Partial characterisation of Ia antigens on murine thymocytes

Synthesis and cell surface display of class II determinants by long-term propagated rat T line cells

We have investigated the capacity of the encephalitogenic BS rat T cell line bs 83 and its variant clone bs 83.III.C6 to synthesize and express RT1.B-specific class II molecule subsets defined by monoclonal antibodies (mAb) MRC-OX6 and MRC-OX3. Earlier studies had indicated that mAb MRC-OX6 recognizes three distinct molecular species: an immature oligomeric polypeptide chain complex comprised of the polymorphic subunits alpha, beta and the invariant proteins of the gamma group; a biosynthetic intermediate composed of post-translationally modified alpha, beta and gamma chain (denoted p35) and a fully glycosylated alpha, beta two-chain complex derived from the plasma membrane. MRC-OX3 was shown to recognize a serologically distinct alpha, beta two-chain complex that coexists with the MRC-OX6-specific heterodimer at the cell surface. Here we show that premutant bs 83 cells were unable to synthesize class II molecules of either set. In contrast endogeneous synthesis by mutant cells of MRC-OX6-specific molecules was demonstrated. Unlike control spleen cells variant cells failed to synthesize the mature MRC-OX3-reactive class II subset. Instead a three-polypeptide chain complex comprised of the terminally glycosylated subunits alpha, beta and invariant chain p35 was present at the cell surface. This complex appears to represent the preserved biosynthetic intermediate that failed to release invariant chain p35 upon its transit into the plasma membrane. These latter observations support our notion of gamma chain-induced epitope diversification during post-translational maturation of RT1.B-specific class II molecules.

Expression of Ia antigens in a murine T-lymphoma variant

ESb, a cellular high metastatic variant derived from the murine T-cell lymphoma L5178Y (Eb), was found to synthesize Ia antigens. Ia-specific antibodies reacted with the ESb cells and precipitated Ia-like molecules from them. Two-dimensional gel electrophoretic analysis of immunoprecipitates of metabolically labeled ESb cells indicated that the Ia molecules on ESb were indistinguishable from those on murine B-cells. No Ia antigens were detectable on the parental tumor line Eb. Treatment with recombinant interferon-gamma (IFN-gamma) caused enhancement of class I histocompatibility antigen expression on Eb and ESb tumor lines. In ESb cells the expression of Ia and of Ia-associated invariant chain (Ii) was also increased upon IFN-gamma treatment. No induction of either Ia and Ii antigens was observed upon IFN-gamma treatment of the Eb line. These studies demonstrate a substantial difference between the Eb and ESb tumor lines with respect to: (i) constitutive expression of class II major histocompatibility antigens, and (ii) response to IFN-gamma treatment.

Structural comparison of I-A antigens produced by a cloned murine T suppressor cell line with B-cell-derived I-A

A cloned, antigen-specific T suppressor cell line derived from a CBA mouse expresses large amounts of I-A and I-E antigens. Comparative two-dimensional polyacrylamid gel electrophoresis of biosynthetically labeled I-A antigens immunoprecipitated with a variety of monoclonal I-Ak-specific antibodies suggested that alpha, beta and Ii polypeptide chains are identical with B-cell-derived I-A. Dimeric complexes formed by I-A chains derived from B or T suppressor cells were also similar with two major exceptions. Pulse-labeled T-cell-derived Ia antigen was complexed with two additional unknown components of about 31K. These components were not visible in pulse-chased (processed) materials. In addition, T suppressor-cell-derived I-A antigens did not contain S-S linked dimers consisting of processed alpha and beta chains, which are usually formed during solubilization of B cells. We consider the possibility that in T cells these chains are associated with other structures, thus preventing S-S linkage between alpha and beta chains.

Murine T cell-specific Ia antigens: monoclonal antibodies define an I-A-encoded T lymphocyte structure

T lymphocytes express a unique I-A subregion-controlled surface molecule that is not expressed on B lymphocytes. We produced antisera and monoclonal antibodies recognizing this structure. Exhaustive B cell adsorption (A.TH X B10.HTT)F antiserum, produced against activated A.TL T cells, left antibodies that bind an I-Ak specificity on some B10.A(4R) T lymphocytes (11 +/- 2%, mean +/- SEM). Similarly, exhaustive B cell adsorption of (A/J X B10.MBR)F antiserum, produced against activated B10.A(5R) cells, left antibodies specific for an I-Ab determinant on B10.A(5R)T cells (17 +/- 2%). We fused A.TL-immune (A.TH X B10.HTT)F splenocytes with NS-1 myeloma cells and identified antibody-producing hybrid cells by a fluorescent enzyme-linked immunosorbent microassay described herein. Eight monoclonal antibodies were selected; these lyse 7--26% of peripheral T cells from I-Ak strains. Thymocytes and bone marrow cells do not express the I-Ak T cells determinant. Exhaustive B cell adsorption did not remove I-Ak T cell-specific monoclonal antibodies.

