Clonal deletion of V beta 5+ T cells by transgenic I-E restricted to thymic medullary epithelium

A variety of cell types expressing MHC class II molecules is known to function as APC in vitro. We employed the Ig kappa gene enhancer and promoter to target the class II E alpha gene, and thereby I-E, exclusively to B cells to address their APC function in vivo. Although transgenic I-E was expressed on B lymphocytes, we unexpectedly obtained I-E on thymic medullary epithelium but not macrophages and at low frequency on dendritic cells. Using these transgenic mice, we constructed bone marrow irradiation chimeras with I-E expressed only on medullary epithelium, in order to determine the role of this cell type in tolerance by clonal deletion in the thymus. Although it is accepted that bm-derived cells play a primary role in deletion, and thymic epithelium can delete clones to a lesser degree, the role of cortical vs medullary thymic epithelium has not been directly dissected. We demonstrate that medullary epithelium alone can tolerize by partial deletion of I-E-reactive V beta 5+ T cells. These results indicate a role for medullary epithelium in deletion during the later stages of thymic development, and support the notion that positive and negative selection of developing T cells can occur in distinct temporal and anatomic compartments.

Clonal deletion of V beta 5+ T cells by transgenic I-E restricted to thymic medullary epithelium

A variety of cell types expressing MHC class II molecules is known to function as APC in vitro. We employed the Ig kappa gene enhancer and promoter to target the class II E alpha gene, and thereby I-E, exclusively to B cells to address their APC function in vivo. Although transgenic I-E was expressed on B lymphocytes, we unexpectedly obtained I-E on thymic medullary epithelium but not macrophages and at low frequency on dendritic cells. Using these transgenic mice, we constructed bone marrow irradiation chimeras with I-E expressed only on medullary epithelium, in order to determine the role of this cell type in tolerance by clonal deletion in the thymus. Although it is accepted that bm-derived cells play a primary role in deletion, and thymic epithelium can delete clones to a lesser degree, the role of cortical vs medullary thymic epithelium has not been directly dissected. We demonstrate that medullary epithelium alone can tolerize by partial deletion of I-E-reactive V beta 5+ T cells. These results indicate a role for medullary epithelium in deletion during the later stages of thymic development, and support the notion that positive and negative selection of developing T cells can occur in distinct temporal and anatomic compartments.

Antigen-presenting cells in adoptively transferred and spontaneous autoimmune diabetes

Histological techniques were used to identify antigen-presenting cells (APC) in adoptively transferred diabetes in NOD mice and Ins-HA transgenic mice, and in spontaneously diabetic NOD mice. In adoptively transferred disease, CD4+ T cells and F4/80+ macrophages dominated early infiltrates. By contrast, in spontaneously developing diabetes in NOD mice, lymphocytic infiltrates appeared to be well organized around a network of VCAM-1+ NLDC-145+ ICAM-1+ dendritic cells. Thus, the primary APC spontaneous autoimmune disease appears to be the strongly stimulatory dendritic cell rather than the normally resident macrophage. Next, we used chimeric animals to demonstrate that insulitis and diabetes could occur even when responding T cells were unable to recognize islet-specific antigen directly on beta cells. Altogether, the results demonstrate that immune-mediated damage does not require direct contact between CD4+ T cells and beta cells. Moreover, despite the induction of ICAM-1, VCAM-1, and class II on vascular endothelium near islet infiltrates, these experiments show that recruitment of lymphocytes occurs even when antigen presentation is not possible on vascular endothelium.

Interference with major histocompatibility complex class II-restricted antigen presentation in the brain by herpes simplex virus type 1: a possible mechanism of evasion of the immune response

Host survival of herpes simplex virus type 1 (HSV-1) infection depends on the establishment of latent infections in both peripheral and central nervous systems. Strains of HSV-1 that are successful in escaping the immune response produce a lethal infection. We now report a possible mechanism of immune response evasion used by HSV-1. After intraocular inoculation of mice, HSV-1 strain F established a latent infection in the brain, whereas strain KOS did not. The immune response to HSV-1 infection (strains KOS and F) in the brain was characterized by induction of major histocompatibility complex class II expression and recruitment of CD4+ and CD8+ cells to highly restricted sites of intracerebral viral infection. Major histocompatibility complex class II antigen expression was primarily intracellular in strain KOS infection centers and at the cell surface in strain F infection centers. We propose that major histocompatibility complex class II-restricted viral-antigen presentation to T cells is interrupted during strain KOS infections, thereby allowing KOS infection to evade T-cell-mediated events that would normally protect the host from a lethal infection. Immunocompromised mice (athymic or irradiate mice) could not survive strain F infections; however, latent F infections were established in irradiated mice reconstituted with naive lymph node and spleen cells. These data suggest that class II-restricted presentation of viral antigens is required for the control of HSV-1 infections in the nervous system.

