Acetic Acid Enables Molecular Enumeration of Mycobacterium tuberculosis from Sputum and Eliminates the Need for a Biosafety Level 3 Laboratory

BACKGROUND

Improved monitoring of Mycobacterium tuberculosis response to treatment is urgently required. We previously developed the molecular bacterial load assay (MBLA), but it is challenging to integrate into the clinical diagnostic laboratory due to a labor-intensive protocol required at biosafety level 3 (BSL-3). A modified assay was needed.

METHODS

The rapid enumeration and diagnostic for tuberculosis (READ-TB) assay was developed. Acetic acid was tested and compared to 4 M guanidine thiocyanate to be simultaneously bactericidal and preserve mycobacterial RNA. The extraction was based on silica column technology and incorporated low-cost reagents: 3 M sodium acetate and ethanol for the RNA extraction to replace phenol-chloroform. READ-TB was fully validated and compared directly to the MBLA using sputa collected from individuals with tuberculosis.

RESULTS

Acetic acid was bactericidal to M. tuberculosis with no significant loss in 16S rRNA or an unprotected mRNA fragment when sputum was stored in acetic acid at 25°C for 2 weeks or -20°C for 1 year. This novel use of acetic acid allows processing of sputum for READ-TB at biosafety level 2 (BSL-2) on sample receipt. READ-TB is semiautomated and rapid. READ-TB correlated with the MBLA when 85 human sputum samples were directly compared (R2 = 0.74).

CONCLUSIONS

READ-TB is an improved version of the MBLA and is available to be adopted by clinical microbiology laboratories as a tool for tuberculosis treatment monitoring. READ-TB will have a particular impact in low- and middle-income countries (LMICs) for laboratories with no BSL-3 laboratory and for clinical trials testing new combinations of anti-tuberculosis drugs.

Current sampling and sequencing biases of Lassa mammarenavirus limit inference from phylogeography and molecular epidemiology in Lassa fever endemic regions

Lassa fever (LF) is a potentially lethal viral haemorrhagic infection of humans caused by Lassa mammarenavirus (LASV). It is an important endemic zoonotic disease in West Africa with growing evidence for increasing frequency and sizes of outbreaks. Phylogeographic and molecular epidemiology methods have projected expansion of the Lassa fever endemic zone in the context of future global change. The Natal multimammate mouse (Mastomys natalensis) is the predominant LASV reservoir, with few studies investigating the role of other animal species. To explore host sequencing biases, all LASV nucleotide sequences and associated metadata available on GenBank (n = 2,298) were retrieved. Most data originated from Nigeria (54%), Guinea (20%) and Sierra Leone (14%). Data from non-human hosts (n = 703) were limited and only 69 sequences encompassed complete genes. We found a strong positive correlation between the number of confirmed human cases and sequences at the country level (r = 0.93 (95% Confidence Interval = 0.71-0.98), p < 0.001) but no correlation exists between confirmed cases and the number of available rodent sequences (r = -0.019 (95% C.I. -0.71-0.69), p = 0.96). Spatial modelling of sequencing effort highlighted current biases in locations of available sequences, with increased sequencing effort observed in Southern Guinea and Southern Nigeria. Phylogenetic analyses showed geographic clustering of LASV lineages, suggestive of isolated events of human-to-rodent transmission and the emergence of currently circulating strains of LASV from the year 1498 in Nigeria. Overall, the current study highlights significant geographic limitations in LASV surveillance, particularly, in non-human hosts. Further investigation of the non-human reservoir of LASV, alongside expanded surveillance, are required for precise characterisation of the emergence and dispersal of LASV. Accurate surveillance of LASV circulation in non-human hosts is vital to guide early detection and initiation of public health interventions for future Lassa fever outbreaks.

A pandemic within a pandemic? Admission to COVID-19 wards in hospitals is associated with increased prevalence of antimicrobial resistance in two African settings

BACKGROUND

Patients who develop severe illness due to COVID-19 are more likely to be admitted to hospital and acquire bacterial co-infections, therefore the WHO recommends empiric treatment with antibiotics. Few reports have addressed the impact of COVID-19 management on emergence of nosocomial antimicrobial resistance (AMR) in resource constrained settings. This study aimed to ascertain whether being admitted to a COVID-19 ward (with COVID-19 infection) compared to a non-COVID-19 ward (as a COVID-19 negative patient) was associated with a change in the prevalence of bacterial hospital acquired infection (HAI) species or resistance patterns, and whether there were differences in antimicrobial stewardship (AMS) and infection prevention and control (IPC) guidelines between COVID-19 and non-COVID-19 wards. The study was conducted in Sudan and Zambia, two resource constrained settings with differing country-wide responses to COVID-19.

