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Summary. Background: The development of an inhibitor is the major complication facing patients with hemophilia A treated by administration of factor (F) VIII concentrates. Restoration of tolerance to FVIII can be achieved by prolonged administration of FVIII (immune tolerance induction, ITI). Although ITI has been used for more than 30 years in patients with hemophilia A and inhibitor, its mechanism of action is still poorly understood. Objectives: As administration of high doses of antigen can induce the apoptosis of the T cells recognizing the antigen, a potential mechanism of action of ITI may be the deletion of FVIII-specific T cells. Patients/Methods: We studied the CD4+ T-cell response to FVIII in five (one mild, one moderate and three severe) patients successfully desensitized by administration of FVIII and in control subjects. Results: Following repeated stimulation with autologous dendritic cells loaded with FVIII, FVIII-specific T oligoclonal cell lines were expanded from the blood of one of the successfully desensitized patients. The FVIII-specific T cells produced IL-5, IL-13 and IL-2. By contrast, FVIII-specific T-cell lines could not be derived from three patients with mild hemophilia A without inhibitor or from four normal control subjects. Conclusions: These data represent the first analysis of the cellular mechanisms regulating the induction of tolerance to FVIII. They demonstrate that successful tolerance induction may occur without deletion of FVIII-specific T cells.

BACKGROUND: The inhibitory activity of an anti-factor VIII (FVIII) antibody can be modulated through glycosylation of the antigen binding site, as has recently been described. This offers the opportunity to develop an optimized anticoagulant agent targeting partial FVIII inhibition. OBJECTIVES: We investigated in non-human primates the antithrombotic activity, pharmacokinetics,and pharmacodynamics of a human monoclonal antibody, Mab-LE2E9Q, inhibiting FVIII activity partially. METHODS: The ability of Mab-LE2E9Q to prevent thrombosis was evaluated in baboons after administration of 1.25 and 5 mg kg(-1) antibody or saline as a single intravenous (i.v.) bolus. Thrombus development was recorded in expansion ('venous') and in Dacron ('arterial') thrombosis chambers incorporated in an extracorporeal arteriovenous shunt implanted between the femoral vessels 1 h, 24 h and 7 days after the administration of Mab-LE2E9Q. RESULTS: Mab-LE2E9Q reduced thrombus growth to a similar extend 1 h, 1 day and 1 week after administration of the antibody. Ex vivo pharmacodynamic analysis indicated that the evaluation of the residual FVIII activity was strongly dependent on the type of FVIII assay and on the phospholipid concentration in the assay. No significant difference in bleedings was observed between animals treated with Mab-LE2E9Q or with saline. CONCLUSIONS: Understanding the role of glycosylation in FVIII inhibition by a human monoclonal antibody allowed selection of an antibody inhibiting only moderately FVIII activity while significantly reducing thrombus development in a baboon extracorporeal model. As that antibody did not increase the bleeding tendency, it may represent a novel type of a long-acting antithrombotic agent with an optimal safety/efficacy profile. ispartof: Journal of Thrombosis and Haemostasis vol:7 issue:3 pages:429-437 ispartof: location:England status: published

This study describes the long-term follow-up of the clinical response of 10 adult patients suffering from atopic dermatitis (AD) who were treated by administration of complexes made of Dermatophagoides pteronyssinus (Dpt) allergens and autologous antibodies specific to that allergen. We have already described the clinical improvement observed after 1 year of treatment involving regular injections of complexes; this improvement was maintained throughout a second year, even though the number of injections was greatly reduced. At the end of the second year of treatment, 5 patients were completely free of disease, and 3 had had a short-lasting recurrence of low-severity dermatitis. Using a disease intensity index the mean improvement for these 8 patients was 83% compared to baseline values. One patient showed a significant recurrence of symptoms, and 1 patient left the study for personal reasons when she was in good clinical condition. A significant reduction of specific anti-Dpt IgE antibody titers was observed in 7 out of the 8 patients in clinical remission, while the level of total IgE antibodies was unchanged until the very end of the study. This study not only confirms that clinical benefit can be obtained from the treatment of AD patients hypersensitive to Dpt by injections of allergen-antibody complexes but also indicates that the therapy induces a suppression of IgE antibody production that is specific for the particular allergen.