T-cell tolerance

Despite the acceptance of principles such as clonal deletion in the thymus and peripheral clonal anergy, several new issues have arisen in the study of T-cell tolerance. For example, in the case of thymic tolerance, it is now clear that several distinct components of the thymus, including various subsets of thymic epithelial cells, can all make contributions to the deletion of autoreactive T cells. In the case of peripheral T-cell tolerance, the induction of anergy now has been complicated by the possibility that bone marrow derived antigen presenting cells may, under certain conditions, be tolerogenic rather than stimulatory.

A recessive defect in lymphocyte or granulocyte function caused by an integrated transgene

A line of transgenic mice has been identified with a recessive defect in lymphocyte or granulocyte function, presumably as a result of insertional mutagenesis by the integrated transgene. Transgenic mice homozygous for the transgene integrant showed nearly complete absence of lymphocytes in peripheral lymph nodes and Peyer's patches, a severely diminished thymus medulla, and a greatly enlarged spleen. These animals also developed a syndrome characterized by granulocyte and mononuclear infiltrates in numerous tissues, including skin, liver, and lung, and immunoglobulin deposits in kidney glomeruli. Lung infiltrates were specifically localized around large blood vessels and bronchi, accompanied in some cases by destruction of arterial walls. The light scatter profile of spleen lymphocytes suggested an extremely high percentage of blast cells. Because tissue development and morphology appears to be normal in all other tissues observed, the genetic lesion appears to specifically affect the regulation of lymphocyte or granulocyte activation.

Two subsets of epithelial cells in the thymic medulla

Information was sought on the features of epithelial cells in the murine thymic medulla. The expression of major histocompatibility complex (MHC) molecules on medullary epithelium was defined by light microscopy with the aid of bone marrow chimeras and MHC-transgenic mice. A proportion of medullary epithelial cells was found to show conspicuously high expression of conventional MHC (H-2) class I (K, D, L) and class II (I-A, I-E) molecules. These cells express a high density of the Y-Ae epitope, a complex of an E alpha peptide and I-Ab molecules found on typical bone marrow-derived cells. MHC+ medullary epithelial cells show limited expression of I-O molecules, a class of atypical nonpolymorphic MHC-encoded class II molecules present on B cells. Other medullary epithelial cells express a high density of I-O molecules but show little or no expression of typical MHC class I or II molecules. MHC and I-O expression thus appear to subdivide medullary epithelial cells into two phenotypically distinct subsets. This applies in adults. In the embryonic thymus most medullary epithelial cells express both types of molecules.

Among naive precursor cell subpopulations only progenitors of memory B cells originate germinal centers

Immunization leads to the generation of both antibody-forming cells (AFC) and memory B cells which are thought to arise in germinal centers within lymphoid follicles. The findings that the precursors to memory B cells reside in the J11Dlo subpopulation of the spleens in non-immune mice and that this subpopulation is distinct from conventional AFC precursors, including CD5+ B cells, suggest that the precursors of germinal centers might also reside in the J11Dlo subpopulation. To test this hypothesis, SCID mice were repopulated with CD4+ carrier-primed T cells and T-depleted J11Dlo, J11Dhi or CD5+ B cells and immunized with a hapten-carrier conjugate. Only the J11Dlo population was enriched for cells that produced germinal centers. Thus, the subpopulation of precursors that generates memory B cells also originates germinal centers.

Peripheral tolerance to an islet cell-specific hemagglutinin transgene affects both CD4+ and CD8+ T cells

To study the basis for immunological tolerance of peripheral tissue-specific antigens, a transgenic mouse line was established that expresses the influenza hemagglutinin (HA) on pancreatic islet beta cells (Ins-HA transgenic mice). When followed up to 14 months of age, Ins-HA transgenic mice did not develop spontaneous autoimmune disease. Upon immunization with HA-expressing viruses, high titers of HA-specific circulating antibody were detected; however, T cell responses by both the T helper and T cytolytic compartment were markedly reduced as compared with transgene-negative littermates, and no evidence could be found for islet infiltrates. Adoptive transfer of histocompatible lymphocytes from transgene-negative mice plus virus into irradiated Ins-HA hosts resulted in islet inflammation dominated by CD4+ T cells, indicating that the HA antigen was accessible to activated T cells. These results suggest that T cells can be rendered tolerant of antigens expressed outside the thymus.