METHODS

Patients suspected of having hospital acquired infections were recruited from COVID-19 wards and non-COVID-19 wards. Bacteria were isolated from clinical samples using culture and molecular methods and species identified. Phenotypic and genotypic resistance patterns were determined by antibiotic disc diffusion and whole genome sequencing. Infection prevention and control guidelines were analysed for COVID-19 and non-COVID-19 wards to identify potential differences.

RESULTS

109 and 66 isolates were collected from Sudan and Zambia respectively. Phenotypic testing revealed significantly more multi-drug resistant isolates on COVID-19 wards in both countries (Sudan p = 0.0087, Zambia p = 0.0154). The total number of patients with hospital acquired infections (both susceptible and resistant) increased significantly on COVID-19 wards in Sudan, but the opposite was observed in Zambia (both p = ≤ 0.0001). Genotypic analysis showed significantly more β-lactam genes per isolate on COVID-19 wards (Sudan p = 0.0192, Zambia p = ≤ 0.0001).

CONCLUSIONS

Changes in hospital acquired infections and AMR patterns were seen in COVID-19 patients on COVID-19 wards compared to COVID-19 negative patients on non-COVID-19 wards in Sudan and Zambia. These are likely due to a potentially complex combination of causes, including patient factors, but differing emphases on infection prevention and control, and antimicrobial stewardship policies on COVID-19 wards were highlighted.

Increased outbreaks of monkeypox highlight gaps in actual disease burden in Sub-Saharan Africa and in animal reservoirs

We explain research gaps on Monkeypox (MPX) virus epidemiology in endemic countries and present hypotheses for the recent increase of MPX cases in West Africa as a possible explanation for the current epidemic in Europe, America, and Australia. The detection of >400 MPX cases in less than a month in May 2022, across many countries underscores the epidemic potential of MPX in humans and demonstrates several important research gaps. First, the true burden of MPX in West and Central Africa is poorly understood, although it is critical for prevention and control of future outbreaks. Second, the diversity and extent of the animal reservoir remain unknown. We hypothesize that the synanthropic rodent population has increased in recent years in Africa leading to more human-rodent interactions and thus increased transmission of MPXV. We further hypothesise that nearly 45 years after the end of routine smallpox vaccination, the larger and more interconnected immune-naïve population has crossed a threshold resulting in more sustainable human-to-human transmission of MPXV. The current epidemic in the Western World is possibly a consequence of increased local transmission of MPXV in Africa. A new estimation of the basic and effective reproduction rate (R0 and Re) in different populations is required. National, regional, and international collaborations are needed to address research gaps related to MPX outbreaks.

Culture-Free Enumeration of Mycobacterium tuberculosis in Mouse Tissues Using the Molecular Bacterial Load Assay for Preclinical Drug Development

Background: The turnaround times for phenotypic tests used to monitor the bacterial load of Mycobacterium tuberculosis, in both clinical and preclinical studies, are delayed by the organism’s slow growth in culture media. The existence of differentially culturable populations of M.tuberculosis may result in an underestimate of the true number. Moreover, culture methods are susceptible to contamination resulting in loss of critical data points. Objectives: We report the adaptation of our robust, culture-free assay utilising 16S ribosomal RNA, developed for sputum, to enumerate the number of bacteria present in animal tissues as a tool to improve the read-outs in preclinical drug efficacy studies. Methods: Initial assay adaptation was performed using naïve mouse lungs spiked with known quantities of M. tuberculosis and an internal RNA control. Tissues were homogenised, total RNA extracted, and enumeration performed using RT-qPCR. We then evaluated the utility of the assay, in comparison to bacterial counts estimated using growth assays on solid and liquid media, to accurately inform bacterial load in tissues from M. tuberculosis-infected mice before and during treatment with a panel of drug combinations. Results: When tested on lung tissues derived from infected mice, the MBL assay produced comparable results to the bacterial counts in solid culture (colony forming units: CFU). Notably, under specific drug treatments, the MBL assay was able to detect a significantly higher number of M. tuberculosis compared to CFU, likely indicating the presence of bacteria that were unable to produce colonies in solid-based culture. Additionally, growth recovery in liquid media using the most probable number (MPN) assay was able to account for the discrepancy between the MBL assay and CFU number, suggesting that the MBL assay detects differentially culturable sub-populations of M. tuberculosis. Conclusions: The MBL assay can enumerate the bacterial load in animal tissues in real time without the need to wait for extended periods for cultures to grow. The readout correlates well with CFUs. Importantly, we have shown that the MBL is able to measure specific populations of bacteria not cultured on solid agar. The adaptation of this assay for preclinical studies has the potential to decrease the readout time of data acquisition from animal experiments and could represent a valuable tool for tuberculosis drug discovery and development.

