Studies on the cytophilic properties of human beta2-microglobulin. II. The role of histocompatibility antigens

The interaction between beta 2-microglobulin (beta 2m) and purified class-I major histocompatibility (MHC) antigen

The function of MHC class-I molecules is to sample peptides from the intracellular environment and present them to CD8+ cytotoxic T lymphocytes. To understand the molecular details of the assembly (and disassembly) of peptide-beta 2m-class-I complexes a biochemical peptide-class-I binding assay has been generated recently and this paper reports on a similar assay for the interaction between beta 2m and class I. As a model system human beta 2m binding to mouse class I was used. The assay is strictly biochemical using purified reagents which interact in solution and complex formation is determined by size separation. It is specific and highly sensitive. The observed affinity of the interaction, KD, is close to 0.4 nM. The rate of association at 37 degrees C is very fast (the ka is around 5 x 10(4)/M/s) whereas the dissociation is slow (the kd is around 8 x 10(-6)/s); the ratio of dissociation to association yields a calculated KD close to the observed value. At 37 degrees C almost all of the purified class I participates in binding of the exogenously offered beta 2m showing that a considerable exchange of the endogenous beta 2m occurs. Finally, it was demonstrated that exogenous beta 2m enhances binding to MHC class-I of short perfectly-matching peptides as well as longer peptides.

Neutrophil beta-2 microglobulin: an inflammatory mediator

Beta-2 microglobulin (beta 2m) constitutes the light invariant chain of HLA class I antigen, and is a constituent of mobilizable compartments of neutrophils. Two forms of beta 2m exist: native beta 2m and proteolytically modified beta 2m (Des-Lys58-beta 2m), which shows alpha mobility in crossed radioimmuno-electrophoresis. The modification of native beta 2m can be executed by membrane-associated activity of mononuclear cells, and Des-Lys58-beta 2m augments the production of interleukin 2. In this study we present evidence that human neutrophils contain native beta 2m in specific granules, secretory vesicles, and plasma membrane. Beta 2m was released in the native form from neutrophils in response to stimulation with chemotactic stimuli and phorbol ester. The results of experiments designed to study the modification of native beta 2m by neutrophils indicated that neutrophils do not participate in the proteolysis of beta 2m. However, we demonstrated that native beta 2m following degranulation may be transformed to Des-Lys58-beta 2m by lymphocytes. We suggest that neutrophil beta 2m following exocytosis may be transformed to Des-Lys58-beta 2m, acting as an extracellular messenger between granulocytes and lymphocytes in the inflammatory focus.

Expression of the W6/32 HLA epitope by cells of rat, mouse, human and other species: critical dependence on the interaction of specific MHC heavy chains with human or bovine beta 2-microglobulin

The HLA class I epitope W6/32 is conformationally dependent on both heavy chain and beta 2-microglobulin (beta 2M). Previously, the W6/32 epitope has been detected in humans and other primates as well as from bovine sources. Two controversial reports suggest the W6/32 epitope is constitutively expressed by either normal or transformed murine cells expressing the Db allele. Here we show that the appearance of the W6/32 epitope in murine cells results from the association of either the Db or Kd gene products with either bovine or human beta 2M. We use congenic mouse strains and hybrid H-2 class I genes between Db and Kb to map the W6/32 epitope to particular amino acid residues in the alpha 2 domain. Subsequently, we show that beta 2M exchange is not confined to murine or human cells in vitro but can be detected after beta 2M injection into a mouse. The data presented suggests that beta 2M exchange takes place at the cell surface under physiological conditions and indicates that MHC class I heavy chains are in an equilibrium between the bound and unbound form of beta 2M.

Restricted recognition of beta 2-microglobulin by cytotoxic T lymphocytes

Recognition of foreign antigen by cytotoxic T lymphocytes (CTL) is restricted by class I major histocompatibility complex (MHC) products. Class I heavy chains (relative molecular mass (Mr) 45,000-48,000) are reversibly and noncovalently associated with beta 2-microglobulin (beta 2M, Mr = 12,000). Cells expressing human or murine class I heavy chains can exchange their native beta 2M for exogenously added free beta 2M, which is present in serum. Two allelic forms of beta 2M exist among the common laboratory mouse strains, beta 2M-A and beta 2M-B, which are represented in BALB and C57BL mice, respectively. The two forms differ at a single amino acid at position 85, the gene (beta 2m) is located on chromosome 2 linked to a minor histocompatibility (H) region, H-3. It has been proposed that one of the H-3 loci is identical with beta 2m, and that CTL raised across certain H-3 incompatibilities are actually specific for beta 2M. Here we describe CTL raised in such a combination which recognize endogenous as well as exogenous beta 2M-B in the context of H-2Kb. This represents a unique case of CTL recognition, as CTL usually recognize antigens inserted into the membrane, and it is the first molecular identification of the product of a minor H locus.