Observation of XeF (C ? A) laser action in liquid argon

Superradiant and laser emissions of the XeF (C ? A) transition were observed at 530 nm in liquid argon for the first time to our knowledge. The liquid samples were prepared from Ar/Xe/F(2) mixtures in a ratio of 1600:10:1 and pumped by a commercial excimer laser at 351 nm. Dramatic line narrowing of the initially broad (FWHM ~60 nm) C ? A fluorescence spectrum was induced at elevated pump powers. The XeF (C) state excimer number density was estimated to be in excess of 5.26 x 10(17) cm(-3).

Expression of mouse IgA by transgenic mice, pigs and sheep

The development of transgenic animal technology allows the introduction of desired traits into the germ line of mice and other animals. Since the production of antibody to various polysaccharide antigens can be protective against pathogenic bacteria, we generated transgenic mice, sheep and pigs carrying genes encoding the mouse alpha and kappa chains for antibodies against phosphorylcholine (PC) to determine whether transgene antibody might be used to influence susceptibility to disease. High serum levels of mouse IgA were detected in transgenic mice and pigs but not in transgenic sheep. It has been noted that transgene immunoglobulin expression can suppress endogenous immunoglobulin gene rearrangement and expression, and indeed, in one mouse line, expression of the transgene resulted in less than 10% of spleen B cells expressing endogenous IgM. Despite this suppression, significant levels of endogenous IgM were still secreted into the serum. Suppression of endogenous IgM expression was not seen in other mouse lines, nor was it seen in transgenic pigs. In the transgenic pigs, the mouse IgA was detected in the serum despite the absence of an intact mouse kappa transgene, so the secreted antibody presumably included pig light chains. Little if any of the mouse IgA in these sera showed binding specificity for PC. In one of the founder sheep, mouse IgA was detectable in peripheral lymphocytes but not in serum. Mouse kappa expression was not detected in the transgenic sheep harboring an intact kappa transgene. These results illustrate the potential of introducing beneficial traits such as germ-line-encoded immunity into large mammalian species.

Infertility in male transgenic mice: disruption of sperm development by HSV-tk expression in postmeiotic germ cells

Previous experiments revealed that male transgenic mice bearing a cosmid that included the Class II E alpha gene, about 35 kb of 5' flanking DNA, and the cosmid vector sequences were sterile. To ascertain the cause of the sterility, various subfragments of the cosmid were tested in transgenic mice. Only those pieces of DNA that included some of the E alpha flanking chromosomal DNA and the herpes simplex virus (HSV)-thymidine kinase (tk) gene that was in the vector resulted in male sterility. Histological analysis revealed abnormalities in nuclear morphology of elongating spermatids and retention of mature spermatids within the seminiferous epithelium. Immunocytochemical studies showed that the HSV-tk gene was expressed at low levels in postmeiotic round spermatids and at higher levels in more mature elongating spermatids. To determine whether expression of HSV-tk in spermatids might be responsible for the sterility, the protamine gene promoter was used to direct the expression of HSV-tk to postmeiotic germ cells. Since the mice so treated were also sterile, the data suggest that expression of this enzyme in spermatids is responsible for the sterility phenotype.

Tolerance in transgenic mice expressing major histocompatibility molecules extrathymically on pancreatic cells

Transgenic mice with defined expression of major histocompatibility complex (MHC) proteins provide novel systems for understanding the fundamental question of T cell tolerance to nonlymphoid self components. The MHC class II I-E and I-A and class I H-2K molecules expressed specifically on pancreatic islet or acinar cells serve as model self antigens. In these systems, transgenic proteins are not detected in the thymus or other lymphoid tissues. Yet mice are tolerant to the pancreatic MHC products in vivo; this tolerance is not induced by clonal deletion. These studies have been aided by monoclonal antibodies specific for I-E-reactive T cells and indicate that clonal anergy may be an important mechanism of tolerance to peripheral proteins.

Strong T cell tolerance in parent----F1 bone marrow chimeras prepared with supralethal irradiation. Evidence for clonal deletion and anergy

