Characterization of Lyt-2-, L3T4- class I-specific cytolytic clones in C3H-gld/gld mice. Implications for functions of accessory molecules and programmed development

Analysis of major histocompatibility complex class I folding: novel insights into intermediate forms

Folding around a peptide ligand is integral to the antigen presentation function of major histocompatibility complex (MHC) class I molecules. Several lines of evidence indicate that the broadly cross-reactive 34-1-2 antibody is sensitive to folding of the MHC class I peptide-binding groove. Here, we show that peptide-loading complex proteins associated with the murine MHC class I molecule K(d) are found primarily in association with the 34-1-2(+) form. This led us to hypothesize that the 34-1-2 antibody may recognize intermediately, as well as fully, folded MHC class I molecules. To further characterize the form(s) of MHC class I molecules recognized by 34-1-2, we took advantage of its cross-reactivity with L(d) . Recognition of the open and folded forms of L(d) by the 64-3-7 and 30-5-7 antibodies, respectively, has been extensively characterized, providing us with parameters against which to compare 34-1-2 reactivity. We found that the 34-1-2(+) L(d) molecules displayed characteristics indicative of incomplete folding, including increased tapasin association, endoplasmic reticulum retention, and instability at the cell surface. Moreover, we show that an L(d) -specific peptide induced folding of the 34-1-2(+) L(d) intermediate. Altogether, these results yield novel insights into the nature of MHC class I molecules recognized by the 34-1-2 antibody.

Fas- and perforin-independent mechanism of cytotoxic T lymphocyte

Cytotoxic T lymphocytes (CTLs) play an important role in elimination of virus-infected cells (1). Recent studies revealed at least two distinct mechanisms that CTLs utilize to destroy their target cells. Both mechanisms induce target cell apoptosis specifically and directionally, but these processes are totally different. One is pore formation on target cell membrane by perforin secreted from CTLs (perforin-granzyme pathway), and the other is ligation of Fas, which is expressed on the surface of target cells and Fas ligand, on the surface of CTLs (Fas-FasL pathway) (2). Here we review our work and describe CTL clones that have novel lytic mechanisms derived from CD4-CD8- lymph node cells of gld mice.

Gld and lpr mice: single gene mutant models for failed self tolerance

Mice homozygous for the gld or lpr mutations develop autoimmunity, and a lymphoproliferative disorder involving accumulation of huge numbers of unusual CD4-CD8-TCR alpha beta lo T cells. Here we review our past work with gld mice, and attempt to explain lymphoproliferation in terms of current models of T cell maturation and self-tolerance induction. The availability of molecular probes to the gene products of lpr and gld should shortly lead to a better understanding of the acquisition of self tolerance during T cell maturation and of autoimmunity.

Analysis of T cell antigen receptor (TCR) expression by human peripheral blood CD4-8- alpha/beta T cells demonstrates preferential use of several V beta genes and an invariant TCR alpha chain

CD4-CD8- (double negative [DN]) alpha/beta T cells are a largely uncharacterized subpopulation of unknown function. To investigate whether these cells are selected to recognize particular antigens or antigen-presenting molecules, DN alpha/beta T cells were purified from the peripheral blood of five normal donors and their T cell receptor (TCR) alpha and beta chains were examined. Random cloning of TCR alpha chains by single-sided polymerase chain reaction (PCR) amplification identified an invariant rearrangement between V alpha 24 and J alpha Q, with no N region diversity, which was expressed preferentially by DN alpha/beta T cells from all donors. Random cloning also identified a precise V alpha 7.2-J alpha (IGRJa14) rearrangement, with two variable amino acids encoded in the V-J junction, which was enriched in the DN alpha/beta T cell preparations from some, but not all, donors. Analysis of TCR beta chains by quantitative PCR amplification demonstrated that the expression of four V beta gene families, V beta 2, 8, 11, and 13, was markedly increased in these DN alpha/beta T cell preparations. The expression of particular TCRs by DN alpha/beta T cells from multiple donors indicates that these cells, or at least a subpopulation of cells with this phenotype, recognize a limited spectrum of antigens and suggests that they may use nonpolymorphic antigen-presenting molecules.

