THE ARRANGEMENT OF THE PEPTIDE CHAINS IN GAMMA-GLOBULIN

The mosaic puzzle of the therapeutic monoclonal antibodies and antibody fragments - A modular transition from full-length immunoglobulins to antibody mimetics

The use of monoclonal antibodies represents an important and efficient diagnostic and therapeutic tool in disease management and modern science but remains limited by several factors including the uneven distribution in diseased tissues as well as undesired activation of side immune reactions. Major scientific advancements including Recombinant DNA Technology, Hybridoma Technology, and Polymerase Chain Reaction have considerably impacted the use of monoclonal antibodies providing technical and effective solutions to overcome the shortcomings encountered with conventional antibodies. Initially, the introduction of antibody fragments allowed a more uniform and deeper penetration of the targeted tissue and reduced unwanted activation of Fc-mediated immune reactions. On another level, the immunogenicity of murine-derived antibodies was overcome by humanizing their encoding genes with specific sequences of human origin andtransgenic mice able to synthesize fully human antibodies were successfully created. Moreover, the advancement of genetic engineering techniques supported by the modular structure of antibody coding genes paved the way for the development of a new generation of antibody fragments with a wide spectrum of monospecific and bispecific agents. These later could be monovalent, bivalent, or multivalent, and either expressed as a single chain, assembled in multimeric forms or stringed in tandem. This has conferred improved affinity, stability, and solubility to antibody targetting. Lately, a new array of monoclonal antibody fragments was introduced with the engineering of nanobody and antibody mimetics as non-immunoglobulin-derived fragments with promising diagnostic and therapeutic applications. In this review, we decipher the molecular basis of monoclonal antibody engineering with a detailed screening of the antibody derivatives that provides new perspectives to expand the use of monoclonal fragments into previously unexplored fields.

Principles and Current Clinical Landscape of Multispecific Antibodies against Cancer

Building upon the resounding therapeutic success of monoclonal antibodies, and supported by accelerating progress in engineering methods, the field of multispecific therapeutic antibodies is growing rapidly. Over 140 different molecules are currently in clinical testing, with excellent results in recent phase 1-3 clinical trials for several of them. Multivalent bispecific IgG-modified formats predominate today, with a clear tendency for more target antigens and further increased valency in newer constructs. The strategies to augment anticancer efficacy are currently equally divided between disruption of multiple surface antigens, and additional redirection of cytotoxic T or NK lymphocytes against the tumor. Both effects complement other modern modalities, such as tyrosine kinase inhibitors and adoptive cell therapies, with which multispecifics are increasingly applied in combination or merged, for example, in the form of antibody producing CAR-T cells and oncolytics. While mainly focused on B-cell malignancies early on, the contemporary multispecific antibody sector accommodates twice as many trials against solid compared to hematologic cancers. An exciting emerging prospect is the targeting of intracellular neoantigens using T-cell receptor (TCR) fusion proteins or TCR-mimic antibody fragments. Considering the fact that introduction of PD-(L)1 inhibitors only a few years ago has already facilitated 5-year survival rates of 30-50% for per se highly lethal neoplasms, such as metastatic melanoma and non-small-cell lung carcinoma, the upcoming enforcement of current treatments with "next-generation" immunotherapeutics, offers a justified hope for the cure of some advanced cancers in the near future.

[MAbs: the history of a basic research or the curiosity as a source of wealth]

In 2019, monoclonal antibodies are a worldwide annual business worth of more than 100 billions USD (i.e., about 90 billions €). In addition to their use in the clinics, monoclonal antibodies are also used for diagnosis and remain highly valuable tools for academic basic and translational research. Forty-four years after the seminal publication of Georges Köhler and César Milstein, dozens of meetings and seminars focusing on various aspects of mAbs are held annually all around the world. But forty-four years later, the scientific works and efforts that have made possible this scientific breakthrough are gradually forgotten and, for many, monoclonal antibodies are no more than a multi-million USD business alike any other big business, guided by financial markets and the results of on-going clinical trials… Time has now come for acknowledging and paying tribute to all these scientists involved in basic research, to these researchers passionate about science, some famous, some forgotten, scattered all over the world. They explored during the 20^th century the frontiers of unknown and generated a knowledge that allowed the emergence of a technique that translated finally into what is one of the greatest therapeutic revolution of the modern era.

