Increasing CD4+ T cells specific for tuberculosis correlate with improved clinical immunity after highly active antiretroviral therapy

Is mapping the BCG vaccine-induced immune responses the key to improving the efficacy against tuberculosis?

In recent years, the century-old Mycobacterium bovis Bacillus Calmette-Guérin (BCG) vaccine against tuberculosis (TB) has been re-evaluated for its capacity to stem the global tide of TB. There is increasing evidence that the efficacy of BCG can be improved by the modified administration methods and schedules. Here, we first discuss recent approaches of vaccine administration, revaccination or boosting that have been used to try to improve the efficacy of BCG against TB. We then dive deeper into studies investigating the immune correlates of protection and describe studies that have investigated BCG-specific T-cell responses and the influence of environmental exposures. These studies all highlight that there is still a lot to learn about the immune response induced by BCG, both in terms of phenotype and specificity, which has been surprisingly understudied. We argue that several critical gaps in knowledge exist and must be addressed by future research to rationally improve the efficacy of BCG, including comprehensive, proteome-wide understanding of the epitopes derived from BCG recognized by BCG-vaccinated individuals, the phenotype of responding antigen-specific T cells and how previous exposure to environmental mycobacteria affect these parameters and thus influence vaccine efficacy. The development of modern techniques allows us to answer some of these questions to better understand how BCG works in terms of both protection against TB and the immune response that it triggers.

Enhanced IFN-γ, but not IL-2, response to Mycobacterium tuberculosis antigens in HIV/latent TB co-infected patients on long-term HAART

BACKGROUND

HIV-infected individuals with latent TB infection are at increased risk of developing active TB. HAART greatly reduces the incidence rate of TB in HIV-infected patients and reconstitutes Mycobacterium tuberculosis (M. tuberculosis)-specific immune response in the first 12 months of therapy. The durability of the anti-mycobacterial immune restoration after a year of HAART however remains less investigated.

METHOD

A cross-sectional study was conducted to evaluate M. tuberculosis-specific functional immune responses in HIV/latent TB co-infected patients who were on HAART for at least 1.5 up to 9 years as compared to HAART-naïve patients. Three-hundred sixteen HIV-infected patients without active TB were screened by tuberculin skin testing for M. tuberculosis infection and peripheral blood mononuclear cells (PBMCs) were isolated from 61 HIV/latent TB co-infected patients (30 HAART-naïve and 31 HAART-treated). IFN-γ and IL-2 ELISPOT as well as CFSE cell proliferation assays were performed after stimulation with M. tuberculosis antigens PPD and ESAT-6.

RESULT

The median frequency of PPD and ESAT-6 specific IFN-γ secreting cells was significantly higher in the HAART-treated patients as compared to HAART-naïve patients, p = 0.0021 and p = 0.0081 respectively. However, there was no significant difference in the median frequency of IL-2 secreting cells responding to PPD (p = 0.5981) and ESAT-6 (p = 0.3943) antigens between HAART-naïve and-treated groups. Both IFN-γ and IL-2 responses were independent of CD4+ T cell count regardless of the HAART status. Notably, the frequency of PPD and ESAT-6 specific IL-2 secreting cells was positively associated with CD4+ T cell proliferation while inversely correlated with duration of HAART, raising the possibility that M. tuberculosis-specific IL-2 response that promote the antigen-specific CD4+ T cell proliferation diminish with time on antiretroviral therapy in HIV/latent TB co-infected patients.

CONCLUSION

This study shows an increased M. tuberculosis-specific IFN-γ, but not IL-2, response in HIV/latent TB co-infected patients with long-term HAART, consistent with only partial immune restoration. Future studies should, therefore, be done to prospectively define the rate and extent to which functional immune responses to M. tuberculosis are restored after long-term HAART.

The immune response in tuberculosis

There are 9 million cases of active tuberculosis reported annually; however, an estimated one-third of the world's population is infected with Mycobacterium tuberculosis and remains asymptomatic. Of these latent individuals, only 5-10% will develop active tuberculosis disease in their lifetime. CD4(+) T cells, as well as the cytokines IL-12, IFN-γ, and TNF, are critical in the control of Mycobacterium tuberculosis infection, but the host factors that determine why some individuals are protected from infection while others go on to develop disease are unclear. Genetic factors of the host and of the pathogen itself may be associated with an increased risk of patients developing active tuberculosis. This review aims to summarize what we know about the immune response in tuberculosis, in human disease, and in a range of experimental models, all of which are essential to advancing our mechanistic knowledge base of the host-pathogen interactions that influence disease outcome.

