Phong T. Le

T cell receptor excision circle assessment of thymopoiesis in aging mice.

Signal joint T cell receptor delta (TCRD) excision circles (TRECs) are episomal DNA circles generated by the DNA recombination process that is used by T lymphocytes to produce antigen-specific alpha/beta T cell receptors. Measurement of TRECs in thymocytes and peripheral blood T cells has been used to study thymus output in chickens and humans. We have developed a real-time quantitative-PCR assay for the specific detection and quantification of mouse TCRD episomal DNA circles excised from the TCRA locus during TCRA gene rearrangement (mTRECs). We found that the mouse TCRD TRECs detected with this assay were predominantly in naïve phenotype CD4(+) and CD8(+) T cells. In a series of aged mice (range 6-90-week-old) we determined the absolute number of thymocytes and the number of molecules of mTRECs/100,000 thymocytes. We found that the absolute number of thymocytes dramatically decreased with age (P<0.05) and that molecules of mTREC/100,000 thymocytes also declined with mouse age (P<0.05). Splenocytes were isolated from aging mice and the frequency of naïve phenotype CD4 and CD8 cells determined. There was a significant drop in both CD4 and CD8 naïve peripheral T cells in the aged mice over time. mTREC analysis in purified CD4(+) and CD8(+) splenocytes demonstrated a constant level of mTRECs in the CD4 compartment until age 90 weeks, while the mTRECs in the CD8 compartment fell with age (P<0.05). By combining the mouse TREC assay with T cell phenotypic analysis, we demonstrated that IL-7 administration to young mice induced both increased thymopoiesis and peripheral T cell proliferation. In contrast, IL-7 treatment of aged mice did not augment thymopoiesis, nor induce expansion of splenic T cells. Thus, thymus output continues throughout murine adult life, and the thymic atrophy of aging in mice is not reversed by administration of IL-7.

The role of leukocyte adhesion molecules in cellular interactions: implications for the pathogenesis of inflammatory synovitis.

Recently, considerable progress has been made in defining the molecules involved in interactions of hematopoietic cells with non-hematopoietic Cells of various microenvironments. While the events that lead to the initiation of the cascade of immunologic events eventuating in most types of human inflammatory synovitis such as rheumatoid arthritis (RA) are not known, it appears that there are both antigen-specific and antigen-independent phases of activation of cellular immune responses in synovitis. Although the specific antigens are not known that trigger T cell autoreactive responses in RA in humans, several animal models have been described in which a defined antigen of an infectious agent such as Mycoplasma arthriditis, triggers lymphocyte responses that lead to cell-mediated joint inflammation [51]. One cellular synovial immune response are triggered, both T and B lymphocytes as well as monocytes home to synovium by expressing organ-specific homing receptors that are involved in leukocyte binding to synovial microenvironment high endothelial venules (HEV) [5, 14, 20, 37, 38, 66, 79]. Homing lymphocytes and monocytes extravasate into the synovial tissue there interacting with a variety of cell types [9, 20, 37, 38, 43, 48, 75]. Lymphocyte and monocyte interactions with various cell types of the synovial microenvironment likely lead to amplification of all phases of T and B cell immune responses and also to the recruitment of additional leukocytes to inflammed synovium [7, 27, 41, 43, 75]. Other reports in this volume will deal in depth with specific adhesion molecules involved in lymphocyte-endothelial cell interactions (Ziff, pp 199-214; Jalkanen, pp 187-198 of this volume). In this report, we will review our recent research defining the distribution and function of select leukocyte adhesion molecules in synovium (Table 1), and present data regarding the roles that adhesion molecules might play in the pathogenesis of inflammatory synovitis.

Human thymic epithelial cells: adhesion molecules and cytokine production

SummaryThe ability to culture human thymic epithelial cells has greatly facilitated studies of direct cell-cell interaction between thymic epithelial cells and T lymphocytes in vitro, as well as cytokine production and regulation of cytokine production. In vitro, human thymic epithelial cells bind to T lymphocytes via two adhesion pathways: CD2-lymphocyte function-associated antigen-3 and lymphocyte function-associated antigen-1-intercellular adhesion molecule-1. Cultured human thymic epithelial cells produce interleukins-1α, −1β, −3, −6 and −8, granulocyte colony-stimulating factor, macrophage colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, leukemia inhibitory factor and transforming growth factor-α. Production of thymic epithelial cell-drived cytokines is regulated by both adhesion molecules (lymphocyte function-associated antigen-3) and soluble factors via both autocrine (interleukin-1α, transforming growth factor-α) and paracrine (interleukin-4, interferon-γ) pathways. Transforming growth factor-α and epidermal growth factor regulate various cytokine mRNA at a post-transcriptional level by increasing cytokine mRNA stability.

Migration of immature and mature B cells in the aged microenvironment.

Studies in aged mice show that the architecture of B‐cell areas appears disrupted and that newly made B cells fail to incorporate into the spleen. These observations may reflect altered migration of immature and mature B cells. Using adoptive transfer, we tested the effect of the aged microenvironment and the intrinsic ability of donor B cells from aged mice to migrate to spleens of intact hosts. Spleens of aged recipients were deficient in attracting young or old donor immature B cells. In contrast, immature and mature B cells maintained an intrinsic ability to migrate to young recipient spleens, except that as the aged immature B cells matured, fewer appeared to enter the recirculating pool. CXCL13 protein, which is necessary for the organization of B‐cell compartments, was elevated with age and differences in CXCL13 distribution were apparent. In aged spleens, CXCL13 appeared less reticular, concentrated in patches throughout the follicles, and notably reduced in the MAdCAM‐1+ marginal reticular cells located at the follicular edge. Despite these differences, the migration of young donor follicular B cells into the spleens of old mice was not impacted; whereas, migration of young donor marginal zone B cells was reduced in aged recipients. Finally, the aged bone marrow microenvironment attracted more donor mature B cells than did the young marrow. Message for CXCL13 was not elevated in the marrow of aged mice. These results suggest that the aged splenic microenvironment affects the migration of immature B cells more than mature follicular B cells.