The PPD-specific T-cell clonal response in UK and Malawian subjects following BCG vaccination: A new repertoire evolves over 12 months
Introduction
The efficacy of bacille Calmette Guerin (BCG) vaccine against pulmonary tuberculosis differs between populations, being lower in tropical countries such as Malawi than in the UK [1], [2]. One hypothesis to account for this is that prior exposure to environmental mycobacteria, which are more common in tropical areas, may mask or interfere with the immune response to the BCG vaccine [3]. This notion is supported by the observation that a high proportion of young adults in Malawi, who have never received BCG, have high interferon gamma (IFNγ) responses to purified protein derivatives (PPDs) of environmental mycobacteria such as Mycobacterium avium and Mycobacterium intracellullare [4]. Furthermore, unvaccinated teenage individuals in Malawi have higher IFNγ and delayed type hypersensitivity (DTH) skin test responses to Mycobacterium tuberculosis (M.tb) PPD compared to UK schoolchildren [5]. Following BCG vaccination, IFNγ and DTH skin test responses increase more in the UK than in Malawi [5]. Thus, the magnitude of the increase in IFNγ response to M.tb PPD correlates with the level of protection afforded by BCG immunisation at the population level.
The immune response to TB has traditionally been measured using the tuberculin skin test. More recently, IFNγ production in a 6 days whole blood assay or the magnitude of the increase in production following vaccination, have been suggested to provide an immunological correlate of protection [5], [6]. IFNγ secreting cells can also be quantified by ex vivo ELISPOT, in which cells are restimulated with antigen for a short time period and activated effector T-cells detected, or the cultured ELISPOT assay which measures central memory T-cells [7], [8]. These assays measure functions of T-cells responding to M.tb antigens, but do not identify their epitope specificity nor their clonal diversity. Antigen specificity can be mapped using recombinant antigens or overlapping peptides and the response to one peptide epitope followed with MHC-peptide tetramers [9], while the clonal diversity of a response can be crudely assessed by staining responding T-cells with antibodies to TCR Vβ families [10]. However clonal diversity can be assessed more sensitively following restimulation with antigen in vitro, by molecular methods such as spectratyping or heteroduplex analysis (HDA) [11].
This study set out to compare the effect of exposure to an assumed higher level of cross reacting environmental microorganisms in Malawi on the clonal response to BCG vaccination. We hypothesised that BCG vaccination might activate pre-existing memory T-cells in Malawi induced by exposure to environmental mycobacteria, but induce a primary immune response in the UK.
We used heteroduplex analysis to analyse the T-cell clonal repertoire to M.tb PPD pre-vaccination, 3- and 12-month post-BCG vaccination in UK adolescents and Malawian young adults [11]. Whole blood IFNγ and DTH skin test responses to M.tb PPD were recorded in the same individuals to allow comparison of these to clonality following BCG vaccination.
Section snippets
Participants
In Malawi, subjects were recruited between December 2002 and May 2003 during a large comparative study of immune responses to BCG vaccination [5] in rural Karonga District, in northern Malawi. Candidates born between 1976 and 1991 were selected from the project database if they had not received BCG during the Karonga Vaccine Trial [1]. These individuals were recruited with informed written consent from the individual or guardian, including permission to perform an HIV test. Exclusion criteria
TCR Vβ family usage is similar in Malawi to Caucasian populations
To determine the most frequently used TCR Vβ families in Malawi, peripheral blood samples from 14 Malawian young adults (age range 13–23, mean 16 years, 43% male), were stained with a panel of 24 Vβ family-specific monoclonal antibodies and anti-CD3. The mean percentage of CD3+ small lymphocytes was 67% and the percentage of CD3+ cells stained by each TCR Vβ-specific monoclonal antibody is shown in Fig. 1. Comparison of these data with results from 85 Caucasian individuals [10] (data provided
Discussion
The efficacy of vaccination with BCG varies greatly between different countries [14]. One hypothesis to account for this variation is that common environmental mycobacteria or other infectious agents, which may be more prevalent in developing countries, induce cross-reactive responses that mask or interfere with the immune response to BCG [3], [14]. Certainly, exposure to environmental mycobacteria can affect the magnitude and functional characteristics of the subsequent response to BCG
Acknowledgements
This work was supported by the Edward Jenner Institute for Vaccine Research and The Wellcome Trust.
Contributors: In Malawi, we thank L. Kachiwanda, O. Mwanyongo, D. Mulawa and W.B. Sichone for field work, and S. Chagaluka, H. Kanyongoloka, A. Mwanyimbo, R. Ndhlovu and H. Moyo for laboratory assistance, M. Mponda for data entry, L. Bliss, J. Saul and A. Jahn for assistance in data management, the people in Karonga District for their cooperation. In the UK, we thank M. Hameed, S. Luke, C.
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