The convergent epidemiology of tuberculosis and human cytomegalovirus infection

Unmasking the hidden impact of viruses on tuberculosis risk

Tuberculosis (TB) is a leading cause of mortality from an infectious disease. In this opinion article, we focus on accumulating scientific evidence indicating that viral infections may contribute to TB progression, possibly allowing novel preventive interventions. Viruses can remodel the mammalian immune system, potentially modulating the risk of reactivating latent microbes such as Mycobacterium tuberculosis (Mtb). Evidence is mixed regarding the impact of emergent viruses such as SARS-CoV-2 on the risk of TB. Therefore, we posit that important knowledge gaps include elucidating which viral families increase TB risk and whether these provide unique or shared immune mechanisms. We also propose potential future research to define the contribution of viruses to TB pathogenesis.

Sex-based tuberculosis mortality in Newfoundland, 1900-1949: Implications for populations in transition

OBJECTIVE

During the second epidemiological transition, tuberculosis (TB) is one disease that declined substantially enough to reduce all-cause mortality. Sex-based differences in TB mortality may reveal an important dimension of population health transitions between the urbanizing and rural regions of Newfoundland.

MATERIALS AND METHODS

For the island of Newfoundland, yearly age-standardized sex-based TB mortality rates were calculated using individual death records from 1900 to 1949 (n = 30 393). Multiple linear regression models predict the relative rates (RR) of sex-based mortality and the absolute difference between males and females while controlling for time and region (the urbanizing Avalon Peninsula or rural Newfoundland). Multiple linear regression models also predict the median age at death from TB while controlling for time, region, and sex to assess if TB was shifting to an older adult disease compared to those typically afflicted in ages 20-44.

RESULTS

Female TB mortality was relatively and absolutely higher than males; additionally, RR and absolute differences between male and female mortality were significantly lower in rural Newfoundland than the Avalon Peninsula. Median age at death for males was significantly higher than females, and differences in median age at death increased over time.

DISCUSSION

The historically high prevalence of TB throughout Newfoundland, including domestic, social, and public health responsibilities of women, likely contributed to increased exposure and transmission, leading to higher observed mortality. Sex-based TB outcomes should be considered in the discussion of the progression of the epidemiological transition as dynamic inequalities that do not necessarily fit contemporary generalizations of sex-based TB epidemiology.

Human Cytomegalovirus Immunoglobulin G Response and Pulmonary Tuberculosis in Adolescents: A Case-Control Study

Background

Emerging evidence suggests a link between infection with herpes viruses, particularly human cytomegalovirus (HCMV) and Epstein-Barr virus (EBV), and progression to tuberculosis disease.

Methods

An unmatched case-control study was conducted among adolescents aged 10-19 years enrolled in an observational study (Teen TB) between November 2020 and November 2021, in Cape Town, South Africa. Fifty individuals with pulmonary tuberculosis and 51 healthy tuberculosis-exposed individuals without tuberculosis were included. Demographics and clinical data were obtained, and serum samples collected at enrolment were tested for HCMV immunoglobulin G (IgG) and EBV nuclear antigen (EBNA) IgG using 2 automated enzyme immunoassays. Odds ratios were estimated using unconditional logistic regression.

Results

The median age of 101 participants was 15 years (interquartile range, 13-17 years); 55 (54%) were female. All participants were HCMV IgG seropositive, and 95% were EBNA IgG seropositive. Individuals with tuberculosis had higher HCMV IgG titers than healthy controls (P = .04). Individuals with upper-tertile HCMV IgG titers had 3.67 times greater odds of pulmonary tuberculosis than those with IgG titers in the lower tertile (95% confidence interval, 1.05-12.84; P = .04). There was a trend for increasing odds of pulmonary tuberculosis with increasing titers of HCMV IgG (P = .04). In contrast, there was no association between tuberculosis and higher EBNA IgG values.

Conclusions

There is a high prevalence of sensitization to HCMV and EBV among adolescents in this high-tuberculosis-burden setting. Higher HCMV IgG titers were associated with pulmonary tuberculosis in adolescents.

