Influence of H-2 class II antigens on the development of murine AIDS

Inbred strains of mice differ markedly in their relative susceptibility to the development of lymphoproliferation and immunodeficiency, a syndrome termed mouse AIDS (MAIDS), after infection with the LP-BM5 mixture of murine leukemia viruses (MuLV). The etiologic virus in this mixture is replication defective (BM5def) and encodes only a variant gag protein. Genetic determinants of resistance and susceptibility to induction of MAIDS reside both within and outside the MHC. In strains with C57BL background genes, the MHC haplotypes associated with resistance to disease include d and a, whereas haplotypes b, s, and q are associated with sensitivity. Previous studies showed that MHC class I genes (H-2Dd, H-2Ld) mapping in the D end of H-2 and other genes mapping proximal to the D end determine resistance to MAIDS. This paper examines the nature of these non-D end MHC genes using assays of MHC recombinant and transgenic mice. We demonstrate that expression of E alpha d confers significant resistance to MAIDS, even in mice that do not express H-2Dd/H-2Ld. Unexpectedly, we found that E alpha polymorphisms can significantly influence resistance, with H-2b mice bearing E alpha d as a transgene having greater resistance to MAIDS than mice bearing an E alpha k transgene. E alpha d-mediated resistance to MAIDS was associated with decreased levels of the BM5def genome in splenic DNA, suggesting that E alpha genes exert their effect by enhancing antiviral activity.

Influence of H-2 class II antigens on the development of murine AIDS

Inbred strains of mice differ markedly in their relative susceptibility to the development of lymphoproliferation and immunodeficiency, a syndrome termed mouse AIDS (MAIDS), after infection with the LP-BM5 mixture of murine leukemia viruses (MuLV). The etiologic virus in this mixture is replication defective (BM5def) and encodes only a variant gag protein. Genetic determinants of resistance and susceptibility to induction of MAIDS reside both within and outside the MHC. In strains with C57BL background genes, the MHC haplotypes associated with resistance to disease include d and a, whereas haplotypes b, s, and q are associated with sensitivity. Previous studies showed that MHC class I genes (H-2Dd, H-2Ld) mapping in the D end of H-2 and other genes mapping proximal to the D end determine resistance to MAIDS. This paper examines the nature of these non-D end MHC genes using assays of MHC recombinant and transgenic mice. We demonstrate that expression of E alpha d confers significant resistance to MAIDS, even in mice that do not express H-2Dd/H-2Ld. Unexpectedly, we found that E alpha polymorphisms can significantly influence resistance, with H-2b mice bearing E alpha d as a transgene having greater resistance to MAIDS than mice bearing an E alpha k transgene. E alpha d-mediated resistance to MAIDS was associated with decreased levels of the BM5def genome in splenic DNA, suggesting that E alpha genes exert their effect by enhancing antiviral activity.

Intracellular distribution of kinesin in chromaffin cells

In this paper we examined the association of the microtubule motor protein kinesin with organelles in chromaffin cells. Approximately 15% of kinesin was associated with membranes as determined by differential and equilibrium centrifugation on sucrose gradients. Kinesin was not enriched in a particular organelle fraction but cofractionated with a variety of organelle markers including markers for early and late endosomes, smooth and rough endoplasmic reticulum (ER) and the Golgi apparatus. Surprisingly, low amounts of kinesin were present in fractions of purified chromaffin granules. The absence of kinesin from the bulk of chromaffin granules was also indicated by immunostaining of tissue sections. A polyclonal antibody that specifically recognized the 120 kDa kinesin heavy chain labeled predominantly a perinuclear region that is typical for most of the kinesin-binding organelles identified by cell fractionation (endosomes, Golgi, ER). Since these organelles are compartments with high membrane turnover, we speculate that kinesin might be involved in certain aspects of trafficking of these membrane systems.

