Expansion of a B-lymphocyte clone producing IgM auto-antibodies encoded by a somatically mutated VHI gene in the spleen of an autoimmune patient

Abnormalities of B cell subsets in patients with systemic lupus erythematosus

The prototypic autoimmune disease, SLE, is known to be associated with polyclonal B cell hyperreactivity. Developing an understanding of the complex nature of human B cell differentiation, largely through the application of multiparameter flow cytometry to an analysis of circulating B cells has permitted an assessment of whether specific stages of B cell maturation are affected by the tendency for polyclonal B cell activation. Moreover, the analysis of perturbations of the specific stages of B cell maturation has generated new information on whether abnormalities in B cell differentiation are primarily involved in autoimmune disease immunopathology or, rather, are secondary to the inflammatory environment characteristic of subjects with this autoimmune disease. Multivariant analysis has begun to document abnormalities in B cell maturation that are primarily associated with lupus, or, alternatively related to disease duration, disease activity and concomitant medication. Together, these analyses have provided new insights on the role of B cell over-reactivity in SLE.

B cells in autoimmunity

B-cell development is tightly regulated, including the induction of B-cell memory and antibody-secreting plasmablasts and plasma cells. In the last decade, we have expanded our understanding of effector functions of B cells as well as their roles in human autoimmune diseases. The current review addresses the role of certain stages of B-cell development as well as plasmablasts/plasma cells in immune regulation under normal and autoimmune conditions with particular emphasis on systemic lupus erythematosus. Based on preclinical and clinical data, B cells have emerged increasingly as both effector cells as well as cells with immunoregulatory potential.

Platelet-reactive IgG antibodies cloned by phage display and panning with IVIG from three patients with autoimmune thrombocytopenia

Autoimmune thrombocytopenic purpura (AITP) is a severe disease in children with a still unknown aetiology. It is not known why AITP can either be transient and self limiting or become chronic. The beneficial use of intravenous immunoglobulins (IVIG) in certain groups of AITP patients has been proven. It is, however, not clear how IVIG functions. To analyse patient-derived monoclonal IgG platelet autoantibodies that interact with IVIG in an anti-idiotypic manner, the combinatorial antibody phage display system was applied. From three different patients a large number of clones specifically reacting with IVIG molecules were derived. Many of these IVIG binders also reacted strongly with platelets in ELISA and FACS, in contrast to IVIG binders derived from a healthy individual. The heavy and light chain variable regions were sequenced and compared with each other and with databases. In all three AITP patients clones with a striking complementarity-determining region (CDR) sequence homology to each other and to many of the known anti-platelet antibodies were observed. Selected Fab-phages representing the characteristic variable regions that occurred in the investigated patients with AITP may now be used to clone potentially regulatory anti-idiotypes from healthy donors by phage display.

Possible presence of enhancing antibodies in idiopathic thrombocytopenic purpura

It is difficult to detect IgG anti-platelet autoantibodies in idiopathic thrombocytopenic purpura (ITP). Recently, it was reported that reactivity with glycoprotein IIb/IIIa was lost when IgG anti-GPIIb/IIIa antibodies from seven ITP patients were digested with pepsin to yield F(ab')2 fragments. These findings suggested that some IgG antiplatelet autoantibodies in ITP may be of low affinity and thus require the presence of 'enhancing' anti-IgG antibodies (i.e. rheumatoid factors, RFs) for detection. To test this hypothesis, we used a phage display technique to isolate five IgG RFs from an ITP patient (patient 1). Sequence analysis revealed that these RFs consisted of two clones, represented by GG3 and GG48. Both representative RFs bound specifically to IgG Fc fragments with apparent dissociation constants of 8.2 x 10(-8) M and 8.8 x 10(-7) M, respectively. Moreover, IgG RFs were subsequently found in a serum sample from patient 1. Combined, these results suggest that IgG RFs may occur in ITP, and may be required for the detection of some IgG anti-platelet autoantibodies and for the corresponding antibody-mediated platelet destruction in autoimmune ITP.

Expression of monovalent fragments derived from a human IgM autoantibody in E. coli. The input of the somatically mutated CDR1/CDR2 and of the CDR3 into antigen binding specificity