Inhibition of abnormal T cell development and autoimmunity in gld mice by transgenic T cell receptor beta chain

Mice homozygous for the gld (generalized lymphoproliferative disease) mutation developed systemic autoimmune disease and severe lymphadenopathy due to an age-related accumulation in the peripheral lymphoid organs of polyclonal T cells bearing a unique phenotype (CD4-CD8-TCR alpha beta+B220+). These T cells overexpress T cell receptor (TcR) alpha beta chain RNA, proto-oncogenes c-myb and fyn, and proliferate poorly in response to TcR-mediated stimulation. The origin of these T cells is poorly understood. To study the influence of a functionally rearranged TcR beta chain on the T cell developmental abnormality of the gld mutation and autoimmunity, we have backcrossed TcR V beta 8.1-transgenic mice to C3H-gld/gld to homozygosity (transgenic gld mice). In transgenic gld mice, lymphadenopathy was markedly inhibited and the accumulation of CD4-CD8- T cells did not occur, although the remaining T cells overexpressed c-myb and proliferated poorly in response to TcR occupancy. These features indicate that the pattern of proto-oncogene expression and abnormal function persist in phenotypically normal T cells in transgenic gld mice, and that these characteristics can be dissociated from the accumulation of CD4-CD8- T cells. The hypergammaglobulinemia and anti-double-stranded DNA (anti-dsDNA) antibody production was partially improved in transgenic gld mice, supporting the critical role of T cells in abnormal B cell activation described in autoimmunity-prone mice. To investigate further the mechanisms underlying the inhibition of CD4-CD8- T cell accumulation in transgenic gld mice, the fetal ontogeny of T cells in transgenic mice was compared with that of non-transgenic mice. In transgenic thymus, development of TcR alpha beta+ cells was accelerated as detected by earlier expression of CD4, CD8 and TcR in fetal thymus. In contrast, the number of TcR gamma delta+ cells was reduced. We suggest that altered T cell development in transgenic mice directly or indirectly inhibits the accumulation of abnormal T cells in gld mice.

Mechanisms of autoimmunity in the context of T-cell tolerance: insights from natural and transgenic animal model systems

There are a number of mechanisms which cooperate to produce and maintain T-cell tolerance. First, and perhaps most important, is the clonal deletion in the thymus of T cells with high affinity for self antigens. However, to ensure that a wide repertoire of T cells is available in the periphery to combat foreign antigens, the threshold of clonal deletion may be set low enough so that T cells whose TCR's have sub-threshold affinity for self antigens mature and migrate to the periphery. T cells which recognize self antigen-derived peptides not expressed or presented in the thymus will also fail to be deleted. For those self-reactive T cells which are not deleted in the thymus, other mechanisms may produce tolerance, including an undefined alteration of signalling pathways which produces clonal anergy, and lowering the avidity of the TCR for its ligand by downregulating coreceptor and accessory molecules. Active suppression of T-cell responses in another well-described phenomenon whose mechanism is undefined. From our observations with the model systems discussed here, we have observed three distinct mechanisms by which T-cell tolerance can be circumvented, allowing autoimmune phenomena to occur. These mechanisms may have relevance for different types of autoimmune diseases seen in humans. In gld mice, the autoimmune disease seems to be related to a global defect in T-cell differentiation and function, which allows for the expansion of autoimmune B cells. While we showed that clonal deletion of V beta-bearing T cells is appropriate in certain cases, aberrant lymphokine secretion by the abnormal T cells or disruption of immune system regulation are most probably responsible for allowing autoantibody production. While human lupus erythematosis shares much of the pathology of lpr and gld mice, there is no expansion of T cells with a similar phenotype in human lupus. There are environmental factors which must play a role in the development of human lupus, since the incidence of the disease does not follow an absolute genetic pattern. The escape from clonal deletion and subsequent reactivation of autoimmune T cells which we observed in V beta 8.1 TCR-transgenic mice can be a model for human autoimmune diseases such as multiple sclerosis and type I diabetes, in which T cells are directed against a specific autoantigen. According to this model, susceptibility loci for autoimmune disease such as the MHC would function by producing different repertoires of T cells which in some cases could gain autoreactivity following activation.(ABSTRACT TRUNCATED AT 400 WORDS)

