CD1.1 expression by mouse antigen-presenting cells and marginal zone B cells.

Mouse CD1.1 is an MHC class I-like, non-MHC-encoded, surface glycoprotein that can be recognized by T cells, in particular NK1.1+ T cells, a subset of alphabeta T cells with semiinvariant TCRs that promptly releases potent cytokines such as IL-4 and IFN-gamma upon stimulation. To gain insight into the function of CD1.1, a panel of nine mAbs was generated and used to biochemically characterize and monitor the surface expression of CD1.1 on different cell types. CD1.1 is a heavily glycosylated, beta2-microglobulin-associated surface protein. Its recognition by a panel of 12 V alpha14-positive and -negative CD1-specific alphabeta T cell hybridomas was blocked by two groups of mAbs that bound to adjacent clusters of epitopes, indicating that different alphabeta TCRs bind to the same region of CD1.1, presumably above the groove. Remarkably, CD1.1 was mainly expressed by dendritic cells, B cells, and macrophages, suggesting a function in Ag presentation to Th cells. Furthermore, the cell type that expressed the highest levels of CD1.1 was the splenic marginal zone B cell, a distinct subset of B cells that also expresses CD21 (the C3d receptor) and may be involved in natural responses to bacterial Ags. Altogether, the results support the idea that CD1.1 may function in recruiting a form of innate help from specialized cytokine producer alphabeta T cells to APCs, a role that might be important at the preadaptive phase of immune responses to some microbial pathogens.

Tissue-specific recognition of mouse CD1 molecules.

Although there is evidence that some members of the CD1 gene family may present particular types of foreign Ags, such as mycobacterial lipid Ags or synthetic hydrophobic peptides, to alphabeta T cells, most CD1 isotypes share the unusual property of being recognized by a high frequency of naturally autoreactive alphabeta T cells. In the case of mouse CD1.1 and its human counterpart CD1d, a significant fraction of the autoreactive T cells express semi-invariant TCRs. CD1.1-specific T cells have a restricted tissue distribution and very promptly secrete a large panel of potent cytokines, including IL-4 and IFN-gamma, upon primary activation through their TCR, suggesting that they might regulate some immune responses in these tissues. We show here that their autorecognition of mouse CD1.1 is highly dependent upon the cell type in which CD1.1 is expressed. For example, some of these T cells only respond to CD1.1 expressed by splenic dendritic cells, some respond preferentially to cortical thymocytes, and others respond to splenic B cells. Tissue specificity of CD1.1 recognition is also observed with various cell lines transfected with CD1.1 cDNA. These results show that different CD1.1 self Ags are expressed in different tissues and can be specifically recognized by autoreactive T cells. They suggest that CD1.1 may be naturally associated with a variety of self ligands that overlap only partially in different cell types.

Self-reactive B cells in nonautoimmune and autoimmune mice.

The defining feature of autoimmune disease is the presence of specific autoreactive lymphocytes. Systemic lupus erythematosus (SLE), for example, is characterized by a discrete set of antibodies directed to nuclear antigens; these include autoantibodies to DNA and snRNPs that are diagnostic for SLE. The murine model of SLE, the MRL-lpr/lpr mouse, likewise, has a similar autoantibody profile. To understand how SLE-associated autoantibodies are regulated in healthy individuals and to identify mechanisms underlying their expression in autoimmunity, we have developed a transgenic (tg) model system using multiple sets of tgs. The development of B cells bearing these tgs has been studied in BALB/c and MRL-lpr/lpr autoimmune backgrounds, and the relative fates of anti-ssDNA and anti-dsDNA tg B cells when they are a part of a diverse as well as monoclonal B cell repertoire have been evaluated.

Characterization of anti-single-stranded DNA B cells in a non-autoimmune background.

Anti-DNA Abs are prevalent in the serum of systemic lupus erythematosus (SLE) patients and in the MRL-lpr/lpr mouse model of SLE, but are generally absent in normal individuals. We have studied the regulation of anti-ssDNA B cells in a non-autoimmune (BALB/c) background by using Ig transgenes (Tgs) encoding anti-DNA Abs. In one case, they are present with other non-DNA-binding B cells (the VH3H9 Tg with endogenous light chains); in the other, they are present as an essentially monospecific population (VH3H9/Vkappa8). We have previously observed that serum anti-ssDNA levels in these Tg mice were no higher than those of non-Tg mice, despite the fact that anti-ssDNA B cells dominate the peripheral B cell repertoire. These results suggested that the anti-ssDNA Tg B cells present are functionally inactivated. In this paper, we isolate B cells from VH3H9/Vkappa8 Tg mice to show that this is indeed the case and go on to further define this state. We demonstrate that VH3H9/Vkappa8 Tg B cells have diminished Ig secretion in response to both T-independent and T-dependent stimuli compared with non-Tg controls. VH3H9/Vkappa8 Tg B cells also show suboptimal proliferation in response to anti-IgM F(ab)'2 fragments and LPS, and are phenotypically distinct in expressing decreased total surface Ig. Despite their functional defects, however, VH3H9/Vkappa8 Tg B cells have an in vivo turnover rate comparable to non-Tg B cells, suggesting that they are long lived.

