Flow cytometric analysis of vaccine responses: how many colors are enough?

The past 5 years have seen an explosion in technological advances related to measuring immunogenicity. Specifically, two distinct areas of development have led to considerably more detailed analysis of T cell responses: first, the ability to measure over a dozen distinct antigens expressed by individual cells simultaneously (12-color flow cytometry); and second, a host of assays that rapidly and viably identify antigen-specific T cells. Together, these technologies reveal the complex heterogeneity of an immune response generated during infection or after vaccine challenge. The next 5 years will see the determination of which underlying variables will be most important to quantifying vaccine efficacy. In this manuscript, we discuss these technologies, with a focus on assisting in the design and implementation of immunogenicity trials for future vaccine efforts.