T cell tolerance induction was examined in long-term H-2-heterozygous parent----F1 chimeras prepared with supralethal irradiation (1,300 rad). Although these chimeras appeared to be devoid of host-type APC, the donor T cells developing in the chimeras showed marked tolerance to host-type H-2 determinants. Tolerance to the host appeared to be virtually complete in four assay systems: (a) primary mixed lymphocyte reactions (MLR) of purified lymph node (LN) CD8+ cells (+/- IL-2); (b) primary MLR of CD4+ (CD8-) thymocytes; (c) skin graft rejection; and (d) induction of lethal graft-vs.-host disease by CD4+ cells. Similar tolerance was observed in chimeras given double irradiation. The only assay in which the chimera T cells failed to show near-total tolerance to the host was the primary MLR of post-thymic CD4+ cells. In this assay, LN CD4+ cells regularly gave a significant antihost MLR. The magnitude of this response was two- to fourfold less than the response of normal parental strain CD4+ cells and, in I-E(-)----I-E+ chimeras, was paralleled by approximately 70% deletion of V beta 11+ cells. Since marked tolerance was evident at the level of mature thymocytes, tolerance induction in the chimeras presumably occurred in the thymus itself. The failure to detect host APC in the thymus implies that tolerance reflected contact with thymic epithelial cells (and/or other non-BM-derived cells in the thymus). To account for the residual host reactivity of LN CD4+ cells and the incomplete deletion of V beta 11+ cells, it is suggested that T cell contact with thymic epithelial cells induced clonal deletion of most of the host-reactive T cells but spared a proportion of these cells (possibly low affinity cells). Since these latter cells appeared to be functionally inert in the thymus (in contrast to LN), we suggest that the thymic epithelial cells induced a temporary form of anergy in the remaining host-reactive thymocytes. This anergic state disappeared when the T cells left the thymus and reached LN.

T-cell tolerance by clonal anergy in transgenic mice with nonlymphoid expression of MHC class II I-E

T-cell reactivity to the class II major histocompatibility complex I-E antigen is associated with T-cell antigen receptors containing the V beta gene segments V beta 17a and V beta 5. Mice expressing I-E with the normal tissue distribution (on B cells, macrophages, dendritic cells and thymic epithelium) induce tolerance to self I-E by clonal deletion in the thymus. By contrast, we find that transgenic INS-I-E mice that express I-E on pancreatic beta-cells, but not in the thymus or peripheral lymphoid organs, are tolerant to I-E but have not deleted V beta 5- and V beta 17a-bearing T cells. Moreover, whereas T-cell populations from nontransgenic mice proliferate in response to receptor crosslinking with V beta 5- and V beta 17a-specific antibodies, T cells from INS-I-E mice do not. Thus, our experiments provide direct evidence that T-cell tolerance by clonal paralysis does occur during normal T-cell development in vivo.

Tolerance to class II MHC in transgenic mice

Transgenic mice offer new possibilities in experimental techniques for understanding the familiar questions of T cell development and the selection of the T cell repertoire. As a powerful method to manipulate gene expression in the whole animal, precisely defined in vivo models can be developed. In our own studies, we have used transgenic mice with targeted expression of I-E and new monoclonal antibodies defining T cell receptors specific for class II I-E molecules. In the thymus, our results suggest that thymic epithelium has significant tolerance inducing capability, but the mechanism may be different from the clonal deletion induced by bone marrow derived cells. In the periphery, our results suggest that tolerance to tissue restricted antigens is not induced by clonal deletion. Instead, clonal paralysis may be an important mechanism for both inducing and maintaining peripheral tolerance.

Targeted correction of a major histocompatibility class II E alpha gene by DNA microinjected into mouse eggs

DNA molecules containing the 5' end of a functional major histocompatibility class II E alpha gene were injected into mouse eggs bearing E alpha genes with 630-base-pair (bp) deletions encompassing the promoter and first exon. The deletion was corrected by homologous recombination in 1 of about 500 transgenic mice that incorporated the injected DNA. The corrected E alpha gene was transmitted to progeny, which were bred to homozygosity. Southern blot analysis, polymerase chain reaction amplification of the DNA spanning the deletion, and sequence analysis revealed that the corrected allele resembles the wild-type E alpha gene. At sites of single-base-pair polymorphisms, there was apparently random conversion to either the donor or recipient sequence. In addition, many point mutations were introduced. mRNAs were produced from the corrected allele in a tissue-specific manner, but their sizes were different from the wild-type allele, and they did not produce detectable E alpha protein. This experiment demonstrates the feasibility of targeting foreign DNA to a gene that is completely inactive in fertilized mouse eggs.

Tolerance in transgenic mice expressing class II major histocompatibility complex on pancreatic acinar cells

To study the nature of tolerance to antigens not expressed by cells of the lymphoid system, expression of class II MHC I-E was targeted to the acinar cells of the exocrine pancreas in transgenic mice (elastase [EL]-I-E). Despite the absence of detectable I-E in the thymus of EL-I-E transgenic mice, both thymocytes and peripheral T lymphocytes were tolerant to I-E, and the pancreas was free of autoimmune infiltrates. Nontolerant T cells adoptively transferred into irradiated or T-depleted transgenic mice rapidly destroy the I-E+ components of the pancreas; however, adoptive transfer of nontolerant T lymphocytes into nonirradiated transgenic mice do not. These results suggest that tolerance in transgenic mice is maintained by some peripheral tolerance mechanism. However, further studies indicate that tolerance in transgenic mice is not maintained by specific Ts cells. For example, cell mixing experiments both in vitro and in vivo fail to reveal dominant unresponsiveness. Furthermore, nontolerant T cells injected into otherwise unmanipulated EL-I-E mice can be primed in situ (by injections of I-E+ spleen cells) to destroy the I-E+ acinar cells.