Bone marrow fat and the decline of B lymphopoiesis in rabbits

B lymphopoiesis is necessary to generate a diverse pool of naïve B cells that are able to respond to a broad spectrum of antigens during immune responses to pathogens and to vaccination. Rabbits have been utilized for many years to generate high affinity monoclonal and polyclonal antibodies. Specific antibodies generated in rabbits have greatly advanced scientific discoveries, but the unique qualities of rabbit B cell development have been underappreciated. Unlike in humans and mice, where B lymphopoiesis declines in mid to late life, B lymphopoiesis in rabbits arrests early in life, between 2 and 4 months of age. This review focuses on the early loss of B cell development in rabbits and the contribution of the bone marrow microenvironment to this process. We also propose directions for future research in this area, and discuss how the rabbit can be used as a model to understand the decline of B lymphopoiesis that occurs in humans late in life. Such studies will be important for developing therapeutics targeted to prevent and/or reverse declining B lymphopoiesis in the elderly, as well as boosting immunity and antibody responses after infection or vaccination.

Characterization of a novel hemolytic activity of human IgG fractions arising from diversity in protein and oligosaccharide components

Human IgG is a well-established multifunctional antigen specific immunoglobulin molecule of the adaptive immune system. However, an antigen nonspecific immunological function of human IgG has never been reported. In this study, human IgG was isolated using ammonium sulfate fractional precipitation and diethylaminoethanol (DEAE) cellulose 52 ion exchange chromatography, from which h-IgG and hs-IgG fractions were purified on the basis of their differential binding to rabbit anti-shrimp hemocyanin antibody (h) and rabbit anti-shrimp hemocyanin's small subunit antibody (hs), respectively. We found that h-IgG had a higher hemolytic activity than hs-IgG against erythrocytes from humans, rabbits, mice and chickens, whereas the control IgG showed negligible activity. h-IgG could interact directly with erythrocyte membranes, and this interaction was suppressed by high molecular weight osmoprotectants, showing that it may follow a colloid-osmotic mechanism. In comparative proteomics and glycomics studies, h-IgG and hs-IgG yielded 20 and 5 significantly altered protein spots, respectively, on a 2-D gel. The mean carbohydrate content of h-IgG and hs-IgG was approximately 3.6- and 2-fold higher than that of IgG, respectively, and the α-d-mannose/α-d-glucose content was in the order of h-IgG>hs-IgG>IgG. In this study, a novel antigen nonspecific immune property of human IgG was investigated, and the diversity in the protein constituents and glycosylation levels may have functional signficance.

On the roles of polyvalent binding in immune recognition: perspectives in the nanoscience of immunology and the immune response to nanomedicines

Immunology often conveys the image of large molecules, either in the soluble state or in the membrane of leukocytes, forming multiple contacts with a target for actions of the immune system. Avidity names the ability of a polyvalent molecule to form multiple connections of the same kind with ligands tethered to the same surface. Polyvalent interactions are vastly stronger than their monovalent equivalent. In the present review, the functional consequences of polyvalent interactions are explored in a perspective of recent theoretical advances in understanding the thermodynamics of such binding. From insights on the structural biology of soluble pattern recognition molecules as well as adhesion molecules in the cell membranes or in their proteolytically shed form, this review documents the prominent role of polyvalent interactions in making the immune system a formidable barrier to microbial infection as well as constituting a significant challenge to the application of nanomedicines.