O03 The impact of COVID-19 infection prevention and control measures on transmission of hospital-acquired infections and antimicrobial resistance in Africa

Background

Patients who develop serious illness due to COVID-19 are more likely to have bacterial coinfections, for which WHO recommends treatment with antibiotics. As a result, many countries are observing a change in antimicrobial stewardship (AMS), in addition to changes in infection prevention and control (IPC) practices such as the use of personal protective equipment, on COVID-19 wards. Few data on COVID-19 and its impact on nosocomial infections and antimicrobial resistance (AMR) are available from low and middle-income countries (LMICs). As these countries often have high rates of AMR, it is vital to report the effects of COVID-19 on AMS so as to inform clinical practice and IPC guidelines. This study aims to compare prevalence of AMR in COVID-19 wards with general non-COVID-19 hospital wards.

Methods

This pilot hospital-based study is being conducted in two sites in both Sudan and Zambia. IPC and AMS guidelines for COVID-19 and non-COVID-19 wards were identified for each institution. This study is comparing bacterial isolates and AMR patterns of nosocomial associated infections acquired on COVID-19 and non-COVID-19 wards were compared, using microbiological and sequencing methods. A total of 200 patients have been recruited: 100 per country, 50 COVID-19 patients and 50 non-COVID-19 patients. AMR transmission patterns are being identified using Oxford Nanopore Technologies sequencing for phylogenetic analysis.

Results

The study began recruiting in May 2021 and completed recruitment of patients in October 2021. The majority of microbiological laboratory work will be completed within Q3 2021, with analysis of the results and sequencing completed in Q4 2021. A half-way point summary analysis of the data suggests differences in patient profiles, both between COVID-19 and non-COVID-19 wards at both sites, as well as differences between the two countries. Preliminary analysis also suggests a significant difference between the prevalence of MDR infections in Gram-negatives seen between COVID-19 (53% in Sudan and 83% in Zambia) and non-COVID-19 (14% Sudan, 33% Zambia) (t-test, P=0.0032 Sudan, P=0.0455 Zambia) ward patients in both countries (see Figure 1).

Conclusions

The study is providing evidence to inform policy on IPC and AMS measures to be implemented on COVID-19 wards. In addition, the outcomes of the study will be used to create a pragmatic sequencing pipeline for potential AMR outbreaks suitable for use in LMICs clinical settings.

Validation of Differentially Expressed Immune Biomarkers in Latent and Active Tuberculosis by Real-Time PCR

Tuberculosis (TB) remains a major global threat and diagnosis of active TB ((ATB) both extra-pulmonary (EPTB), pulmonary (PTB)) and latent TB (LTBI) infection remains challenging, particularly in high-burden countries which still rely heavily on conventional methods. Although molecular diagnostic methods are available, e.g., Cepheid GeneXpert, they are not universally available in all high TB burden countries. There is intense focus on immune biomarkers for use in TB diagnosis, which could provide alternative low-cost, rapid diagnostic solutions. In our previous gene expression studies, we identified peripheral blood leukocyte (PBL) mRNA biomarkers in a non-human primate TB aerosol-challenge model. Here, we describe a study to further validate select mRNA biomarkers from this prior study in new cohorts of patients and controls, as a prerequisite for further development. Whole blood mRNA was purified from ATB patients recruited in the UK and India, LTBI and two groups of controls from the UK (i) a low TB incidence region (CNTRLA) and (ii) individuals variably-domiciled in the UK and Asia ((CNTRLB), the latter TB high incidence regions). Seventy-two mRNA biomarker gene targets were analyzed by qPCR using the Roche Lightcycler 480 qPCR platform and data analyzed using GeneSpring™ 14.9 bioinformatics software. Differential expression of fifty-three biomarkers was confirmed between MTB infected, LTBI groups and controls, seventeen of which were significant using analysis of variance (ANOVA): CALCOCO2, CD52, GBP1, GBP2, GBP5, HLA-B, IFIT3, IFITM3, IRF1, LOC400759 (GBP1P1), NCF1C, PF4V1, SAMD9L, S100A11, TAF10, TAPBP, and TRIM25. These were analyzed using receiver operating characteristic (ROC) curve analysis. Single biomarkers and biomarker combinations were further assessed using simple arithmetic algorithms. Minimal combination biomarker panels were delineated for primary diagnosis of ATB (both PTB and EPTB), LTBI and identifying LTBI individuals at high risk of progression which showed good performance characteristics. These were assessed for suitability for progression against the standards for new TB diagnostic tests delineated in the published World Health Organization (WHO) technology product profiles (TPPs).