The mechanism of equilibrium binding of microtubule-associated protein 2 to microtubules. Binding is a multi-phasic process and exhibits positive cooperativity

The mechanism of binding of microtubule-associated protein 2 (MAP2) to taxol-stabilized microtubules (MTs) was examined through Scatchard analysis of equilibrium binding and by immunoelectron microscopy. We demonstrate the following. 1) Binding is a cooperative process as indicated by sigmoidal binding curves, prominent humps in Scatchard plots, and an all-or-none response in binding during ligand titrations. At high tubulin/MAP2 ratios, the Kd for noncontiguous binding (5-25 microM) is estimated to be 100-1500 times greater than that predicted for contiguous binding, suggesting a high degree of cooperativity. 2) Cooperativity is indicated independently by a highly clustered or patchy distribution of MAP2 on MTs as revealed by immunoelectron microscopy. 3) The binding of truncated constructs of mouse MAP2 protein suggests that a domain of MAP2 conferring cooperativity is located in or near the MT binding site near the carboxyl terminus. We speculate that in the cell, cooperativity may generate MTs with uniform biochemical properties and contribute to the segregation of MAPs in neuronal cell processes.

T cell mediated immunosuppression

Suppressor T cells down-regulate the activity of other cells in the immune system, and, albeit controversial, are believed to play a role in immunological tolerance and immunoregulation. Significant progress has been made in characterizing suppressor T cells and their receptors, and in elucidating mechanisms of immunosuppression. This knowledge is important for understanding the immune system and certain disease states and for favorably manipulating immunity.

Dynamics of microtubules from erythrocyte marginal bands

Microtubules can adjust their length by the mechanism of dynamic instability, that is by switching between phases of growth and shrinkage. Thus far this phenomenon has been studied with microtubules that contain several components, that is, a mixture of tubulin isoforms, with or without a mixture of microtubule-associated proteins (MAPs), which can act as regulators of dynamic instability. Here we concentrate on the influence of the tubulin component. We have studied MAP-free microtubules from the marginal band of avian erythrocytes and compared them with mammalian brain microtubules. The erythrocyte system was selected because it represents a naturally stable aggregate of microtubules; second, the tubulin is largely homogeneous, in contrast to brain tubulin. Qualitatively, erythrocyte microtubules show similar features as brain microtubules, but they were found to be much less dynamic. The critical concentration of elongation, and the rates of association and dissociation of tubulin are all lower than with brain microtubules. Catastrophes are rare, rescues frequent, and shrinkage slow. This means that dynamic instability can be controlled by the tubulin isotype, independently of MAPs. Moreover, the extent of dynamic behavior is highly dependent on buffer conditions. In particular, dynamic instability is strongly enhanced in phosphate buffer, both for erythrocyte marginal band and brain microtubules. The lower stability in phosphate buffer argues against the hypothesis that a cap of tubulin.GDP.Pi subunits stabilizes microtubules. The difference in dynamics between tubulin isotypes and between the two ends of microtubules is preserved in the different buffer systems.

Kinesin-mediated vesicular transport in a biochemically defined assay

Here we have described simple and reproducible methods to observe kinesin-mediated vesicle and microtubule movements under defined conditions using video microscopy. We are optimistic that this assay will provide a useful tool to study kinesin function, regulation, and dynamic physical interactions with membranous organelles and microtubules.

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.

Monoclonal antibody detection of a major self peptide. MHC class II complex

MHC class I and class II molecules transport foreign and self peptides to the cell surface and present them to T lymphocytes. Detection of these peptide:MHC complexes has thus far been limited to analysis of the response of a T cell. Previously, we showed that a mAb, Y-Ae, reacts with 10 to 15% of class II molecules on peripheral B lymphocytes and on cells in the thymus medulla but not thymus cortex in mice that express both I-Ab and I-Eb molecules. Elsewhere, we show that Y-Ae detects a self E alpha peptide bound to I-Ab molecules. Data presented here suggest that the antibody binds over the peptide binding groove of class II molecules, and, like a TCR, appears to recognize both the self peptide and polymorphic class II residues. In addition to B lymphocytes, the Y-Ae determinant is expressed at comparable levels on other APC, including macrophages and dendritic cells. Finally, the antibody does not react with invariant chain-associated class II complexes, thus providing direct evidence that invariant chain:class II complexes and peptide:class II complexes are mutually exclusive. These data provide further evidence that immunologic self is of limited complexity, and have important implications for T cell selection, self tolerance, and autoreactivity.