Joint Modeling of Longitudinal Outcome and Competing Risks: Application to HIV/AIDS Data

BACKGROUND

Tuberculosis (TB) and human immunodeficiency virus (HIV) are major public health challenges globally, and the number of TB infections and death caused by HIV are high because of HIV/ TB co-infection. On the other hand, CD4 count plays a significant role in TB/HIV co-infections. We used a joint model of longitudinal outcomes and competing risks to identify the potential risk factors and the effect of CD4 cells on TB infection and death caused by HIV in HIV-infected patients.

STUDY DESIGN

This was a retrospective cohort study.

METHODS

The current study was performed on 1436 HIV+patients referred to Behavioral Diseases Counseling Centers in Kermanshah Province during 1998-2019. In this study, joint modeling was used to identify the effect of potential risk factors and CD4 cells on TB and death caused by HIV.

RESULTS

The results demonstrated that the decreasing CD4 cell count was significantly associated with an increased risk of death, while it had no significant relation with the risk of TB. In addition, patients with TB were at a higher risk of death. Based on the results, a significant relationship was found between CD4 count and sex, marital status, education level, antiretroviral therapy (ART), time, and the interaction between time and ART. Further, people infected with HIV through sexual relationships were at higher risk of TB, while those with a history of imprisonment who received ART or were infected with HIV through drug injection had a lower risk of TB.

CONCLUSION

The findings revealed that the decreasing CD4 count had a significant association with an increased risk of death caused by HIV. However, it was not significantly related to the risk of TB. Finally, patients with TB were at higher risk of death caused by HIV.

Indirect effects of cytomegalovirus infection: Implications for vaccine development

Development of a cytomegalovirus (CMV) vaccine is a high priority due to its significant global impact-contributing to mortality in immunosuppressed individuals, neurodevelopmental delay in infected neonates and non-genetic sensorineural hearing loss. The impact of CMV on the general population has been less well studied; however, a wide range of evidence indicates that CMV may increase the risk of atherosclerosis, cancer, immunosenescence, and progression of tuberculosis (TB) and human immunodeficiency virus. Due to the high seroprevalence of CMV worldwide, any modulation of risk by CMV is likely to have a significant impact on the epidemiology of these diseases. This review will evaluate how CMV may cause morbidity and mortality outside of the neonatal and immunosuppressed populations and consider the potential impact of a CMV vaccine on these outcomes.

Tissue-localized immune responses in people with cystic fibrosis and respiratory nontuberculous mycobacteria infection

Nontuberculous mycobacteria (NTM) are an increasingly common cause of respiratory infection in people with cystic fibrosis (PwCF). Relative to those with no history of NTM infection (CF-NTMNEG), PwCF and a history of NTM infection (CF-NTMPOS) are more likely to develop severe lung disease and experience complications over the course of treatment. In other mycobacterial infections (e.g. tuberculosis), an overexuberant immune response causes pathology and compromises organ function; however, since the immune profiles of CF-NTMPOS and CF-NTMNEG airways are largely unexplored, it is unknown which if any immune responses distinguish these cohorts or concentrate in damaged tissues. Here we evaluated lung lobe-specific immune profiles of three cohorts (CF-NTMPOS, CF-NTMNEG, and non-CF adults) and found that CF-NTMPOS airways are distinguished by a hyper-inflammatory cytokine profile. Importantly, the CF-NTMPOS airway immune profile was dominated by B cells, classical macrophages and the cytokines which support their accumulation. These and other immunological differences between cohorts, including the near absence of NK cells and complement pathway members, were enriched in the most damaged lung lobes. The implications of these findings for our understanding of lung disease in PwCF are discussed, as are how they may inform the development of host-directed therapies to improve NTM disease treatment.

Association Between Cytomegalovirus Infection and Tuberculosis Disease: A Systematic Review and Meta-Analysis of Epidemiological Studies

BACKGROUND

Tuberculosis is one of the leading causes of mortality worldwide from an infectious disease. This review aimed to investigate the association between prior cytomegalovirus infection and tuberculosis disease.

METHODS

Six bibliographic databases were searched from their respective inception to 31 December 2021. Data were pooled using random-effects meta-analysis.