Background: The cysteine peptidase activity of group 1 house dust mite allergens is important for their allergenicity and may offer new therapeutic targets for allergy treatment. Hitherto, the design of specific inhibitors has been impeded because the availability of pure, fully active allergens has limited the implementation of drug screening campaigns. Similarly, investigation of the mechanisms by which peptidase allergens promote sensitization has also been restricted. Our aim was to compare the enzymology of recombinant and native forms of Der p 1 to establish if an easily expressed recombinant form of Der p 1 could be used as a drug discovery tool. Methods: Enzymatic activity of natural and recombinant Der p 1 was compared fluorimetrically using a novel specific substrate (ADZ 50,059) and a novel specific active site titrant (ADZ 50,000). The effect of recombinant Der p 1 prodomain on the catalytic activity of both Der p 1 preparations was also examined. Results: Although differing substantially in molecular weight, the enzymological properties of recombinant and native Der p 1 were indistinguishable. Our data show clearly by experiment that, in contrast to some suggestions, Der p 1 is not an enzyme of bifunctional mechanism. Conclusion: The catalytic activity of Der p 1 is tolerant of glycosylation differences that occur at N150 when the protein is expressed in Pichia pastoris. This suggests that this recombinant protein may be suitable for drug design studies and in the elucidation of how peptidase activity promotes sensitization to peptidase and nonpeptidase bystander allergens.

We have prepared antigen-antibody complexes from grass pollen allergens and autologous specific antibodies isolated by immunoadsorption from the serum of allergic patients. These complexes were inoculated into patients in a double-blind trial to evaluate their effect on grass pollen-related rhinitis and bronchial asthma. Thirty-eight grass pollen-hypersensitive patients were allocated to three groups; patients in the first two groups were treated with antigen-antibody complexes at different ratios and dosages and were compared with the third group who received the placebo carrier buffer alone. In addition, we treated a fourth group who had already received antigen-antibody complex inoculation during the previous pollen season. Injections were given every 2 weeks during the pollen season, starting 5 weeks prior to it. Tolerance was excellent with no signs of local or systemic side effects. The treatment prevented nasal symptoms while enabling the patients to reduce antihistamine intake. Bronchial asthma was virtually absent in the treated groups even though no bronchodilators or corticosteroids had to be taken. Specific IgE antibodies did not increase during the pollen season nor did IgG "blocking" antibodies. Inoculation of allergen-antibody complexes could provide a valuable alternative for the treatment of immediate hypersensitivity to airborne allergens as it appears to be safe and rapidly efficacious. This treatment offers several advantages compared to conventional hyposensitization and is characterized by the absence of an increase in specific IgG antibodies.

Mutations responsible for mild/moderate hemophilia A were extensively characterized over the last 15 years and more than 200 mutations have been identified. However, most of the molecular mechanisms responsible for the reduced factor (F)VIII levels in patients' plasma were determined only recently. Recent progresses in the study of the FVIII molecule three-dimensional structure provided a major insight for understanding molecular events leading to mild/moderate hemophilia A. This allowed prediction of mutations impairing FVIII folding and intracellular processing, which result in reduced FVIII secretion. Mutations potentially slowing down FVIII activation by thrombin were also identified. A number of mutations were also predicted to result in altered stability of activated FVIII. Biochemical analyses allowed identification of mutations reducing FVIII production. Mutations impairing FVIII stability in plasma, by reducing FVIII binding to von Willebrand factor (VWF) were also characterized. Defects in FVIII activity, notably slow activation by thrombin, or abnormal interaction with FIXa, were also recently demonstrated. Biochemical analysis of FVIII variants provided information regarding the structure/function relationship of the FVIII molecule and validated predictions of the three-dimensional structure of the molecule. These observations also contributed to explain the discrepant activities recorded for some FVIII variants using different types of FVIII assays. Altogether, the study of the biochemical properties of FVIII variants and the evaluation of the effects of mutations in three-dimensional models of FVIII identified molecular mechanisms potentially explaining reduced FVIII levels for a majority of patients with mild/moderate hemophilia A. It is expected that these studies will improve diagnosis and treatment of this disease.