Tuberculosis bacillary load, an early marker of disease severity: the utility of tuberculosis Molecular Bacterial Load Assay

In this comparative biomarker study, we analysed 1768 serial sputum samples from 178 patients at 4 sites in Southeast Africa. We show that tuberculosis Molecular Bacterial Load Assay (TB-MBLA) reduces time-to-TB-bacillary-load-result from days/weeks by culture to hours and detects early patient treatment response. By day 14 of treatment, 5% of patients had cleared bacillary load to zero, rising to 58% by 12th week of treatment. Fall in bacillary load correlated with mycobacterial growth indicator tube culture time-to-positivity (Spearmans r=−0.51, 95% CI (−0.56 to −0.46), p<0.0001). Patients with high pretreatment bacillary burdens (above the cohort bacillary load average of 5.5log₁₀eCFU/ml) were less likely to convert-to-negative by 8th week of treatment than those with a low burden (below cohort bacillary load average), p=0.0005, HR 3.1, 95% CI (1.6 to 5.6) irrespective of treatment regimen. TB-MBLA distinguished the bactericidal effect of regimens revealing the moxifloxacin—20 mg rifampicin regimen produced a shorter time to bacillary clearance compared with standard-of-care regimen, p=0.008, HR 2.9, 95% CI (1.3 to 6.7). Our data show that the TB-MBLA could inform clinical decision making in real-time and expedite drug TB clinical trials.

Monkeypox - Enhancing public health preparedness for an emerging lethal human zoonotic epidemic threat in the wake of the smallpox post-eradication era

The identification of monkeypox in 3 separate patients in the United Kingdom in September raised media and political attention on an emerging public health threat. Nigeria, whose last confirmed case of monkeypox was in 1978, is currently experiencing an unusually large and outbreak of human monkeypox cases, a 'One Human-Environmental-Animal Health' approach is being effectively used to define and tackle the outbreak. As of 13th October 2018, there have been one hundred and sixteen confirmed cases the majority of whom are under 40 years. Over the past 20 years ten Central and West African countries have reported monkeypox cases which have risen exponentially. We review the history and evolution of monkeypox outbreaks in Africa and USA, the changing clinical presentations, and discuss possible factors underlying the increasing numbers being detected including the cessation of smallpox vaccination programs. Major knowledge gaps remain on the epidemiology, host reservoir, and emergence, transmission, pathogenesis and prevention of monkeypoz.

Optimising molecular diagnostic capacity for effective control of tuberculosis in high-burden settings

The World Health Organization's 2035 vision is to reduce tuberculosis (TB) associated mortality by 95%. While low-burden, well-equipped industrialised economies can expect to see this goal achieved, it is challenging in the low- and middle-income countries that bear the highest burden of TB. Inadequate diagnosis leads to inappropriate treatment and poor clinical outcomes. The roll-out of the Xpert(®) MTB/RIF assay has demonstrated that molecular diagnostics can produce rapid diagnosis and treatment initiation. Strong molecular services are still limited to regional or national centres. The delay in implementation is due partly to resources, and partly to the suggestion that such techniques are too challenging for widespread implementation. We have successfully implemented a molecular tool for rapid monitoring of patient treatment response to anti-tuberculosis treatment in three high TB burden countries in Africa. We discuss here the challenges facing TB diagnosis and treatment monitoring, and draw from our experience in establishing molecular treatment monitoring platforms to provide practical insights into successful optimisation of molecular diagnostic capacity in resource-constrained, high TB burden settings. We recommend a holistic health system-wide approach for molecular diagnostic capacity development, addressing human resource training, institutional capacity development, streamlined procurement systems, and engagement with the public, policy makers and implementers of TB control programmes.