Isolation and partial characterization of rat lymphoid cell surface histocompatibility antigens and immunoglobulins

Cell surface molecules of rat normal lymphoid cells were selectively labelled by lactoperoxidase catalysed iodination or by a galactose oxidase tritiated sodium borohydride technique, subsequently detergent solubilized, isolated by indirect immunoprecipitation and analysed on SDS-polyacrylamide gel electrophoresis. Four polypeptide chains were isolated by using the alloantiserum DA anti-Lewis. The molecular weights of the antigens were calculated as 41,000, 33,000, 27,000 and 12,000. Based on functional in vitro characteristics of the antiserum used and on the physiochemical properties as well as genetics of inheritance and tissue distribution, the polypeptide chains were identified as being subunits of Ag-B and Ia antigens. Two types of immunoglobulin heavy chains exhibiting the molecular weight 70,000 and 64,000 were isolated from unfractionated normal spleen cells by use of a polyvalent rabbit anti-rat immunoglobulin serum and tentatively identified as mu and delta chain. Using the same anti-immunoglobulin serum, no molecules could be precipitated from the lysated of Lewis thymocytes or peripheral T cells.

Cellular requirements for the induction of cytotoxic T cells in vitro

The stimulator and accessory cell requirements for the induction of cytotoxic T cell responses to alloantigens in vitro are reviewed. The ability of lymphocytes to stimulate was acquired early in development and was found to be a property of both Ig+ and Ig- cells. The presence of Fc receptors, complement receptors or Ia antigens on lymphocytes did not confer superior stimulator capacity. In contrast to lymphocytes mature macrophages were poor stimulators. When resting lymphocytes were used as a source of stimulator cells there was an additional requirement for an adherent accessory cell population for the induction of cytotoxic T cells. The accessory cell population functioned whether syngeneic or allogeneic with the responder and was required for the induction rather than the maintenance of the response. It was further characterized as an Ig+, theta, FcR+/-, CR +/-, Ia- cell which was not a mature macrophage. Accessory cells could be replaced to some extent by supernatants from spleen cell cultures. A model for T cell induction is proposed where only cells which produce unique inductive stimuli are able to function as stimulator cells. Stimulator cells are defined as those cells capable of stimulating cytotoxic T cell precursors directly and independently. Stimulator capacity is suggested to be a function of activated cells only, thus lymphocytes only stimulate following activation by accessory cells. Cells lacking innate stimulator capacity but exhibiting Ia antigens are suggested to stimulate via an alternative means.

Conjugation of antibodies with fluorochromes: modifications to the standard methods

A number of modification to the standard procedures for coupling of fluorochromes to antibodies are described. The suggested procedures result in economies of time, labour and materials, and allow the reliable production of high quality conjugates. The modifications include the use of staphylococcal protein A-Sepharose for (a) a simple one-step procedure for preparation of IgG from whole serum, (b) removal of undigested IgG after pepsin treatment, and (c) concentration of dilute solutions of IgG. Many other details of coupling procedures are discussed, and modifications suggested. Optimally coupled antibodies were separated by linear salt gradient elution from DEAESephadex, and the effects of fluorescein and tetramethyl rhodamine on the antibody isoelectric point studied.

Association between Ia antigens and the Fc receptors of certain T lymphocytes

The Fc receptors of thymic and splenic T lymphocytes were detected using indirect immunofluorescence and soluble antigen-antibody complexes. 10-20% of thymocytes and 40-50% of Thy-1-positive splenic lymphocytes bound antigen-complexed Ig. The binding to thymocytes was partially inhibited (45-74%) by antibodies against antigens determined by the I region of the H-2 complex, but not by antibodies against K- or D-region antigens or Thy-1 antigen. The inhibition did not require the Fc portion of the inhibiting antibody. These results provide evidence that Ia antigens and the Fc receptors of some T lymphocytes are associated, and that the populations of T cells which bear these moieties at least partially overlap.