Evolving concepts of specificity in immune reactions

Our goal is to provide a perspective on current understanding of the origins of specificity in immune reactions, a topic that has intrigued scientists for over a century. A fundamental property of adaptive immune responses is the ability to discriminate among an immense variety of substances by means of antibodies (Abs) and Ab-like receptors on T lymphocytes [T-cell receptors (TCRs)], each able to bind a particular chemical structure [the antigen (Ag)] and not, or only weakly, similar alternatives. Evidence has long existed, however, and has grown, especially recently, that while exhibiting remarkable specificity, many individual Abs and TCRs can also bind a variety of very different ligands. How can Ag recognition by these receptors exercise the great specificity for which they are renowned and yet react with a variety of different ligands (degeneracy)? We critically consider the mechanistic bases for this specificity/degeneracy enigma and also compare and contrast Ag recognition by Abs and TCRs.

The crystal structure of rabbit IgG-Fc

We report the structure of the Fc fragment of rabbit IgG at 1.95 A (1 A=0.1 nm) resolution. Rabbit IgG was the molecule for which Porter established the four-chain, Upsilon-shaped structure of the antibody molecule, and crystals of the Fc ('Fragment crystallisable') were first reported almost 50 years ago in this journal [Porter, R. R. (1959) Biochem. J. 73, 119-126]. This high-resolution analysis, apparently of the same crystal form, reveals several features of IgG-Fc structure that have not previously been described. More of the lower hinge region is visible in this structure than in others, demonstrating not only the acute bend in the IgG molecule that this region can mediate, as seen in receptor complexes, but also that this region has a tendency to adopt a bent structure even in the absence of receptor. As observed in other IgG-Fc structures, the Cgamma2 domains display greater mobility/disorder within the crystals than the Cgamma3 domains; unexpectedly the structure reveals partial cleavage of both Cgamma2 intra-domain disulphide bonds, whereas an alternative conformation for one of the cysteine residues in the intact bridge within the more ordered Cgamma3 domains is observed. The N-linked oligosaccharide chains at Asn(297) are well-defined and reveal two alternative conformations for the galactose units on each of the alpha(1-6)-linked branches. The presence of this galactose unit is important for stabilizing the structure of the entire branched carbohydrate chain, and its absence correlates with the severity of autoimmune conditions such as rheumatoid arthritis in both human clinical studies and in a rabbit model of the disease. Rabbit IgG, through this high-resolution structure of its Fc region, thus continues to offer new insights into antibody structure.

The disulphide bonds of human and rabbit gamma-globulins

1. The reaction of the disulphide bonds of the predominant species of human and rabbit gamma-globulins (the 7s gamma-globulins) with sulphite was studied in the presence and absence of denaturing agents and heavy-metal reagents. 2. The total number of bonds reacting/mol. of mol.wt. 160000 was approx. 18 for human and 20 for rabbit gamma-globulin. 3. Six S.S bonds/mol. of human and 6.5 S.S bonds/mol. of rabbit gamma-globulin reacted with sulphite alone at pH6. These appeared to include all the interchain S.S bonds. 4. The number of free SH groups was less than 0.2/mol. of human and less than 0.3/mol. of rabbit gamma-globulin.

Direct radiation damage is confined to a single polypeptide in rabbit immunoglobulin G

Frozen rabbit immunoglobulin G was exposed to high-energy electrons. The surviving polypeptide subunits were determined and analyzed by radiation target analysis. Each subunit was independently damaged by radiation whether or not they were bound by disulfide bridges to other subunits, demonstrating that in IgG radiation-deposited energy did not travel across disulfide bonds.