Pediatric tuberculosis-human immunodeficiency virus co-infection in the United Kingdom highlights the need for better therapy monitoring tools: a case report

Background

Tuberculosis is an infection that requires at least 6 months of chemotherapy in order to clear the bacteria from the patient’s lungs. Usually, therapeutic monitoring is dependent on smear microscopy where a decline in acid-fast bacilli is observed. However, this might not be indicative of the actual decline of bacterial load and thus other tools such as culture and molecular assays are required for patient management.

Case presentation

Here, we report the case of a 12-year-old Black African boy co-infected with tuberculosis and human immunodeficiency virus who remained smear culture positive and liquid culture negative for a prolonged period of time following chemotherapy. In order to determine whether there was any live bacteria present in his specimens, we applied the newly developed molecular bacterial load assay that detects the presence of 16S ribosomal ribonucleic acid derived from the bacteria. Using this methodology, we were able to quantify his bacterial load and inform the management of his treatment in order to reduce the disease burden. Following this intervention he went on to make a complete recovery.

Conclusions

This case report highlights the value of improved biomarkers for monitoring the treatment of tuberculosis and the role of molecular assays such as the molecular bacterial load assay applied here. The molecular bacterial load assay detects bacterial ribonucleic acid which corresponds closely with the number of live bacilli as compared with polymerase chain reaction that detects deoxyribonucleic acid and may include dead bacteria.

The use of digital PCR to improve the application of quantitative molecular diagnostic methods for tuberculosis

BACKGROUND

Real-time PCR (qPCR) based methods, such as the Xpert MTB/RIF, are increasingly being used to diagnose tuberculosis (TB). While qualitative methods are adequate for diagnosis, the therapeutic monitoring of TB patients requires quantitative methods currently performed using smear microscopy. The potential use of quantitative molecular measurements for therapeutic monitoring has been investigated but findings have been variable and inconclusive. The lack of an adequate reference method and reference materials is a barrier to understanding the source of such disagreement. Digital PCR (dPCR) offers the potential for an accurate method for quantification of specific DNA sequences in reference materials which can be used to evaluate quantitative molecular methods for TB treatment monitoring.

METHODS

To assess a novel approach for the development of quality assurance materials we used dPCR to quantify specific DNA sequences in a range of prototype reference materials and evaluated accuracy between different laboratories and instruments. The materials were then also used to evaluate the quantitative performance of qPCR and Xpert MTB/RIF in eight clinical testing laboratories.

RESULTS

dPCR was found to provide results in good agreement with the other methods tested and to be highly reproducible between laboratories without calibration even when using different instruments. When the reference materials were analysed with qPCR and Xpert MTB/RIF by clinical laboratories, all laboratories were able to correctly rank the reference materials according to concentration, however there was a marked difference in the measured magnitude.

CONCLUSIONS

TB is a disease where the quantification of the pathogen could lead to better patient management and qPCR methods offer the potential to rapidly perform such analysis. However, our findings suggest that when precisely characterised materials are used to evaluate qPCR methods, the measurement result variation is too high to determine whether molecular quantification of Mycobacterium tuberculosis would provide a clinically useful readout. The methods described in this study provide a means by which the technical performance of quantitative molecular methods can be evaluated independently of clinical variability to improve accuracy of measurement results. These will assist in ultimately increasing the likelihood that such approaches could be used to improve patient management of TB.

Profiling persistent tubercule bacilli from patient sputa during therapy predicts early drug efficacy

BACKGROUND

New treatment options are needed to maintain and improve therapy for tuberculosis, which caused the death of 1.5 million people in 2013 despite potential for an 86 % treatment success rate. A greater understanding of Mycobacterium tuberculosis (M.tb) bacilli that persist through drug therapy will aid drug development programs. Predictive biomarkers for treatment efficacy are also a research priority.