Self-reactive T cells can escape clonal deletion in T-cell receptor V beta 8.1 transgenic mice

To study the mechanisms of tolerance in detail, we have constructed transgenic mice expressing a V beta 8.1-D beta 2-J beta 2.3-C beta 2 T-cell receptor (TCR) gene. Since expression of V beta 8.1 is known to correlate with reactivity of CD4+CD8- T cells to minor lymphocyte-stimulating locus 1a (Mls-1a), we expected to induce tolerance in most CD4+CD8- T cells in V beta 8.1 transgenic mice of the Mls-1a allele. In one line of Mls-1b V beta 8.1 transgenic mice, the V beta 8.1 TCR was expressed on greater than 98% of mature T cells and their response to Mls-1a was highly enriched. In Mls-1a V beta 8.1 transgenic mice, CD4+CD8- T cells in these mice were severely reduced among both peripheral T cells and thymocytes. However, the deletion of these cells was not complete, and most of the residual CD4+CD8- mature T cells still expressed normal densities of V beta 8.1 TCR. The residual CD4+CD8- T cells did not respond to Mls-1a but were still able to proliferate in response to other stimuli via the TCR. Interestingly, CD4+CD8- V beta 8.1+ T-cell clones isolated from Mls-1a V beta 8.1 transgenic mice could respond to Mls-1a. We suggest that these types of T cells escape clonal deletion in the thymus.

CD4-CD8- human T cells: phenotypic heterogeneity and activation requirements of freshly isolated "double-negative" T cells

In the present study we have analyzed the in vitro activation requirements of freshly isolated CD4-CD8- "double-negative" (DN) human peripheral blood T cells. DN cells were isolated from E+ cells by removal of CD4+, CD8+, and CD16+ cells through consecutive steps of C'-mediated lysis and panning. While the majority (79.0 +/- 12.0%) of DN cells were TCR gamma delta+ as shown by staining with mAb TCR delta-1, a minor fraction (6.7 +/- 4.7%) expressed TCR alpha beta as revealed by staining with mAb BMA031. Within the gamma delta+ DN fraction, most cells reacted with mAb Ti gamma A which delineates a V gamma 9JPC gamma 1 epitope, whereas a minor fraction stained with mAb delta TCS-1 which identifies a V delta 1J delta 1 epitope. Functional studies performed at low cell number (1000) per microculture indicated that DN cells can be activated by anti-CD3 mAb, PHA and allogeneic stimulator cells, provided that exogenous growth factors are supplied. Both rIl-2 and rIl-4 acted as efficient growth factors for DN cells, and a synergistic stimulatory effect of rIl-2 and rIl-4 was observed when DN cells were cocultured with allogeneic LCL stimulator cells. As compared to unseparated E+ cells, isolated DN responder cells had a reduced capacity to secrete Il-2 upon PHA stimulation in the presence of LCL feeder cells. The majority of DN cells maintained their CD3+ CD4-CD8- phenotype upon coculture with allogeneic LCL stimulator cells. These data demonstrate that CD3+ DN cells in human peripheral blood are heterogeneous with respect to TCR expression. In addition, they show that freshly isolated DN cells are deficient in Il-2 production but may be normally stimulated by anti-CD3, PHA, or alloantigen if exogenous growth factors (rIL-2 and/or rIl-4) are provided.

