Specificity and Function of T- and B-Cell Recognition in Tuberculosis. In: Bloom BR, editor. Tuberculosis. Washington: ASM Press; 1994. pp. 437-58. [DOI: 10.1128/9781555818357.ch26]

Function and Potentials of M. tuberculosis Epitopes

Study of the function of epitopes of Mycobacterium tuberculosis antigens contributed significantly toward better understanding of the immunopathogenesis and to efforts for improving infection and disease control. Characterization of genetically permissively presented immunodominant epitopes has implications for the evolution of the host–parasite relationship, development of immunodiagnostic tests, and subunit prophylactic vaccines. Knowledge of the determinants of cross-sensitization, relevant to other pathogenic or environmental mycobacteria and to host constituents has advanced. Epitope-defined IFNγ assay kits became established for the specific detection of infection with tubercle bacilli both in humans and cattle. The CD4 T-cell epitope repertoire was found to be more narrow in patients with active disease than in latently infected subjects. However, differential diagnosis of active TB could not be made reliably merely on the basis of epitope recognition. The mechanisms by which HLA polymorphism can influence the development of multibacillary tuberculosis (TB) need further analysis of epitopes, recognized by Th2 helper cells for B-cell responses. Future vaccine development would benefit from better definition of protective epitopes and from improved construction and formulation of subunits with enhanced immunogenicity. Epitope-defined serology, due to its operational advantages is suitable for active case finding in selected high disease incidence populations, aiming for an early detection of infectious cases and hence for reducing the transmission of infection. The existing knowledge of HLA class I binding epitopes could be the basis for the construction of T-cell receptor-like ligands for immunotherapeutic application. Continued analysis of the functions of mycobacterial epitopes, recognized by T cells and antibodies, remains a fertile avenue in TB research.

Passive protection with immunoglobulin A antibodies against tuberculous early infection of the lungs

We report on a new approach toward protection against tuberculosis, based on passive inoculation with immunoglobulin A (IgA) antibodies. In a mouse model of tuberculous lung infection, intranasal inoculations of mice with an IgA monoclonal antibody (mAb) against the alpha-crystallin antigen of Mycobacterium tuberculosis reduced up to 10-fold the lung bacterial counts at nine days after either aerosol- or intranasal challenge. This effect involved synergism between mAb inoculations shortly before and 3 days after infection. Monomeric IgA reduced the colony-forming unit counts to the same extent as the polymeric IgA, suggesting antibody targeting to Fcalpha, rather than poly-immunoglobulin receptors on infected lung macrophages. The protective effect was of short duration, presumably due to the rapid degradation of the intranasally applied IgA. Our results provide evidence of an alternative approach which could be further developed toward immunoprophylaxis against tuberculosis in immunocompromised subjects.

Lymphocyte subtypes in experimentally induced early-stage bovine tuberculous lesions

The identity of the lymphocyte subtypes constituting the lymphocytic mantle within developing early-stage lesions of bovine tuberculosis was investigated immunohistochemically in calves inoculated intranasally with 2 x 10(7) colony-forming units of a field isolate of Mycobacterium bovis. Pulmonary lesions were examined 7, 14, 21, 28 and 42 days after inoculation, and bronchial lymph node lesions at 35 days. The immunolabelling results reported were obtained with monoclonal antibodies against two T-cell epitopes (WC1+ gamma delta and CD2+) and against B-cell epitopes. Large numbers of CD2+ T-lymphocytes were observed around developing areas of necrosis throughout the study; WC1+ gamma delta cells, however, were more numerous at these sites up to and including day 21. On the other hand, aggregates of B lymphocytes did not become prominent in areas adjacent to lesions until day 42. The results suggest that these lymphocyte phenotypes play a role in the pathogenesis of early-stage lesions.

Immunologic diagnosis of tuberculosis

Evaluation of new vaccines against tuberculosis requires diagnostic tools for accurately identifying asymptomatic individuals infected with Mycobacterium tuberculosis and persons with active tuberculosis. This article discusses limitations of current methods for the immunologic diagnosis of latent infection and active disease and presents novel approaches to developing skin tests and serodiagnostic assays based on "cocktails" of multiple antigens of M. tuberculosis.

