Purification of staphylococcal protein A using immunosorbents

The gene for staphylococcal protein A

Protein A from Staphylococcus aureus has become an important tool in immunology and molecular biology due to its specific binding to the constant region of immunoglobulins (Igs) from most mammalian species(1). Many qualitative and quantitative techniques have been developed which take advantage of this 'pseudo-immune' reaction(2). In addition, solid state protein A has recently been introduced in medical therapy to decrease the amount of circulating immune complexes in sera(3). In this article Mathias Uhlén, Martin Lindberg and Lennart Philipson describe the structure of the protein A molecule and its gene. They also discuss the possibilities for fusing the protein A gene to other genes.

Human immunoglobulin A (IgA)-specific ligands from combinatorial engineering of protein A

Affinity reagents capable of selective recognition of the different human immunoglobulin isotypes are important detection and purification tools in biotechnology. Here we describe the development and characterization of affinity proteins (affibodies) showing selective binding to human IgA. From protein libraries constructed by combinatorial mutagenesis of a 58-amino-acid, three-helix bundle domain derived from the IgG-binding staphylococcal protein A, variants showing IgA binding were selected by using phage display technology and IgA monoclonal antibodies (myeloma) as target molecules. Characterization of selected clones by biosensor technology showed that five out of eight investigated affibody variants were capable of IgA binding, with dissociation constants (K(d)) in the range between 0.5 and 3 microm. One variant (Z(IgA1)) showing the strongest binding affinity was further analyzed, and showed that human IgA subclasses (IgA(1) and IgA(2)) as well as secretory IgA were recognized with similar efficiencies. No detectable cross-reactivity towards human IgG, IgM, IgD or IgE was observed. The potential use of the Z(IgA1) affibody as a ligand in affinity chromatography applications was first demonstrated by selective recovery of IgA protein from a spiked Escherichia coli total cell lysate, using an affinity column containing a divalent head-to-tail Z(IgA1) affibody dimer construct as a ligand. In addition, efficient affinity recovery of IgA from unconditioned human plasma was also demonstrated.

Purification of genus-specific chlamydial antigen and its separation into several components by ion-exchange chromatography

Sodium deoxycholate-extracted genus-specific chlamydial antigen was purified from contaminating substances by ion-exchange chromatography with DEAE-sephacel, resulting in a decrease in the complement-fixing activity of the antigen, whereas the enzyme-linked immunosorbent assay activity increased. By successive elution with 0.5, 1.0, and 1.5 M acetate buffer at least three clearly separated components were consistently recovered in 14 trials. The identity of these components as genus-specific chlamydial antigen was demonstrated in enzyme-linked immunosorbent assay tests with specific antisera. The antigenic activity of these components was not diminished by prior treatment of the chlamydial particles with pronase. Antiserum prepared by immunization of rabbits with the antigenic component I of an egg-propagated antigen reacted predominantly with the antigenic components I and II of a cell culture-propagated antigen.

Identifying immunoglobulin isotypes, allotypes and idiotypes by a hemolytic assay in gel

We developed a hemolytic radial immunodiffusion assay for identifying immunoglobulin (Ig) isotypes, allotypes and idiotypes by using gels containing erythrocytes coated with anti-Ig antibody or erythrocytes coated with Staphylococcal protein A. These indicator cells lysed specifically when treated sequentially with Ig antigen, the appropriate anti-Ig antiserum (developer) and complement. To identify these Ig subpopulations, we used monospecific indicator cells, e.g. erythrocytes coated with antibody specific for an Ig isotype, and developers with broader specificities ('multispecific'), e.g. antiserum to Fab. Alternatively, we used 'multispecific' indicator cells, e.g. erythrocytes coated with antibody to Fab and monospecific developers, e.g. antiserum to Ig idiotype. To identify Ig subpopulations specifically, either the indicator cells or the developer need to be monospecific. When both the indicator cells and the developer were monospecific, e.g. to allotype and to isotype, the specificity was determined by both reagents and ultimately restricted by the reagent with the narrower specificity, that is, reacting with the smallest Ig subpopulation. This sensitive hemolytic assay may be used to quantitate subpopulations of Ig molecules and may be modified into a reverse plaque forming cell assay to count lymphocytes secreting a given Ig class, type, allotype and idiotype.

Single-tube mixed agglutination test for the detection of staphylococcal protein A

A simple, rapid mixed agglutination test using sheep erythrocytes (SRBC) sensitized with rabbit hemolysin and intact viable staphylococci is described for the detection of bound staphylococcal protein A. Soluble protein A was heat extracted from 50 clinical isolates as well as the Cowan I and Wood 46 strains of Staphylococcus aureus and titered by a hemagglutination test using sensitized SRBC and dilutions of soluble protein A. Protein A could be detected in all of these supernatants including that of S. aureus Wood 46, a strain generally considered to be protein A negative. These organisms were later retested by the mixed agglutination test and even those staphylococcal isolates expressing very low heat-extractable soluble protein A concentrations (1:2 titers) were positive, confirming the sensitivity of the test. In a screen of clinical isolates, only 4 of 235 (1.8%) coagulase-positive isolates were negative in the mixed agglutination test. Of 25 coagulase-negative isolates, none yielded a positive reaction.

Purification of immunolobulin G Fc-reactive factor from Streptococcus azgazardah

A factor from Streptococcus azgazardah (Lancefield's group C) capable of reacting with the Fc fragment of human IgG was partially purified and characterized. The bacteria were lysed by infection with phage, and the IgG Fc-reactive factor was purified from the crude lysate by affinity chromatography on a IgG-coupled Sepharose 4B column. The column was eluted with 3 M KSCN and the eluate gel-filtered on a Sephadex G 75 column. A 30-fold purification was obtained. The molecular weight of the IgG Fc-reactive factor was estimated to be 60,000. The factor was sensitive to trypsin treatment and to heating 95 degrees C, 10 min at pH 2.0. On polyacrylamide electrophoresis only one band was obtained. The trypsin-and heat-sensitivity, and the estimated molecular weight of the factor was similar to an IgG Fc-reactive factor preparation obtained from group A, type M 56 streptococci by heat treatment at 120 degrees C for 30 min.