RESULTS

Of 5476 identified articles, 15 satisfied the inclusion criteria with a total sample size of 38 618 patients. Pooled findings showed that individuals with cytomegalovirus infection had a higher risk of tuberculosis disease compared to those not infected with cytomegalovirus (odds ratio [OR], 3.20; 95% confidence interval [CI], 2.18-4.70). Age was the only covariate that exerted a significant effect on the result of the association. Meta-analysis of risk estimates reported in individual studies showed a marked and significant correlation of cytomegalovirus infection with active tuberculosis (adjusted hazard ratio, 2.92; 95% CI, 1.34-4.51; adjusted OR, 1.14; 95% CI, .71-1.57). A clear dose-response relation was inferred between the levels of cytomegalovirus antibodies and the risks of tuberculosis events (OR for high levels of cytomegalovirus antibodies, 4.07; OR for medium levels of cytomegalovirus antibodies, 3.58).

CONCLUSIONS

The results suggest an elevated risk of tuberculosis disease among individuals with a prior cytomegalovirus infection.

Mice infected with Mycobacterium tuberculosis are resistant to acute disease caused by secondary infection with SARS-CoV-2

Mycobacterium tuberculosis (Mtb) and SARS-CoV-2 (CoV2) are the leading causes of death due to infectious disease. Although Mtb and CoV2 both cause serious and sometimes fatal respiratory infections, the effect of Mtb infection and its associated immune response on secondary infection with CoV2 is unknown. To address this question we applied two mouse models of COVID19, using mice which were chronically infected with Mtb. In both model systems, Mtb-infected mice were resistant to the pathological consequences of secondary CoV2 infection, and CoV2 infection did not affect Mtb burdens. Single cell RNA sequencing of coinfected and monoinfected lungs demonstrated the resistance of Mtb-infected mice is associated with expansion of T and B cell subsets upon viral challenge. Collectively, these data demonstrate that Mtb infection conditions the lung environment in a manner that is not conducive to CoV2 survival.

Mycobacterium tuberculosis (Mtb) and SARS-CoV-2 (CoV2) are distinct organisms which both cause lung disease. We report the surprising observation that Mtb-infected mice are resistant to secondary infection with CoV2, with no impact on Mtb burden and resistance associating with lung T and B cell expansion.

Understanding the interaction between cytomegalovirus and tuberculosis in children: The way forward

Over 1 million children develop tuberculosis (TB) each year, with a quarter dying. Multiple factors impact the risk of a child being exposed to Mycobacterium tuberculosis (Mtb), the risk of progressing to TB disease, and the risk of dying. However, an emerging body of evidence suggests that coinfection with cytomegalovirus (CMV), a ubiquitous herpes virus, impacts the host response to Mtb, potentially influencing the probability of disease progression, type of TB disease, performance of TB diagnostics, and disease outcome. It is also likely that infection with Mtb impacts CMV pathogenesis. Our current understanding of the burden of these 2 diseases in children, their immunological interactions, and the clinical consequence of coinfection is incomplete. It is also unclear how potential interventions might affect disease progression and outcome for TB or CMV. This article reviews the epidemiological, clinical, and immunological literature on CMV and TB in children and explores how the 2 pathogens interact, while also considering the impact of HIV on this relationship. It outlines areas of research uncertainty and makes practical suggestions as to potential studies that might address these gaps. Current research is hampered by inconsistent definitions, study designs, and laboratory practices, and more consistency and collaboration between researchers would lead to greater clarity. The ambitious targets outlined in the World Health Organization End TB Strategy will only be met through a better understanding of all aspects of child TB, including the substantial impact of coinfections.

Cytomegalovirus acquisition in infancy and the risk of tuberculosis disease in childhood: a longitudinal birth cohort study in Cape Town, South Africa

BACKGROUND

The risk of tuberculosis disease after recent exposure is greatest before age 5 years; however, the mechanisms explaining this increased risk are not well elucidated. Acquisition of viral infections, such as cytomegalovirus, in early life might modulate the immune system. We aimed to evaluate the acquisition of cytomegalovirus infection in infancy and the development of tuberculosis disease in children.

