Research Project

An assessment of humoral and cellular immunity against yellow fever in HIV+ and HIV- children vaccinated during infancy

Vaccination of residents in yellow fever endemic countries and visitors travelling to these areas is a straightforward way of preventing yellow fever [1, 2]. In endemic countries, the yellow fever vaccine (YFVac) is routinely administered to infants at 9-12 months of age. Prior to 2014, vaccination at 9-12 months of age was followed by a booster dose every 10 years. In 2014, the World Health Assembly and the Centre for Disease Control adopted the recommendation to discontinue the 10-year boosting on the basis that a single dose confers long-lasting immunity[3]. This recommendation was made without supporting evidence [4] for infants receiving vaccine at 9-12 months of age or in HIV+ persons [2, 3]. Infants vaccinated at 9-12 months age may not develop an immune response similar to adults or may lose immunity more rapidly [5]. A recent serosurvey found that although ~95% of infants developed neutralizing antibodies to yellow fever virus 4wks post-vaccination, by year 6 post-vaccination, ~50% of these infants no longer had any detectable neutralizing antibodies [6]. It would be expected that in the absence of neutralizing antibodies (absence of protection), more yellow fever cases would be reported but that is not the case. This raises the question: does the vaccine induce any long-lasting immune response in infants vaccinated at 9-12 months and are neutralizing antibodies the best correlate to assess this long-lasting protection? Especially, as it has been observed with some vaccines that some vaccinated individuals without measure of neutralizing antibodies remain protected [7]. We propose to characterize the yellow fever vaccine induced humoral (B cells) and cellular (T cell) immune response over time in infants. We hypothesize that the yellow fever vaccine administered at 9-12months elicits a polyfunctional initial and long-lasting antibody and T cell response and that measuring neutralizing antibodies only cannot define this immune response. Specifically, we propose to Aim #1. Determine the presence and persistence of IgG (including IgG subclasses) and IgM antibodies to yellow fever virus in these infants before vaccination to 11-15 years after vaccination. Aim #2. Determine the presence and persistence of neutralizing antibodies to yellow fever virus in these infants before vaccination to 11-15 years after vaccination. Aim #3. Determine the presence of yellow fever specific T cells and T cell response in these infants 7-15 years post vaccination. To investigate our hypothesis, we would use archived biological samples (plasma, Whole blood PBMCs) collected in the PEDIACAM study [8] (ethical approvals No 2021/10/1399/CE/CNERSH/SP) in Cameroon. The PEDIACAM study is a prospective cohort of infants born live to HIV+ and HIV- mothers, included between 2007 – 2011 mostly during their first week of life and followed to date. These children received YFVac at 9-12months of age. Plasma, sera, and subsequently whole blood samples from these infants were archived.

Research Significance

At present, the yellow fever vaccine is the only effective measure for preventing yellow fever. Unfortunately, to date, it remains uncertain if vaccinated infants who receive a single dose at 9-12 months of age will develop a long-lasting immunity. This study will provide key information on the vaccine induced immune response and its persistence in both HIV- and HIV+ infants who received the vaccine at 9-12 months of age.

Publications

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Mentors

• • Vivek Nerurkar, MSc, DMLT, PhD
  • Rose Leke, MS, PhD 
  • Lishomwa Ndhlovu, MD, PhD
  • Mathurin Tejiokem, MD, MPH, PhD
  • Dr. Paul Tagnouokam 
  • Sara Eyangoh, PhD, HDR

Research Project

Phenotype and Genotype of Mycobacterium tuberculosis in HIV-positive Cameroonian Adults 

In 2012, the estimated tuberculosis (TB) incidence rate in Cameroon was 238 per 100,000 inhabitants. Diagnosis of TB relies primarily on the identification of Mycobacterium bacilli on sputum smears of symptomatic patients using conventional light microscopy. Smear- positive patients are also screened for HIV infection. Screening is offered free of charge after obtaining informed consent. Smear- positive patients are placed on the standardized 6-month treatment regimen of Isoniazid Rifampicin, Pyrazinamide and Ethambutol. Patients receive these drugs daily for 6 months. A patient is considered cured if smear-negative in the 6th or at least 5th month of treatment. Overall treatment success rate of new smear-positive cases is ~ 68.4%. Drug susceptibility testing (DST) capacity is limited. Only a small fraction of high risk patients (including HIV+) are tested for drug resistant tuberculosis. The National Tuberculosis Program of Cameroon (NTPC) plans to expand DST facilities in order to increase diagnosis. This will lead to better treatment and limit spread of resistance. Based on epidemiological studies, there is 5-8% rate of multidrug resistant TB and 10.7-27.7% drug resistance to a single drug of all first-line drugs.

Specific aims
Aim 1: Determine whether the current concentration of Rifampicin, Isoniazid and Ethambutol used in DST are appropriate. We hypothesize that the critical concentration of these drugs will be too high and that some individuals thought to have drug resistant Mtb will be misclassified.
Aim 2: To test the hypothesis that four hot-spot mutations in the rpoB gene for Rifampicin and one hot-spot mutation in the katG gene and two hot-spot mutations in the inhA gene for Isoniazid will correlate with phenotypic resistance.

Research Significance

The prevalence of drug resistant tuberculosis is on the rise world-wide. Accordingly, to provide adequate treatment, a critical need exists for susceptibility testing of TB isolates from patients, especially in HIV+ individuals who are highly susceptible to the severe effects of TB. In 2012, approximately 37% of TB cases in the Africa region were HIV+. DST of Mycobacterium tuberculosis (Mtb) is determined using a variety of laboratory techniques, most of which are based on phenotypic (growth) or genotypic (gene mutations) assays.
Phenotypic assays culture sputum of patients in the presence of anti-TB drugs and observed inhibition of growth or metabolism. The indirect proportion method, a phenotypic assay, is commonly used in developing countries, including Cameroon.

Unfortunately, there are several problems with this method. First, the standard critical drug concentration of the anti-TB drugs used in the assay is based on the 1963 WHO recommendation. These critical concentrations were assigned based on consensus and experience, rather than on scientific evidence. Thus, it is unclear if these concentrations are appropriate. Second, culturing takes at least six weeks before DST can be done, rendering diagnosis slow. In Cameroon, DST is only performed in high risk groups, including HIV+ patients. The National Tuberculosis Program of Cameroon (NTPC) made a recommendation for the rapid genotypic assays to be used for early detection of multidrug-resistant TB . However, these methods have not yet been implemented. The use of genotypic assays has paralleled the identification of mutations in key genes that confer resistance to anti-TB drugs. For example, mutations within an 81bp region of the rpoB gene of Mtb has been reported to confer Rifampicin resistance in 90%–95% of all clinical isolates examined. Thus, there is a need to determine if genetic mutations can be used to rapidly detect cases of drug resistance in Cameroon.

Publications

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Mentors

• Diane W. Taylor, PhD
• Rose Leke, PhD
• Guy Vernet, PhD
• Eric Pefura Yone
• Sara Eyangoh, PhD, HDR
• Veronique Penlap
• Wilfred Mbacham, DS, DSc