METHODS AND RESULTS

Genome-wide transcriptional profiling was used to map the mRNA signatures of M.tb from the sputa of 15 patients before and 3, 7 and 14 days after the start of standard regimen drug treatment. The mRNA profiles of bacilli through the first 2 weeks of therapy reflected drug activity at 3 days with transcriptional signatures at days 7 and 14 consistent with reduced M.tb metabolic activity similar to the profile of pre-chemotherapy bacilli. These results suggest that a pre-existing drug-tolerant M.tb population dominates sputum before and after early drug treatment, and that the mRNA signature at day 3 marks the killing of a drug-sensitive sub-population of bacilli. Modelling patient indices of disease severity with bacterial gene expression patterns demonstrated that both microbiological and clinical parameters were reflected in the divergent M.tb responses and provided evidence that factors such as bacterial load and disease pathology influence the host-pathogen interplay and the phenotypic state of bacilli. Transcriptional signatures were also defined that predicted measures of early treatment success (rate of decline in bacterial load over 3 days, TB test positivity at 2 months, and bacterial load at 2 months).

CONCLUSIONS

This study defines the transcriptional signature of M.tb bacilli that have been expectorated in sputum after two weeks of drug therapy, characterizing the phenotypic state of bacilli that persist through treatment. We demonstrate that variability in clinical manifestations of disease are detectable in bacterial sputa signatures, and that the changing M.tb mRNA profiles 0-2 weeks into chemotherapy predict the efficacy of treatment 6 weeks later. These observations advocate assaying dynamic bacterial phenotypes through drug therapy as biomarkers for treatment success.

Targeting of a CD8 T cell env epitope presented by HLA-B*5802 is associated with markers of HIV disease progression and lack of selection pressure

In HIV-infected persons, certain HLA class I alleles are associated with effective control of viremia, while others are associated with rapid disease progression. Among the most divergent clinical outcomes are the relatively good prognosis in HLA-B*5801 expressing persons and poor prognosis with HLA-B*5802. These two alleles differ by only three amino acids in regions involved in HLA-peptide recognition. This study evaluated a cohort of over 1000 persons with chronic HIV clade C virus infection to determine whether clinical outcome differences associated with B*5801 (n = 93) and B*5802 ( n = 259) expression are associated with differences in HIV-1-specific CD8 (+) T cell responses. The overall breadth and magnitude of HIV-1-specific CD8(+) T cell responses were lower in persons expressing B*5802, and epitope presentation by B*5802 contributed significantly less to the overall response as compared to B*5801-restricted CD8 (+) T cells. Moreover, viral load in B*5802-positive persons was higher and CD4 cell counts lower when this allele contributed to the overall CD8 (+) T cell response, which was detected exclusively through a single epitope in Env. In addition, persons heterozygous for B*5802 compared to persons homozygous for other HLA-B alleles had significantly higher viral loads. Viral sequencing revealed strong selection pressure mediated through B*5801-restricted responses but not through B*5802. These data indicate that minor differences in HLA sequence can have a major impact on epitope recognition, and that selective targeting of Env through HLA-B*5802 is at least ineffectual if not actively adverse in the containment of viremia. These results provide experimental evidence that not all epitope-specific responses contribute to immune containment, a better understanding of which is essential to shed light on mechanisms involved in HIV disease progression.

Compensatory mutation partially restores fitness and delays reversion of escape mutation within the immunodominant HLA-B*5703-restricted Gag epitope in chronic human immunodeficiency virus type 1 infection

HLA-B*5703 is associated with effective immune control in human immunodeficiency virus type 1 (HIV-1) infection. Here we describe an escape mutation within the immunodominant HLA-B*5703-restricted epitope in chronic HIV-1 infection, KAFSPEVIPMF (Gag 162-172), and demonstrate that this mutation reduces viral replicative capacity. Reversion of this mutation following transmission to HLA-B*5703-negative recipients was delayed by the compensatory mutation S165N within the same epitope. These data may help explain the observed association between HLA-B*5703 and long-term control of viremia.

Control of human immunodeficiency virus type 1 is associated with HLA-B*13 and targeting of multiple gag-specific CD8+ T-cell epitopes

To better understand relationships between CD8+ T-cell specificity and the immune control of human immunodeficiency virus type 1 (HIV-1), we analyzed the role of HLA-B*13, an allele associated with low viremia, in a cohort of 578 C clade-infected individuals in Durban, South Africa. Six novel B*13-restricted cytotoxic T lymphocyte epitopes were defined from analyses of 37 B*13-positive subjects, including three Gag epitopes. These B*13-restricted epitopes contribute to a broad Gag-specific CD8+ response that is associated with the control of viremia. These data are consistent with data from studies of other HLA-class I alleles associated with HIV control that have shown that the targeting of multiple Gag epitopes is associated with relative suppression of viremia.

