Immunity and the Animation of the Genome

Not-so-great expectations: re-assessing the essence of T-cell memory

We are often taught that secondary, or memory, responses by lymphocytes are more vigorous than primary responses. An expectation commonly associated with this notion is that the initial encounter with a pathogen should result in immunity to re-infection. Although this outcome is sometimes the case, it is not universally true. In this review, we propose a unified model of T-cell memory to explain the apparent successes and failures of eliciting vaccine-like protection from prior encounters with pathogens. We speculate that memory T cells arise as an invariant consequence of clonal selection during an immune response. The quality of memory T cells, however, seems to vary in the degree to which they have acquired effector characteristics and, thus, their ability to confer immunity to re-infection. Although not all memory T cells possess the embellished attributes of fully developed effector cells, they all seem to share the rudimentary quality of preserving an antigen specificity that has proven itself useful. We suggest that the ability to maintain the integrity of the T-cell repertoire, more than establishing immunity to re-infection, may represent the fundamental form of memory for the adaptive immune system.

Epigenetic control in the immune response

Helper T cells engaged in an immune response confront a prevalent challenge for developmentally regulated gene expression: How does a cell give rise to daughter cells with different fates? Additionally, lymphocyte function is intimately associated with the processes of cell division and migration. This imposes an additional burden for daughter cells, to remember inductive events from which they are temporally and spatially removed. An emerging view is that helper T cells use epigenetic mechanisms tied to the structure of chromatin and its covalent modifications to achieve at least two important features of their programmed gene expression. Epigenetic effects organize the ability of signal transduction pathways to generate a restricted set of progeny from a multi-potent progenitor. In addition, epigenetic effects seem to allow dividing cells to memorize, or imprint, signaling events that occurred earlier in their development. Beyond helper T cells, the use of epigenetic effects is emerging as a common strategy in development and function of the mammalian immune system, suggesting that epigenetic effects may play a more prominent role in metazoan cell differentiation than previously appreciated. Lymphocytes are, thus, becoming a tractable system for genetic and biochemical dissection of the ways in which the genome is embedded with regulatory information to achieve developmental complexity.