Pulmonary manifestations of the immune reconstitution inflammatory syndrome

PURPOSE OF REVIEW

Immune reconstitution inflammatory syndrome (IRIS) is a common occurrence in HIV patients starting antiretroviral therapy (ART), and pulmonary involvement is an important feature of tuberculosis-IRIS and pneumocystis-IRIS. Pulmonologists need an awareness of the timing, presentation and treatment of pulmonary IRIS.

RECENT FINDINGS

Case definitions for tuberculosis-IRIS and cryptococcal-IRIS have been published by the International Network for the Study of HIV-associated IRIS (INSHI). A number of studies have addressed validation of clinical case definitions and the optimal time to commence ART after diagnosis of an opportunistic infection in HIV patients. The pathogenesis of IRIS is being assessed at a molecular level, increasing our understanding of mechanisms and possible targets for future preventive and therapeutic strategies.

SUMMARY

Tuberculosis-IRIS, nontuberculosis mycobacterial-IRIS and pneumocystis-IRIS occur within days to weeks of starting ART, causing substantial morbidity, but low mortality. Cryptococcal-IRIS usually occurs later in the course of ART, and may be associated with appreciable mortality. Early recognition of unmasking and paradoxical IRIS affecting the lung allows timely initiation of antimicrobial and/or immunomodulatory therapies.

Hypercytokinaemia accompanies HIV-tuberculosis immune reconstitution inflammatory syndrome

Increased access to combination antiretroviral therapy in areas co-endemic for tuberculosis (TB) and HIV-1 infection is associated with an increased incidence of immune reconstitution inflammatory syndrome (TB-IRIS) whose cause is poorly understood. A case-control analysis of pro- and anti-inflammatory cytokines in TB-IRIS patients sampled at clinical presentation, and similar control patients with HIV-TB prescribed combined antiretroviral therapy who did not develop TB-IRIS. Peripheral blood mononuclear cells were cultured in the presence or absence of heat-killed Mycobacterium tuberculosis for 6 and 24 h. Stimulation with M. tuberculosis increased the abundance of many cytokine transcripts with interleukin (IL)-1β, IL-5, IL-6, IL-10, IL-13, IL-17A, interferon (IFN)-γ, granulocyte-macrophage colony-stimulating factor (GM-CSF) and tumour necrosis factor (TNF) being greater in stimulated TB-IRIS cultures. Analysis of the corresponding proteins in culture supernatants, revealed increased IL-1β, IL-2, IL-6, IL-8, IL-10, IL-12p40, IFN-γ, GM-CSF and TNF in TB-IRIS cultures. In serum, higher concentrations of TNF, IL-6, and IFN-γ were observed in TB-IRIS patients. Serum IL-6 and TNF decreased during prednisone therapy in TB-IRIS patients. These data suggest that cytokine release contributes to pathology in TB-IRIS. IL-6 and TNF were consistently elevated and decreased in serum during corticosteroid therapy. Specific blockade of these cytokines may be rational approach to immunomodulation in TB-IRIS.

Specific T cell frequency and cytokine expression profile do not correlate with protection against tuberculosis after bacillus Calmette-Guérin vaccination of newborns

RATIONALE

Immunogenicity of new tuberculosis (TB) vaccines is commonly assessed by measuring the frequency and cytokine expression profile of T cells.

OBJECTIVES

We tested whether this outcome correlates with protection against childhood TB disease after newborn vaccination with bacillus Calmette-Guérin (BCG).

METHODS

Whole blood from 10-week-old infants, routinely vaccinated with BCG at birth, was incubated with BCG for 12 hours, followed by cryopreservation for intracellular cytokine analysis. Infants were followed for 2 years to identify those who developed culture-positive TB-these infants were regarded as not protected against TB. Infants who did not develop TB disease despite exposure to TB in the household, and another group of randomly selected infants who were never evaluated for TB, were also identified-these groups were regarded as protected against TB. Cells from these groups were thawed, and CD4, CD8, and γδ T cell-specific expression of IFN-γ, TNF-α, IL-2, and IL-17 measured by flow cytometry.

MEASUREMENTS AND MAIN RESULTS

A total of 5,662 infants were enrolled; 29 unprotected and two groups of 55 protected infants were identified. There was no difference in frequencies of BCG-specific CD4, CD8, and γδ T cells between the three groups of infants. Although BCG induced complex patterns of intracellular cytokine expression, there were no differences between protected and unprotected infants.