CD8+ T-cell responses to different HIV proteins have discordant associations with viral load

Selection of T-cell vaccine antigens for chronic persistent viral infections has been largely empirical. To define the relationship, at the population level, between the specificity of the cellular immune response and viral control for a relevant human pathogen, we performed a comprehensive analysis of the 160 dominant CD8(+) T-cell responses in 578 untreated HIV-infected individuals from KwaZulu-Natal, South Africa. Of the HIV proteins targeted, only Gag-specific responses were associated with lowering viremia. Env-specific and Accessory/Regulatory protein-specific responses were associated with higher viremia. Increasing breadth of Gag-specific responses was associated with decreasing viremia and increasing Env breadth with increasing viremia. Association of the specific CD8(+) T-cell response with low viremia was independent of HLA type and unrelated to epitope sequence conservation. These population-based data, suggesting the existence of both effective immune responses and responses lacking demonstrable biological impact in chronic HIV infection, are of relevance to HIV vaccine design and evaluation.

Differential selection pressure exerted on HIV by CTL targeting identical epitopes but restricted by distinct HLA alleles from the same HLA supertype

HLA diversity is seen as a major challenge to CTL vaccines against HIV. One current approach focuses on "promiscuous" epitopes, presented by multiple HLA alleles from within the same HLA supertype. However, the effectiveness of such supertype vaccines depends upon the functional equivalence of CTL targeting a particular epitope, irrespective of the restricting HLA. In this study, we describe the promiscuous HIV-specific CTL epitopes presented by alleles within the B7 supertype. Substantial differences were observed in the ability of CTL to select for escape mutation when targeting the same epitope but restricted by different HLA. This observation was common to all six promiscuous B7 epitopes identified. Moreover, with one exception, there were no significant differences in the frequency, magnitude, or immunodominance of the CTL responses restricted by different HLA alleles to explain these discrepancies. This suggests that the unique peptide/MHC complexes generated by even closely related HLA induce CTL responses that are qualitatively different. This hypothesis is supported by additional differences observed between CTL targeting identical epitopes but restricted by different HLA: first, the occurrence of distinct, HLA-specific escape mutation; second, the recruitment of distinct TCR repertoires by particular peptide/MHC complexes; and, third, significant differences in the functional avidity of CTL. Taken together, these data indicate that significant functional differences exist between CTL targeting identical epitopes but restricted by different, albeit closely related HLA. These findings are of relevance to vaccine approaches that seek to exploit HLA supertypes to overcome the problem of HLA diversity.

Proliferative capacity of epitope-specific CD8 T-cell responses is inversely related to viral load in chronic human immunodeficiency virus type 1 infection

The relationship between the function of human immunodeficiency virus (HIV)-specific CD8 T-cell responses and viral load has not been defined. In this study, we used a panel of major histocompatibility complex class I tetramers to examine responses to frequently targeted CD8 T-cell epitopes in a large cohort of antiretroviral-therapy-naïve HIV type 1 clade C virus-infected persons in KwaZulu Natal, South Africa. In terms of effector functions of proliferation, cytokine production, and degranulation, only proliferation showed a significant correlation with viral load. This robust inverse relationship provides an important functional correlate of viral control relevant to both vaccine design and evaluation.

Motif inference reveals optimal CTL epitopes presented by HLA class I alleles highly prevalent in southern Africa

HIV-specific CTL play a central role in immune control of HIV. The basis for understanding the success or failure of this immune response requires identification of the specific epitopes targeted by CTL. However, in populations most severely affected by the global epidemic, this fundamental knowledge is hindered by the lack of characterization of many of the HLA class I alleles highly prevalent in such populations. Overall, the peptide-binding motif has been determined for a small minority (9%) of HLA class I alleles, with a strong bias toward those alleles prevalent in Caucasoid populations. These studies therefore set out to define, in a South African Zulu/Xhosa population at the epicenter of the epidemic, the epitopes presented by alleles highly prevalent, but for which the peptide-binding motif had not been characterized. Using a method of motif inference, epitopes presented by four such alleles prevalent in the Zulu/Xhosa population of Durban, South Africa, namely, B*3910, B*4201, B*8101, and Cw*1801, are described. Importantly, this approach may additionally facilitate optimization of epitopes in certain instances where conflicting reports in the literature exist regarding the peptide-binding motif, such as for HLA-A*2902, also highly prevalent in southern African populations. These data indicate that the previously anomalous position of HLA-A*2902 among HLA-A alleles, outside any recognized HLA-A supertype, is artifactual, and the true position of the A*2902 motif overlaps those of the A1 and A24 supertypes.