Identification of genes encoding exported Mycobacterium tuberculosis proteins using a Tn552'phoA in vitro transposition system

Secreted and cell envelope-associated proteins are important to both Mycobacterium tuberculosis pathogenesis and the generation of protective immunity to M. tuberculosis. We used an in vitro Tn552'phoA transposition system to identify exported proteins of M. tuberculosis. The system is simple and efficient, and the transposon inserts randomly into target DNA. M. tuberculosis genomic libraries were targeted with Tn552'phoA transposons, and these libraries were screened in M. smegmatis for active PhoA translational fusions. Thirty-two different M. tuberculosis open reading frames were identified; eight contain standard signal peptides, six contain lipoprotein signal peptides, and seventeen contain one or more transmembrane domains. Four of these proteins had not yet been assigned as exported proteins in the M. tuberculosis databases. This collection of exported proteins includes factors that are known to participate in the immune response of M. tuberculosis and proteins with homologies, suggesting a role in pathogenesis. Nine of the proteins appear to be unique to mycobacteria and represent promising candidates for factors that participate in protective immunity and virulence. This technology of creating comprehensive fusion libraries should be applicable to other organisms.

Enhancement of the T cell response to a mycobacterial peptide by conjugation to synthetic branched polypeptide

A peptide-based approach towards improving the immunodiagnosis of, and vaccination against, tuberculosis faces the problems of MHC restriction of T cell recognition and the poor immunogenicity of peptides in the absence of adjuvant. We sought to compensate this by the use of synthetic branched polypeptides of the poly[Lys-(Xi-DL-Alam)] type, containing a glutamic acid residue (EAK), and further modified either by succinylation (SucEAK) or acetylation (AcEAK). These carriers were conjugated to two permissively recognized peptides of Mycobacterium tuberculosis. The 38p350 - 369-SucEAK conjugate enhanced IFN-gamma production more than 13-fold (from 22.6 to 294 pg / ml, p = 0.001) in peripheral blood mononuclear cells from healthy subjects, and 8.7-fold (p = 0. 012) in cells from tuberculosis patients. The effect was dependent on the carrier used and on covalent linkage of SucEAK to 38p350 - 369. An increased response occurred best in cells from subjects bearing at least one HLA-DR allele for which 38p350 - 369 had high binding affinity and required cellular processing of the conjugate as inhibitors (chloroquine and wortmannin) blocked the IFN-gamma response. SucEAK conjugation of peptide 16p91 - 110 did not significantly increase IFN-gamma production, indicating that the ability of conjugation to enhance the response was peptide structure dependent. These data indicate that the use of SucEAK polymer coupled with permissively recognized peptides could contribute to the development of an improved immunodiagnostic or vaccine reagent for tuberculosis.

Influence of HLA-DR on the phenotype of CD4+ T lymphocytes specific for an epitope of the 16-kDa alpha-crystallin antigen of Mycobacterium tuberculosis

T helper phenotype may be influenced by cytokine milieu, the differential expression of co-stimulatory molecules, antigen dose, and by differences in affinity at the TCR-peptide-MHC interface. We investigated the latter hypothesis by examining the response of six HLA-DR-restricted CD4+ T cell lines specific for the immunodominant and permissively recognized p91-110 epitope of the 16-kDa alpha-crystallin protein of Mycobacterium tuberculosis. Each line was generated from a sensitized HLA-DR-heterozygous donor and all proliferated when peptide was presented by autologous irradiated peripheral blood mononuclear cells. However, when HLA-DR-matched homozygous Epstein-Barr-virus-transformed B cell lines (L-BCL) were used as peptide-presenting cells there was heterogeneity in the response. The most pronounced proliferative response, and the highest IFN-gamma secretion and cytolytic activity was stimulated by L-BCL expressing molecules (DRB1*0101, *1501 and *0401) with high affinity (IC50 < 10 microM) for the 16p91-110 peptide. By comparison, IL-4 secretion or a lower proliferative response could occur when peptide was presented by alleles of high, or of intermediate (10 microM < IC50 < 100 microM), affinity. These data support the hypothesis that the host MHC can influence CD4+ phenotype and have implications for subunit vaccination against tuberculosis.

