Date Published: July 1, 2008
Publisher: Public Library of Science
Author(s): Willem A Hanekom, Hazel M Dockrell, Tom H. M Ottenhoff, T. Mark Doherty, Helen Fletcher, Helen McShane, Frank F Weichold, Dan F Hoft, Shreemanta K Parida, Uli J Fruth
Abstract: Willem Hanekom and colleagues make recommendations on assay harmonization for novel tuberculosis vaccine trials.
Partial Text: The mechanisms of immune protection against human TB, a disease that causes 2 million deaths world-wide each year, are not fully known. T cell immunity is critical for protection [1,2]; therefore, the current TB vaccine, bacille Calmette-Guérin (BCG), and most new vaccines under development aim to induce this immunity. Most of these developmental vaccines [1–4] are designed to boost pre-existing immunity induced by BCG; however, some candidates aim to ultimately replace BCG as the priming vaccine. Following phase I/IIa trials of the vaccines, safety and immunogenicity results will be critical to decide which vaccine candidates should move into efficacy trials. For this choice, the ability to compare immunogenicity would be an important asset. Potential comparisons are confounded by variation in individual laboratory approaches, logistics, and the diverse populations studied in vaccine trials. Some comparison may be achieved by harmonisation of assays (see below); however, even then, antigen components of vaccines and therefore antigens in assays may differ. Further, the desired character of induced immunity may differ according to vaccine candidate, making choice of an assay to be harmonised difficult.
Some T cell assays use whole blood, while others use isolated peripheral blood mononuclear cells (PBMCs). PBMCs may either be used fresh, or after cryopreservation. Assays may have relatively short (less than 24 hours), intermediate (one to three days), or longer (five to seven days) periods of incubation. Each assay approach may have distinct advantages, as summarised in Table S1.
Delaying incubation of whole blood, or delaying isolation of PBMCs after blood collection, may affect assay outcomes (see Text S3 for more detail). Overall, available evidence suggests that sub-optimal outcomes of shorter-term assays are likely when delays occur from the time of blood collection to incubation, or to PBMC isolation and cryopreservation for later incubation. The panel therefore recommended that until further evidence becomes available, PBMCs for later ELISPOT and short-term intracellular cytokine assays be isolated as soon as possible after blood collection and never more than eight hours after collection, preferably at the same time point after collection in all participants of a specific study. The same principles apply to short-term, undiluted whole-blood intracellular cytokine assays; incubation later than two hours after collection should not be considered. In contrast, longer-term assays, such as a six to seven-day whole-blood assay, appear to be less affected by delays in incubation (H. M. Dockrell, personal communication); we hypothesise that these assays measure expansion of specific T cells, and are therefore less affected than shorter-term assays that measure direct ex vivo function quantitatively.
Multiple variables in the PBMC isolation, cryopreservation, and thawing process affect ultimate recovery of viable, functional cells (; Smith and Dockrell, unpublished data). Although most labs use very similar procedures, conflicting results regarding fine details such as freezing media composition have emerged (T. Kollmann, personal communication; ). However, most researchers now agree that assay results of increased quality may be obtained when PBMCs are “rested” for at least four hours after thawing, prior to adding antigens for functional assays. In shorter-term assays, this procedure may decrease assay background and increase functional response [9,10].
“Harmonisation” refers to a consensus in assay standard operating procedures for multiple testing sites. “Standardisation” comprises all measures necessary to obtain comparable results, in terms of both time and place. Optimal standardisation will result in comparable results when a test is performed at different times and by different technicians in different laboratories. To achieve such results, standardised materials, reagents, and equipment are important. “Validation” refers to a detailed characterisation of assay performance. Typical validation characteristics include accuracy, repeatability, specificity, detection limit, quantitation limit, linearity, and range. Regulatory authorities require that investigators introduce a validated assay as the primary immunological outcome in new vaccine trials, if the data are intended to be used for licensure. “Qualification” is a term sometimes used to describe partial validation, and refers to an experimental protocol that demonstrates that an accepted method will provide meaningful data, given specific conditions and samples.
It is likely that investigators and sponsors will continue to introduce their “favourite” assays in new TB vaccine trials. However, a single, harmonised assay common to all vaccine trials would be ideal to allow comparison of immunogenicity results between different vaccine candidates, and the use of such an assay is strongly recommended by members of this expert panel. Ideally, such an assay should be widely implementable, even at remote field sites, while delivering informative results. The panel judged that the seven-day whole-blood IFN- assay best meets these criteria and recommended that it be introduced into all new TB vaccine trials. Excellent performance of this assay has been demonstrated in multiple large clinical studies. Additionally, GC6-74 (“Biomarkers for TB”) has standardised this method to screen new TB antigens at field sites. A harmonised protocol has been developed (Text S4). It will also be important to standardise reagents and the equipment that measures cytokine levels.
All current assays described here use the magnitude and, to some extent, the qualitative character of the immune response to measure “vaccine take”. Without a complete knowledge of immune correlates of vaccination-induced protection against TB, all assays may be described as vaccine take assays. Regardless, the current assays focus on T cell immunity, particularly IFN-γ production, which is thought to be important for protection. Because emerging evidence suggests that IFN-γ production alone is not necessarily an immune correlate of vaccination-induced protection against TB (W. A. Hanekom, unpublished observations; ) it is important to define these correlates in complementary projects. Multiple ongoing projects aim to define immune correlates of protection, which may ultimately be validated as surrogates of protection in phase IIb/III trials of effective TB vaccines. Until these correlates/surrogates are available, it would be extremely useful to also store blood products in a manner that is efficient and that would allow an excellent functional yield of cells or products when thawed at a later stage. These blood products would then be available to measure newly described immune correlates/surrogates of protection, in retrospective studies or for application of newer technologies.