Isolation and characterization of human polyreactive pneumococcal polysaccharide antibodies

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.

Homeostasis and regulation of autoreactive B cells

In contrast to the previous belief that autoreactive B cells are eliminated from the normal repertoire of B cells,many autoreactive B cells actually escape clonal deletion and develop into mature B cells.These autoreactive B cells in healthy individuals perform some beneficial functions in the host and are homeostatically regulated by regulatory T and B cells or other mechanisms to prevent autoimmune diseases.Autoreactive B-1 cells constitutively produce polyreactive natural antibodies for tissue homeostasis.Recently,autoreactive follicular B cells were reported to participate actively in the germinal center reaction.Furthermore,the selection and usefulness of autoreactive marginal zone(MZ)B cells found in autoimmune diseases are not well understood,although the repertoire of MZ B-cell receptors(BCRs)is presumed to be biased to detect bacterial antigens.In this review,we discuss the autoreactive B-cell populations among all three major B-cell subsets and their regulation in immune responses and diseases.