Abstract
Natural antibodies serve as the body’s first line of defense against pneumococcal challenge. Polyreactive human pneumococcal polysaccharide IgG antibodies have not been extensively studied. We analyzed human polyreactive antibodies that bind multiple pneumococcal polysaccharides, including PPS14 and PPS23F. These antibodies were isolated from single pneumococcal polysaccharide specific B cells allowing for the analysis of human immunoglobulins with natively paired variable regions. Although isolated individually, these antibodies demonstrated similar characteristics. Most antibodies possessed a variable light chain with a CDR3 length made up of nine amino acids and relatively high number of flexible amino acids in combined VH/VL. While these antibodies were polyreactive and structurally alike, kinetic analysis revealed unique KD values. Variable chains are responsible for antigen recognition whereas antibody fine specificity is affected by isotype structure. To investigate the contribution of the constant region of these isotypes and their effect on antibody avidity to pneumococcal polysaccharide, the polyreactive variable regions were expressed as IgG1 or IgG2 and subjected to kinetic analysis. The IgG1 antibodies uniformly had a stronger avidity to PPS14 and PPS23F compared to IgG2. To further document the importance of the constant region in antibody avidity and fine specificity, analysis of antibody F(ab)’2 fragment binding to PPS14 and PPS23F resulted in similar KD values. These studies suggest that antigen recognition by polyreactive antibodies is determined by a conserved variable light chain CDR3 length and longer, more flexible variable heavy CDR3s when compared to pneumococcal polysaccha-ride-specific sequences while differences in specific avidities are modulated by antibody isotype.
Highlights
Streptococcus pneumoniae is a human bacterial pathogen that colonizes the nasopharynx and is a major cause of pneumonia, meningitis and acute otitis media [1,2]
The expressed immunoglobulins were tested in pneumococcal polysaccharide (PPS)-specific ELISA
All immunoglobulins were absorbed with cell wall polysaccharide (CWPS) and PPS22F as described in Materials and Methods
Summary
Streptococcus pneumoniae is a human bacterial pathogen that colonizes the nasopharynx and is a major cause of pneumonia, meningitis and acute otitis media [1,2]. Antibodies against pneumococcal polysaccharide (PPS) provide protection against disease. Natural or nonspecific antibodies are believed to provide a first line of defense against pneumococcal invasion. These antibodies are part of the early phase of the immune reaction and protect against infection and/or decrease the bacterial load if infection is already present [4]. Little is known about inherent human nonspecific PPS antibodies. It is thought that human polyreactive antibodies are selected into the marginal zone B cell compartment, as demonstrated in immunoglobulin (Ig) transgenic mice [5,6,7,8]
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