CONCLUSIONS

The frequency and cytokine profile of mycobacteria-specific T cells did not correlate with protection against TB. Critical components of immunity against Mycobacterium tuberculosis, such as CD4 T cell IFN-γ production, may not necessarily translate into immune correlates of protection against TB disease.

Tuberculosis-associated immune reconstitution inflammatory syndrome and unmasking of tuberculosis by antiretroviral therapy

The tuberculosis-associated immune reconstitution inflammatory syndrome (TB-IRIS) is a frequent early complication of antiretroviral therapy (ART), used to treat HIV-1 infection, especially in countries where TB is prevalent. TB-IRIS is characterized by an exaggerated inflammatory response toward the antigens of Mycobacterium tuberculosis that results in clinical deterioration in patients experiencing immune recovery during early ART. Two forms of TB-IRIS are recognized: paradoxical; and unmasking. Paradoxical TB-IRIS manifests with new or recurrent TB symptoms or signs in patients being treated for TB during early ART, and unmasking TB-IRIS is characterized by an exaggerated, unusually inflammatory initial presentation of TB during early ART. In this review the incidence, clinical features, risk factors, treatment, and prevention of TB-IRIS in adult and pediatric patients are discussed.

Tuberculosis-associated immune restoration syndrome in HIV-1-infected patients involves tuberculin-specific CD4 Th1 cells and KIR-negative gammadelta T cells

Tuberculosis (TB)-associated immune restoration syndrome (IRS) is a frequent event (10 to 30%) in HIV-1-infected patients receiving antiretroviral treatment and is associated with an increased number of IFN-gamma-producing tuberculin-specific cells. To further understand the immune mechanisms of TB-IRS and to identify predictive factors, we prospectively analyzed the Th1 and TCRgammadelta T cells known to be involved in mycobacterial defenses and dendritic cells at baseline and after antiretroviral and TB treatment in 24 HIV-1(+) patients, 11 with and 13 without IRS. At baseline, these two groups differed by significantly lower proportions of TCRgammadelta and Vdelta2(+) T cells displaying the inhibitory receptors CD94/NKG2 and CD158ah,b in IRS patients. The two groups did not differ in the baseline characteristics of CD8 or CD4 T cells or TLR-2 expression on monocytes or myeloid/plasmacytoid dendritic cells. During IRS, the increase in tuberculin-specific IFN-gamma-producing cells involved only highly activated effector memory multifunctional (IFN-gamma(+)TNF-alpha(+)IL-2(-)) CD4 T cells, whereas activated HLA-DR(+) CD4(+) T cells also increased during IRS. In contrast, dendritic cells decreased significantly during IRS and there were no changes in TLR-2 expression. Finally, the Vdelta2(+) T cells, mostly killer Ig-related receptor (KIR) (CD94/NKG2(-) and CD158(-)), significantly peaked during IRS but not in non-IRS patients. In conclusion, IRS is associated with an increase in the number of activated tuberculin-specific effector memory CD4 T cells and of KIR(-)Vdelta2(+) TCRgammadelta(+) T cells. Higher proportions of Vdelta2(+)TCRgammadelta(+) T cells lacking KIR expression are present as baseline and distinguish patients who will develop IRS from those who will not.

Type 1 helper T cells and FoxP3-positive T cells in HIV-tuberculosis-associated immune reconstitution inflammatory syndrome

RATIONALE

Tuberculosis-associated immune reconstitution inflammatory syndrome (TB-IRIS) induced by combination antiretroviral therapy (cART) has been attributed to dysregulated expansion of tuberculin PPD-specific IFN-gamma-secreting CD4(+) T cells.

OBJECTIVES

To investigate the role of type 1 helper T cell expansions and regulatory T cells in HIV-TB IRIS.

METHODS

Longitudinal and cross-sectional studies of Mycobacterium tuberculosis-specific IFN-gamma enzyme-linked immunospot responses and flow cytometric analysis of blood cells from a total of 129 adults with HIV-1-associated tuberculosis, 98 of whom were prescribed cART.

