FUNCTIONAL CAPACITIES OF CLONAL ANTIBODIES TO HAEMOPHILUS-INFLUENZAE TYPE-B POLYSACCHARIDE

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 phosp hate. 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-K gen e named A2, acid the remaining anti-Hib-PS Abs use one of many other V L genes. In order to understand the structural basis for the variabili ty in the function of these Abs, we prepared 18 clonally pure Abs from adults and studied their affinity, avidity, bactericidal potency in v itro, and ability to reduce bacteremia in newborn rats, Affinities and avidities were determined as the inverse of the concentrations of sho rt (3 repeating units) and long (20 repeating units) ligands which cou ld bind 50% of anti-Hib-PS Ab in solution, respectively, No significan t 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 t o have less bactericidal activity (r = 0.38). This suggests that avidi ty may be a better predictor of Ab function than affinity. Affinity an d avidity results were negatively correlated (r = 0.76, P = 0.0022), a nd Abs that had A2 V-K gene products had higher avidity (P < 0.05) and lower affinity (P = 0.06) than Abs that had other VL genes. A possibl e explanation of these observations is that the epitope for Abs with t he 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.