Monoclonal antibody detection of a major self peptide. MHC class II complex

MHC class I and class II molecules transport foreign and self peptides to the cell surface and present them to T lymphocytes. Detection of these peptide:MHC complexes has thus far been limited to analysis of the response of a T cell. Previously, we showed that a mAb, Y-Ae, reacts with 10 to 15% of class II molecules on peripheral B lymphocytes and on cells in the thymus medulla but not thymus cortex in mice that express both I-Ab and I-Eb molecules. Elsewhere, we show that Y-Ae detects a self E alpha peptide bound to I-Ab molecules. Data presented here suggest that the antibody binds over the peptide binding groove of class II molecules, and, like a TCR, appears to recognize both the self peptide and polymorphic class II residues. In addition to B lymphocytes, the Y-Ae determinant is expressed at comparable levels on other APC, including macrophages and dendritic cells. Finally, the antibody does not react with invariant chain-associated class II complexes, thus providing direct evidence that invariant chain:class II complexes and peptide:class II complexes are mutually exclusive. These data provide further evidence that immunologic self is of limited complexity, and have important implications for T cell selection, self tolerance, and autoreactivity.

Multiple sites of crossing over within the Eb recombinational hotspot in the mouse

The Eb gene of the mouse major histocompatibility complex (MHC) contains a well-documented hotspot of recombination. Twelve cases of intra-Eb recombination derived from the b, d, k and s alleles of the Eb gene were sequenced to more precisely position the sites of meiotic recombination. This analysis was based on positioning recombination breakpoints between nucleotide polymorphisms found in the sequences of parental haplotypes. All twelve cases of recombination mapped within the second intron of the Eb gene. Six of these recombinants, involving the k and s haplotypes, mapped to two adjoining DNA segments of 394 and 955 base pairs (bp) in the 3' half of the intron. In an additional two cases derived by crossing over between the d and s alleles, breakpoints were positioned to adjoining segments of 28 and 433 bp, also in the 3' half of the intron. Finally, four b versus k recombinants were mapped to non-contiguous segments of DNA covering 2.9 kb and 1005 bp of the intron. An analysis of the map positions of crossover breakpoints defined in this study suggests that the second intron of the Eb gene contains a recombinational hotspot of approximately 800-1000 bp which contains at least two closely linked recombinationally active sites or segments. Further examination of the sequence data also suggests that the postulated location for the recombinational hotspot corresponds almost precisely to an 812 bp sequence that shows nucleotide sequence similarity to the MT family of middle repetitive DNA.

On the complexity of self

Self peptides bound to self major histocompatibility complex (MHC) molecules have been implicated both in positive and in negative selection of T cells during intrathymic development. We report here that the novel MHC-restricted monoclonal antibody Y-Ae detects the MHC class II bound form of a major self peptide. Y-Ae binds approximately 12% of the relevant MHC class II molecules on self antigen presenting cells. The peptide detected by Y-Ae is one of several major peptides eluted from the MHC molecule. These data suggest that self peptides presented by self MHC class II molecules at densities sufficient to signal a CD4 T cell are of very limited complexity. Furthermore, as Y-Ae stains antigen presenting cells that mediate negative selection but not thymic cortical epithelial cells that drive positive selection, differential expression of self peptide:self MHC class II complexes may be a key feature of intrathymic selection.

Purified kinesin promotes vesicle motility and induces active sliding between microtubules in vitro

We examined the ability of kinesin to support the movement of adrenal medullary chromaffin granules on microtubules in a defined in vitro system. We found that kinesin and ATP are all that is required to support efficient (33% vesicle motility) and rapid (0.4-0.6 micron/s) translocation of secretory granule membranes on microtubules in the presence of a low-salt motility buffer. Kinesin also induced the formation of microtubule asters in this buffer, with the plus ends of microtubules located at the center of each aster. This observation indicates that kinesin is capable of promoting active sliding between microtubules toward their respective plus ends, a movement analogous to that of anaphase b in the mitotic spindle. The fact that vesicle translocation, microtubule sliding, and microtubule-dependent kinesin ATPase activities are all enhanced in low-salt buffer establishes a functional parallel between this translocator and other motility ATPases, myosin, and dynein.

