Marilyn Thoman, Ph.D., Associate Professor

Immunology Program

T CELL DEVELOPMENT AND MATURATION IN THE AGED: STRATEGIES FOR INTERVENTION

Team Thoman
Marilyn Thoman, PhD
Joy Phillips, PhD
Elizabeth Virts, PhD
Edward Morgan,PhD

Major goals of our research program are to uncover the mechanisms resulting in diminished immune activity in the aged ( immunosenescence); and by understanding the underlying mechanistic issues, to then improve immune reactivity in the elderly. Immunosenescence results from functional changes intrinsic to immune cells as well as from alterations in the microenvironment. Fifteen years ago we described a dramatic shift in the subset profile of T lymphocytes (one type of immune cell) that occurs with aging, with an increase in the proportion of antigen-experienced “memory” cells and a corresponding decrease in the naïve subset. This changing T cell profile is driven in part by repeated exposure to germs, but declining production of new T cells with age also contributes to this process. One of our research programs is directed to delineating the mechanisms controlling the loss in potential to create new T cells with age. In adult mammals T cell differentiation primarily takes place in the thymus and requires the physical interaction between lymphocyte progenitors and the structural components of the thymus. The appropriate function of each population requires interaction with the other, so called “cross-talk”. Aging, disease, chemotherapy and irradiation all have deleterious effects on cross-talk and result in the shrinking of the thymus. We have developed a cell-based gene product delivery system by which we can study the impact on T cell differentiation of local (intrathymic) delivery of specific gene products. Using this delivery system, we demonstrated that increasing intrathymic IL-7 levels, leads to the prevention of the age-associated decline in early T cell development. Conversely, raising intrathymic levels of sonic hedgehog (an important cell-fate regulatory molecule) inhibits the this process, decreasing T cell differentiation. Current investigations are focused on modulating thymic epithelial cell function by delivery of keratinocyte growth factor and wnt. Future work will both identify and assess other “cross-talk” molecules to define their role in maintaining T cell differentiation as well as investigate the feasibility of utilizing tissue-resident thymic stromal stem cells to reverse involution.
Our cell gene therapy delivery system is also being evaluated as a means to treat single gene diseases such as hemophilia. It could potentially be used in any patient without the customization normally required for a cell based therapy. This research is focused on refining the therapeutic approach and assessing feasibility.
The increase in morbidity and mortality associated with infectious disease as well as the increase in incidence of neoplastic disease bespeak of the decline in protective immune responsiveness in the aged. Intrinsic alterations in the functional capacity of various lymphoid cell classes contribute to this decline, manifested as changes in cytokine production profile and reduced proliferative capacity. It is likely that some of these functions can by enhanced or restored by means of providing additional or unique cell-activating stimuli. Another research program of our team focuses on enhancing immunoreactivity in the aged. We are examining a group of unique molecular adjuvants, developed by our collaborators at the University of Nebraska Medical Center that increase vaccine-mediated immunity even in the immunosenescent elderly population. In contrast to some others, this molecular adjuvant is a potent inducer of a Th1-like immune responses, characterized by an IgG1a and IgGb enriched immunoglobulin profile and IL-2 and IFN-? cytokine production. At present we are characterizing the mechanism of action of this adjuvant and its efficacy in inducing protective anti-influenza responses in aged mice. In the near future the scope of these investigations will be expanded to include vaccine development for difficult organisms, such as fungal pathogens.
These research efforts are supported by five NIH-funded grants, awarded from the National Institutes of Aging, Autoimmune and Infectious Disease, and Biomedical Imaging and Bioengineering. Our operational paradigm is collaboration amongst a group of senior scientists, each bringing their particular expertise to bear on a common set of questions, will result in a more rapid pace of investigation and a more stable funding environment.

Advancing age is accompanied by an increasing risk of infectious disease and cancer, thought to be in part due to alterations in protective immune function. Amongst the cells which comprise the immune system, the helper ( CD4+) T lymphocytes are particularly susceptible to age-related changes and display a marked shift in their subset composition with age. At least three functionally distinct subpopulations of CD4+ T cells can be distinguished. These populations are; antigen-inexperienced naive cells, activated effector cells, and memory cells. Each population can be identified by a unique profile of cell surface marker expression and pattern of cytokine production. In young individuals the relative distribution of these subsets favors a high percentage of naive T cells, with a much lower fraction of either effectors or memory cells. As the individual ages, this balance shifts, such that the predominate subset becomes the memory, or memory/effector. This change alters the type and amount of cytokines which are released upon contact with antigen, which in turn directs the subsequent cellular responses. The decline in the number of naive CD4+ cells suggests that the repertoire becomes restricted with age limiting the ability of the individual to respond to antigenic determinants newly encountered in later life.

Over the past two years our work has focused on regenerating a CD4+ T cell population in aged mice which more closely resembles that found in young animals, ie. enriched in naive cells. Our model system employs bone marrow transplantation following high dose irradiation. By means of this regimen peripheral T cells are depleted by greater that 90%, but over the course of 8 to 12 weeks regenerate to numbers approaching pre-treatment levels both by expansion of radioresistant host T cells , and new T cell differentiation from donor bone marrow-derived precursors. However, in contrast to our expectations, the T lymphocyte compartment in these aged mice was not enriched in naive cells. As in untreated control mice, the CD4+ population contained a majority of CD44hiCD45RBlo L-selectinlo cells which upon activation produce large amounts of gamma-IFN and IL-1 , characteristic of memory cells.

Investigations now focus on answering the question of why aged animals regenerate a memory-enriched population rather than a naive-enriched population . Mechanisms which have been investigated include: age-related changes in the differentiative environment of the thymus, or alterations within the peripheral environment of the aged animals which accelerate the maturation of the naive T cell to the memory state. While these studies are still in progress, our preliminary evidence suggests that it is the second of these two proposed mechanisms which is responsible. It is likely that activation through the T cell receptor complex plays a role in this process. It is anticipated that the results of these studies will suggest ways in which the recovery of immune function following irradiation and/or chemotherapy can be accelerated.

A second line of investigation in my laboratory concerns the process of T cell differentiation. The production of T lymphocytes occurs primarily in the thymus. However, the thymus undergoes with age a process known as involution, in which the structure of the organ becomes disorganized and the lymphoid portion is lost, significantly reducing the production and release of new T cells during adult life. This too contributes to the decline in the fraction of naive T cells in older individuals. We have sought to determine how advancing age affects T cell differentiation and have identified several transition points which are particularly age-sensitive. The aquisition of CD25 by the earliest T cell precursors is one step which shows reduced activity with aging. Another is the entry of cells into cycle which occurs at the DN to DP transition, which is also curtailed in the aging thymus. These steps are targets for intervention in the involution process which could result in the maintenance of vigorous T cell production into later life.

Our goal is to understand how aging alters the differentiation and function of cells of the immune system and identify means by which to prevent or reverse this process. Such interventions will be particularly valuable not only for the general population, but also in clinical situations where immune function is compromised, such as in patients with AIDS or following chemotherapy.