METHODS

In this prospective, birth cohort study we enrolled pregnant women who were between 20 and 28 weeks of gestation attending antenatal care in Paarl, a periurban setting outside of Cape Town, South Africa. Participants were recruited from two clinics (TC Newman and Mbekweni). Infants were given Bacillus Calmette-Guérin vaccination at birth as per national policy. Nasopharyngeal swabs for cytomegalovirus detection using qPCR were done for infants at birth, age 3 and 6 weeks, and age 3, 6, 12, and 24 months. Children were prospectively followed up for tuberculosis disease until age 9 years using tuberculin skin testing, radiographic examinations, GeneXpert, and sputum testing. Tuberculin skin tests were done at the 6-month visit and then at age 12, 24, 36, 48, and 60 months, and at the time of lower respiratory tract infection. We compared tuberculosis disease incidence after age 1 year or after age 6 months in children with and without cytomegalovirus infection using Cox regression and hazard ratios (HRs) with 95% CIs.

FINDINGS

Between March 5, 2012, and March 31, 2015, 1225 pregnant women were recruited and enrolled in the birth cohort. 88 (7%) women were excluded because of loss to antenatal follow-up or pregnancy losses. Of 1143 livebirths, 68 (6%) mother-infant pairs were excluded. In total, 963 children were serially tested for cytomegalovirus (7186 cytomegalovirus measurements taken; median six tests per child, IQR 2-11). The prevalence of congenital cytomegalovirus at age younger than 3 weeks was 2% (18 of 816). Cytomegalovirus positivity increased continuously with age from 3% (27 of 825) by age 6 weeks to 21% (183 of 882) by 3 months, 35% (315 of 909) by 6 months, and 42% (390 of 933) by 12 months. Mother-infant pairs were followed up for a median of 6·9 years (IQR 6·0-7·8). The risk of tuberculosis disease in children after age 1 year was higher in those with cytomegalovirus infection by age 6 weeks (adjusted HR 4·1, 95% CI 1·2-13·8; p=0·022), 3 months (2·8, 1·4-5·8; p=0·0040), 6 months (3·6, 1·7-7·3; p<0·0001), 12 months (3·2, 1·6-6·4; p=0·0010), and 24 months (4·2, 2·0-8·8; p<0·0001). The risk of microbiologically confirmed tuberculosis disease was also higher among children acquiring cytomegalovirus infection before age 3 months (adjusted HR 3·2, 95% CI 1·0-10·6; p=0·048), 6 months (3·9, 1·2-13·0; p=0·027), 12 months (4·4, 1·2-16·3; p=0·027), and 24 months (6·1, 1·3-27·9; p=0·020). In children older than 1 year, the risk of tuberculosis disease was consistently greater in those with high cytomegalovirus loads than in those with low cytomegalovirus loads that were acquired before age 3 months (adjusted HR 2·0 vs 3·7; ptrend=0·0020; both groups compared with cytomegalovirus negative reference) and before age 12 months (2·7 vs 3·7; ptrend=0·0009).

INTERPRETATION

Infants that acquire cytomegalovirus in the first year of life are at high risk of subsequently developing tuberculosis disease. Efforts to prevent tuberculosis in early childhood in high-burden countries might need to deter or delay acquisition of cytomegalovirus perinatally or in the first months of life.

FUNDING

Bill & Melinda Gates Foundation, MRC South Africa, National Research Foundation South Africa, and Wellcome Trust.

Mice infected with Mycobacterium tuberculosis are resistant to secondary infection with SARS-CoV-2

Mycobacterium tuberculosis (Mtb) and SARS-CoV-2 (CoV2) are the leading causes of death due to infectious disease. Although Mtb and CoV2 both cause serious and sometimes fatal respiratory infections, the effect of Mtb infection and its associated immune response on secondary infection with CoV2 is unknown. To address this question we applied two mouse models of COVID19, using mice which were chronically infected with Mtb. In both model systems, Mtb-infected mice were resistant to secondary CoV2 infection and its pathological consequences, and CoV2 infection did not affect Mtb burdens. Single cell RNA sequencing of coinfected and monoinfected lungs demonstrated the resistance of Mtb-infected mice is associated with expansion of T and B cell subsets upon viral challenge. Collectively, these data demonstrate that Mtb infection conditions the lung environment in a manner that is not conducive to CoV2 survival.