A hybridoma producing a polyspecific human monoclonal IgM antibody (named CB03) has been derived from a fusion of mouse myeloma cells with human spleen lymphocytes obtained from an autoimmune patient suffering from chronic idiopathic thrombocytopenia. The antibody was found to be encoded by somatically mutated VHI and VlambdaIII genes. To study the input of mutated complementarity regions (CDRs) into antibody specificity, the antigen binding features of the purified complete IgM antibody were compared with (i) a Fab fragment by hot tryptic digestion and (ii) recombinant monovalent fragments expressed in E. coli. In detail, vectors were constructed encoding for (i) rFab03 and single chain Fv03 fragments containing the VH and VL genes connected by a linker sequence, (ii) scFc1.1. fragments containing the VH germline equivalent and the CB03 wild-type CDR3 region, and (iii) scFv fragments containing the CDR1 and CDR2 in germline configuration and the CDR3 expressed in the CB253 human fetal B cell hybridoma producing a polyspecific IgM antibody. The expression vectors contained at the 3' end either a (His)6 motif allowing purification on Ni(2+)-agarose or a c-myc tag for specifically detecting the expression products by a murine monoclonal antibody. Western blotting and ELISA analyses of the expression products indicate: (i) recombinant Fab fragments were found in the bacterial periplasm in extremely low amounts (1-10 micrograms from 1 litre bacterial culture), (ii) scFv fragments were obtained in suitable amounts from bacterial periplasm (800-1000 micrograms/l), (iii) the monovalent recombinant fragments as well as the Fab obtained by tryptic digestion reflected the polyspecific antigen binding features of the complete IgM antibody, but did bind to the antigens with much lower affinity, and (iv) the CDR3 was found to be of critical importance for the antigen binding pattern of this particular IgM. We discuss the expression of recombinant scFv fragments in E. coli as a suitable method in studying the role of the somatic mutation in autoantibody generation.

Anti-DNA and anti-platelet specificities of SLE-derived autoantibodies: evidence for CDR2H mutations and CDR3H motifs

Although polyreactivity appears to be a characteristic feature of natural autoantibodies, polyreactive anti-DNA autoantibodies can be derived both from patients with autoimmune disease and from normal individuals. It is unclear whether these autoantibodies differ depending on their origin, but previous studies from our laboratory have suggested that polyreactive systemic lupus erythematosus (SLE)-derived platelet-binding anti-DNA autoantibodies have more restricted antigen reactivity and greater functional activity than normal-derived polyreactive autoantibodies. The objective of the present study was to characterize the VH and VL region sequences of 10 human hybridoma anti-DNA autoantibodies derived from peripheral blood lymphocytes of different origins [SLE, rheumatoid arthritis (RA), or normal] to determine whether there are structural differences between these autoantibodies. We show that although some unmutated germline structures (VH and VL) are represented, these are not restricted to anti-DNA autoantibodies from normal individuals and that two normal-derived anti-DNA antibodies showed quite extensively mutated VH genes. However, these mutations, unlike those found in the CDR2H of several of the SLE-derived antibodies, did not appear to be antigen-selected. Three different amino acid motifs, putatively involved in antigen binding specificity, were observed in the CDR3H segments of some of the autoantibodies. One was the previously described YYGSG motif, which was found in a normal-derived anti-DNA autoantibody, while two new potential motifs were observed only in SLE-derived platelet-binding anti-DNA autoantibodies. These data suggest that antigenic and functional differences between SLE-derived and normal-derived platelet-binding anti-DNA autoantibodies may be due to antigen-selected mutations in the CDR2H and specific amino acid motifs in the CDR3H.

Tumour cell binding by a human monoclonal IgM antibody from the spleen of a non-tumour-associated patient is due to somatic mutations in the VH gene

Recently we described the occurrence of B cells producing polyspecific natural IgM with anti-tumour specificity in the spleen of non-tumour-bearing individuals as well as in fetal organisms. Immunoprecipitation and 2-D electrophoresis showed the binding of such antibodies to a 55-kD (pI 6.0) membrane surface glycoprotein. In vitro cultivation of human cancer cell lines in the presence of the purified IgM antibodies resulted in growth inhibition and complement-mediated cell lysis. Furthermore, the antibodies were shown to be able to induce MHC class I molecule expression on tumour cells. Because of this, a role for naturally occurring antibodies with anti-tumour specificity in preventing neoplasias had been suggested. We have constructed and expressed in Escherichia coli single-chain fragments (scFv: VH-linker-VL) derived from a polyspecific human monoclonal IgM autoantibody produced by a human x mouse heterohybridoma which was obtained from the spleen of an autoimmune patient. The mutated complementarity determining region (CDR) gene segments were replaced by the equivalent germ-line sequences and the CDR3 region was swapped for that from another polyspecific human natural antibody with no binding to tumours. Using these four scFv constructs for binding analyses and in vitro cultivation experiments we found: (i) scFv containing the mutated VH region of the original antibody were able to bind to tumour cells, to induce MHC class I molecule expression, and to inhibit tumour growth in a way similar to what had been described for the complete antibody; (ii) replacement of the mutated by the germ-line VH gene independently of the CDR3 to which it had been recombined, resulted in failure to bind to tumour cells. Nevertheless, other antigens (ssDNA, tetanus toxin) were still recognized, although with lower affinity. We discuss the significance of the replacement mutations in the VH gene CDRs, selected probably by B cell contact to an (auto)antigen, for generating a tumour binding capacity, not encoded by the germ-line gene.