Cellular and Molecular Regulation of Immune Response

The Role of Costimulatory Receptors in Lymphocyte Activation. A primary area of interest in this laboratory is the study of costimulatory molecules and their function. Costimulation has been extensively studied in the T cell receptor (TCR)-mediated activation of mature T cells. In recent efforts, we have focused on analysis of costimulatory function in T cell development, homeostasis, and effector function. Findings include striking effects of costimulation on critical check-points in thymic development and are using genetic manipulation to elucidate the molecular mechanisms mediating these effects.

We recently identified a critical role of TRAF3 in mediating the requirement for cross-talk between SP thymocytes and thymic epithelial cells (TEC). Using conditional gene inactivation, we found that deleting TRAF3 from TEC completely eliminated the requirement for SP thymocytes in medullary TEC (mTEC) development. The normal function of SP T cells is thus to overcome this TRAF3-dependent inhibition, through multiple signaling pathways capable of TRAF3 inactivation and NFkB alternative pathway activation.

Regulation of Chromosomal Integrity and Lymphocyte Replicative Capacity. This work analyzed the role of telomerase and telomere length regulation. Recent findings included:

• Telomerase activity is highly regulated during activation and development of T and B lymphocytes, and this regulation is mediated by both transcriptional and posttranslational mechanisms.

• Telomere length in normal somatic cells is genetically regulated. We have shown that differences in telomere length between mouse species are regulated by a single recessive locus on distal chromosome 2. These findings demonstrated that increases in telomere length can occur during normal differentiation.

• Telomere length in vivo is regulated by telomerase expression. We have recently shown that telomere length is regulated by expression of telomerase RNA template (TER), and have demonstrated haploinsufficiency in telomere maintanence in heterozgous TER knock-out mice. We generated both knock-out and transgenic models of telomerase reverse transcriptase (TERT) expression to further assess function in vivo.

We have recently extended studies of chromosomal stability and control of lymphocyte proliferation to study two critical components of these regulatory pathways: ATM and tumor suppressor p53.

A recombinase-associated gene (Rag)-dependent mechanism mediates TCR translocation in thymic lymphomagenesis in mice deficient for the ataxia telangiectasia mutated (ATM) gene. ATM plays a role in repair responses to double-strand DNA breaks and influences telomere maintenance. We have characterized the role of ATM in Rag-dependent and Rag-independent recombination in tumorigenesis and have shown a role for ATM in Ig class switch recombination.

We have recently reported that ATM functions to regulate rearrangement of TCR beta and TCR alpha and to regulate the corresponding stages of TCR-dependent thymocyte development. In the absence of ATM, we found a notable instability of RAG-mediated TCR break complexes, with resulting increase of trans rearrangements between TCR loci on different chromosomes. These trans rearrangements result in expression of hybrid TCR chains, which have in turn allowed elucidation of the role of TCR in determining lineage and MHC restriction, two basic attributes of T cell development and function.

We have recently analyzed the role of tumor suppressor p53 in regulating proliferative responses of T cells to antigen-specific and nonspecific signals. These studies led to the striking finding that antigen-specific proliferative responses of na? and memory CD4 T cells require the down-modulation of tumor suppressor p53. In the absence of TCR signal, IL-2 induces a sustained increase in p53 protein, which prevents proliferative responses despite strong signaling through the IL-2 receptor. In contrast, TCR signaling results in early termination of p53 protein expression by decreasing p53 mRNA as well as by strong transcriptional induction of the p53-regulating protein Mdm2. Down-modulation of p53 in response to antigen stimulation is in fact critical for antigen-specific T cell proliferation; and preventing p53 degradation by inhibiting Mdm2 results in sustained p53 protein levels and prevents antigen-specific T cell proliferation. These studies elucidate a critical role of p53 as a negative regulator of T cell proliferation. It is the termination of p53 elevation by TCR signaling that allows proliferative responses to occur, enforcing antigen specificity.