AUTHOR SUMMARY

Mycobacterium tuberculosis (Mtb) and SARS-CoV-2 (CoV2) are distinct organisms which both cause lung disease. We report the surprising observation that Mtb-infected mice are resistant to secondary infection with CoV2, with no impact on Mtb burden and resistance associating with lung T and B cell expansion.

Cytomegalovirus Infection Is Associated With Increased Prevalence of Latent Tuberculosis Infection

Cytomegalovirus (CMV) increases tuberculosis (TB) risk, but its relationship with latent TB infection (LTBI) is unknown. Using US nationally representative data, we report that CMV was independently associated with LTBI (odds ratio, 2.94; 95% CI, 1.19–7.28; P=.02). CMV and LTBI were associated with higher C-reactive protein, suggesting chronic inflammation.

Sexual Behavior Surveys Should Ask More: Covering the Diversity of Sexual Behaviors That May Contribute to the Transmission of Pathogens

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Human Cytomegalovirus-Induced Interleukin-10 Production Promotes the Proliferation of Mycobacterium massiliense in Macrophages

Human cytomegalovirus (HCMV) exploits the interleukin-10 (IL-10) pathway as a part of its infection cycle through the manipulation of the host IL-10 signaling cascade. Based on its immunomodulatory nature, HCMV attenuates the host immune response and facilitates the progression of co-infection with other pathogens in an immune-competent host. To investigate the impact of HCMV infection on the burden of non-tuberculous mycobacteria (NTM), whose prevalence is growing rapidly worldwide, macrophages were infected with HCMV and further challenged with Mycobacterium massiliense in vitro. The results showed that HCMV infection significantly increased host IL-10 synthesis and promoted the proliferation of M. massiliense in an IL-10-dependent manner. Transcriptomic analysis revealed that HCMV infection dampened the regulatory pathways of interferon gamma (IFN-γ), tumor necrosis factor alpha (TNF-α), and interleukin-1 (IL-1), consequently abrogating the immune responses to M. massiliense coinfection in macrophages. These findings provide a mechanistic basis of how HCMV infection may facilitate the development of pathogenic NTM co-infection by upregulating IL-10 expression.

Gastrointestinal Bleeding in a Patient With Gastric Lymphoma, Tuberculosis Enteritis, and Cytomegalovirus Enteritis

Bleeding from the small bowel can be challenging to identify by endoscopic or radiographic evaluation. We present the case of a patient with incompletely treated latent tuberculosis and medical history of T-cell lymphoma who developed gastrointestinal bleeding because of concurrent Burkitt lymphoma, tuberculosis enteritis, and cytomegalovirus enteritis. The interplay of these 3 diagnoses is discussed.

A systematic review of the impact of psychosocial factors on immunity: Implications for enhancing BCG response against tuberculosis

BACKGROUND

Tuberculosis (TB) remains an urgent global public health priority, causing 1.5 million deaths worldwide in 2018. There is evidence that psychosocial factors modulate immune function; however, how this may influence TB risk or BCG vaccine response, and whether this pathway can be modified through social protection, has not been investigated. This paper aims to: a) systematically review evidence of how psychosocial factors influence the expression of biomarkers of immunity, and b) apply this general evidence to propose plausible TB-specific pathways for future study.

METHODS

Papers reporting on the impact of psychosocial stressors on immune biomarkers in relation to infectious disease risk were identified through a search of the databases MEDLINE, PsycINFO, Global Health and PsycEXTRA alongside reference list and citation searching of key papers. Data extraction and critical appraisal were carried out using a standardised form. The findings were tabulated and synthesised narratively by infectious disease category, and used to propose plausible mechanisms for how psychosocial exposures might influence immune outcomes relevant to TB and BCG response.

RESULTS

27,026 citations were identified, of which 51 met the inclusion criteria. The literature provides evidence of a relationship between psychosocial factors and immune biomarkers. While the direction and strength of associations is heterogenous, some overarching patterns emerged: adverse psychosocial factors (e.g. stress) were generally associated with compromised vaccine response and higher antibody titres to herpesviruses, and vice versa for positive psychosocial factors (e.g. social support).