Three-dimensional structure of beta 2-microglobulin

The three-dimensional structure of beta 2-microglobulin, the light chain of the major histocompatibility complex class I antigens, has been determined by x-ray crystallography. An electron density map of the bovine protein was calculated at a nominal resolution of 2.9 A by using the methods of multiple isomorphous replacement and electron density modification refinement. The molecule is approximately 45 X 25 X 20 A in size. Almost half of the amino acid residues participate in two large beta structures, one of four strands and the other of three, linked by a central disulfide bond. The molecule thus strongly resembles Ig constant domains in polypeptide chain folding and overall tertiary structure. Amino acid residues that are the same in the sequences of beta 2-microglobulin and Ig constant domains are predominantly in the interior of the molecule, whereas residues conserved among beta 2-microglobulins from different species are both in the interior and on the molecular surface. In the crystals studied, the molecule is clearly monomeric, consistent with the observation that beta 2-microglobulin, unlike Ig constant domains, apparently does not form dimers in vivo but associates with the heavy chains of major histocompatibility complex antigens. Our results demonstrate that, at the level of detailed three-dimensional structure, the light chain of the major histocompatibility class I antigens belongs to a superfamily of structures related to the Ig constant domains.

On the heterologous interaction between beta 2-microglobulin and the heavy chain of rat major histocompatibility complex class 1 antigens

The heterologous interaction between beta 2-microglobulin (beta 2m) and rat major histocompatibility complex (MHC) (RT1) antigens was measured in a two-step binding assay consisting of binding of radiolabelled beta 2m to RT1 antigens and immunoprecipitation of beta 2m-RT1 antigen complexes with RT1 antisera. The effects of varying the concentrations of the three reactants involved were studied. The molecular events taking place in the two steps were analysed by gel chromatography. The beta 2m-RT1 antigen complex had the apparent size of albumin and reacted completely with specific alloantisera. RT1 antigens prepared from Wistar/Furth (RT1u) and Brown Norway (RT1n), respectively, both effectively bound heterologous beta 2m. The times for association and dissociation, respectively, at 37 degrees C, were of the same order, but dissociation was slightly slower. Association was markedly temperature-dependent and was considerably slower at low temperatures. All these processes were slower for RT1n than for RT1u antigens. The association constant for the interaction between RT1u antigens and 125I-human beta 2m was estimated by Scatchard analysis to be about 10(9) M-1. Contribution to the heterologous interaction by products from various rat MHC subloci (A, B, and C) was investigated by the introduction of sublocus-specific antisera in step 2. The reaction apparently involved neither class 2 antigens (sublocus B) nor the presumed rat Qa homologue (sublocus C). Classical class 1 antigens (sublocus A) clearly contributed to the binding. However, a monoclonal antibody against products from rat MHC class 1 genes only precipitated less than half of the RT1 antigen-complexed beta 2m. Thus, at least two RT1u class 1 alloantigen molecules seem to participate in the reaction. This, in turn, indicates that the rat genome may contain multiple class 1 genes, as is the case for most other mammals investigated.

Beta 2-microglobulin from serum associates with MHC class I antigens on the surface of cultured cells

Beta 2-microglobulin (beta 2-m) is a highly conserved polypeptide (12,000 molecular weight; 12K) noncovalently associated with the heavy chain (45-48K) of the major histocompatibility complex (MHC) class I antigens. Its synthesis is required for expression of the HLA-A/B and H-2K/D heavy chains at the cell surface; beta 2-m is also associated with the human cell-surface antigens T6 and M241 isolated from thymocytes. However, on the T leukaemic cell line MOLT-4 some of the T6 antigens contain a different 12K subunit, termed beta t (refs 3, 7, 8). Purified human beta 2-m can exchange partially both with human beta 2-m associated with HLA-antigens, and with mouse beta 2-m associated with murine alloantigens. As MOLT-4 cells were grown in the presence of fetal calf serum (FCS) and as serum is known to contain some free beta 2-m, we examined whether beta t was bovine beta 2-m which had replaced endogenous beta 2-m on the surface of the cell. Here we show both that beta 2-m from FCS or human serum (HuS) used in cell culture can exchange with beta 2-m on the cell surface, and that beta t is in fact bovine beta 2-m.

Binding of monoclonal antibodies inhibits dissociation but not exchange of HLA and beta 2-microglobulin

Hyafil and Strominger (Proc Nat Acad Sci USA 76:5834, 1979) showed that the complex of papain-solubilized HLA heavy chain and beta 2-microglobulin (beta 2-m) will measurably dissociate and exchange with free beta 2-m at 37 degrees C but not at 4 degrees C. We have investigated the affects of monoclonal antibodies on these processes. The dissociation reaction at 37 degrees C is inhibited by the presence of HLA-A,B,C specific monoclonal antibodies. Monomorphic antibodies such as BB7.7 against combinatorial determinants are most potent though antibodies against many different determinants of the HLA heavy chain are effective. This suggests that dissociation of beta 2-m from HLA involves a conformational change of the HLA heavy chain which is prevented by antibody binding. BB7.7 also increased the rate of association of HLA heavy chain and beta 2-m. This affect was significantly greater at 37 degrees C and 25 degrees C than at 4 degrees C. Monoclonal antibodies that inhibited dissociation did not inhibit the beta 2-m exchange reaction. If beta 2-m exchange proceeds through a simple dissociation/association reaction then the affect of antibody inhibition on dissociation is almost exactly compensated by its enhancement of association. Alternatively B 2-m exchange may not proceed via a simple dissociation/association reaction.

