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The identification and quantification of T cells specific for a particular antigen is a highly effective means for characterizing a subject's reaction to an immunogenic stimulus, such as from a pathogen or a vaccine. Unfortunately, this can be a daunting process; some effective techniques exist, such as cell proliferation assays, but these can be time-consuming and are ineffective for the conduct of large-scale experiments.

In the body, antigens are presented to the immune system as peptides bound to cell-surface major histocompatibility complex (MHC) proteins. In 2003, Stanford University investigators developed an array-based system that used such immobilized MHC-peptide complexes to bind and isolate T cells that target a specific epitope (Soen et al., 2003), demonstrating new promise for high-throughput T-cell analysis. Now, work by University of Massachusetts Medical School investigator Lawrence Stern and postdoctoral fellow Jennifer Stone takes this technology another step forward, allowing investigators not only to isolate specific T-cell populations, but also to determine the extent and the nature of activation in response to antigen binding (Stone et al., 2005).

Each array spot contains MHC complexed with different peptides, alongside costimulatory antibodies; however, the spots also include antibodies specific for cytokines released by T cells following activation (Fig. 1). After binding cells to the array, the results of the experiment are visualized with fluorescent antibodies that also recognize those cytokines. “The actual advance here,” says Stern, “is using the capture antibodies together with our MHCs, so that we can not only attract, bind and count the cells, but also look at their functional responses on the microarray.” Because the detection process is specific for activation, only functional interactions should be observed; furthermore, by simultaneously screening for several different cytokines, Stern's team shows that one can very specifically characterize the nature of T-cell activation.

Figure 1: The Stern group's MHC-peptide array strategy.
figure 1

(a) Each spot contains immobilized MHC proteins (red), which present different peptides, and costimulatory antibodies (not shown). (b) T cells that recognize a given epitope will bind the array and release cytokines (yellow), which are bound by capture antibodies on the array. (c) Fluorescently tagged antibodies (blue) against these same cytokines allow detection.

Stern is looking to apply this new technology to study immune responses to viral infection—specifically, vaccinia and dengue—and he sees this as a potentially powerful tool for quickly identifying immunogenic peptides from these pathogens. “My personal feeling is that the technology will be most useful if we can pick up responses in blood,” says Stern, “...and the sensitivity appears to be enough to pick up cells at a frequency that you might expect to find them in blood from a person who's had a vaccine or been exposed to an infectious agent.” But he adds, “we haven't actually tested peripheral blood samples from individuals [yet],” and this remains a priority for future development.

Nevertheless, this pilot study highlights the method's promise in terms of both efficiency and economy. “The advantage of this technology in general is really that we're parallelizing and miniaturizing existing assays, so they can be done on a larger scale,” says Stern. Initial experiments using a nonfluorescent, precipitatable detection substrate also suggest that even facilities without expensive array-scanning equipment, could avail themselves of this high-throughput diagnostic tool. “Potentially... you could put some blood or a blood product on it, process it, and just read the results by eye.” In the fight to stay one step ahead of the latest pathogenic threats, this could offer an important edge to clinicians and researchers on the front lines.