A Severe Combined Immunodeficient (SCID) mouse model has been established to evaluate experimental conditions leading to the production of factor VIII (FVIII) autoantibodies. To this end, we humanized 10 groups of 7 mice with peripheral blood mononuclear cells of 10 unrelated healthy blood donors (15 x 10(6) cells/mouse). Mice were injected with saline or immunized i.p. with 50 IU of a plasma derived human FVIII 24 h after reconstitution. Further immunization was made with 25 IU of FVIII every fortnight during 6 weeks and animals were sacrificed after 8 weeks. All reconstituted mice showed a spontaneous production of anti-FVIII antibodies in the absence of immunization with the corresponding antigen. However, no differences were observed regarding the quantity or the quality of these antibodies produced in the immunized or the saline group, indicating that tolerance to FVIII had been transferred with cell reconstitution. Affinity purified FVIII specific antibodies were capable of inhibiting FVIII activity and preventing the binding of FVIII to phospholipids in a dose-dependent manner. Immunoprecipitation experiments showed that the antibodies recognized only the C1 and C2 light chain domains. Since antibodies of interest can be found in the SCID mouse model and, moreover, since they are qualitatively comparable with the source donor's antibodies, this model provides a tool to study the regulation of tolerance against self antigens in normal subjects and in acquired haemophilia patients.

Peptides containing major T cell epitopes have the capacity to induce T cell anergy and have therefore been proposed for the treatment of allergic and autoimmune diseases. Such peptides should not be immunogenic, i. e. should not contain a B cell recognition site. We have evaluated in BALB/c mice the therapeutic potential of a 15-mer peptide (p21 - 35) derived from Der p2, a major allergen of the house dust mite Dermatophagoides pteronyssinus, which contains a dominant T cell epitope but is not recognized by antibodies to Der p2. Unexpectedly, p21 - 35 elicited strong immune responses, suggesting the presence of a cryptic B cell epitope. Similar results were obtained with mice of three additional MHC haplotypes. A core sequence of four amino acids (Ile-Ile-His-Arg) corresponding to residues 28 - 31 was shared by the B and T cell epitopes. Critical residues for B cell recognition were Arg31 and Lys33, while Ile28 was essential for T cell recognition. A Lys33Ala mutant of p21 - 35 still activated T cells but had much reduced immunogenic properties, making it a suitable alternative peptide for T cell anergy induction. Careful investigation of the immunogenic potential of peptides used to induce T cell anergy should be carried out prior to their clinical application.

The development of an immune response towards factor VIII (fVIII) remains a major complication for hemophilia A patients receiving fVIII infusions. The design of a specific therapy to restore unresponsiveness to fVIII has been hampered by the diversity of the anti-fVIII antibody. Molecular analysis of the specific immune response is therefore required. To this end, we have characterized an fVIII-specific human IgG4kappa monoclonal antibody (BO2C11) produced by a cell line derived from the memory B-cell repertoire of a hemophilia A patient with inhibitor. BO2C11 recognizes the C2 domain of fVIII and inhibits its binding to both von Willebrand factor (vWF) and phospholipids. It completely inhibits the procoagulant activity of native and activated fVIII, with a specific activity of approximately 7,000 Bethesda units/mg. vWF reduces the rate of fVIII inactivation by BO2C11. The antibody-fVIII association rate constant (kass approximately 7.4 x 10(5) M-1 s-1) is eightfold lower than that for vWF-fVIII association, whereas its dissociation rate constant (kdissor = 1 x 10(-5) s-1) is 100-fold lower than that for the vWF-fVIII complex, which suggests that BO2C11 almost irreversibly neutralizes fVIII after its dissociation from vWF. BO2C11 is the first human monoclonal anti-fVIII IgG antibody that has been isolated and allows the study of fVIII inactivation at the molecular level.

Antibodies to factor VIII (inhibitors) are usually produced at the beginning of treatment with factor VIII and are rare in multitransfused patients. Such antibodies are deemed to be patient-related, as supported by the description of a number of associated risk factors. However, a second category of inhibitors has recently been identified, namely antibodies occurring in multitransfused patients as a result of exposure to a particular factor VIII concentrate. A first outbreak of product-related inhibitors was recently described. The present paper describes the second well-documented occurrence of such inhibitors. Eight out of 140 multitransfused patients with severe haemophilia A developed an inhibitor to factor VIII shortly after changing treatment to a double-virus inactivated plasma-derived factor VIII concentrate. In addition to solvent-detergent treatment, this concentrate was pasteurised at 63 degrees C for 10 hours. Exposure to the pasteurised product before inhibitor detection ranged from 9 to 45 days. Inhibitor titers varied between 2.2 and 60 Bethesda Units and recovery of transfused factor VIII ranged from 0.21 to 0.68 (expressed as i.u./dl factor VIII rise per i.u./kg administered). In contrast to usual inhibitors in haemophilia A patients, these product-related inhibitors showed complex inhibition kinetics. They were found specific for the factor VIII light chain. The inhibitors gradually declined when exposure to the pasteurised product was stopped, despite further treatment with other factor VIII concentrates. The present data stress the importance of carefully monitored clinical studies, both in previously treated and previously untreated patients, before introduction of a new or modified clotting factor concentrate.