Interference of beta 2-microglobulin specific autoantibodies with EA-binding of human peripheral lymphocytes; inhibition of B-cell and enhancement of T-lymphocyte Fc-receptors

The effect of anti-beta 2 m-specific autoantibodies was investigated on the FcRs of human PBMCs. Anti-beta 2 m autoantibodies inhibited the FcRs of the lymphocyte subpopulation detectable by rosetting with EA(hu). On the contrary, when EA(ox) indicator system was used, in the majority of the cases an enhancement of EA rosette formation was detected. Using separated lymphocyte subpopulations we found that the binding of anti-beta 2 m autoantibodies increased the number of FcR+ non-B-cells and inhibited that of B-lymphocytes.

Interspecies exchange of beta 2-microglobulin and associated MHC and differentiation antigens

Radiolabeled human beta 2-microglobulin (beta 2m) can bind to mouse histocompatibility (H-2) antigens on the cell surface or to partially purified H-2 antigens in solution. The complexes containing human beta 2m and H-2 antigens from C3H (H-2k) mice could be immunoprecipitated specifically with alloantisera, rabbit anti-H-2 xenoantisera, and with monoclonal H-2-specific antibodies. Specific association with H-2 antigens was also observed with other haplotypes. The only exception was B10.D2 (H-2d) from which complexes containing human beta 2m could only be precipitated with anti-H-2-xenosera. Thus radiolabeled human beta 2m can be used as a specific label for mouse H-2 antigens in precipitation and radioimmunoassays. The application of this finding extends to major histocompatibility complex antigens of other species, and to differentiation antigens with primary association with beta 2m.

Binding of beta 2-microglobulin by heterologous sera

Purified human, rat or guinea-pig beta 2-microglobulin (beta 2m) was mixed with sera from guinea-pig, rat, mouse, rabbit, horse, goat, cow, rhesus monkey or man. The mixtures were incubated at 37 degrees C for various lengths of time. When the sera were separated by gel-chromatography on Sephadex G-200, beta 2m was traced not only in 'free' form but also in fractions with higher molecular weights. Evidence is presented suggesting that heterologous beta 2m binds to beta 2m-containing molecules in sera by exchange with the homologous counterpart.

Serum beta 2-microglobulin in acute and chronic leukaemia

The serum concentration of beta 2-microglobulin (beta 2-m) was measured in 69 patients with acute or chronic lympho- and myeloproliferative disorders. Serum beta 2-m was found significantly increased in 12 out of 14 patients with chronic lymphatic leukaemia. The serum concentration was proportional to the estimated lymphatic infiltration of tissues but inversely related to the number of circulating lymphocytes. Cytostatic treatment was followed by a decrease in serum beta 2-m, but normalization of the serum concentration was not observed. 11 patients with chronic granulocytic leukaemia all had significantly elevated serum concentrations of beta 2-m and increased serum concentrations were also found in patients with acute leukaemias. Thus, 12 out of 25 patients with acute myeloid leukaemia and all of 5 patients with acute myelomonocytic leukaemia as well as 4 out of 5 patients with acute lymphatic leukaemia had increased serum beta 2-m levels. In acute leukaemia no correlation could be demonstrated between the blood lymphocyte concentration and serum beta 2-m. Also no significant changes in serum beta 2-m were found in either remission or relapse of the acute leukaemia. It is concluded that serum beta 2-m in patients with chronic leukaemia may reflect the total amount or turn-over of leukaemic cells in the body and that repeated determinations of serum beta 2-m in these patients might be useful as an estimate of the residual leukaemic cell mass after therapy. Apart from this the determination of serum beta 2-m seems to be of little, if any, clinical use in leukaemia.

Isolation and partial characterization of a murine cell surface glycoprotein with affinity for exogenously added beta 2-microglubulin

Exogenously added beta 2-microglobulin (beta 2m) binds to a variety of murine cell types. The 'receptor' for beta 2m has been isolated. The purified 'receptor' comprised a 48,000-dalton chain and occasionally a 25,000-dalton component. Direct crosslinking of beta 2m to the receptor on intact cells gave rise to a single 60,000-dalton beta 2m-'receptor' complex. The molecular characteristics of the 'receptor' were considerably changed on binding beta 2m. The size of the beta 2m-'receptor' complex was increased partly due to enhanced binding of deoxycholate. The 'receptor' was less easily degraded by proteases when beta 2m was bound then when free. The solubilized 'receptor' reacted with a heteroantiserum raised against H-2K and D antigens but did not exhibit any alloantigenic determinants shared with H-2K, D or Ia antigens.