Inhibition of immunoglobulin gene rearrangement by the expression of a lambda 2 transgene

The rearrangement of Ig genes is known to be regulated by the production of H and kappa L chains. To determine whether lambda L chains have a similar effect, transgenic mice were produced with a lambda 2 gene. It was necessary to include the H chain enhancer, since a lambda gene without the added enhancer did not result in transgene expression. The lambda 2 transgene with the H enhancer was expressed in lymphoid cells only. The majority of the B cells of newborn transgenic mice produced lambda, whereas kappa + cells were reduced. Concomitantly, serum levels of kappa and kappa mRNA were diminished. By 2 wk after birth the proportion of kappa-expressing cells was dramatically increased. Adults had reduced proportions of B cells that produced lambda only, but the levels of lambda were still higher than in normal littermates. Also, kappa + cells were still lower than in normal mice. Analysis of hybridomas revealed that reduction of kappa gene rearrangement was the basis for the decreased frequency of kappa + cells. Furthermore, many cells also contained an unrearranged H chain allele. It was concluded that feedback inhibition by the lambda 2 together with endogenous H protein may have inhibited recombinase activity in early pre-B cells, leading to inhibition of both H chain and kappa gene rearrangement. Thus, lambda 2 can replace kappa in a feedback complex. The levels of serum lambda 1 and, to a lesser degree, of spleen lambda 1 mRNA were reduced in the lambda 2 transgenic mice. However, the proportion of hybridomas with endogenous lambda gene rearrangement was at least as high as in normal mice. It was therefore concluded that the suppression of functional lambda 1 may be a consequence of decreased selection of endogenous lambda-producing cells because of the excess of transgenic lambda. The escape of kappa-producing cells from feedback inhibition may be the result of several mechanisms that operate to varying degrees, among them: (a) kappa rearrangement during a period in which the recombinase is still active after appearance of a lambda 2/mu stop signal; (b) a B cell lineage that is not feedback inhibited at the pre-B cell stage; (c) subthreshold levels of transgenic lambda 2 in some pre-B cells; and (d) loss of the lambda 2 transgenes in rare pre-B cells.

A novel MHC class II epitope expressed in thymic medulla but not cortex

The repertoire of receptors expressed by peripheral T cells is the result of two selective events that occur during intrathymic development. Positive selection expands cells able to recognize foreign peptides presented by self MHC molecules, and negative selection eliminates cells reactive to self MHC molecules and associated self peptides. Chimaera studies suggest that, at least in the case of T cells recognizing MHC class II, interaction with thymic cortical epithelial cells is responsible for the former, whereas thymic medullary cells, of bone marrow origin, mediate the latter. This view of thymic development is supported by recent morphometric analyses, showing that autoreactive cells are found in thymic cortex but not medulla. Although numerous studies have shown that MHC class II molecules are expressed in both sites, none provides any explanation for the differential selection of T cells that is observed. Here, we describe a novel MHC class II epitope which is found on cells in thymic medulla but not cortex. The antibody to this epitope reacts with about 10% of class II molecules on B cells and may be recognizing a self peptide-MHC complex. These results provide the first evidence for differential expression of class II epitopes in different tissues and are compatible with the hypothesis that different ligands, rather than different affinity thresholds for the same ligand, are involved in positive and negative selection of the T-cell repertoire.

Selective expression of class II E alpha d gene in transgenic mice

Class II genes of the MHC must be expressed by APC for activation of CD4+ T cells and efficient delivery of T cell help to B lymphocytes. Class II genes have restricted tissue expression and are under complex regulation. By using various deletion constructs of the class II E alpha d gene in transgenic mice we have mapped different 5' flanking regions which control E alpha d gene expression in distinct cell types. We demonstrate dissociate expression of E alpha d within the macrophage lineage as well as within the B cell lineage, and present evidence for a repressive element operative in B cells and macrophages. We describe the generation of novel transgenic lines with limited constitutive and inducible E alpha mRNA and I-E protein.

Selective expression of class II E alpha d gene in transgenic mice

Class II genes of the MHC must be expressed by APC for activation of CD4+ T cells and efficient delivery of T cell help to B lymphocytes. Class II genes have restricted tissue expression and are under complex regulation. By using various deletion constructs of the class II E alpha d gene in transgenic mice we have mapped different 5' flanking regions which control E alpha d gene expression in distinct cell types. We demonstrate dissociate expression of E alpha d within the macrophage lineage as well as within the B cell lineage, and present evidence for a repressive element operative in B cells and macrophages. We describe the generation of novel transgenic lines with limited constitutive and inducible E alpha mRNA and I-E protein.