Recognition of MHC TL gene products by gamma delta T cells

We have studied the ligand specificity of a gamma delta T-cell receptor (TCR) derived from a mouse T-cell hybridoma (KN6). KN6 cells reacted with syngeneic (C57BL/6) cells from various origins (splenocytes, thymocytes, peritoneal exudate cells, etc.) and cells from many different mouse strains. KN6 reactivity against cells from a panel of congenic and recombinant mouse strains demonstrated that the ligand recognized by KN6 is controlled by an MHC-linked gene that most probably maps in the TL region. We cloned this gene and formally proved that it does map in the TL region. This gene turned out to be a novel class I gene (designated T22b) belonging to a hitherto unidentified cluster of TL region genes in strain C57BL/6. This gene was expressed in many different tissues and cell types. We also examined the tissue expression of several other TL genes. One of these, the structural gene (T3b) encoding the thymus leukemia (TL) antigen from C57BL/6 mice, was specifically expressed in the epithelium of the small intestine. Since the intestinal epithelium of the mouse is known to be the homing site for a subset of gamma delta T cells (i-IEL) bearing diverse TCR with V7 rearranged gamma chains, we propose that the T3b gene product is part of the ligand recognized by some of the i-IEL. Our data support the idea that gamma delta T cells might be specific for non-classical class I or class I-like molecules and suggest that gamma delta TCR and non-classical MHC co-evolved for the recognition of a conserved set of endogenous or foreign peptides.

Functions of tubulin isoforms

The biological significance of tubulin isotypes lies in their ability to function in different chemical and physical environments. Recent papers document the origin and distribution of several new tubulin isotypes and suggest new ways for studying their assembly and function in specialized cells.

Recognition of the product of a novel MHC TL region gene (27b) by a mouse gamma delta T cell receptor

The gamma delta T cell receptor (TCR) derived from the mouse KN6 T cell hybridoma recognizes an autologous determinant encoded by a broadly expressed gene mapping in the TL region of the major histocompatibility complex (MHC). We have cloned the gene and demonstrated that it is a novel class I gene (designated 27b) belonging to a hitherto undescribed TL region gene cluster in strain C57BL/6. The BALB/c allele of 27b, gene T17c, is defective because it lacks an appropriate splice acceptor site, which explains the lack of recognition of BALB/c stimulator cells by the KN6 cells. We propose that gamma delta TCR and nonclassical MHC and MHC-related class I molecules have coevolved to recognize a conserved set of endogenous and foreign determinants.

Structural plugs at microtubule ends may regulate polymer dynamics in vitro

Microtubules contain in their lumens distinct structures (plugs) that influence their dynamic behavior in vitro. As observed by electron microscopy, plugs are stain-occluding structures 10-30 nm in length that occur along the lengths and at the ends of microtubules. Plugs occur at a frequency of 20-40% at the ends of microtubules assembled from cycled microtubule protein containing MAPs. While the composition of plugs is not known, preliminary evidence suggests that they are accretions of tubulin, that they are labile, and that they are more common in preparations containing MAPs. When polymers are induced to depolymerize by endwise subunit dissociation, the frequency of plugged microtubule ends increases transiently, suggesting that plugs temporarily stabilize microtubules. The functional significance of plugs may be that they prevent the sudden complete loss of microtubules through catastrophic disassembly. It is possible that plugs, by slowing the rate of disassembly, enable a polymer to add GTP-tubulin subunits, thereby forming a stabilizing GTP-cap. These observations suggest that plugs may stabilize polymers and account for the frequent transitions from shortening to growing phases that characterize dynamic instability.

Copolymerization of two distinct tubulin isotypes during microtubule assembly in vitro

