Purpose: The quality of an antibody response is determined by both the concentration and the strength of antigen-binding, or avidity, of the antibodies produced. Currently, only antibody concentration is routinely evaluated in the clinical assessment of humoral immunity. Here we studied correlations of avidities and concentrations of antibodies to pneumococcal polysaccharides with immunologic and clinical characteristics of patients with recurrent infections.
Methods: We measured concentration and avidity of antibodies to 12 pneumococcal serotypes in 78 children aged 0.6-18 years with recurrent bacterial respiratory infections, and in 80 individuals who were being tested for peanut allergy, ages 0.4-15 years, serving as a comparison group. Avidity was assessed by measuring antibody binding in the presence of thiocyanate.
Results: Antibody concentrations and avidities correlated positively for very few types contained in the conjugated pneumococcal vaccine (PCV7) in both patients and controls with some dependence on age; there were even fewer correlations for non-PCV7 types. Antibody concentrations and avidities negatively correlated with age for most of the PCV7 types. There was no consistent correlation of total IgG or IgG subclasses with either concentrations or avidities. Overall, antibody concentrations were higher and avidities were lower in patients compared to controls. Patients requiring chronic antibiotic use tended to have higher antibody concentrations and lower avidities for most serotypes than patients who did not. We identified several patients having many infections with apparent good antibody concentrations with low avidity for many types.
Conclusion: Antibody concentration and avidity correlate with patient clinical characteristics and distinguish patients from controls. Measurement of antibody avidity may provide another dimension for the clinical assessment of pneumococcal polysaccharide antibody response.