Expression of immunoglobulin genes in transgenic mice and transfected cells

Immunoglobulin (Ig) genes are expressed sequentially (first H-, then L-chain genes) during the development of B lymphocytes. These studies, performed with transgenic mice and transfected cells, were aimed at the regulation of turning on and off the rearrangement of Ig genes. The specific recombinase is active in pre-B cells, but not in plasma cells. Production of membrane mu, but not secreted mu or gamma-2b, turns off rearrangement of H genes. Feedback inhibition of kappa-gene rearrangement requires kappa and membrane mu. Kappa alone or in combination with secreted mu does not stop recombination. Mouse lambda genes were mapped by deletion analysis and pulsed-field gel electrophoresis. The gene order is V2-C2,4-V1-C3,C1. The distance between V2 and C2 is 74 kb, but that between V1 and C3, 1 is only 20 kb. V2 and C3, 1 are over 190 kb apart. Lambda genes appear to be rearranged in a subset of B cells that do not respond to feedback inhibition at the pre-B cell stage. Lambda and kappa genes are both rearranged and potentially functional in these cells. Kappa genes may then be deleted by recombination of a sequence (described by Selsing and Siminovitch et al.) downstream of C-kappa with sequences upstream of C-kappa. Presumably the recombinase is eventually inactivated in kappa-lambda cells by a mechanism that is different from H-kappa feedback.

Antigen presenting function of class II MHC expressing pancreatic beta cells

Class II major histocompatibility complex (MHC) gene expression in the mouse is generally limited to thymic epithelium and bone marrow-derived cells such as B lymphocytes and cells of the macrophage/dendritic cell lineage (M phi/DC). Class II-bearing B lymphocytes and M phi/DC possess antigen presenting cell (APC) function; that is, they can stimulate T lymphocytes reactive to either antigen plus MHC or foreign MHC alone. To assess whether non-bone-marrow-derived cells can acquire APC function and elicit graft rejection through expression of class II, we studied transgenic pancreatic islet beta cells that express a foreign class II (I-E) molecule. In vivo, grafts of I-E+ transgenic islets into I-E- naive hosts are not rejected unless the host is primed by an injection of I-E+ spleen cells. In vitro, the I-E+ beta cells are unable to stimulate T lymphocytes reactive to I-E plus a peptide antigen. Paradoxically, they induce antigen specific unresponsiveness in the T cells. We propose that expression of class II on non-lymphoid cells may serve as an extrathymic mechanism for maintaining self tolerance.

The effect of thymus environment on T cell development and tolerance

During development in the thymus, T cells are deleted if their receptors are able to recognize self major histocompatibility complex (MHC) proteins. We show that such clonal deletion can occur because of interaction between receptors on T cells and MHC expressed on bone marrow-derived cells. In addition, development in the thymus picks out T cells to mature if their receptors will be restricted for antigen recognition in association with self MHC alleles expressed on thymus epithelial cells. This process is usually thought to involve positive selection of T cells bearing receptors with high and low affinity for MHC on thymus epithelium, and subsequent deletion of high affinity cells by interaction with bone marrow-derived cells. Our data do not fit such a model, but rather suggest that MHC molecules on thymus epithelium and bone marrow-derived cells may not be seen identically by T cell receptors.

Diabetes and tolerance in transgenic mice expressing class II MHC molecules in pancreatic beta cells

Insulin-dependent diabetes is caused by the loss of insulin-producing beta cells in pancreatic islets. It has been proposed that aberrant expression of Class II Major Histocompatibility Complex (MHC) molecules on beta cells stimulates an autoimmune attack against beta cell antigens. To test this hypothesis, we generated transgenic mice that express Class II MHC molecules (E alpha d/E beta b, or I-Eb) on beta cells. Diabetes was found in 100% of transgenic progeny from three expressing transgenic mouse lines, but without evidence for lymphocytic infiltrates. Furthermore, T lymphocytes appeared to be tolerant to the transgene I-Eb molecule, despite the absence of expression of I-Eb in the thymus or any other lymphoid tissue. The results suggest that novel expression of Class II MHC molecules on nonlymphoid cells is by itself insufficient to initiate autoimmune responses against tissue-specific antigens.

