Do CD1-restricted T cells contribute to antibody-mediated immunity against Mycobacterium tuberculosis?

Lipoarabinomannan in Active and Passive Protection Against Tuberculosis

Glycolipids of the cell wall of Mycobacterium tuberculosis (Mtb) are important immunomodulators in tuberculosis. In particular, lipoarabinomannan (LAM) has a profound effect on the innate immune response. LAM and its structural variants can be recognized by and activate human CD1b-restricted T cells, and emerging evidence indicates that B cells and antibodies against LAM can modulate the immune response to Mtb. Anti-LAM antibodies are induced during Mtb infection and after bacille Calmette-Guerin (BCG) vaccination, and monoclonal antibodies against LAM have been shown to confer protection by passive administration in mice and guinea pigs. In this review, we describe the immune response against LAM and the potential use of the mannose-capped arabinan moiety of LAM in the construction of vaccine candidates against tuberculosis.

A toolbox for tuberculosis (TB) diagnosis: an Indian multi-centric study (2006-2008); evaluation of serological assays based on PGL-Tb1 and ESAT-6/CFP10 antigens for TB diagnosis

BACKGROUND

The aim of this multi-centric prospective study in India was to assess the accuracy of a serological test as an additional tool for diagnosing active tuberculosis (ATB). In particular, an assay based on ELISA using a phenolic glycolipid (PGL-Tb1) or a fusion protein (ESAT-6/CFP10) was compared to the tuberculin skin test (TST) and the microbiological results according to HIV status.

METHODS

Individuals with and without ATB and HIV infection were enrolled. Serology and TST results were analyzed per se and in combination with the microbiological data.

RESULTS

Among the 778 ATB patients, 102 were HIV-infected, 316 HIV-uninfected and 360 had an HIV-unknown status. Of the 945 non-ATB subjects, 559 were at low risk (community adults) and 386 at high risk of M. tuberculosis exposure. Among those with ATB, the sensitivity of ELISA-PGL-Tb1 for ATB was higher than that of ELISA-ESAT-6/CFP10, both in HIV-infected (72.3% versus 63.7%, p = 0.29) and HIV-uninfected/HIV-unknown groups (40.5% versus 28.6%; p<0.0001), whereas the specificity was around 91% for both tests. Sensitivity for ATB increased when the results of the two ELISA were combined, reaching 75.5% in the HIV-infected and 50.9% in the group of HIV-uninfected/HIV-unknown ATB, with a significant decrease of the global specificity (83.9%). Analyzing the ELISA results with the microbiological results, we observed that the sensitivity of both serology tests was independent of the ATB patients' smear microscopy (SM) status and grade. Combining the results of SM with both ELISA, the detection of ATB patients significantly increased (p<0.0001), particularly in those with extrapulmonary TB (up to 45.1%) or HIV infection (up to 83.3%). No significant association was observed between TST and serology results.

CONCLUSIONS

In this prospective multi-centric study, the combination of two rapid tests, such as SM and serology, might be useful in detecting ATB, especially in HIV-infected patients.

Evaluation of the humoral immune response and cross reactivity against Mycobacterium tuberculosis of mice immunized with liposomes containing glycolipids of Mycobacterium smegmatis

Mycobacterium smegmatis (Ms) is a nonpathogenic mycobacteria of rapid growth, which shares many characteristics with Mycobacterium tuberculosis (MTB), the major causative agent of tuberculosis. MTB has several cell wall glycolipids in common with Ms, which play an important role in the pathogenesis of tuberculosis and the induction of a protective immune response against MTB infection in some animal models. In this study, the humoral immune response and cross reactivity against MTB, of liposomes containing a mixture of cell wall glycolipids of Ms and commercial lipids was evaluated, in order to study its possible use as a component of a vaccine candidate against tuberculosis. Liposomes containing total lipids extracted from Ms, distearoyl phosphatidyl choline and cholesterol were prepared by the dehydration-rehydration technique. Balb/c mice were immunized with the liposomes obtained and the antibody response and cross reactivity against MTB were tested by ELISA. Total lipids extract from Ms showed the presence of several polar glycolipids in common with MTB, such as phosphatidylinositol mannosides. Liposomes that contained glycolipids of Ms were capable of inducing a specific IgG antibody response that allowed the recognition of surface antigens of MTB. The results of this study demonstrated the presence of immunogenic glycolipids in Ms, which could be included to enhance the protective effects of subunit vaccine formulations against tuberculosis.