Symptoms of atopic dermatitis (AD) can be provoked by exposure to airborne allergens. We have previously shown that patients hypersensitive to D. pteronyssinus (Dpt) allergens were improved by administration of complexes composed of specific antibodies and allergen, which reduce the allergen-specific immune response. We now report that similar results can be achieved by using F(ab')2 fragments of specific antibodies instead of whole antibody molecules. Eight adult patients with severe AD were included in a single-blind study. During the first 11 months patients were maintained on injections of carrier buffer alone, in an effort to evaluate the extent of spontaneous improvement. They were then treated with intradermal injections of allergen-F(ab')2 complexes made from autologous specific antibodies and Dpt allergens. The majority of the patients improved spontaneously during the summer months, with an average 30% reduction of symptoms. However, a much more pronounced improvement was observed after 3 months on active therapy, corresponding to a cumulative amount of 60 micrograms F(ab')2 and 15 micrograms allergens. The patients continued to improve over the next 5 months, showing an average 83% reduction of severity scores. The use of F(ab')2 antibody fragments reduces the risk of inducing an anti-allotypic immune response, and raises the possibility of adding adjuvants to allergen-antibody complexes and/or using specific antibodies isolated from pooled gammaglobulins.

Several approaches have recently been put forward describing attempts to suppress the IgE immune response towards allergens, which is thought to be the key event in allergic diseases. In a series of clinical trials we have shown that injections of complexes made up from allergen and specific antibodies are an effective treatment for allergic bronchial asthma and atopic dermatitis. In the work presented here we have examined the humoral immunity changes associated with the use of such complexes in a group of 19 adult patients suffering from atopic dermatitis and hypersensitive to Dermatophagoides pteronyssinus (Dp), and in whom a significant clinical improvement was observed. By comparing serum samples taken prior to and after 4 months of therapy, we show that the administration of immune complexes is associated with: (i) a significant and selective reduction of IgG and IgE antibodies specific for Dp allergens; (ii) a down-regulation that affects only the antibodies present in the complexes; (iii) the induction of corresponding anti-idiotypic antibodies. To our knowledge, this is the first demonstration in humans that an anti-allergen antibody response can be down-regulated in a highly selective manner and that this is accompanied by significant clinical improvement. Moreover, the selective reduction of IgG antibodies could be of value in the treatment of some forms of auto-immune diseases.

A significant proportion of hemophilia A patients receiving transfusions of factor VIII (FVIII) develop a specific antibody response towards FVIII. These antibodies are usually detected by assays in which they inhibit the function of the molecule, such as the Bethesda clotting test. We have prepared anti-FVIII antibodies by specific immunoadsorption from the plasma of four hemophiliacs with stable inhibitor levels. The isotypic distribution of such antibodies was determined and their capacity to bind to insolubilized FVIII was compared with their inhibitory activity in two functional assays, namely, the Bethesda assay and a chromogenic assay. In addition, the FVIII epitope specificity was determined by competition with monoclonal antibodies for the binding to insolubilized FVIII. We show here that (1) anti-FVIII antibodies are not isotypically restricted; thus, a significant proportion of specific IgG2 was found; (2) antibodies are frequently directed towards epitopes of FVIII that are not directly involved in the function of the molecule and therefore escape detection in the Bethesda method or chromogenic assay; and (3) each patient shows a unique pattern of FVIII epitope recognition. We conclude that evaluation of anti-FVIII antibodies by a functional method does not provide an accurate evaluation of the specific antibody response. These findings have important implications for the comparison of the immunogenicity of FVIII molecules produced by different technologies and for the development of methods to control anti-FVIII antibody production.

Twenty-three adult patients suffering from chronic atopic dermatitis (AD) have been treated by regular injections of complexes made of D. pteronyssinus allergens and specific autologous antibodies. A double-blind placebo-controlled protocol was followed for 4 months, then the patients were treated openly to complete a full year on active therapy. Preliminary results are presented for the first 8 months. Seventy-three percent of patients improved when treated with complexes, showing a mean improvement of more than 70% after 4 months. This study suggests that injections of allergen-antibody complexes is an effective treatment of at least some forms of AD and confirms that airborne allergens are significant exacerbating factors of AD.

