Thymic epithelial cell abnormalities in (NZB x H-2u)F1 mice

Reduced thymic Aire expression and abnormal NF-kappa B2 signaling in a model of systemic autoimmunity

The thymic stromal niche normally directs the production and export of a self-tolerant T cell repertoire. Many models of spontaneous autoimmunity, however, develop thymic architectural abnormalities before disease onset. Although this is suspected to affect central tolerance induction, creating an autoimmune predisposition, in-depth analysis of the microenvironment within these thymi is lacking, such that the mechanisms and likely direct effects on the T cell repertoire are unknown or speculative. Here we show that NZB mice, the first described model for systemic autoimmunity, demonstrate a complex thymic phenotype, including a lack of the autoimmune regulator (Aire), early defects in thymic epithelial cell (TEC) expansion, and evidence for altered NF-kappaB2 signaling. Analysis of medullary TEC revealed a numerical loss of the Aire-expressing MHC class II(high) (mTEC-high) subset as well reduced Aire protein and mRNA per cell. RelB expression was also reduced, while chemokines CCL19 and CCL21 were increased. Unexpectedly, the proportion of cortex and medulla in the NZB mice was normal from 36 wk, despite worsening architectural abnormalities. These data show that the NZB defect is more complex than previously appreciated, segregating into early numerical TEC deficiencies that correct with age, late degeneration of the niche architecture that does not affect TEC number, and a persistent reduction in Aire and RelB expression per cell acquired upon mTEC-high differentiation.

Comparative immunobiology of thymic DC mRNA in autoimmune-prone mice

New Zealand Black (NZB) mice have multiple defects in both innate and acquired immunity. A fundamental defect, described more than 25 years ago, is premature thymic involution. Subsequent studies have disclosed multiple defects in thymic epithelial cells, and it has been proposed that thymic dendritic cells (DCs) play an important role not only in thymic involution but also in the appearance of immunopathology. However, the number of available thymic DCs makes this population extremely difficult to study. We have taken advantage of our ability to isolate pure populations of thymic DCs and have examined several key mRNA levels of enzymes involved in signal transduction. Our data on NZB mice was compared to that of NZB x NZW F1 (B/WF1), BXSB-Yaa, MRL/lpr, NOD and control mice. Importantly, we demonstrate herein that a common feature in autoimmune-prone mice is an increase of thymic DC c-met mRNA. Indeed, the increase in c-met mRNA levels appeared specific to the thymus and was not noted in the spleen. Additionally, we demonstrate that E-cadherin, a downstream molecule of c-met, is also reduced. Finally, we note that the levels of HGF mRNA are normal in the autoimmune strains examined herein, confirming that the abnormality of c-met mRNA is not due to primary defects in thymic stromal cells. We submit that these results highlight the possibility of a selective defect in thymic DCs which will be a pivotal step in loss of tolerance, and suggest that future studies focus on adoptive cell transfer involving this population.

Thymic nurse cells: a microenvironment for thymocyte development and selection

Thymic nurse cells (TNCs) represent a unique microenvironment in the thymus for MHC restriction and T cell repertoire selection composed of a cortical epithelial cell surrounding 20-200 immature thymocytes. TNCs have been isolated from many classes of animals from fish to humans. Studies performed using TNC lines showed that TNCs bind viable alphabetaTCRlow CD4(+)CD8(+)CD69(-) thymocytes. A subset of the bound cells is internalized, proliferates within the TNC, and matures to the alphabetaTCRhigh CD4(+)CD8(+)CD69(+) stage, indicative of positive selection. A subset of the internalized population is released while cells that remain internalized undergo apoptosis and are degraded by lysosomes within the TNC. A TNC-specific monoclonal antibody added to fetal thymic organ cultures resulted in an 80% reduction in the number of thymocytes recovered, with a block at the double positive stage of development. Together these data suggest a critical role for TNC internalization in thymocyte selection as well as the removal and degradation of negatively selected thymocytes. Recent studies have shown that in addition to thymocytes, peripheral circulating macrophages are also found within the TNC complex and can present antigens to the developing thymocytes. These circulating macrophages could provide a source of self-antigens used to ensure a self-tolerant mature T cell repertoire. A reduction in TNC numbers is associated with a variety of autoimmune diseases including thyroiditis and systemic lupus erythematosis.