Anti-phospholipid antibody levels as biomarker for monitoring tuberculosis treatment response

Standard methods to monitor tuberculosis (TB) treatment response rely on sputum microscopy and culture conversion. Alternatives to these methods are needed for those patients whose sputum tests are smear or culture negative. Here, we examine anti-phospholipid IgM antibody level changes as a biomarker for treatment response in smear positive TB patients. Serum samples were obtained from 40 pulmonary TB patients at the start and end of the intensive phase treatment (IPT) from the CDC-TB Trials Consortium randomized clinical trial in Kampala, Uganda. Samples were screened by ELISA for IgM levels against five phospholipids found in Mycobacterium tuberculosis and host cells. Lipid antigens included cardiolipin (CL), phosphatidyl inositol (PI), phosphatidyl ethanolamine (PE), phosphatidyl choline (PTC), and sphingolipid (SL). Levels of IgM against all phospholipids significantly decreased (p = 0.034, 0.001, 0.008 0.008, 0.040, respectively) following anti-TB drug treatment in patients without lung cavitary disease at baseline. The mean sensitivity of this test in these patients was 83% when the IgM response to a single lipid antigen was used; it was >90% when responses to 2 or more lipids were assessed. In contrast, cavitary TB patients showed an overall IgM increase, with a significant rise against PE (p = 0.025). There was no significant difference in the change in antibody levels between patients who remained culture-positive and those who culture-converted after 40 doses of drug therapy. The measurement of IgM anti-phospholipid antibodies may be a useful biomarker to monitor treatment response in non-cavitary TB patients.

Immune response of cattle immunized with a conjugate of the glycolipid glucose monomycolate and protein

Strong anti glycolipid IgG responses can occur in humans and animals, but contrary to anti protein responses and anti glycoprotein responses, the exact mechanism of induction is unknown. We have previously shown that experimental immunization with the glycolipid glucose monomycolate (GMM) causes the development of specific T cell responses, but not of anti GMM antibodies. However, cattle naturally infected with Mycobacterium avium ssp. paratuberculosis produce high levels of anti GMM IgG. In the present study, we tested whether vaccination with GMM conjugated to a protein mimics natural infection in its capacity to induce the production of antibodies against GMM. Cattle were immunized (n=5 per group) with GMM conjugated to a protein, or GMM and protein non-conjugated and administered at contralateral locations, or carrier only. Although immunization with the GMM-protein conjugate vaccine and the non-conjugated vaccine induced protein specific antibody responses, GMM specific antibodies were not detected in either of the groups. In conclusion, the generation of isotype-switched anti lipid antibodies appears to require more than providing peptide epitopes for T helper cells to support glycolipid specific B cells in antibody production.

Exogenous control of the expression of Group I CD1 molecules competent for presentation of microbial nonpeptide antigens to human T lymphocytes

Group I CD1 (CD1a, CD1b, and CD1c) glycoproteins expressed on immature and mature dendritic cells present nonpeptide antigens (i.e., lipid or glycolipid molecules mainly of microbial origin) to T cells. Cytotoxic CD1-restricted T lymphocytes recognizing mycobacterial lipid antigens were found in tuberculosis patients. However, thanks to a complex interplay between mycobacteria and CD1 system, M. tuberculosis possesses a successful tactic based, at least in part, on CD1 downregulation to evade CD1-dependent immunity. On the ground of these findings, it is reasonable to hypothesize that modulation of CD1 protein expression by chemical, biological, or infectious agents could influence host's immune reactivity against M. tuberculosis-associated lipids, possibly affecting antitubercular resistance. This scenario prompted us to perform a detailed analysis of the literature concerning the effect of external agents on Group I CD1 expression in order to obtain valuable information on the possible strategies to be adopted for driving properly CD1-dependent immune functions in human pathology and in particular, in human tuberculosis.