Atopic dermatitis (AD) can be exacerbated by contact with airborne allergens, amongst which Dermatophagoides pteronyssinus (Dpt) appears to be potentially important. Specific IgE antibodies towards Dpt are often found in AD and it can therefore be speculated that suppression of the production of anti-Dpt IgE might result in a significant clinical improvement. Complexes of antigen and specific antibodies have been shown to suppress the production of antibody in other systems; we report here the evaluation in an open trial of the capacity of such complexes to improve symptoms of AD. Ten adult patients were enrolled in this study. In addition to satisfying the criteria of AD, they all suffered from a severe disease (more than 20% of the body surface involved) that had been stable for at least the last 2 years. The patients had high titers of total IgE antibodies and specific anti-Dpt antibodies. Allergen-antibody complexes were prepared from Dpt allergens and an excess of autologous specific anti-Dpt antibodies obtained by immunoadsorption. The patients received regular injections of these complexes throughout 1 year, during which clinical parameters of disease intensity, percentage of body surface affected and intensity of pruritus were regularly monitored. A significant clinical improvement was obtained after 3–4 months of therapy and was maintained through the 9th month. After 1 year of treatment 2 patients were completely free of disease 4 had residual lesions which continued to improve and 4 patients had a partial recurrence of dermatitis. In addition to the demonstration that injection of antigen-antibody complexes results in a clinical benefit, these results suggest that the immune response towards Dpt is a relevant factor in AD.

The effectiveness and safety of injections of complexes made of specific anti-allergen antibodies and allergen was confirmed in a double-blind placebo-controlled trial with adult patients suffering from atopic dermatitis.

Specific antibodies isolated by immunoabsorption on four main insolubilized allergens from Dermatophagoides pteronyssinus (DPT) had the following isotypic distribution: in 16 atopic patients, 52% IgG, 40% IgM, 8% IgA, 0.1% IgE and, in 12 non-atopic individuals, 48% IgG, 46% IgM, 6% IgA, 0.03% IgE. The ratios between geometric means of antibody values in each class (atopic vs non-atopics) were 2.4 for IgG, 2.0 for IgM, 2.8 for IgA and 66.7 for IgE. The amount of anti-DPT antibodies in IgG subclasses did not follow the usual distribution of total IgG subclasses, i.e., IgG1 greater than IgG2 greater than IgG3 greater than IgG4. In atopics the order was IgG2 greater than IgG1 greater than IgG4 greater than IgG3 and in non-atopics, IgG4 greater than IgG1 = IgG2 greater than IgG3 although 6/12 of the latter had no detectable (less than 0.5 micrograms/ml plasma) IgG4.

Activation of the classical pathway of human complement by monoclonal IgE from patient DES was demonstrated by using IgE(DES) coupled to latex particles. This material depleted human serum of C1 and C4 hemolytic activities. In addition, C3bi was deposited in a calcium-dependent way onto the insolubilized IgE as shown by the agglutination of latex by conglutinin. The alternative pathway was also activated. These anticomplementary activities were dose and time dependent. Moreover, we confirmed that another monoclonal IgE, IgE(PS), activated the alternative pathway exclusively. Particular attention was paid to exclude contamination by other immunoglobulins or C-reactive protein, generation of artifacts due to the chemical coupling, and the presence of proteolytic enzymes in the IgE(DES) preparation. Moreover, evidence is also presented against the involvement of IgG or IgM anti-IgE autoantibodies that could activate the classical pathway after their binding to insolubilized IgE(DES). Although one cannot exclude the possibility that IgE(DES) or IgE(PS) are abnormal proteins, these findings suggest the existence of an isotypic or allotypic variation of IgE.

Activation of the classical pathway of human complement by monoclonal IgE from patient DES was demonstrated by using IgE(DES) coupled to latex particles. This material depleted human serum of C1 and C4 hemolytic activities. In addition, C3bi was deposited in a calcium-dependent way onto the insolubilized IgE as shown by the agglutination of latex by conglutinin. The alternative pathway was also activated. These anticomplementary activities were dose and time dependent. Moreover, we confirmed that another monoclonal IgE, IgE(PS), activated the alternative pathway exclusively. Particular attention was paid to exclude contamination by other immunoglobulins or C-reactive protein, generation of artifacts due to the chemical coupling, and the presence of proteolytic enzymes in the IgE(DES) preparation. Moreover, evidence is also presented against the involvement of IgG or IgM anti-IgE autoantibodies that could activate the classical pathway after their binding to insolubilized IgE(DES). Although one cannot exclude the possibility that IgE(DES) or IgE(PS) are abnormal proteins, these findings suggest the existence of an isotypic or allotypic variation of IgE.

