CD21int CD23+ IgM+ mouse follicular B cells comprise the bulk of the mature B-cell compartment, but it is not known whether these cells contribute to the humoral antibody response. We show using a direct RT-PCR method for antigen-specific VH, that FACS-sorted mouse CD21int CD23+ B cells express specific secretory IgM VH transcripts in response to immunization and also exhibit a memory response. The secretory IgM expressed is distinct from the IgG expressed by cells of this phenotype, which we also analyse here, having a distinct broader distribution of CDR-H3 sequences and zero or low levels of somatic mutation in the region analysed. These results imply that cells of the CD21int CD23+ phenotype have distinct IgM+ and IgG+ populations that contribute directly to the humoral antibody and memory responses by expressing antigen-specific secretory immunoglobulin. We also argue that the more diverse CDR-H3 sequences expressed by antigen-experienced IgM+ CD21int CD23+ follicular B cells would place them at the bottom of a recently hypothesized memory B-cell hierarchy.

During maturing antibody responses the increase in affinity for target antigens is achieved by genetic diversification of antibody genes followed by selection for improved binding. The effect this process has on the specificity of antibody for variants of the antigen is not well-defined, despite the potential role of antibody diversification in generating enhanced protection against pathogen escape mutants, or novel specificities after vaccination. To investigate this, a library of single amino-acid substitution epitope variants has been screened with serum obtained at different time-points after immunization of mice with the HIV gp41 peptide epitope ELDKWA. The serum IgG response is shown to mature and increase affinity for ELDKWA, and the titre and affinity of IgG against most epitope variants tested increases. Furthermore there is a bias towards high affinity serum IgG binding to variant epitopes with conservative substitutions, although underlying this trend there is also significant binding to many epitopes with non-conservative substitutions. Thus, maturation of the antibody response to a single epitope results in a broadening of the high-affinity response toward variant epitopes. This implies that many pathogen epitope escape variants that could manifest as single amino-acid substitutions would not emerge by escaping immune surveillance.

Development and maintenance of cells in the murine follicular and marginal zone compartments is thought to involve differing levels of stimulation of the BCR, although it is still not clear which BCR ligands mediate these events. How the delineation between naive and Ag experienced B cell populations relates to cell phenotype and how precise or blurred this delineation is, is also not well understood. In this study, using PCR to analyze the Ab response to phenyl-oxazolone in the mouse, we show that the Ab repertoire of CD21(hi)/CD23(-) marginal zone B cells shows persistent increase in levels of particular IgM after immunization with foreign Ag. Further, we show that these IgMs have different but related VH/CDR3 sequences from those seen in the class-switched response to oxazolone that we have also analyzed. We also detect an effect of Ag on the follicular B cell repertoire that is less persisting. These results provide evidence consistent with the signal-strength model of mature B cell development being extended to include stimulation by foreign Ag, and also further the known zone of influence of foreign Ag on the B cell compartment.

BACKGROUND: X-linked severe combined immunodeficiency (SCID-X1) is caused by mutations in the common cytokine-receptor gamma chain (gamma(c)), resulting in disruption of development of T lymphocytes and natural-killer cells. B-lymphocyte function is also intrinsically compromised. Allogeneic bone-marrow transplantation is successful if HLA-matched family donors are available, but HLA-mismatched procedures are associated with substantial morbidity and mortality. We investigated the application of somatic gene therapy by use of a gibbon-ape-leukaemia-virus pseudotyped gammaretroviral vector. METHODS: Four children with SCID-X1 were enrolled. Autologous CD34-positive haemopoietic bone-marrow stem cells were transduced ex vivo and returned to the patients without preceding cytoreductive chemotherapy. The patients were monitored for integration and expression of the gamma(c) vector and for functional immunological recovery. FINDINGS: all patients have shown substantial improvements in clinical and immunological features, and prophylactic medication could be withdrawn in two. No serious adverse events have been recorded. T cells responded normally to mitogenic and antigenic stimuli, and the T-cell-receptor (TCR) repertoire was highly diverse. Where assessable, humoral immunity, in terms of antibody production, was also restored and associated with increasing rates of somatic mutation in immunoglobulin genes. INTERPRETATION: Gene therapy for SCID-X1 is a highly effective strategy for restoration of functional cellular and humoral immunity.

Immune reconstitution after conventional allogeneic transplantation is a major determinant of survival. We conducted a detailed investigation of T- and B-cell immune reconstitution and clinical outcome in 19 patients with multiple myeloma undergoing reduced-intensity stem cell transplantation using in vivo T-cell depletion with alemtuzumab. These patients experienced delayed T-cell recovery, particularly in the naïve (CD45 RA+) CD4 compartment. T-cell receptor spectratype analysis showed a reduced repertoire diversity, which improved rapidly after the administration of donor leucocyte infusions and subsequent conversion to full donor T-cell chimaerism. Post-transplant recovery of CD19+ B cells was also delayed for up to 18 months. Spectratype analysis of IgH CDR3 repertoire revealed a gradual normalization in IgM spectratype complexity by 6-12 months after transplant. There was a high incidence of viral infection, particularly cytomegalovirus reactivation, but the regimen-related mortality was low, perhaps because of the very low incidence of acute graft-versus-host disease (GVHD; grade I-II skin GVHD was seen in 5/19 patients). Over 80% of all patients have relapsed at a median of 283 (range 153-895) d after transplant, suggesting that the initially low rate of GVHD comes at a high price with regard to the desired graft-versus-myeloma effect.

