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

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.

Dissociation of the genetic loci leading to b1a and NKT cell expansions from autoantibody production and renal disease in B6 mice with an introgressed New Zealand Black chromosome 4 interval

Previous mapping studies have linked New Zealand Black (NZB) chromosome 4 to several lupus traits, including autoantibody production, splenomegaly, and glomerulonephritis. To confirm the presence of these traits, our laboratory introgressed homozygous NZB chromosome 4 intervals extending from either 114 to 149 Mb or 32 to 149 Mb onto the lupus-resistant C57BL/6 background (denoted B6.NZBc4S and B6.NZBc4L, respectively). Characterization of aged cohorts revealed that B6.NZBc4L mice exhibited a striking increase in splenic B1a and NKT cells in the absence of high titer autoantibody production and significant renal disease. Tissue-specific expansion of these subsets was also seen in the peritoneum and liver for B1a cells and in the bone marrow for NKT cells. Staining with CD1d tetramers loaded with an alpha-galactosylceramide analog (PBS57) demonstrated that the expanded NKT cell population was mainly CD1d-dependent NKT cells. The lack of both cellular phenotypes in B6.NZBc4S mice demonstrates that the genetic polymorphism(s) that result in these phenotypes are on the proximal region of NZB chromosome 4. This study confirms the presence of a locus that promotes the expansion of B1a cells and newly identifies a region that promotes CD1d-restricted NKT cell expansion on NZB chromosome 4. Taken together, the data indicate that neither an expansion of B1a cells and/nor NKT cells is sufficient to promote autoantibody production and ultimately, renal disease.

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.

A Defect in Deletion of Nucleosome-Specific Autoimmune T Cells in Lupus-Prone Thymus: Role of Thymic Dendritic Cells

To study central tolerance to the major product of ongoing apoptosis in the thymus, we made new lines of transgenic (Tg) mice expressing TCR of a pathogenic autoantibody-inducing Th cell that was specific for nucleosomes and its histone peptide H4(71-94). In the lupus-prone (SWR x NZB)F1 (SNF1) thymus, introduction of the lupus TCR transgene caused no deletion, but marked down-regulation of the Tg TCR and up-regulation of endogenous TCRs. Paradoxically, autoimmune disease was suppressed in the alphabetaTCR Tg SNF1 mice with induction of highly potent regulatory T cells in the periphery. By contrast, in the MHC-matched, normal (SWR x B10. D2)F1 (SBF1), or in the normal SWR backgrounds, marked deletion of transgenic thymocytes occurred. Thymic lymphoid cells of the normal or lupus-prone mice were equally susceptible to deletion by anti-CD3 Ab or irradiation. However, in the steady state, spontaneous presentation of naturally processed peptides related to the nucleosomal autoepitope was markedly greater by thymic dendritic cells (DC) from normal mice than that from lupus mice. Unmanipulated thymic DC of SNF1 mice expressed lesser amounts of MHC class II and costimulatory molecules than their normal counterparts. These results indicate that apoptotic nucleosomal autoepitopes are naturally processed and presented to developing thymocytes, and a relative deficiency in the natural display of nucleosomal autoepitopes by thymic DC occurs in lupus-prone SNF1 mice.

A defective NF-kappa B/RelB pathway in autoimmune-prone New Zealand black mice is associated with inefficient expansion of thymocyte and dendritic cells

New Zeland Black (NZB) mice develop an autoimmune disease involving an abnormal B cell response to peripheral self Ags. This disease is associated with defects in other cell types and thymic stromal organization. We present evidence that NZB cells of various lineages, including thymocytes, fibroblasts, and dendritic precursor cells, show impaired proliferation and enhanced cell death in culture upon stimulation compared with non-autoimmune-prone mice such as C57BL/6. This phenotype explains the reduced efficiency of maturation of bone marrow-derived dendritic cells and the loss of TNF- or IL-1-dependent thymocyte costimulation. Upon TNF-induced activation of NZB thymocytes, nuclear translocation and DNA binding of RelA- and RelB-dependent NF-kappaB heterodimers are significantly reduced. This phenotype has a transcriptional signature, since the NZB, but not the nonobese diabetic, thymic transcriptome shows striking similarities with that of RelB-deficient thymuses. This partial NF-kappaB deficiency detected upon activation by proinflammatory cytokines could explain the disorganization of thymic microenvironments in NZB mice. These combined effects might reduce the efficiency of central tolerance and expose apoptotic debris generated during inflammatory processes to self recognition.

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.

B cells are selectively associated with thymic cortical but not medullary pathology in NZB mice

Abnormal expansion of autoantibody-synthesizing B cells and self-reactive T cells, which most likely escape negative selection within the thymus, have both been characterized and reasoned to play a role in the pathogenesis of autoimmunity in NZB mice. Support for this thesis has been our observation that NZB mice have severe cortical and medullary thymic microarchitectural defects. As a means to dissect the roles of T and B cells in the induction of such abnormalities, B cell-deficient NZB mice were bred by backcrossing the Igh6(null)allele on to the NZB background (NZB-muMT mice). Such mice showed undetectable levels of autoantibodies. NZB-muMT mice, as compared to wild-type NZB mice, had lower absolute numbers of CD4(+)T cells. Furthermore, thymic abnormalities in NZB-muMT mice were restricted to the medulla. These data suggest that, while B cells may play a role in thymic cortical abnormalities, the medullary abnormalities are induced by other mechanisms.

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.