Synthetic iNKT cell-agonists as vaccine adjuvants--finding the balance

The unique position of invariant natural killer T (iNKT) cells at the interface of the innate and adaptive arms of the immune response, combined with their ability to modulate the activity of antigen-presenting cells, has led to their intensive investigation as a means of augmenting the immune response both in vaccination strategies for microbial infections and in tumor immunotherapy. Several synthetic iNKT-cell agonists that have potential as vaccine adjuvants have been identified, but these are not without their limitations-strong agonists can lead to the undesirable effects associated with overstimulation of the immune system, whereas too weak agonists may provide insufficient iNKT cell help to stimulate maturation of dendritic cells and differentiation of B cells. In this article we explore strategies being investigated as means of increasing the specificity of and controlling the magnitude of the immune response generated by activation of iNKT cells with synthetic agonists.

The mycobacterial glycolipid glucose monomycolate induces a memory T cell response comparable to a model protein antigen and no B cell response upon experimental vaccination of cattle

Glycolipids are presented to T cells by human group 1 CD1 proteins, but are not used as subunit vaccines yet. Experimental immunizations with pure mycobacterial glucose monomycolate (GMM) and keyhole limpet haemocyanin (KLH) in cattle, a species which, unlike mice, expresses group 1 CD1, showed that GMM was equally efficient as KLH in generating T cell responses in blood, but not in the draining lymph node. Also, KLH induced strong antibody responses whereas GMM did not. These data suggest that non-overlapping T cell populations are targeted and demonstrate the potential of glycolipids as a special class of subunit vaccine candidates.

Role of marginal zone B lymphocytes in invariant NKT cell activation

Splenic marginal zone B (MZB) lymphocytes represent, along with dendritic cells (DC) a first line of defense against blood-borne pathogens. MZB cells express high levels of MHC class II and CD1d molecules but so far their ability to activate and orientate conventional and innate-like T lymphocytes, such as invariant NKT (iNKT) cells, is still elusive. In the present study, we show that murine MZB cells proliferate, mature phenotypically, and secrete cytokines in response to TLR (except TLR3) agonists. When pulsed with OVA peptide (but not whole OVA), MZB cells promote the release of IFN-gamma and IL-4 by Ag-specific CD4(+) T lymphocytes and their stimulation with the TLR9 agonist CpG oligodeoxynucleotide (ODN), a potent MZB cell activator, biases them toward more Th1 inducers. Unlike DC, CpG ODN-stimulated MZB cells fail to stimulate iNKT cells. Although able to activate iNKT hybridomas, MZB cells sensitized with free alpha-galactosylceramide (alpha-GalCer), a CD1d-restricted glycolipid Ag, do not directly activate ex vivo sorted iNKT cells unless DC are added to the culture system. Interestingly, MZB cells amplify the DC-mediated activation of iNKT cells and depletion of MZB cells from total splenocytes strongly reduces iNKT cell activation (cytokine production) in response to alpha-GalCer. Thus, DC and MZB cells provide help to each other to optimize iNKT cell stimulation. Finally, in vivo transfer of alpha-GalCer-loaded MZB cells potently activates iNKT and NK cells. This study confirms and extends the concept that MZB cells are important players in immune responses, a property that might be exploited.