Ia antigens: isolation, chemical modification and structural characterization

The results described above, with respect to the similarity of basic structural features of Ia and H-2 antigens (see Table V), led to the evolutionary concept presented in Figure 10 according to which these antigens are the products of tandem-duplicated genes. The evolution of MHC-gene products is reflected by the basic structure of these molecules. This structure is compatible with basic repetitive units of molecular weight 12,000 as suggested by the selective cleavage of the intact protein by, for example, papain.

Genetic polymorphism of IgD-like cell surface immunoglobulin in the mouse

Lymphocyte surface antigens from spleen cells of several mouse strains were studied by cell surface radioiodination, extraction with detergent incubation with various antisera, and separation of complexes using protein A-containing staphylococci as a solid phase adsorbent. Complexes were then dissociated and analyzed by polyacrylamide gel electrophoresis in the presence of sodium diodecyl sulfate. Using this technique and an alloantiserum prepared in C57BL mice against CBA spleen cells, four distinct specific peaks of radioactivity were found with CBA spleen cells. These corresponded to H-2 and Ia antigens, immunoglobulin light chain, and a heavy chain previously proposed to be the murine homolog of the human delta chain. With the same serum, B10.BR spleen cells revealed only H-2 and Ia antigens, whereas C57BL.Ige (allotype congenic) spleen cells showed only the light chain and "delta" chain peaks. Depletion of immunoglobulin from the surface-iodinated preparations resulted in removal of the light chain and "delta" chain peaks. The tissue distribution and membrane expression of this "delta" chain antigen was then studied by indirect immunofluorescence with various C57BL derived alloantisera and lymphoid cells from C57.Ige allotype congenic mice. Significant numbers of positive cells were found in spleen, lymph nodes, and Peyer's patches, whereas few if any positive were found in bone marrow or thymus. No reaction was found between this molecule and alloantisera to any of the previously described immunoglobulin allotypes. It is proposed that these alloantisera to spleen cells recognise one allelic form of the murine "delta" chain coded for by a gene locus closely linked to the known structural genes for mouse immunoglobulin heavy chains. The designation Ig-5 is proposed for this new immunoglobulin heavy chain locus.

The Ia antigens

The Ia antigens constitute a polymorphic series of cell surface determinants. At present, their definition is mainly a genetic one, and thus any cell surface antigen which can be demonstrated to be encoded by a gene in the Ir region of the H-2 complex may be classified as an Ia antigen. There are presently three subregions of the I region defined on the basis of available recombinant haplotypes, and designated at I-A, I-B, AND I-C. Mapping of individual Ia specificities indicates that numerous specificities are determined by genes in the I-A subregion, several in the I-C subregion, and few, if any, in the I-B subregion. This may be a reflection of the state of the art, however, rather than an accurate assessment of the extent of polymorphism. The Ia antigens appear to be expressed preferentially on the B-cell subpopulation of lymphoid cells. However, with the use of sensitive techniques they have also been demonstrated on some T cells, on macrophages, on sperm cells, and on epidermal cells. The Ia antigens have also been demonstrated on several T-cell factors which appear to be involved in the immune response. Whether or not all of the Ia antigens thus localized are identical or represent overlapping specificities within the same sera remains in many cases to be determined. There are presently three ways of defining Ia specificities serologically: (1) by direct immunization between strains differing only in the I region; (2) by detection of shared Ia determinants using polyspecific sera which contain H-2K region and H-2D region antibodies but which are nevertheless specific only for Ia antigens when tested on target cells of other strains; and (3) by selective absorption of H-2K region and H-2D region antibodies from an H-2 antiserum by cells bearing these antigens but lacking (or relatively lacking Ia antigens. All three of these methods produce anti-Ia reagents of reasonable titer for use in both serological and functional experimentation. The definition of the specificity as an Ia specificity in each case requires the availability of appropriate recombinant strains to map the specificity to the Iregion. In addition, there are several correlative criteria which have been developed in order to detect Ia activity in alloantisera in the absence of the availability of appropriate recombinants for mapping of the specificity. These include the tissue distribution of the Ia antigens (namely, their predominant expression on the B-cell subpopulation), their characteristics molecular size, their association on the B-cell surface with the Fc receptor, and their lack of association with other products of the major histocompatibility complex as distinguished either chemically or by cocapping studies. These correlative criteria make it possible to distinguish probable anti-Ia reactivity in a variety of serological reactions, but the results must still be interpreted with caution until appropriate recombinants have been obtained which can map the specificities to the I region...