Ab avidity was measured by determining the concentration of a Hib-PS oligomer that abrogates half of the binding of immunoglobulin G anti-Hib-PS Abs to microwells. The PRP-OMPC vaccine induced lower-avidity Abs than the prelicensure HbOC vaccine (P = 0.05). When we compared anti-Hib-PS Abs expressing V kappa Ia, V kappa II, and V lambda subgroups, a greater Ab response was induced by the prelicensure HbOC vaccine than other vaccines (P < 0.05). When anti-Hib-PS Abs with the V kappa III subgroup were compared, however, both PRP-T and prelicensure HbOC vaccines induced a comparable response, which in turn was greater than those induced by the PRP-OMPC or the postlicensure HbOC vaccine (P < 0.001). The VL repertoire of Abs induced with the prelicensure HbOC or PRP-T vaccine in young children is dominated (about 80%) by anti-Hib-PS Abs using subgroup V kappa II. However, anti-Hib-PS using V kappa II VL accounts for only about 40% of the total anti-Hib-PS Abs induced with the PRP-OMPC vaccine or the postlicensure HbOC. Our data suggest that immunogenic properties of Hib vaccines in young children vary depending on the vaccine preparations as well as the vaccine types.

Nonhuman primates are often used as a model for studying vaccines for humans. However, it is not always clear how closely the antibody responses in these species mimic human responses. Recent studies have characterized the human antibody response to Haemophilus influenzae type b (Hib) in great detail. In this study, we have compared the antibody response to Hib of humans with those of other primates. Studies of isoelectric points and V kappa subgroup usage show that, like humans, nonhuman primates produce oligoclonal antibodies. Also, monkey antibodies to the Hib polysaccharide are as protective as human antibodies in an in vivo model of Hib infection. Thus, we conclude that nonhuman primates produce antibodies to Hib polysaccharide that are structurally and functionally similar to human antibodies and are a good model for testing human vaccines.