Impaired development of T lymphoid precursors from pluripotent hematopoietic stem cells in New Zealand Black mice

Bone marrow cells from autoimmune-prone New Zealand Black (NZB) mice are less efficient at colonizing fetal thymic lobes than cells from normal strains. This study demonstrates that the reduced capacity of NZB bone marrow cells to repopulate the thymus does not result from their inability to migrate to or enter the thymus. Rather, the T lymphopoietic defect of NZB mice is due to an impaired ability of pluripotent hematopoietic stem cells (PHSCs) to generate more committed lymphoid progeny, which could include common lymphoid precursors and/or other T cell-committed progenitors. Although PHSCs from NZB mice were not as efficient at thymic repopulation as comparable numbers of PHSCs from control strains, the ability of common lymphoid precursors from NZB mice to repopulate the thymus was not defective. Similarly, more differentiated NZB T cell precursors included in the intrathymic pool of CD4(-)CD8(-) cells also exhibited normal T lymphopoietic potential. Taken together, the results identify an unappreciated defect in NZB mice and provide further evidence that generation of lymphoid progeny from the PHSCs is a regulated event.

NZB mice exhibit a primary T cell defect in fetal thymic organ culture

Defects in T cell development have been suggested to be a factor in the development of systemic autoimmunity in NZB mice. However, the suggestion of a primary T cell defect has often been by extrapolation, and few direct observations of T cell precursors in NZB mice have been performed. Moreover, the capacity of NZB bone marrow T cell precursors to colonize the thymus and the ability of the NZB thymic microenvironment to support T lymphopoiesis have not been analyzed. To address this important issue, we employed the fetal thymic organ culture system to examine NZB T cell development. Our data demonstrated that NZB bone marrow cells were less efficient at colonizing fetal thymic lobes than those of control BALB/c or C57BL/6 mice. In addition, NZB bone marrow cells did not differentiate into mature T cells as efficiently as bone marrow cells from BALB/c or C57BL/6 mice. Further analysis revealed that this defect resulted from an intrinsic deficiency in the NZB Lin-Sca-1+c-kit+ bone marrow stem cell pool to differentiate into T cells in fetal thymic organ culture. Taken together, the data document heretofore unappreciated deficiencies in T cell development that may contribute to the development of the autoimmune phenotype in NZB mice.

Apoptosis and the thymic microenvironment in murine lupus

The thymus of New Zealand black (NZB) mice undergoes premature involution. In addition, cultured thymic epithelial cells from NZB mice undergo accelerated preprogrammed degeneration. NZB mice also have distinctive and well-defined abnormalities of thymic architecture involving stromal cells, defined by staining with monoclonal antibodies specific for the thymic microenvironment. We took advantage of these findings, as well as our large panel of monoclonal antibodies which recognize thymic stroma, to study the induction of apoptosis in the thymus of murine lupus and including changes of epithelial architecture. We studied NZB, MRL/lpr, BXSB/Yaa, C3H/gld mice and BALB/c and C57BL/6 as control mice. Apoptosis was studied both at basal levels and following induction with either dexamethasone or lipopolysaccharide (LPS). The apoptotic cells were primarily found in the thymic cortex, and the frequency of apoptosis in murine lupus was less than 20% of controls. Moreover, all strains of murine lupus had severe abnormalities of the cortical network. These changes were not accentuated by dexamethasone treatment in cultured thymocytes. However, the thymus in murine lupus was less susceptible to LPS-induced apoptosis than control mice. Finally we note that the number of thymic nurse cells (TNC) was lowest in NZB mice. Our findings demonstrate significant abnormalities in the induction of apoptosis and the formation of TNC-like epithelial cells in SLE mice, and suggest that the abnormalities of the thymic microenvironment have an important role in the pathogenesis of murine lupus.

Thymic microenvironment and NZB mice: the abnormal thymic microenvironment of New Zealand mice correlates with immunopathology

There are distinct microenvironmental abnormalities of thymic architecture in several murine models of SLE defined using immunohistochemistry and a panel of mAb dissected at thymic epithelial markers. To address the issue of the relationship between the thymic microenvironment and autoimmunity, we studied backcross (NZB x NZW) F1 x NZW mice in which 50% of offspring develop nephritis associated with proteinuria and anti-DNA antibodies. We reasoned that if thymic abnormalities are associated with development of disease, the correlation of abnormalities with lupus-like disease in individual backcross mice will form the foundation for identification of the mechanisms involved. In parallel, we directed a genetic linkage analysis, using markers previously shown to be linked to nephritis and IgG autoantibody production, to determine if such loci were similarly associated with microenvironmental changes. Our data demonstrate that all (NZB x NZW) F1 x NZW backcross mice with disease have microenvironmental defects. Although the microenvironmental defects are not sufficient for development of autoimmune disease, the severity of thymic abnormalities correlates with titers of IgG autoantibodies to DNA and with proteinuria. Consistent with past studies of (NZB x NZW) F1 x NZW mice, genetic markers on proximal chromosome 17 (near MHC) and distal chromosome 4 showed trends for linkage with nephritis. Although the markers chosen only covered about 10-15% of the genome, the results demonstrated trends for linkage with thymic medullary abnormalities for loci on distal chromosome 4 and distal chromosome 1. We believe it will be important to define the biochemical nature of the molecules recognized by these mAbs to understand the relationships between thymic architecture and immunopathology.