Antigen persistence is required throughout the expansion phase of a CD4(+) T cell response.

For CD8(+) T cells, a relatively short antigen pulse seems sufficient for antigen-presenting cells to drive clonal expansion and differentiation. It is unknown whether the requirement for antigen is similarly ephemeral for CD4(+) T cells. To study the dependence of a CD4(+) T cell response on antigen persistence in a quantitatively and temporally controlled manner in vivo, we engineered a mouse line expressing a major histocompatibility complex class II-restricted epitope in dendritic cells under the control of a tetracycline-inducible promoter. Experiments tracking the proliferation of CD4(+) T cells exposed to their cognate antigen in various amounts for different time periods revealed that the division of such cells was contingent on the presence of antigen throughout their expansion phase, even in the presence of an inflammatory stimulus. This previously unrecognized feature of a CD4(+) T cell response contrasts with the proliferative behavior of CD8(+) T cells that has been documented, and it implies that the two T cell subsets might require different strategies for efficient vaccination.

Number of T reg cells that differentiate does not increase upon encounter of agonist ligand on thymic epithelial cells.

It has been reported that the differentiation of CD4+CD25+ regulatory T cells (T reg cells) can be induced by agonist peptide/major histocompatibility complex ligands in the thymus. Exploiting a transgenic mouse line wherein expression of a particular T cell epitope can be controlled temporally and quantitatively, we found that diversion of differentiating thymocytes into the FoxP3 T reg cell pathway by this agonist ligand was essentially nonexistent. However, CD4+CD25+ thymocytes were much less sensitive than their CD4+CD25- companions, by two to three orders of magnitude, to agonist-induced clonal deletion, such that their proportion increased, giving the false impression of induced differentiation. To account for these and prior observations, one can propose that differentiation along the CD4+CD25+ pathway is induced by cues other than recognition of self-agonist cues, which are poorly read by thymocytes, whose T cell receptors are conducive to selection toward the conventional CD4+CD25- lineage. Thus, selective survival, rather than induced differentiation, may explain the apparent enrichment observed here and in previous studies.

Sustained antigen presentation can promote an immunogenic T cell response, like dendritic cell activation.

Activation of dendritic cells (DCs) enhances their ability to prime naïve T cells. How activation renders them immunogenic rather than tolerogenic is unclear. Here, we show, using temporally regulated expression of a transgene-encoded neoself antigen in DCs, that either prolonged antigen presentation or DC activation could elicit full expansion, effector cytokine production, and memory-cell differentiation. Microarray analysis of gene expression in T cells showed that all changes linked to DC activation through CD40 could be reproduced by persistent antigen delivery, suggesting that stabilization of antigen presentation is an important consequence of DC activation in vivo. In this system, DC activation by CD40 engagement indeed extended their ability to present antigen to CD4(+) T cells in vivo, although different results were obtained with antigen delivered to DCs by means of endocytosis from the cell surface. These results suggest that antigen persistence may be an important discriminator of immunogenic and tolerogenic antigen exposure.