T cell specificity in twice-irradiated F1----parent bone marrow chimeras: failure to detect a role for immigrant marrow-derived cells in imprinting intrathymic H-2 restriction

In an attempt to resolve the issue of whether H-2-restricted T cell specificity is controlled by thymic epithelial cells or by cells of the macrophage/dendritic cell (M phi/DC) lineages, long-term F1----parent chimeras were subjected to secondary irradiation and reconstitution with F1 marrow cells. The rationale was that if F1 M phi/DC enter the thymus only quite slowly after irradiation, as claimed by other investigators, leaving F1----parent chimeras for a period of several months before re-irradiation would ensure that the new wave of T cells generated in the thymus of the chimeras would have no difficulty in making contact with donor-derived F1 M phi/DC. According to the view that M phi/DC rather than epithelial cells control H-2 restriction, the T cells differentiating in these chimeras would be expected to show H-2 restriction to both parental strains. In practice, T cells from twice-irradiated (1000 + 800 rad) chimeras showed strong restriction to host (thymic) H-2 determinants, the degree of restriction to host determinants being as marked as with T cells from once-irradiated chimeras. This finding applied both to T proliferative responses to KLH assayed in vitro and to T helper function for sheep erythrocytes measured in vivo. Preliminary experiments established that the initial dose of irradiation used for preparing the chimeras (1000 rad) resulted in almost total replacement of intrathymic M phi/DC by donor-derived cells within 4 wk of irradiation; M phi/DC were typed by determining their capacity to stimulate mixed-lymphocyte reactions. Collectively, the data imply that, at least under the conditions used, H-2-restricted T cell specificity is controlled by epithelial cells rather than by M phi/DC.

Exogenous control of I-A expression in fetal thymus explants

With the use of a system in which 14 day fetal thymus lobes were cultured in vitro with bone marrow or other lymphoid cells, evidence was obtained that entry of macrophage/dendritic cells (M phi/DC) into the thymus causes a marked rise in the density of endogenous I-A molecules expressed in the cortex, presumably on epithelial cells. High cortical I-A expression also occurred when thymus lobes were cultured with supernatants containing IFN-gamma; addition of anti-IFN-gamma antibody blocked I-A induction. The working hypothesis for these findings is that cellular interactions occurring in the medullary region between M phi/DC and a subset of thymocytes leads to production of IFN-gamma, which then diffuses into the cortex and promotes epithelial I-A expression. The possible relevance of this scheme to the process of thymic "education" is discussed.

Exogenous control of I-A expression in fetal thymus explants

With the use of a system in which 14 day fetal thymus lobes were cultured in vitro with bone marrow or other lymphoid cells, evidence was obtained that entry of macrophage/dendritic cells (M phi/DC) into the thymus causes a marked rise in the density of endogenous I-A molecules expressed in the cortex, presumably on epithelial cells. High cortical I-A expression also occurred when thymus lobes were cultured with supernatants containing IFN-gamma; addition of anti-IFN-gamma antibody blocked I-A induction. The working hypothesis for these findings is that cellular interactions occurring in the medullary region between M phi/DC and a subset of thymocytes leads to production of IFN-gamma, which then diffuses into the cortex and promotes epithelial I-A expression. The possible relevance of this scheme to the process of thymic "education" is discussed.

T cell specificity in twice-irradiated F1----parent bone marrow chimeras: failure to detect a role for immigrant marrow-derived cells in imprinting intrathymic H-2 restriction

In an attempt to resolve the issue of whether H-2-restricted T cell specificity is controlled by thymic epithelial cells or by cells of the macrophage/dendritic cell (M phi/DC) lineages, long-term F1----parent chimeras were subjected to secondary irradiation and reconstitution with F1 marrow cells. The rationale was that if F1 M phi/DC enter the thymus only quite slowly after irradiation, as claimed by other investigators, leaving F1----parent chimeras for a period of several months before re-irradiation would ensure that the new wave of T cells generated in the thymus of the chimeras would have no difficulty in making contact with donor-derived F1 M phi/DC. According to the view that M phi/DC rather than epithelial cells control H-2 restriction, the T cells differentiating in these chimeras would be expected to show H-2 restriction to both parental strains. In practice, T cells from twice-irradiated (1000 + 800 rad) chimeras showed strong restriction to host (thymic) H-2 determinants, the degree of restriction to host determinants being as marked as with T cells from once-irradiated chimeras. This finding applied both to T proliferative responses to KLH assayed in vitro and to T helper function for sheep erythrocytes measured in vivo. Preliminary experiments established that the initial dose of irradiation used for preparing the chimeras (1000 rad) resulted in almost total replacement of intrathymic M phi/DC by donor-derived cells within 4 wk of irradiation; M phi/DC were typed by determining their capacity to stimulate mixed-lymphocyte reactions. Collectively, the data imply that, at least under the conditions used, H-2-restricted T cell specificity is controlled by epithelial cells rather than by M phi/DC.