Cells contain multiple tubulin isotypes that are the products of different genes and posttranslational modifications. It has been proposed that tubulin isotypes become segregated into different classes of microtubules each adapted to specific activities and functions. To determine if mixtures of tubulin isotypes segregate into different classes of polymers in vitro, we used immunoelectron microscopy to examine the composition of microtubule copolymers that assembled from mixtures of purified tubulin subunits from chicken brain and erythrocytes, each of which has been shown to exhibit distinct assembly properties in vitro. We observed that (a) the two isotypes coassemble rapidly and efficiently despite the fact that each isotype exhibits its own unique biochemical and assembly properties; (b) at low monomer concentrations the ratio of tubulin isotypes changes along the lengths of elongating copolymers resulting in gradients in immuno-gold labeling; (c) two distinct classes of copolymers each containing a distinct ratio of isotypes assemble simultaneously in the same subunit mixture; and (d) subunits and polymers of different isotypes associate nearly equally well with each other, there being only a slight bias favoring interactions among subunits and polymers of the same isotype. The observations agree with previous studies on the homogeneous distribution of multiple isotypes within cells and suggest that if segregation of isotypes does occur in vivo, it is most likely directed by cell-specific microtubule-associated proteins (MAPs) or specialized intracellular conditions.

Recognition of a self major histocompatibility complex TL region product by gamma delta T-cell receptors

Ligand specificity of a murine gammadelta T-cell receptor-expressing hybridoma (KN6) derived from adult thymocytes has been analyzed in detail. The molecule recognized by the KN6 gammadelta T-cell receptor is expressed on syngeneic cells of various sources (peritoneal macrophages, thymocytes, spleen cells, and Abelson murine leukemia virus-transformed cell lines) and on transformed cells arrested at an early stage of development (e.g., PCC3 embryonal carcinoma cells). Linkage of the gene coding for the KN6 ligand to the major histocompatibility complex genes could be demonstrated by testing KN6 hybridoma reactivity to cells from congenic strains that differ only at H-2. In addition, analysis of recombinant strains indicates that the gene controlling the KN6 ligand is located in or distal to the TL region. Involvement of the KN6 gammadelta T-cell receptor in this recognition process could be directly demonstrated by transferring the KN6 TL specificity after introduction of the productively rearranged KN6 gamma and delta genes into an alphabeta T-cell clone or into the germ line in transgenic mice. These observations raise the possibility that at least some gammadelta cells regulate hemopoietic cell maturation and activation.

Monoclonal antibodies to murine CD3 epsilon define distinct epitopes, one of which may interact with CD4 during T cell activation

The TCR is comprised of two variable chains that confer specificity, called alpha:beta or gamma:delta, physically associated with five different molecules that comprise the complex known as CD3. Antibodies to this complex are very useful, as they react with all T lymphocytes. A rat mAb to mouse CD3 has been prepared. It reacts with 100% of T cells in all mouse strains tested but with no other cell type. It binds to the CD3 epsilon chain. This antibody activates cloned T cell lines and normal T cells, provided suitable accessory cells and signals are present. This antibody detects a determinant similar to but not identical with those detected by two previously reported hamster anti-CD3 epsilon antibodies. This antibody fixes C efficiently, and it is thus useful for depletion of T cells from bulk populations. Activation of T cells by one of the three different anti-CD3 epsilon antibodies was inhibited by the Fab fragment of anti-CD4, similar to the effects of anti-CD4 Fab on two previously reported anti-TCR V region antibodies that bind a CD3 epsilon-associated epitope. This further defines a site involving TCR V regions and CD3 epsilon with which CD4 appears to associate during T cell activation.

Monoclonal antibodies to murine CD3 epsilon define distinct epitopes, one of which may interact with CD4 during T cell activation

The TCR is comprised of two variable chains that confer specificity, called alpha:beta or gamma:delta, physically associated with five different molecules that comprise the complex known as CD3. Antibodies to this complex are very useful, as they react with all T lymphocytes. A rat mAb to mouse CD3 has been prepared. It reacts with 100% of T cells in all mouse strains tested but with no other cell type. It binds to the CD3 epsilon chain. This antibody activates cloned T cell lines and normal T cells, provided suitable accessory cells and signals are present. This antibody detects a determinant similar to but not identical with those detected by two previously reported hamster anti-CD3 epsilon antibodies. This antibody fixes C efficiently, and it is thus useful for depletion of T cells from bulk populations. Activation of T cells by one of the three different anti-CD3 epsilon antibodies was inhibited by the Fab fragment of anti-CD4, similar to the effects of anti-CD4 Fab on two previously reported anti-TCR V region antibodies that bind a CD3 epsilon-associated epitope. This further defines a site involving TCR V regions and CD3 epsilon with which CD4 appears to associate during T cell activation.