Making sense of serotype-specific pneumococcal antibody measurements

Abstract

In 2012, a specialist group of the American Academy of Allergy, Asthma & Immunology (AAAI) collaborated to produce guidelines on the use and interpretation of diagnostic vaccination in primary immunodeficiency. Within this document there is extensive evaluation of the use of serotype-specific pneumococcal serotype testing for diagnosis of immune deficiency. A titre of 1.3 mg/mL is reported to confer protection for each serotype. This concentration has generated much discussion: in the UK these tests are only available in a few centres and all use 0.35 mg/mL as a putative protective titre. In an attempt to help clarify this discrepancy, this article summarizes the literature describing the development of these assays. Although widely quoted, a ‘protective titre’ of 1.3 mg/mL may not have a sound evidence base. The evolution of assays to measure antibodies to serotype specific pneumococcus, particularly the unit of measurement conversions, has played a significant part in the confusion. A sensitive radioimmunoassay was developed by Schiffman et al. in 1980 which measured IgG antibodies to C polysaccharide generating antibodies in units of ng antibody nitrogen/mL. Later enzyme-linked immunosorbent assays (ELISAs) were developed which were more sensitive but also generated results in ng antibody nitrogen/mL. In 1995, the World Health Organisation (WHO) produced reference and calibration sera to standardize assays. The first ELISAs showed poor correlation of antibodies with vaccine efficacy studies. This was due to the presence of C polysaccharide and 22F. Antibodies to C polysaccharide, although naturally occurring in humans, are not opsonic and therefore do not protect against pneumococcal infection; antibodies to serotype 22F are cross reactive to a number of other serotypes. Reabsorbing sera with C polysaccharide and 22F led to the development of 2nd and 3rd generation assays respectively with increased specificity for measuring protective antibodies; these assays gave results in mg/ mL. From this point onwards there was a divergence in interpretation of assay results depending on special interests such as whether the assays were used to assess vaccination response from a public health perspective or to compare base line and postvaccination response to help diagnose T-cell independent antibody deficiency. The WHO approved the ELISA for monitoring response to newly produced vaccines. WHO published data showing that pneumococcal conjugated vaccines should induce titres of 0.35mg/mL or above four weeks after vaccination to confer protection against invasive pneumococcal disease. This putative protective level was based on a 2nd generation ELISA. When reassessed on a 3rd generation (more sensitive) ELISA only a small decline in putative protection level was found (0.32 mg/mL); this meta-analysis was based on three clinical efficacy trials with in excess of 60,000 control and vaccinated infants. A decision to stick to the original value (for ease) of 0.35mg/mL was made. Although derived from data in children, this level of protection has been extrapolated for adult use. Development and interpretation of ELISA results in the adult immune deficiency world took a different approach. Landesman and Schiffman used the radioimmunoassay to assess pneumococcal antibody concentration with systemic pneumococcal infection >250– 300 ng antibody nitrogen/mL for adults. This figure