Functions of purified L3T4+ and Lyt-2+ cells in vitro and in vivo

The expression of L3T4 and Lyt-2 cell surface molecules separates T cells into two broad, non-overlapping subsets: typical T helper cells are L3T4+ Lyt-2- whereas most T killer cells and their precursors are L3T4- Lyt-2+. This review compares highly purified populations of unprimed L3T4+ and Lyt-2+ cells for their capacity to respond to class I vs. class II H-2 alloantigens. Various parameters are considered, including generation of mixed lymphocyte reactions (MLR) and cell mediated lympholysis (CML) in vitro, proliferative responses in irradiated mice, graft-versus-host reactions and skin allograft rejection. In all of these assays the two T cell subsets exhibit marked specificity in their response to H-2 alloantigens, L3T4+ cells responding only to class II and not class I differences and Lyt-2+ cells showing reciprocal specificity. Contrary to current dogma, the bulk of the evidence suggests that primary responses of Lyt-2+ cells do not depend on exogenous help provided by other T cells.

Properties of purified T cell subsets. II. In vivo responses to class I vs. class II H-2 differences

Highly purified populations of C57BL/6 (B6) L3T4+ and Lyt-2+ T cell subsets were compared for their capacity to exert alloreactivity to class I vs. class II H-2 differences in vivo. B6 Lyt-2+ cells responded strongly to the class I different mutant, bm1, as manifested by DNA synthesis in the spleen of irradiated mice followed by entry of blast cells into thoracic duct lymph, induction of splenomegaly in newborn mice, production of lethal GVHD in irradiated mice, and skin allograft rejection. By all of these parameters, B6 Lyt-2+ cells showed almost total unresponsiveness to the class II-different mutant, bm12. Reciprocal results were observed with B6 L3T4+ cells, these cells responding strongly against bm12 but not against bm1. In the case of purified T cell subsets from other strains, CBA/Ca and B10.BR L3T4+ cells both responded well to a full H-2 difference. Responses by Lyt-2+ cells from these strains were weaker, especially for CBA/Ca cells. The implications of these findings are discussed.

Identity of cells that imprint H-2-restricted T-cell specificity in the thymus

The thymus has two important roles in controlling the specificity of T lymphocytes. First, T cells differentiating in the thymus are rendered tolerant of 'self' antigens, particularly antigens encoded by the major histocompatibility complex, the H-2 complex in mice. Second, the thymus imbues T cells with the property of H-2-restricted recognition of antigen, that is, the capacity of T cells to react with foreign antigens presented in association with self H-2 gene products. Until recently it has generally been assumed that self-tolerance and H-2-restricted specificity both reflect early T-cell contact with self H-2 determinants expressed on thymic epithelial cells. Recent evidence suggests, however, that intrathymic cells of the macrophage/dendritic cell (Mphi/DC) lineage also have a role in shaping T-cell specificity. In particular, it has been found that the tolerance to graft-type H-2 determinants which normally ensues when T cells differentiate in an H-2-different thymus fails to occur when the thymus is pretreated with deoxyguanosine (dGuo), a procedure that selectively destroys Mphi/DC but spares epithelial cells. In contrast to these findings on tolerance induction, evidence is presented here that dGuo-treated thymus grafts do imprint T cells with H--2-restricted specificity for antigen. It appears, therefore, that induction of tolerance and H--2 restriction are controlled by different cells in the thymus.

The role of postnatal age and magnesium on parathyroid hormone responses during "exchange" blood transfusion in the newborn period

One hundred and eight "exchange" blood transfusions were done on 61 newborn infants. Baseline serum PTH concentrations and the PTH rise in response to citrate-induced hypocalcemia were studied. Baseline PTH values increased with postnatal age, particularly after the first three days of life. The acute response of PTH to citrate-induced hypocalcemia appears within ten minutes following the initiation of exchange transfusion and was shortlived in spite of further decline of serum ionized calcium. The dominant effect of postnatal age over gestational age was demonstrated: postnatally older but gestationally less mature infants exhibited greater responsiveness than postnatally younger, but gestationally more mature, infants. The PTH response during exchange transfusion was blunted in hypomagnesemic infants. Since lower serum magnesium concentrations were also present during the first three days of life, a separate effect of serum magnesium concentrations on parathyroid responsiveness cannot be ruled out in this study.

Cryptococcosis in the Northern Territory

This report represents a review of cryptococcosis in the Northern Territory from 1957 to 1975. There were 26 cases over a 19-year period; 25 of these were in full-blooded Aborigines. The disease occurred throughout the rural areas of the Territory as isolated cases. There were 24 cases of cryptococcal meningitis and only two with solitary lung involvement. The overall mortality was 50%. In all five untreated cases the disease was fatal. There were eight deaths among the 20 patients receiving chemotherapy, a mortality of 40%. Lung resection was performed in six cases of localized pulmonary cryptococcosis. The outcome for these was excellent.