Investigations on the formation of immunological memory

Heightened protection from subsequent exposure to pathogens is a key feature of immune responses, and is the hallmark of the phenomenon known as immunological memory. The adaptive immune system, consisting of T and B lymphocytes, mediates such long-term protection through the maintenance of specialized lymphocyte subsets, termed memory cells, that persist in the host long after resolution of the infection. T lymphocytes provide protection against intracellular pathogens, while B lymphocytes secrete antigen-specific antibodies that help to recognize and clear microbial invaders from blood and interstitial spaces. In this dissertation, two aspects concerning the development of immunological memory were investigated.; The identification of memory B cells has long been problematic due to a lack of specific cell surface markers that discriminate between naive and memory B cell populations. To address this issue, we generated transgenic mice in which permanent expression of a tractable marker was induced in germinal center B cells, the precursors to memory B lymphocytes. Upon secondary antigen exposure, two populations of responding B cells were identified based upon (i) transgene expression and (ii) the presence of somatic hypermutation within immunoglobulin light chains. These results suggest that, contrary to traditional belief, secondary antibody responses may not be borne exclusively by germinal center-derived memory B cells, but rather involve substantial participation by B cells derived from outside the germinal center.; DNA methylation is a critical component of gene regulation that is required for early gastrulation and embryonic development. However, the requirement of DNA methylation for the development of antigen-specific CD8 T cell populations is unknown. To investigate this, we selectively ablated expression of DNA methyltransferase 1 (Dnmt1), the enzyme required to maintain DNA methylation during cell replication, in activated CD8 T cells during a live viral infection. Our experiments revealed that functional effector and memory CD8 T cells developed in the absence of Dnmt1, however the expansion of effector CD8 T cells was severely impaired. These results indicate that Dnmt1 activity in adult CD8 T lymphocytes is not critical for induction of proper gene expression patterns, and suggest that additional epigenetic mechanisms are responsible for gene regulation in this system.