NK T cells provide lipid antigen-specific cognate help for B cells

The mechanisms of T cell help for production of antilipid antibodies are largely unknown. This study shows that invariant NK T cells (iNK T cells) and B cells cooperate in a model of antilipid antigen-specific antibody responses. We use a model haptenated lipid molecule, 4-hydroxy-3-nitrophenyl-alphaGalactosylCeramide (NP-alphaGalCer), to demonstrate that iNK T cells provide cognate help to lipid-antigen-presenting B cells. B cells proliferate and IgG anti-NP is produced from in vivo-immunized mice and in vitro cocultures of B and NK T cells after exposure to NP-alphaGalCer, but not closely related control glycolipids. This B cell response is absent in CD1d(-/-) and Jalpha18(-/-) mice but not CD4(-/-) mice. The antibody response to NP-alphaGalCer is dominated by the IgM, IgG3, and IgG2c isotypes, and marginal zone B cells stimulate better in vitro lipid antigen-driven proliferation than follicular B cells, suggesting an important role for this B cell subset. iNK T cell help for B cells is shown to involve cognate help from CD1d-instructed lipid-specific iNK T cells, with help provided via CD40L, B7-1/B7-2, and IFN-gamma, but not IL-4. This model provides evidence of iNK T cell help for antilipid antibody production, an important aspect of infections, autoimmune diseases, and vaccine development. Our findings also now allow prediction of those microbial antigens that would be expected to elicit cognate iNKT cell help for antibody production, namely those that can stimulate iNKT cells and at the same time have a polar moiety that can be recognized by antibodies.

Anti-PGL-Tb1 responses as an indicator of the immune restoration syndrome in HIV-TB patients

A prospective and multi-centre study has allowed us to analyse antibody responses and Mycobacterium tuberculosis clinical isolate genotypes on 24 consecutive HIV-TB co-infected patients treated with Highly Active Antiretroviral Therapy (HAART) who either went on to develop a TB Immune Restoration Syndrome (TB-IRS), or not. Circulating free and immune-complexed antibodies against ManLAM, ESAT-6/CFP10 and PGL-Tb1 in HIV-TB co-infected patients were measured by ELISA at the initiation of anti-TB treatment, at the date of HAART initiation and thereafter. Presence of circulating B cells was also monitored by in vitro antibody production (IVAP) against ESAT-6/CFP10 and PGL-Tb1. Finally, 16 out of 24M. tuberculosis clinical isolates from patients with TB-IRS were genotyped using spoligotyping and MIRUs-VNTR typing. Eleven patients (45.8%) experienced TB-IRS (TB-IRS+). Significantly, lower anti-PGL-Tb1 antibody levels were identified in TB-IRS+ compared to TB-IRS-negative patients prior to TB-IRS development. These very low levels were neither related to CD4 counts nor with complexed antibodies. No difference in antibody levels was observed with the other tested antigens. In addition, no specific strain genotype was associated with TB-IRS. The presence of specific anti-PGL-Tb1 antibodies only in TB-IRS-negative patients represents for the first time an indicator of a potential protective response or a diagnostic biomarker for the detection of non-progression to TB-IRS in HIV-TB co-infected patients starting HAART.

Cell wall-associated alpha-glucan is instrumental for Mycobacterium tuberculosis to block CD1 molecule expression and disable the function of dendritic cell derived from infected monocyte

We previously described an escape mechanism exploited by Mycobacterium tuberculosis (Mtb) to prevent the generation of fully competent dendritic cells (DC). We have now tested the effect of isolated mycobacterial components on human monocyte differentiation into DC and demonstrated that cell wall (CW)-associated alpha-glucan induces monocytes to differentiate into DC (Glu-MoDC) with the same altered phenotype and functional behaviour of DC derived from Mtb-infected monocytes (Mt-MoDC). In fact, Glu-MoDC lack CD1 molecule expression, fail to upregulate CD80 and produce IL-10 but not IL-12. We also showed that Glu-MoDC are not able to prime effector T cells or present lipid antigens to CD1-restricted T-cell clones. Thus, we propose a mechanism of Mtb-monocyte interaction mediated by CW-associated alpha-glucan, which allows the bacterium to evade both innate and acquired immune responses.