We have studied immune reconstitution in a patient with paediatric-onset polyarteritis nodosa treated with high-dose immunosuppressive agents followed by stem cell rescue. The patient developed several new autoimmune phenomena over the 18 months after immunosuppression and stem cell rescue. Flow cytometry, reverse transcription-polymerase chain reaction (RT-PCR) heteroduplex and isotype-specific RT-PCR analysis of immunoglobulin expression showed that the T- and B-cell repertoires were highly restricted in the first few months after treatment. The dominant T-cell clones seen after reconstitution were persistently expanded, were different from those which could be demonstrated before autologous stem cell transplantation, and were in the CD8(+) population. Our data also show that 12 months after treatment these expanded T-cell clones were within the CD45RA(+) population, suggesting that reversion from the CD45RO(+) to the CD45RA(+) phenotype had occurred in vivo.

The distribution of immunoglobulin (Ig) isotypes within specific B cell clones in vivo after immunization is not well defined. Using an IgV(H)/CDR3- and isotype-specific reverse transcription polymerase chain reaction method, we have carried out a survey of the diversification of the isotype in a splenic response to phenyl-oxazolone (phOx) on a chicken serum albumin carrier. The phOx-specific V(H) (V(H)Ox-1 with specific CDR3 motif) is associated with all of the heavy chains (mu, delta, alpha, gamma, and epsilon) after simple immunization with antigen in alum. The kinetics of expression of each isotype are distinct and reproducible. Focusing mainly on the expression of secretory Ig transcripts, IgM, IgG1, and IgE are found after priming, whereas IgD and IgA appear after boosting. Secretory IgD transcripts are found reproducibly at moderate levels and may, therefore, contribute significantly to the secreted Ig response in mice. Most crucially, we find enhanced levels of secretory IgM/V(H)Ox-1 transcripts (with 'phOx-specific' CDR3) after boosting, strongly indicating the existence of IgM memory cells that give rise to an enhanced specific IgM secretion in the secondary response.

Pneumococcal serotype-specific anti-capsular polysaccharide antibodies protect against invasive pneumococcal disease. Within an individual the diversity of these antibodies is limited. To evaluate the repertoire of antibodies to pneumococcus and determine whether oligoclonality is seen both between serotypes and between individuals, we sampled the B cell repertoire induced by polysaccharide and conjugate vaccine in adult volunteers. Fifteen hybridomas secreting pneumococcus-specific monoclonal antibodies were generated from five volunteers. Ten were isotype switched, six were IgG2 and four were IgA. These included two isotype switch variants of the same clone. V(H)3 and V(kappa)2 were used by 10/15 and 7/13 of the sequenced clones, respectively, with identical genes, V(H)3-48 and V(kappa)2-A17 used by a number of volunteers to a variety of serotypes. VDJ junctional characteristics and complementarity-determining region (CDR) 3 length were variable. High levels of somatic mutation in CDR1 and 2, inconsistent with a primary response, were found in 10/11 of the isotype-switched antibodies, including those induced by plain polysaccharide antigens. These data suggest that wild-type infection or nasopharyngeal carriage of Streptococcus pneumoniae in adults may induce memory and the response to subsequent immunization with plain polysaccharide or conjugate pneumococcal vaccines may have the characteristics of a secondary response.

The cytokine receptor common gamma chain mutation in X-linked SCID results in a failure of T and NK cell development and an as yet undefined defect of B cells. Using immunoglobulin isotype-specific reverse transcription-PCR we show that although hematopoietic stem cell transplantation restores a diverse repertoire of class-switched B cell clones, on further analysis these are almost all of donor origin. This suggests that host B cells, which predominate after unconditioned transplantation, are still defective even in the presence of normal T cells. These studies imply that effective humoral reconstitution can only be achieved by the engraftment of normal donor B cells.

Here we describe a method for fingerprinting the mouse immunoglobulin heavy chain variable region (VH) repertoire. Using a novel combination of existing techniques, large numbers of expressed VH genes can be simultaneously displayed as a fingerprint of VH gene fragments on a sequencing gel. This is achieved using isotype-specific reverse transcription-PCR amplification, restriction digestion and end-labeled primer run-offs. This technique (Res-PCR) allows analysis of the immune response, in this case to phenyloxazolone, in several different tissues and enables molecular cloning and sequencing of the VH genes involved in vivo. In addition, with Res-PCR, a "global" picture of expressed immunoglobulin genes is represented. This elucidates B cell clonality in different tissues and isotypes and detects a preference for shorter complementarity-determining region 3 in IgM-expressing cells. Res-PCR is a rapid and revealing method which will allow analysis of the complexity and sequence composition of the B cell immunoglobulin repertoire and so perhaps better define the B cell memory compartment and immune responses in vivo.