Humoral response in spontaneous and experimental autoimmunity to the ribosomal P proteins

Humoral Epitope Spreading in Autoimmune Bullous Diseases

Autoimmune blistering diseases are characterized by autoantibodies against structural adhesion proteins of the skin and mucous membranes. Extensive characterization of their autoantibody targets has improved understanding of pathogenesis and laid the basis for the study of antigens/epitopes diversification, a process termed epitope spreading (ES). In this review, we have reported and discussed ES phenomena in autoimmune bullous diseases and underlined their functional role in disease pathogenesis. A functional ES has been proposed: (1) in bullous pemphigoid patients and correlates with the initial phase of the disease, (2) in pemphigus vulgaris patients with mucosal involvement during the clinical transition to a mucocutaneous form, (3) in endemic pemphigus foliaceus, underlining its role in disease pathogenesis, and (4) in numerous cases of disease transition associated with an intermolecular diversification of immune response. All these findings could give useful information to better understand autoimmune disease pathogenesis and to design antigen/epitope specific therapeutic approaches.

Humoral Antigenic Targets of the Ribosomal P0 Lupus Autoantigen Are Not Limited to the Carboxyl Region

Autoantibodies binding the ribosomal P phosphoproteins are highly specific for systemic lupus erythematosus (SLE) and can be found in precipitating levels in approximately 15% of these patients. Anti-ribosomal P antibodies are directed against three proteins, and the primary autoimmune target of this response has been described as a common 22-amino acid sequence. Sera from 31 anti-ribosomal P immunodiffusion-positive SLE patients were tested for C-terminal P-peptide reactivity by ELISA. Sera from three patients (9.7%) were negative for the peptide ELISA, despite having anti-ribosomal P by immunodiffusion and Western blot. In addition, inhibition experiments showed that the common P-peptide response accounts for a variable amount of anti-ribosomal P0 reactivity (52-89% of the response dependent upon the patient serum). Based upon these findings, fine-specificity sequential humoral epitope mapping of ribosomal P0 was performed. Several common sequential antigenic targets were defined dispersed throughout the molecule. The most commonly targeted epitopes included RDMLLANKVPAAARA (amino acids 99-113, 11 of 12 patients reactive) and QALGITTKISRGT (amino acids 139-151, 9 of 12 patients reactive). This study confirms that the P22 ribosomal peptide is commonly targeted in SLE and accounts for a variable percentage of the anti-ribosomal P response. Additional anti-ribosomal P humoral epitopes are described.

Production and characterization of a human monoclonal anti-idiotype to anti-ribosomal P antibodies

Autoantibodies to ribosomal P protein (anti-P) are a specific hallmark of systemic lupus erythematous (SLE). Several authors found significant associations of anti-P antibodies with neuropsychiatric, hepatic, and renal disease. We now report the isolation by phage display of human anti-idiotype (Id) monoclonal antibody fragments as single-chain Fv fragment (scFv) against anti-P antibodies. The V gene repertoires were derived from the RNA obtained from the B cells of a SLE patient. Affinity-purified anti-P antibodies were used for the selection of bacterial clones producing anti-P-specific scFv antibody fragments and little reactivity with normal IgG and other IgG antibodies. The anti-Id antibody recognizes a public idiotope broadly cross-reactive with polyclonal anti-P antibodies and inhibited binding of anti-P to ribosomal P antigen in immunoassays and on Jurkat cells. The anti-Id scFv antibody fragment may have therapeutic implications in SLE. They may also be used as probes in the study of the structure of the idiotype.