MEASUREMENTS AND MAIN RESULTS

In cross-sectional analysis the frequency of IFN-gamma-secreting T cells recognizing early secretory antigenic target (ESAT)-6, alpha-crystallins 1 and 2, and PPD of M. tuberculosis was higher in patients with TB-IRIS than in similar patients treated for both HIV-1 and tuberculosis who did not develop IRIS (non-IRIS; P <or= 0.03). The biggest difference was in the recognition of alpha-crystallin molecules: peptide mapping indicated a polyclonal response. Flow cytometric analysis indicated equal proportions of CD4(+) and CD8(+) cells positive for activation markers HLA-DR and CD71 in both patients with TB-IRIS and non-IRIS patients. The percentage of CD4(+) cells positive for FoxP3 (Forkhead box P3) was low in both groups (TB-IRIS, 5.3 +/- 4.5; non-IRIS, 2.46 +/- 2.46; P = 0.13). Eight weeks of longitudinal analysis of patients with tuberculosis who were starting cART showed dynamic changes in antigen-specific IFN-gamma-secreting T cells in both the TB-IRIS and non-IRIS groups: the only significant trend was an increased response to PPD in the TB-IRIS group (P = 0.041).

CONCLUSIONS

There is an association between helper T-cell type 1 expansions and TB-IRIS, but the occurrence of similar expansions in non-IRIS brings into question whether these are causal. The defect in immune regulation responsible for TB-IRIS remains to be fully elucidated.

Reconstitution of antimycobacterial immune responses in HIV-infected children receiving HAART

OBJECTIVE

Recent epidemiological studies in adults suggest that HAART can prevent the development of tuberculosis in HIV-infected individuals, but the mechanisms are incompletely understood and no data exist in children. We investigated whether changes in mycobacterial-specific immune responses can be demonstrated in children after commencing antiretroviral therapy.

DESIGN

We measured mycobacterial growth in vitro using a novel whole-blood assay employing reporter-gene tagged bacillus Calmette-Guérin (BCG) in a prospective cohort study in the tuberculosis-endemic environment of South Africa. Key cytokines were measured in supernatants collected from the whole-blood assay using cytometric bead array.

PATIENTS

A cohort of 15 BCG-vaccinated HIV-infected children was evaluated prospectively for in-vitro antimycobacterial immune responses before and during the first year of HAART. All children had advanced HIV disease. Nine children completed all study timepoints.

RESULTS

Before HAART, blood from children showed limited ability to restrict the growth of mycobacteria in the functional whole-blood assay. The introduction of HAART was followed by rapid and sustained reconstitution of specific antimycobacterial immune responses, measured as the decreased growth of mycobacteria. IFN-gamma levels in culture supernatants did not reflect this response; however, a decline in TNF-alpha was observed.

CONCLUSION

This is the first study using a functional in-vitro assay to assess the effect of HAART on immune responses to mycobacteria in HIV-infected children. Our in-vitro data mirror the in-vivo observation of decreased susceptibility to tuberculosis in HIV-infected adults receiving antiretroviral agents. This model may be useful for further characterizing immune reconstitution after HAART.

Explosion of tuberculin-specific Th1-responses induces immune restoration syndrome in tuberculosis and HIV co-infected patients

OBJECTIVES

To test the hypothesis that an acute exacerbation of mycobacteria-specific Th1 response after HIV infection control by HAART causes immune restoration syndrome (IRS) in HIV-tuberculosis (TB) coinfected patients.

DESIGN

Prospective, multicenter study of 19 consecutive untreated HIV-TB coinfected patients included when initiating antimycobacterial therapy and sequentially evaluated during HAART and at time of IRS. IRS was defined according to classical clinical diagnostic criteria. Patients were declared IRS- if no IRS occurred within 3 months after HAART initiation.

METHODS

Mycobacteria-specific [purified protein derivative (PPD), ESAT-6, 85B] Th1 cells producing interferon (IFN)-gamma quantified by ELISpot, in vitro production of 25 cytokines/chemokines in antigen-stimulated peripheral blood mononuclear cell (PBMC) supernatants quantified by chemiluminescence.

RESULTS

Seven patients (37%) experienced IRS (IRS+). Mycobacteria-specific (PPD) Th1 IFN-gamma-producing cells increased sharply during IRS (median, 2970 spot forming cells/10 PBMC), but not the cytomegalovirus-specific responses tested as control. Only three IRS+ patients had low ESAT-6- but no 85B-specific responses. IRS- patients did not develop acute PPD-specific responses except in one case. In addition, at time of IRS a peak of PPD-specific Th1 cytokines/chemokines [interleukin (IL)-2, IL-12, IFN-gamma, IP10 and monokine-induced by IFN-gamma] without Th2 cytokines, and a peak of non-specific inflammatory cytokines/chemokines (TNF-alpha, IL-6, IL-1beta, IL-10, RANTES and MCP-1) occurred. These findings were independent from CD4 cell count, viral loads or time of HAART initiation.