Human T- and B-cell reactivity to the 16kDa alpha-crystallin protein of Mycobacterium tuberculosis

The attributes of immunodominance, predominant expression during mycobacterial dormancy and restriction to the Mycobacterium tuberculosis complex make the 16 kDa protein an important candidate for the study of the immune response in humans. We therefore investigated the relationship between T- and B-cell reactivity to the recombinant antigen and disease in a total of 127 subjects. The percentage of T-cell responders towards both the intact antigen and its permissively recognised peptide 16p91-110 was highest in healthy bacillus Calmette-Guérin (BCG)-sensitized controls (96% and 68%, respectively) and lowest in those with extensive untreated tuberculosis (26% and 18%) (P < 0.001). By contrast, antibody levels (ABT50 > 100) were highest in patients with extensive disease (46-50%) (P < 0.005). There was significantly higher production of IFN-gamma in the BCG-sensitized controls by comparison with untreated patients (P < 0.05), but complete antituberculous chemotherapy abolished this deficit in patients. The significance of these findings to immunodiagnosis and protective immunity is discussed.

Synthesis and in vitro T-cell immunogenicity of conjugates with dual specificity: attachment of epitope peptides of 16 and 38 kDa proteins from Mycobacterium tuberculosis to branched polypeptide

T-cell epitope containing peptides covalently attached to macromolecular carriers can be considered as synthetic immunogens for the development of skin-test diagnostics and of vaccines. As a carrier, an amphoteric branched chain polypeptide, poly[Lys-(Glui-DL-Alam)] (EAK) with poly(L-lysine) backbone has been used. This polypeptide with free alpha-amino and gamma-carboxyl groups at the end of the side chains was conjugated with peptides representing two immunodominant regions of the 16 and 38 kDa proteins of Mycobacterium tuberculosis, respectively. Peptide C91SEFAYGSFVRTVSLPVGADE110 was elongated by Cys at the N-terminal and attached to the carrier containing protected SH groups to form disulfide bridges. Peptide 65FNLWGPAFHERYPNVTITA83 was conjugated to the 3-(2-pyridyldithio)propionic acid N-hydroxysuccinimide ester (SPDP) modified and acetylated EAK by introducing amide bond between the free alpha-amino group of peptide and the free gamma-COOH group of Glu at the terminal position of the branches. This strategy lead to chemically well-defined synthetic immunogens that contain two different epitopes in multiple copies covalently linked to a synthetic branched polypeptide carrier. In vitro T-cell immunogenicity of a prototype conjugate was studied using T-cell hybridomas, lymph node cells from 38 kDa protein immunized mice, and human peripheral blood mononuclear cell (PBMC) cultures from sensitized individuals. These data document that the specific T-cell stimulatory effect of each mycobacterial epitope was maintained in this conjugate. Taken together, these findings suggest that it is feasible to use a biodegradable polymeric polypeptide for producing macromolecular bioconjugates for the stimulation of T-cell responses.

Site-directed mutagenesis of the 19-kilodalton lipoprotein antigen reveals No essential role for the protein in the growth and virulence of Mycobacterium intracellulare

The mycobacterial 19-kilodalton antigen (19Ag) is a highly expressed, surface-associated glycolipoprotein which is immunodominant in infected patients and has little homology with other known proteins. To investigate the pathogenic significance of the 19Ag, site-directed mutagenesis of the Mycobacterium intracellulare 19Ag gene was carried out by using a suicide vector-based strategy. Allelic replacement of the 19Ag gene of a mouse-avirulent M. intracellulare strain, 1403, was achieved by double-crossover homologous recombination with a gentamicin resistance gene-mutated allele. Unfortunately, an isogenic 19Ag was not achievable in the mouse-virulent strain, D673. However, a 19Ag mutant was successfully constructed in M. intracellulare FM1, a chemically mutagenized derivative of strain D673. FM1 was more amenable to genetic manipulation and susceptible to site-directed mutagenesis of the 19Ag gene yet retained the virulent phenotype of the parental strain. No deleterious effects of 19Ag gene mutation were observed during in vitro growth of M. intracellulare. Virulence assessment of the isogenic 19Ag mutants in a mouse infection model demonstrated that the antigen plays no essential role in the growth of M. intracellulare in vivo. Site-directed mutagenesis of the 19Ag gene demonstrated that it plays no essential role in growth and pathogenicity of M. intracellulare; however, the exact nature of its biological function remains unknown.