Type A chrysotile fibres (white asbestos) were tested in vitro for activation of the complement system. Fibres were incubated in normal human serum (NHS), factor B-depleted human serum, and normal and C4-deficient guinea-pig sera; the supernates were assayed for the remaining complement activity. Activation of the alternative pathway (AP) was shown in three ways. First, quantitative measurement of factor B; second, kinetic analysis of rabbit red blood cell lysis in whole alternative pathway (AP) and factor B lytic assays; third, qualitative measurement of C3 and factor B conversion by crossed immunoelectrophoresis. No C3 convertase activity could be demonstrated on the fibres but other possible mechanisms of AP activation are discussed. Magnesium itself is not responsible for this activation because acid-treated fibres retain this property. The early classical pathway is not involved as shown by normal whole complement activity of a factor B-depleted human serum and the absence of decrease of C4 functional activity. Knowing that complement proteins are present in pulmonary alveoli, mainly provided by cell synthesis, we suggest that complement activation in vivo may be relevant to the genesis of the chronic inflammation and fibrosis in the lung.

From the serum of 10 allergic subjects we have prepared IgG antibodies recognizing idiotopes carried by specific antibodies to Dermatophagoides pteronyssinus (Dpt) allergens, and studied cross-reactivity of anti-Dpt IgG bystander and antigen-binding site-associated idiotopes by latex agglutination assays. Idiotopes of specific anti-Dpt IgE were evaluated by radioimmunoassays. Depending on the assay, a binding or inhibition of more than 50%, as compared to the reactivity of specific antibodies with the corresponding anti-idiotypic (anti-Id) IgG, was considered significant. Cross-reactivity of antigen-binding site-associated idiotopes attained a mean proportion of 6/10 for IgG and 9.6/10 for IgE. By contrast, bystander idiotopes cross-reacted only occasionally with a mean proportion of 2/10 for both IgG and IgE antibodies. Anti-Id antibodies from two subjects have been isolated by adsorption on insolubilized anti-Dpt antibodies of the corresponding patient. Using this purified material we have confirmed that (a) the majority of anti-Id antibodies carry an "internal image" of the initial antigen and compete in a dose-dependent manner with Dpt allergens for the binding to the anti-Dpt antibodies and (b) paratope-associated idiotopes of anti-Dpt antibodies are shared by unrelated individuals.

IgG isolated from the plasma of seven individuals hypersensitive to the common house dust mite Dermatophagoides pteronyssinus (DPT) was exhaustively adsorbed onto insolubilized DPT. The unbound fraction was found by radioimmunoassay to contain antibodies recognizing the variable region of both anti-DPT IgG and IgE antibodies. This recognition was idiotype (Id)-specific as it persisted after passage over insolubilized polyclonal IgG of unrelated specificity. Most of these anti-Id IgG carried the internal image of the initial antigen in that they competitively inhibited the binding of anti-DPT antibodies to DPT. Immunoadsorption of anti-Id IgG onto insolubilized anti-DPT IgG antibodies from the same individual completely eliminated their reaction with anti-DPT IgG but not with anti-DPT IgE, suggesting that idiotopes included in the antigen-binding site of specific IgG and IgE antibodies were not identical. Anti-Id IgG recognizing idiotopes located outside the antigen-binding site (bystander idiotopes) were also completely removed by passage over insolubilized anti-DPT IgG; in this case the reaction of the anti-Id IgG with both anti-DPT IgG and anti-DPT IgE was inhibited, indicating that, for a given individual, bystander idiotopes were shared between anti-DPT antibodies pertaining to these two isotypes.

Two brothers with ataxia-telangiectasia and recurrent respiratory tract infections are presented. Immunological investigations showed normal immunoglobulin levels (except for the absence of IgA in one child) with absence of haemagglutination inhibiting antibody response to inactivated influenza virus vaccine, normal total lymphocyte counts with decreased T cell numbers, absence of delayed hypersensitivity skin reaction to PPD, in vitro unresponsiveness of T lymphocytes to PHA with presence of a serum inhibitor able to depress normal lymphocyte transformation, and normal phagocytosis and killing of Staph. aureus by peripheral blood polymorphonuclear leucocytes. Complete complement screening revealed a non-functioning alternative pathway in both brothers. The presence of normal factor B lytic activity after isoelectrofocusing the serum and inhibition of the lytic activity of purified components with the patients' serum raised the possibility of a serum inhibitor. The nature of this was not elucidated but it may be one of the reasons why patients with ataxia-telangiectasia have an increased incidence of infections.