CONCLUSION

An acute exacerbation of Th1 responses against mycobacterial antigens appears to cause IRS in patients co-infected with HIV and TB. This key event provides new evidence valuable for the diagnosis and treatment of IRS.

How effectively does HAART restore immune responses to Mycobacterium tuberculosis? Implications for tuberculosis control

Use of highly active antiretroviral treatment (HAART) has had a major impact on HIV-associated morbidity and mortality in industrialized countries. Access to HAART is now expanding in low-income countries where tuberculosis (TB) is the most important opportunistic disease. The incidence of TB has been fueled by the HIV epidemic and in many countries with high HIV prevalence current TB control measures are failing. HAART reduces the incidence of TB in treated cohorts by approximately 80% and therefore potentially has an important role in TB control in such countries. However, despite the huge beneficial effect of HAART, rates of TB among treated patients nevertheless remain persistently higher than among HIV-negative individuals. This observation raises the important question as to whether immune responses to Mycobacterium tuberculosis (MTB) are completely or only partially restored during HAART. Current data suggest that full restoration of circulating CD4 cell numbers occurs only among a minority of patients and that, even among these, phenotypic abnormalities and functional defects in lymphocyte subsets often persist. Suboptimal restoration of MTB-specific immune responses may greatly reduce the extent to which HAART is able to contribute to TB control at the community level because patients receiving HAART live much longer and yet would maintain a chronically heightened risk of TB.

Use of a T Cell–Based Assay for Monitoring Efficacy of Antituberculosis Therapy

Monitoring the efficacy of antituberculosis therapy is crucial both for the individual patient and for better control of the spread of tuberculosis. We studied 18 patients with microbiologically confirmed tuberculosis, both at the time of diagnosis and 3 months after they started therapy, using an in vitro assay that detects T cell-mediated interferon- gamma response to selected peptides of Mycobacterium tuberculosis-specific early secretory antigenic target 6 (ESAT-6) protein. All patients had positive results at diagnosis; however, 3 months later, the response to ESAT-6 peptides was still detectable only in the 5 patients with microbiological isolation and/or absence of clinical improvement after treatment. On the basis of these data, we conclude that our assay is a useful tool in monitoring the efficacy of antituberculosis therapy.

Intracellular expression of interleukin-4 and interferon-gamma by a Mycobacterium tuberculosis antigen-stimulated CD4+ CD57+ T-cell subpopulation with memory phenotype in tuberculosis patients

In some chronic pathological conditions, antigen persistence activates and expands the CD4+ CD57+ T-cell subset. The host immune response against tuberculosis infection is maintained through the continuous presence of antigen-stimulated effector/memory helper T cells. To determine whether CD4+ CD57+ T cells were also expanded in human tuberculosis, we analysed (by flow cytometry) the phenotype of peripheral blood CD4+ T cells from 30 tuberculosis patients and 30 healthy controls. We observed a significant increase in the CD4+ CD57+ T-cell subset in tuberculosis patients in comparison to healthy controls (P < 0.001). Most CD4+ CD57+ T cells exhibited a CD28- CD45RO+ CD62L- phenotype, which is associated with memory cells. In vitro, a higher number of antigen-stimulated CD4+ CD57+ T cells produced intracellular interferon-gamma and interleukin-4 compared with antigen-stimulated CD4+ CD57- T cells (P < 0.001). These findings suggest that the majority of CD4+ CD57+ T cells correspond to a phenotype of activated memory T cells.

Immune Restoration Inflammatory Syndromes: The Dark Side of Successful Antiretroviral Treatment

The prevalence of cellular immunodeficiency has increased due to rising use of immunosuppressive therapies and the pandemic spread of HIV infection. More recently, the introduction of highly active antiretroviral therapy (HAART) in HIV has led to significant immune reconstitution, even in patients with previously long-lasting secondary immunodeficiency. HAART reduces morbidity and mortality in HIV infection and also changes the clinical course of prevalent subclinical opportunistic infections or autoimmune diseases. Atypical inflammatory disorders develop after initiation of HAART and have been summarized as "immune reconstitution syndrome," "immune restoration disease," and "immune restoration inflammatory syndrome." However, diagnostic criteria and standards of therapy are yet to be defined. The awareness for these diseases is of increasing importance from a clinical point of view. This review summarizes the variety of immunoreconstitution disorders and describes possible diagnostic pitfalls. We also propose possible therapeutic options.