BCG-infected adherent mononuclear cells release cytokines that regulate group 1 CD1 molecule expression

Increasing evidence is now available showing that CD1-restricted T cell responses against non-peptide mycobacterial antigens could play a role in the immune resistance against tuberculosis. BCG, widely used in anti-tubercular vaccination, shares various constituents with Mycobacterium tuberculosis, but does not provide full protection. In the present study we have investigated the pattern of group 1 CD1 molecule expression in adherent mononuclear cells (AMNC) of human peripheral blood, infected in vitro with BCG. Shortly after exposure to BCG, both BCG-positive and BCG-negative AMNC showed a moderate CD1 expression elicited by BCG-induced release of GM-CSF presumably acting through an autocrine and a paracrine mechanism. This was demonstrated using two-color flow cytometry with green fluorescent BCG and anti-CD1 PE-labeled antibodies. However, high CD1 expression induced by exogenously added GM-CSF in AMNC was reduced if target cells were cocultivated with BCG. Monoclonal antibodies against IL-10 partially restored CD1 expression, thus showing that IL-10, released from infected AMNC, is involved, at least in part, in CD1 negative modulation. Therefore, through a complex cytokine network, including not yet identified factor(s), BCG triggers but does not allow full expression of CD1 on AMNC. It cannot be excluded that this mechanism could play a role in the limited efficiency of BCG vaccination.

B-cell immune responses in HIV positive and HIV negative patients with tuberculosis evaluated with an ELISA using a glycolipid antigen

The diagnostic value of the PGL-Tb1 enzyme-linked immunosorbent assays (ELISA) was established following a survey study using sera from 220 Tuberculosis patients (including 69 HIV coinfected) and 324 controls. A higher percentage (76.8%) of the HIV-seropositive compared to the HIV-seronegative (58.9%) TB patients were ELISA positive (p=0.02) with a specificity of 94%. In HIV-positive TB patients, ELISA sensitivity was identical for all sites of disease and antibody levels were not affected by the CD4+ counts, PPD results, age or bacterial yield. Combining data for both the smear microscopy and ELISA maximized sensitivity. The kinetics of anti-PGL-Tb1 antibody was evaluated in cohort studies using sera collected before, during and after treatment for clinical TB for 79 TB patients (including 39 HIV coinfected). Statistically significant ELISA signals were observed in 51.3% of HIV-seropositive TB patients prior to the diagnosis of clinical TB and elevated antibody levels persisting 18 months after the end of antituberculous chemotherapy. Asymptomatic development of antibody also occurred in 22.7% of a cohort of 44 HIV-positive patients with a high risk of tuberculosis, but no correlation was found between persisting elevated antibody levels and progression to active disease. This antibody response in absence of disease, might reflect the control of an incipient tuberculosis infection by antituberculous prophylaxis or through an improved protective immune response associated with antiretroviral therapy.

The CD1d-binding glycolipid alpha-galactosylceramide enhances humoral immunity to T-dependent and T-independent antigen in a CD1d-dependent manner

Specific interaction of class II/peptide with the T-cell receptor (TCR) expressed by class II-restricted CD4+ T helper (Th) cells is essential for in vivo production of antibodies reactive with T-dependent antigen. In response to stimulation with CD1d-binding glycolipid, Valpha14+ TCR-expressing, CD1d-restricted natural killer T (NKT) cells may provide additional help for antibody production. We tested the hypothesis that the CD1d-binding glycolipid alpha-galactosylceramide (alpha-GC) enhances production of antibodies reactive with T-dependent antigen in vivo. alpha-GC enhanced antibody production in vivo in a CD1d-dependent manner in the presence of class II-restricted Th cells and induced a limited antibody response in Th-deficient mice. alpha-GC also led to alterations in isotype switch, selectively increasing production of immunoglobulin G2b. Further analysis revealed that alpha-GC led to priming of class II-restricted Th cells in vivo. Additionally, we observed that alpha-GC enhanced production of antibodies reactive with T-independent antigen, showing the effects of NKT cells on B cells independently of Th cells. Our data show that NKT cells have multiple effects on the induction of a humoral immune response. We propose that NKT cells could be exploited for the development of novel vaccines where protective antibody is required.