The molecular basis for a polyspecific antibody
Following the seminal work of Landsteiner and colleagues [1], who demonstrated that specific antiserum could be raised to different haptens, it was generally agreed that antibodies were specific for the immunizing antigen. This view has subsequently been strengthened by the advent of highly specific mouse monoclonal antibodies. Contrary to this point of view, were reports of myeloma proteins, the MoAb produced by human B cell lymphomas, which were able to recognize multiple antigens [2]. Such 'polyreactive' antibodies have also been subsequently described in healthy individuals or unimmunized animals [3,4]. Limiting dilution analysis following polyclonal activation and transformation of human peripheral blood B lymphocytes with Epstein-Barr virus (EBV), revealed that between 3% and 13% of the IgM-secreting clones produced antibodies reactive against multiple autoantigens including ssDNA, IgG Fc, thyroglobulin or insulin [5]. These antibodies also recognize protein and carbohydrate antigens on bacteria and viruses. Whilst many of these antibodies are of the IgM isotype, IgG and IgA polyreactive antibodies can also be readily detected in the serum of normal healthy animals and man. The affinities of polyreactive antibodies tend to be low, with a dissociation constant (Kd) of between 1 x 10"' and 1 X 10~̂ M, though some have a Kd within the range expected for monospecific antibodies, i.e. 1 x 10" -1 x 10~ M. However, since many of these polyspecific antibodies are IgM, existing as pentamers, the actual avidity may be significantly higher. Sequence analysis of the variable (V) regions of polyreactive antibodies has revealed that they are often in germ-line configuration, without N-nucleotide insertions at the VH-DJH junctional regions, and have limited or absent point mutations at the complementarity determining regions (CDR). This suggests that these antibodies have not undergone the antigendriven somatic mutations which are associated with achieving high-affinity antibodies. Analysis of the B lymphocytes producing these antibodies identified CD5, CD45RA'° as a marker for these cells, which constitute up to 25% of the circulating B lymphocyte population [6]. Additionally, CD5"CD45RA' ° B lymphocytes have also been shown to produce polyspecific antibodies, and these two populations have been described as Bl lymphocytes, to distinguish them from the B2 lymphocytes which are CD5" CD45RA''' and tend to secrete high-affinity monospecific antibodies following contact with antigen.