Substitution at residue 227 of H-2 class I molecules abrogates recognition by CD8-dependent, but not CD8-independent, cytotoxic T lymphocytes

The CD8 (Lyt 2) molecule is a phenotypic marker for T lymphocytes that recognize and react with major histocompatibility complex (MHC) class I molecules. Antibody blocking experiments and gene transfection studies indicate that CD8 binds to a determinant on MHC class I molecules on the target cells, facilitating interaction between effector T lymphocytes and the target cell. The CD8 molecule may also be involved in transmembrane signalling during T-cell activation. The existence of CD8- cytotoxic T lymphocytes (CTL) and class I-reactive CTL that are not inhibited by antibody to CD8 suggests that at least some CTL do not require the CD8 molecule to interact with and lyse target cells. We have recently demonstrated that cells transfected with an H-2Dd gene that carries a mutation at residue 227 are not killed by primary CTL8. Here we show that although this mutation abrogates recognition by primary CTL, it does not affect recognition by CD8-independent CTL, suggesting that residue 227 of class I molecules might contribute to a determinant that is the ligand of the CD8 molecule.

Novel class I-like molecule expressed on a murine leukemia virus-transformed cell line

A retrovirus-induced tumor cell line, which expresses no H-2K or H-2D class I molecules, appears to express a tumor-specific transplantation antigen which induces tumor rejection in vivo and cytotoxic T lymphocyte generation in vitro without prior immunization and thus resembles class I molecules. In addition, although these tumor cells express no detectable class I molecules, they do express beta 2 microglobulin and a 55- to 60-kDa beta 2 microglobulin-associated protein. Northern analysis demonstrated that these cells express no RNA hybridizing to class I probes, suggesting that neither the tumor-specific transplantation antigen nor the beta 2 microglobulin-associated protein, if these are different, are encoded by known class I genes.

The Lyt-2 molecule recognizes residues in the class I alpha 3 domain in allogeneic cytotoxic T cell responses

The involvement of the different domains of the MHC class I molecule in CTL recognition was investigated. mAbs specific for the alpha 1/alpha 2 domains of H-2Ld interfered with both the primary and secondary generation and effector function of in vitro Ld-specific CTL. mAbs specific for the alpha 3 domain of H-2Ld interfered with the generation and function of primary in vitro Ld-specific CTL; however, there was no effect on the in vitro generation of secondary CTL and only partial inhibition of their function. In vivo treatment with graft-specific antibodies to both the alpha 3 domain and the alpha 1/alpha 2 domains together resulted in a dramatic enhancement of Ld- or Dd-disparate skin grafts, whereas the individual mAbs showed minimal effects. This suggested that the class I alpha 3 domain is recognized by alloreactive CTL. Several approaches were undertaken to examine whether recognition of the alpha 3 domain determinants is mediated by the Lyt-2 molecule. When mAbs specific for the alpha 3 domain of either H-2Ld or H-2Dd were used in vivo and in vitro, the resulting CTL population was not inhibited by antibody to the alpha 3 domain and was only partially inhibited by antibody to Lyt-2. We therefore observed a correlation between the effects of antibody to the class I alpha 3 domain of the target molecule and antibody to the Lyt-2 molecule on the CTL. To further test the relationship between CTL recognition of the alpha 3 domain and the involvement of Lyt-2, we used a cell expressing a mutation in the alpha 3 domain of the Dd molecule. The mutation resulted in a single amino acid substitution of glu to lys at residue 227 of the alpha 3 domain. Consistent with an earlier report, cells expressing the mutant Dd lys molecule were not lysed by CTL from a primary stimulation against the wild-type Dd glu molecule. However, this same cell line was killed by the Lyt-2-independent secondary Dd-specific CTL generated in the presence of antibody to the alpha 3 domain in vivo and in vitro. Furthermore, cells expressing the mutant Dd lys molecule failed to stimulate a primary response. In conclusion, several independent lines of evidence indicate that residues in the alpha 3 domain of the class I molecule are involved in recognition by the Lyt-2 molecule, and that Lyt-2-mediated recognition can be specifically blocked using mAb to determinants in the alpha 3 domain.