Cross-reactive monoclonal antibodies for diagnosis of pneumococcal meningitis

Capsule does not block antibody binding to PspA, a surface virulence protein of Streptococcus pneumoniae

Of the proteins on the surface of Streptococcus pneumoniae, one of those best able to elicit protection against pneumococcal infection is pneumococcal surface protein A (PspA). Although this protein is attached to the membrane molecule, lipoteichoic acid, which is well beneath the capsule, PspA's ability to inhibit complement deposition and killing by apolactoferrin, suggests that it must have surface exposure. This study provides quantitative data showing that the capsular polysaccharide on types 2 and 3 pneumococci provides little or no masking ability of antibodies to bind PspA. Capsule was even observed to enhance, rather than inhibit the binding of two protective monoclonal antibodies to their epitopes on cell surface PspA. These results with antibodies to PspA are in contrast to binding by antibodies to the phosphocholine (PC) epitope of the lipoteichoic and teichoic acids. The binding of antibody to PC was largely, but not completely, blocked by capsular polysaccharide.

Pneumococcal diversity: considerations for new vaccine strategies with emphasis on pneumococcal surface protein A (PspA)

Streptococcus pneumoniae is a problematic infectious agent, whose seriousness to human health has been underscored by the recent rise in the frequency of isolation of multidrug-resistant strains. Pneumococcal pneumonia in the elderly is common and often fatal. Young children in the developing world are at significant risk for fatal pneumococcal respiratory disease, while in the developed world otitis media in children results in substantial economic costs. Immunocompromised patients are extremely susceptible to pneumococcal infection. With 90 different capsular types thus far described, the diversity of pneumococci contributes to the challenges of preventing and treating S. pneumoniae infections. The current capsular polysaccharide vaccine is not recommended for use in children younger than 2 years and is not fully effective in the elderly. Therefore, innovative vaccine strategies to protect against this agent are needed. Given the immunogenic nature of S. pneumoniae proteins, these molecules are being investigated as potential vaccine candidates. Pneumococcal surface protein A (PspA) has been evaluated for its ability to elicit protection against S. pneumoniae infection in mouse models of systemic and local disease. This review focuses on immune system responsiveness to PspA and the ability of PspA to elicit cross-protection against heterologous strains. These parameters will be critical to the design of broadly protective pneumococcal vaccines.

Diagnosis of Streptococcus pneumoniae pneumonia by quantitative enzyme linked immunosorbent assay of C-polysaccharide antigen

AIMS

To evaluate the use of a quantitative enzyme linked immunosorbent assay (ELISA) detecting C-polysaccharide (PnC) antigen in sputum for the diagnosis of Streptococcus pneumoniae infection.

METHODS

Specimens of sputum from 60 patients with acute community and hospital acquired pneumonia and infective exacerbations of obstructive airways disease were examined by semiquantitative culture and antigen ELISA.

RESULTS

Using a cutoff value of 1 microgram/ml PnC antigen for a positive result, the sensitivity of this assay was 90.3%, specificity 93.1%, predictive value of a positive result was 93.5%, and the predictive value of a negative result 89.6%.

CONCLUSIONS

Quantitation of C-polysaccharide antigen in sputum by ELISA distinguishes between carriage of oral bacteria which express PnC-like antigen and infection with S pneumoniae and compares favourably with other diagnostic methods.

Quantitation of pneumococcal C polysaccharide in sputum samples from patients with presumptive pneumococcal pneumonia by enzyme immunoassay

Although the Gram stain and culture of expectorated sputum are considered standard methods for the diagnosis of presumptive pneumococcal pneumonia, these methods remain relatively insensitive and nonspecific. We developed an enzyme immunoassay (EIA) for the quantitation of pneumococcal C polysaccharide (PnC) in the sputum of patients with presumptive pneumococcal pneumonia. Of 34 patient sputum samples collected within 24 h of the first radiographic report of pneumonia, 12 grew Streptococcus pneumoniae on culture. By using a cutoff point of 0.5 micrograms of PnC per ml of sputum, all 12 specimens were positive (sensitivity, 100%) by EIA. PnC levels ranged from 1.43 to 57.53 micrograms/ml. Blood samples from 18 of the 34 patients were cultured. S. pneumoniae grew in the culture of a blood sample from one patient, whose sputum also had the highest PnC level. Of 22 sputum samples from patients with pneumonia that did not grow S. pneumonia, two were positive by EIA (specificity, 90.1%). Sputa from both patients had low levels of PnC (2.7 and 4.5 micrograms/ml), and both patients had received antibiotics before sputum collection. The positive predictive value of the quantitative EIA was 85.7%. Quantitation of PnC has the potential for improving the accuracy of sputum examination for S. pneumoniae, monitoring disease severity and the effectiveness of antibiotic therapy, and differentiating between those patients with invasive pneumococcal disease and those who are carriers of S. pneumoniae.

Monoclonal antibodies--their diagnostic potential

Considering the long and excellent performance of 'classical' immune sera in a range of diagnostic applications, the advent of 'tailor-made' highly specific monoclonal antibodies has given even higher hopes of diagnostic accuracy for the future. Understanding what monoclonal antibodies are and how they compare to polyclonal sources of antibodies is absolutely essential in order to appreciate their potential. This paper will briefly focus on the production, characterization and use of monoclonal antibodies and look critically at the future prospects. Specificity and cross-reactivity are key terms which are explained with examples in this paper. It can be anticipated that diagnostic kits in the future will be composite assays using highly specific monoclonals (or oligoclonal cocktails) in conjunction with high-avidity polyclonal sera, thus taking advantage of the best features of both systems.