Differential immunogenicity of HIV-1 clade C proteins in eliciting CD8+ and CD4+ cell responses

BACKGROUND

The relative immunogenicity of human immunodeficiency virus type 1 (HIV-1) proteins for CD8+ and CD4+ cell responses has not been defined.

METHODS

HIV-1-specific T cell responses were evaluated in 65 chronically HIV-1-infected untreated subjects by interferon- gamma flow cytometry with peptides spanning the clade C consensus sequence.

RESULTS

The magnitude of HIV-1-specific CD8+ T cell responses correlated significantly with CD4+ cell responses, but the percentage of CD8+ T cells directed against HIV-1 (median, 2.76%) was always greater than that of CD4+ cells (median, 0.24%). Although CD8+ T cell responses were equally distributed among Gag, Pol, and the regulatory and accessory proteins, Gag was the dominant target for CD4+ cell responses. There was no consistent relationship between virus-specific CD8+ or CD4+ cell response and viral load. However, the median viral load in subjects in whom Gag was the dominant CD8+ T cell target was significantly lower than that in subjects in whom non-Gag proteins were the main target (P=.007).

CONCLUSIONS

Gag-specific responses dominate the CD4+ T cell response to HIV, whereas CD8+ T cell responses are broadly distributed, which indicates differential immunogenicity of these cells against HIV-1. The preferential targeting of Gag by CD8+ T cells is associated with enhanced control of viral load.

Transmission and accumulation of CTL escape variants drive negative associations between HIV polymorphisms and HLA

Human immunodeficiency virus (HIV)-1 amino acid sequence polymorphisms associated with expression of specific human histocompatibility leukocyte antigen (HLA) class I alleles suggest sites of cytotoxic T lymphocyte (CTL)-mediated selection pressure and immune escape. The associations most frequently observed are between expression of an HLA class I molecule and variation from the consensus sequence. However, a substantial number of sites have been identified in which particular HLA class I allele expression is associated with preservation of the consensus sequence. The mechanism behind this is so far unexplained. The current studies, focusing on two examples of “negatively associated” or apparently preserved epitopes, suggest an explanation for this phenomenon: negative associations can arise as a result of positive selection of an escape mutation, which is stable on transmission and therefore accumulates in the population to the point at which it defines the consensus sequence. Such negative associations may only be in evidence transiently, because the statistical power to detect them diminishes as the mutations accumulate. If an escape variant reaches fixation in the population, the epitope will be lost as a potential target to the immune system. These data help to explain how HIV is evolving at a population level. Understanding the direction of HIV evolution has important implications for vaccine development.

Dominant influence of HLA-B in mediating the potential co-evolution of HIV and HLA

The extreme polymorphism in the human leukocyte antigen (HLA) class I region of the human genome is suggested to provide an advantage in pathogen defence mediated by CD8+ T cells. HLA class I molecules present pathogen-derived peptides on the surface of infected cells for recognition by CD8+ T cells. However, the relative contributions of HLA-A and -B alleles have not been evaluated. We performed a comprehensive analysis of the class I restricted CD8+ T-cell responses against human immunodeficiency virus (HIV-1), immune control of which is dependent upon virus-specific CD8+ T-cell activity. In 375 HIV-1-infected study subjects from southern Africa, a significantly greater number of CD8+ T-cell responses are HLA-B-restricted, compared to HLA-A (2.5-fold; P = 0.0033). Here we show that variation in viral set-point, in absolute CD4 count and, by inference, in rate of disease progression in the cohort, is strongly associated with particular HLA-B but not HLA-A allele expression (P < 0.0001 and P = 0.91, respectively). Moreover, substantially greater selection pressure is imposed on HIV-1 by HLA-B alleles than by HLA-A (4.4-fold, P = 0.0003). These data indicate that the principal focus of HIV-specific activity is at the HLA-B locus. Furthermore, HLA-B gene frequencies in the population are those likely to be most influenced by HIV disease, consistent with the observation that B alleles evolve more rapidly than A alleles. The dominant involvement of HLA-B in influencing HIV disease outcome is of specific relevance to the direction of HIV research and to vaccine design.