CONCLUSIONS

The evidence identifies pathways linking psychosocial factors and immune response: co-viral infection and immune suppression, both of which are potentially relevant to TB and BCG response. However, the heterogeneity in the strength and nature of the impact of psychosocial factors on immune function, and lack of research on the implications of this relationship for TB, underscore the need for TB-specific research.

Seroprevalence of Human Cytomegalovirus in Pregnant Women in the Asir Region, Kingdom of Saudi Arabia

BACKGROUND

Human cytomegalovirus (HCMV) infection spreads easily by interpersonal contact.

OBJECTIVE

This study determined the prevalence of seropositivity of cytomegalovirus immunoglobulin G (IgG) in the Asir Region, Kingdom of Saudi Arabia.

METHODS

The study evaluated the seropositivity for cytomegalovirus-specific IgG in 460 females. Collected samples were processed and tested using enzyme-linked immunosorbent assay and specific HCMV IgG.

RESULTS

The study showed that all the respondents aged 15-20 years were seropositive for the HCMV. HCMV seropositive status was recorded in 99.2% of the older patients (>40 years of age). In the remaining age groups, the rate of seropositivity ranged from 95.7 (age range 20-25 years) to 98.9% (age range 30 years).

CONCLUSIONS

In all age groups of females tested, the prevalence of seropositive for HCMV was high, i.e., in the range of 95.7-100%.

Cytomegalovirus infection is a risk factor for tuberculosis disease in infants

Immune activation is associated with increased risk of tuberculosis (TB) disease in infants. We performed a case-control analysis to identify drivers of immune activation and disease risk. Among 49 infants who developed TB disease over the first 2 years of life, and 129 healthy matched controls, we found the cytomegalovirus-stimulated (CMV-stimulated) IFN-γ response to be associated with CD8+ T cell activation (Spearman's rho, P = 6 × 10-8). A CMV-specific IFN-γ response was also associated with increased risk of developing TB disease (conditional logistic regression; P = 0.043; OR, 2.2; 95% CI, 1.02-4.83) and shorter time to TB diagnosis (Log Rank Mantel-Cox, P = 0.037). CMV+ infants who developed TB disease had lower expression of NK cell-associated gene signatures and a lower frequency of CD3-CD4-CD8- lymphocytes. We identified transcriptional signatures predictive of TB disease risk among CMV ELISpot-positive (area under the receiver operating characteristic [AUROC], 0.98, accuracy, 92.57%) and -negative (AUROC, 0.9; accuracy, 79.3%) infants; the CMV- signature was validated in an independent infant study (AUROC, 0.71; accuracy, 63.9%). A 16-gene signature that previously identified adolescents at risk of developing TB disease did not accurately classify case and control infants in this study. Understanding the microbial drivers of T cell activation, such as CMV, could guide new strategies for prevention of TB disease in infants.

A rare case of CMV pneumonia in HIV-infection

Cytomegalovirus (CMV) pneumonia is a rare opportunistic infection in the setting of HIV (Human Immunodeficiency Virus)-infection. Establishing accurate diagnosis of CMV pneumonia in HIV-infection can be challenging. Co-infections by multiple opportunistic pathogens are common and a high degree of clinical vigilance to evaluate for multiple infections, including CMV pneumonia, should be maintained. As there can be a degree of overlap in clinical and radiological features amongst different opportunistic infections affecting the lungs, definitive microbiological and cytohistologic evidences are needed. Reliance on microbiological evidence of CMV in respiratory specimens alone for the diagnosis of CMV pneumonia will lead to an over-diagnosis of the condition and unnecessary treatment.

In our case report, we describe a 53-year-old man with recently diagnosed HIV-infection who presented with non-resolving pneumonia. A diagnosis of CMV pneumonia was reached through consistent clinical, radiological, microbiological and cytologic investigations. The patient made a full clinical recovery after being started on anti-CMV treatment.