Mouse CD1-specific NK1 T cells: development, specificity, and function.

NK1 T cells are a specialized population of alpha/beta T cells that coexpress receptors of the NK lineage and have the unique potential to very rapidly secrete large amounts of cytokines, providing early help for effector cells and regulating the Th1 or Th2 differentiation of some immune responses. NK1 T cells express a restricted TCR repertoire made of an invariant TCR alpha chain, V alpha 14-J alpha 281, associated with polyclonal V beta 8, V beta 7, and V beta 2 TCR beta chains. NK1 T cells recognize the products of the conserved family of MHC class I-like CD1 genes, apparently in the absence of foreign antigens. Thus, this novel regulatory pathway, which straddles the innate and the adaptive immune systems, is unique in that its activation may not require associative recognition of antigen. Here, we review the specificity and function of mouse NK1 T cells, and we discuss the relationship of this lineage to mainstream T cells and NK cells.

Persistence of functionally compromised anti-double-stranded DNA B cells in the periphery of non-autoimmune mice.

Both anti-single-stranded (ss) and anti-double-stranded (ds) DNA antibodies are associated with the autoimmune disease systemic lupus erythematosus (SLE), but only anti-dsDNA antibodies are considered one of the diagnostic criteria. Using Ig transgenes coding for anti-DNA we have determined the fate of anti-dsDNA B cells in a non-autoimmune environment. In a Rag-2 wild-type background, B cells expressing the anti-dsDNA Ig transgenes are present in the spleen but dsDNA specificity is disrupted due to expression of endogenous L chains. In a Rag-2-deficient background where co-expression of endogenous Ig is blocked, splenic B cells expressing only the anti-dsDNA transgene Ig are present, indicating that endogenous Ig expression is not required for bone marrow export. The anti-dsDNA B cells that persist are profoundly crippled in that they are unable to proliferate to lipopolysaccharide or anti-Ig stimulation. Furthermore, these anti-dsDNA Ig transgene B cells show a decreased lifespan relative to non-transgene BALB/c B cells. Persistence of anti-dsDNA B cells in the periphery of non-autoimmune mice raises the possibility that their appearance in the context of SLE is due to their reactivation by T cell help.

B cell selection and allelic exclusion of an anti-DNA Ig transgene in MRL-lpr/lpr mice.

We have used an Ig transgene (VH3H9) that increases the frequency of anti-DNA autoantibodies to address whether the production of antinuclear Abs in systemic lupus erythematosus is the consequence of a breakdown of B cell tolerance. We have shown that nonautoimmune mice regulate anti-DNA B cells, and that lupus-prone MRL-lpr/lpr mice are defective in this regulation. Here we show that a subset of anti-DNA B cells, namely those that stain nuclei in a homogeneous fashion, not only fail to be deleted in MRL-lpr/lpr mice, but undergo preferential clonal expansion. In addition, we describe a surprising finding: the VH3H9 transgene is less efficient at inhibiting endogenous heavy chain gene rearrangement on the autoimmune-prone MRL-lpr/lpr genetic background than on the nonautoimmune BALB/c background.

Breakdown of B cell tolerance in a mouse model of systemic lupus erythematosus.

Anti-DNA antibodies, specifically those that stain nuclei in a homogenous nuclear (HN) fashion, are diagnostic of systemic lupus erythematosus (SLE) and the MRL-lpr/lpr SLE murine model. We have used a heavy chain transgene that increases the frequency of anti-HN antibodies to address whether their production in SLE is the consequence of a defect in B cell tolerance. Anti-HN B cells were undetectable in nonautoimmune-prone transgenic mice, but in MRL-lpr/lpr transgenic mice their Ig was evident in the sera and they were readily retrievable as hybridomas. We conclude that nonautoimmune animals actively delete anti-HN-specific B cells, and that MRL-lpr/lpr mice are defective in this process possibly because of the lpr defect in the fas gene.

Intrathymic islet transplantation in the spontaneously diabetic BB rat.

Recently it was demonstrated that pancreatic islet allografts transplanted to the thymus of rats made diabetic chemically are not rejected and induce specific unresponsiveness to subsequent extrathymic transplants. The authors report that the thymus can also serve as an effective islet transplantation site in spontaneously diabetic BB rats, in which autoimmunity and rejection can destroy islets. Intrathymic Lewis islet grafts consistently reversed hyperglycemia for more than 120 days in these rats, and in three of four recipients the grafts promoted subsequent survival of intraportal islets. In contrast intraportal islet allografts in naive BB hosts all failed rapidly. The authors also show that the immunologically privileged status of the thymus cannot prevent rejection of islet allografts in Wistar Furth (WF) rats sensitized with donor strain skin and that suppressor cells are not likely to contribute to the unresponsive state because adoptive transfer of spleen cells from WF rats bearing established intrathymic Lewis islets fails to prolong islet allograft survival in secondary hosts.