Evaluation of assays for the detection of autoantibodies to the ribosomal P proteins

Sera from systemic lupus erythematosus patients that had antibodies to the ribosomal P proteins were compared in several different assays. The enzyme-linked immunosorbent assay (ELISA) method was compared to the Western immunoblotting method using either affinity purified human or bovine ribosomal P proteins. All 30 normal sera had no significant reactivity with these antigens. The most sensitive test was the ELISA using the human P protein, where 31/32 patients were positive (97%). The assay with bovine proteins in ELISA yielded 28/32 (88%) positive results. Immunoblotting with either bovine or human P protein was equally effective with 30/32 (94%) positive. An ELISA incorporating human P proteins is a more sensitive assay for clinical diagnosis than an ELISA with the bovine protein. Immunoblotting is a sensitive method, but is less convenient and is not quantitative. The ELISA with the human protein appears to be the method of choice.

Heterogeneity of anti-dsDNA antibodies in their cross-reaction with ribosomal P protein

We have investigated the possible cross-reaction of anti-dsDNA antibodies with ribosomal P peptide for several reasons. First, the antibodies frequently occur together, and secondly, they vary similarly with disease activity. Human polyclonal anti-dsDNA antibodies were affinity purified from eight patients and anti-ribosomal P antibodies from two patients with systemic Lupus erythematosus (SLE) who had high titers of anti-dsDNA as well as anti-ribosomal P antibodies. Nine of the 10 sera were totally specific in their reactivity with their cognate antigens. In only one patient did we find a subpopulation of antibodies which cross-reacted with both dsDNA and the carboxyl terminal 22 amino acid peptide. Our results indicate that anti-dsDNA antibodies are heterogeneous and usually do not cross-react with the carboxyl terminal P peptide, but on occasion (1/10) a patient will produce anti-dsDNA antibodies cross-reactive with the carboxyl terminal P peptide.

Determinant spreading: lessons from animal models and human disease

Spreading of the immune response is a common theme in organ-specific and systemic autoimmune diseases. We evaluated whether some of the mixed antinuclear antibody patterns characteristic of systemic autoimmunity might be the result of determinant spreading from a single initiating event. Immunisation of healthy mice with individual protein components of the La/Ro ribonucleoprotein (RNP) targeted in systemic lupus erythematosus and primary Sjögren's syndrome induced autoantibodies recognising Ro60 (SS-A), Ro52 (SS-A) and La (SS-B) and in some cases the molecular chaperones calreticulin and Grp78. The endogenous antigen(s) driving determinant spreading might be derived from physiological apoptosis which could explain the involvement of some chaperone proteins in the autoimmune response. Diversified anti-La/Ro antibody responses were initiated by challenge with a single subdominant T epitope of La even though some self epitopes of La were efficiently tolerised. The pattern of autoantibody responses in primary Sjögren's syndrome was strongly influenced by HLA class II phenotype which we speculate controls activation of T cells recognising defined peptides from the La/Ro RNP. In this way, HLA class II alleles may be critical in influencing initiation and spreading of systemic autoimmune reactions. Molecular mimicry of such determinants by exogenous agents might readily initiate spreading of an autoimmune response in genetically susceptible hosts.

Rapid detection of anticardiolipin antibodies

A rapid screening method for the detection of antiphospholipid antibodies is described. Dense, red dyed polystyrene beads coated with cardiolipin were incubated with test sera for a short period of time, then added to a microtube containing anti-human IgG in a gel provided within a pre-cast card (DiaMed ID Microtyping System). The card was centrifuged at 150g for 5 min and then examined for movement of the beads through the gel. Beads without bound antibody travelled through the gel and formed a pellet on the bottom of the tube. Anti-human IgG within the gel matrix impeded cardiolipin-coated beads when antiphospholipid antibodies bound to the beads. Positivity was indicated by the formation of a layer of beads on the top of the gel matrix. Prospective analysis of 103 samples for the presence of antiphospholipid antibodies by flow cytometry and the gel-card technique showed good correlation between the two methods. All samples found to be positive by flow cytometry (23 of 103) were identified as positive by the gel-card technique. Two samples were identified as positive by the gel-card method but negative by flow cytometry. The technique is simple to perform and should prove useful as a rapid screening method for the detection of antiphospholipid antibodies.