Kimberly D. Klonowski

A primary role of the immune system is to eradicate invading pathogens while preserving the integrity of the host. CD8 T cells are pivotal in the clearance of cells infected with intracellular pathogens such as bacteria or viruses. Prior to their contact with foreign antigen (Ag), the representation of individual CD8 T cell clones capable of responding to a specific infection is low and the migration of these cells is restricted to a continuous loop between the spleen and lymph nodes via the blood and lymphatics. This restricted migration pattern facilitates a greater chance of a particular CD8 T cell interacting with its cognate Ag. However, once the immune response is initiated, the frequency, function, and migratory potential of the Ag-specific CD8 T cells is altered to facilitate the killing of pathogen infected cells at remote sites (i.e. non-lymphoid tissues such as the lung, liver, and intestinal mucosa). More importantly, a portion of these responding T cells will develop into a population of memory cells which "remember" a particular pathogen and continue to protect following subsequent infections. This pool of memory T lymphocytes is maintained through a steady-state balance of death and proliferation mediated by growth factors called cytokines, particularly IL-7 and IL-15.

Because memory CD8 T cells provide life-long immunity to a particular pathogen, understanding their development, behavior, and maintenance is crucial for the development of successful vaccines. My previous research demonstrated that different pathogens require distinct secondary lymphoid tissues (spleen and lymph node) for optimal secondary (memory) CD8 T cell responses. Thus, understanding the trafficking patterns of these memory cells is relevant, particularly for tissue-specific infections where productive responses require memory cells to be in a distinct locale. In terms of the known dynamics of memory CD8 T cell migration, my work has shown that memory cells traffic via the blood and are capable of reseeding numerous tissues, with varying kinetics and effectiveness. One supposition stemming from this work is that perhaps 3 subpopulations of memory cells exist: 1) a blood-borne pool capable of freely moving into and out of certain tissues; 2) a tissue-specific pool in which memory cells become permanent residents of a specific tissue; or 3) a population with a transient tissue residence or preference for certain sites. Most of the current research in the field of lymphocyte migration has focused on the requirements for tissue entry. However, to understand the global aspects of memory cell migration, one must also consider the process of tissue exit as well as how certain memory cell pools maintain their migratory homeostasis.

The research in my laboratory will contribute to our understanding of the migratory path of memory cells by addressing the following questions:

1. What are the homeostatic mechanisms maintaining memory cell migration? The cytokines IL-7 and IL-15 are important for memory CD8 T cell survival and maintenance, respectively. The influence of locally expressed cytokines on memory cell migration has not been explored. Our hypothesis is that differential expression of these resources directly affects memory cell migration through modulation of dwell time or retention at a given site. In vitro migration assays and transgenic technology will be used to test the relevance of local depots of cytokines on memory cell trafficking. Finally, whether or not an ongoing infection perturbs the levels of cytokines in serum, lymph, and within specific tissues and affects lymphocyte trafficking will also be examined.
2. What are the factors regulating the transendothelial migration of memory cells from tissue space into lymphatic vessels? My recent work demonstrated that reactivation of memory cells in non-lymphoid tissues may occur at low levels following challenge with certain bacterial and viral infections. However, efficient activation of memory cells following infection with certain pathogens requires memory cells to be in lymph nodes. Therefore, understanding how memory cells emigrate from non-lymphoid tissues and back to lymph nodes is important. To date, the interaction of lymphocytes with lymphatic endothelium and the potential factors driving migration of memory cells into lymphatics are unknown. By performing phenotypic and genetic analysis on memory cells isolated from lymphatic fluid, we hope to identify potential molecules responsible for entry into lymphatics. Complimentary to this approach, lymphatic lining endothelial cells from different tissues will also be isolated to determine whether there are tissue-specific differences in these vessels which may be important for migration to certain sites. The development of cell lines from these isolates will also be used to study the basic cellular biology behind lymphocyte/ lymphatic endothelial interactions in vitro.