Examining the Complex Relationship Between Tuberculosis and Other Infectious Diseases in Children

Millions of children are exposed to tuberculosis (TB) each year, many of which become infected with Mycobacterium tuberculosis. Most children can immunologically contain or eradicate the organism without pathology developing. However, in a minority, the organism overcomes the immunological constraints, proliferates and causes TB disease. Each year a million children develop TB disease, with a quarter dying. While it is known that young children and those with immunodeficiencies are at increased risk of progression from TB infection to TB disease, our understanding of risk factors for this transition is limited. The most immunologically disruptive process that can happen during childhood is infection with another pathogen and yet the impact of co-infections on TB risk is poorly investigated. Many diseases have overlapping geographical distributions to TB and affect similar patient populations. It is therefore likely that infection with viruses, bacteria, fungi and protozoa may impact on the risk of developing TB disease following exposure and infection, although disentangling correlation and causation is challenging. As vaccinations also disrupt immunological pathways, these may also impact on TB risk. In this article we describe the pediatric immune response to M. tuberculosis and then review the existing evidence of the impact of co-infection with other pathogens, as well as vaccination, on the host response to M. tuberculosis. We focus on the impact of other organisms on the risk of TB disease in children, in particularly evaluating if co-infections drive host immune responses in an age-dependent way. We finally propose priorities for future research in this field. An improved understanding of the impact of co-infections on TB could assist in TB control strategies, vaccine development (for TB vaccines or vaccines for other organisms), TB treatment approaches and TB diagnostics.

The Wonder Years: What Can Primary School Children Teach Us About Immunity to Mycobacterium tuberculosis?

In high burden settings, the risk of infection with Mycobacterium tuberculosis increases throughout childhood due to cumulative exposure. However, the risk of progressing from tuberculosis (TB) infection to disease varies by age. Young children (<5 years) have high risk of disease progression following infection. The risk falls in primary school children (5 to <10 years), but rises again during puberty. TB disease phenotype also varies by age: generally, young children have intrathoracic lymph node disease or disseminated disease, while adolescents (10 to <20 years) have adult-type pulmonary disease. TB risk also exhibits a gender difference: compared to adolescent boys, adolescent girls have an earlier rise in disease progression risk and higher TB incidence until early adulthood. Understanding why primary school children, during what we term the "Wonder Years," have low TB risk has implications for vaccine development, therapeutic interventions, and diagnostics. To understand why this group is at low risk, we need a better comprehension of why younger children and adolescents have higher risks, and why risk varies by gender. Immunological response to M. tuberculosis is central to these issues. Host response at key stages in the immunopathological interaction with M. tuberculosis influences risk and disease phenotype. Cell numbers and function change dramatically with age and sexual maturation. Young children have poorly functioning innate cells and a Th2 skew. During the "Wonder Years," there is a lymphocyte predominance and a Th1 skew. During puberty, neutrophils become more central to host response, and CD4+ T cells increase in number. Sex hormones (dehydroepiandrosterone, adiponectin, leptin, oestradiol, progesterone, and testosterone) profoundly affect immunity. Compared to girls, boys have a stronger Th1 profile and increased numbers of CD8+ T cells and NK cells. Girls are more Th2-skewed and elicit more enhanced inflammatory responses. Non-immunological factors (including exposure intensity, behavior, and co-infections) may impact disease. However, given the consistent patterns seen across time and geography, these factors likely are less central. Strategies to protect children and adolescents from TB may need to differ by age and sex. Further work is required to better understand the contribution of age and sex to M. tuberculosis immunity.

Sex differences in tuberculosis

Tuberculosis is the most prevalent bacterial infectious disease in humans and the leading cause of death from a single infectious agent, ranking above HIV/AIDS. The causative agent, Mycobacterium tuberculosis, is carried by an estimated two billion people globally and claims more than 1.5 million lives each year. Tuberculosis rates are significantly higher in men than in women, reflected by a male-to-female ratio for worldwide case notifications of 1.7. This phenomenon is not new and has been reported in various countries and settings over the last century. However, the reasons for the observed gender bias are not clear, potentially highly complex and discussed controversially in the literature. Both gender- (referring to sociocultural roles and behavior) and sex-related factors (referring to biological aspects) likely contribute to higher tuberculosis rates in men and will be discussed.