Role of oligosaccharide residues of IgG1-Fc in Fc gamma RIIb binding

Engagement of Fc gamma receptors (Fc gamma Rs) with the Fc region of IgG elicits immune responses by leukocytes. The recent crystal structure of Fc gamma RIII in complex with IgG-Fc has provided details of molecular interactions between these components (Sondermann, P., Huber, R., Oosthuizen, V., and Jacob, U. (2000) Nature 406, 267-273). One of the most intriguing issues is that glycosylation of IgG-Fc is essential for the recognition by Fc gamma Rs although the carbohydrate moieties are on the periphery of the Fc gamma RIII-Fc interface. To better understand the role of Fc glycosylation in Fc gamma R binding we prepared homogeneous glycoforms of IgG-Fc (Cri) and investigated the interactions with a soluble form of Fc gamma RIIb (sFc gamma RIIb). A 1:1 complex stoichiometry was observed in solution at 30 degrees C (K(d), 0.94 microm; Delta G, -8.4 kcal mol(-1); Delta H, -6.5 kcal mol(-1); T Delta S, 1.9 kcal mol(-1); Delta C(p), -160 cal mol(-1) K(-1)). Removal of terminal galactose residues did not alter the thermodynamic parameters significantly. Outer-arm GlcNAc residues contributed significantly to thermal stability of the C(H)2 domains but only slightly to sFc gamma RIIb binding. Truncation of 1,3- and 1,6-arm mannose residues generates a linear trisaccharide core structure and resulted in a significantly decreased affinity, a less exothermic Delta H, and a more negative Delta C(p) for sFc gamma RIIb binding, which may result from a conformational change coupled to complex formation. Deglycosylation of the C(H)2 domains abrogated sFc gamma RIIb binding and resulted in the lowest thermal stability accompanied with noncooperative unfolding. These results suggest that truncation of the oligosaccharides of IgG-Fc causes disorder and a closed disposition of the two C(H)2 domains, impairing sFc gamma RIIb binding.

Monomeric Complement-Activating IgG Paraproteins

Three patients presented a unique syndrome of recurrent panniculitis with an IgGκ paraprotein and depletion of the early components of the classical pathway of complement. The IgGκ paraproteins were monomers with a normal structure, and with no evidence for aggregation, as assessed by electron microscopy and ultracentrifugation. Both heavy and light chains were of normal molecular size (SDS-PAGE), and the paraproteins were not heavily glycosylated. However, the paraproteins from all three patients had unusual features that included abnormal behavior on gel filtration chromatography and a heavy chain of high pI. When analyzed by fast protein liquid chromatography (Superdex 200), elution of the paraproteins was retarded, particularly when the ionic strength was increased. This retardation was partially reversed in 20% alcohol, and fully reversed in 6 M guanidine-HCl. Neither anti-C1 inhibitor nor anti-C1q autoantibodies were found in any of the patients’ sera. However, the paraproteins bound to the globular heads of C1q at normal ionic strength. They activated C4 in normal human serum, but not in C1q-deficient serum. Activation led to the formation of C1s-C1 inhibitor complexes. Taken together, the data suggest that the unusual paraproteins have the capacity to bind C1q, which then leads to activation of C1. The ability of these paraproteins to activate C1, in spite of their being soluble monomers, is likely to be related to their unique physicochemical features.

Separation method of IgG fragments using protein L

Protein L (IgG kappa-chain-binding bacterial protein) showed a precipitate line(pseudo-immuno-reaction) with IgG and F(ab')2 fragment, but did not show any line with the Fab fragment, the Fc fragment and free kappa-chains in the micro-Ouchterlony method. The IgG and Fab fraction obtained from pa-pain-digested IgG (from the sera of patients with chronic thyroiditis), followed by Protein A-Sepharose, were separated by Protein L-Sepharose affinity chromatography. The unbound fraction (UF) consisted of IgG(lambda) or Fab(lambda) and the bound fraction (BF) consisted of IgG(kappa) or Fab(kappa) were obtained. Anti-thyroglobulin and anti-thyroid peroxidase antibody activities were found equally in both the UF and the BF. When Fab(kappa) was reduced with dithiothreitol (DTT), the Fd fragment in the UF could be separated from the free kappa-chain and the unreduced Fab(kappa) in the BF with a Protein L-Sepharose column. A separation method of human IgG fragments such as free kappa-chain, combined forms of kappa-chain [Fab or F(ab')2], and the Fd region, using Protein L, is described.

H CHAIN SUBGROUPS OF MYELOMA PROTEINS AND NORMAL 7S GAMMA-GLOBULIN

Through the use of a variety of antisera to isolated myeloma proteins, four subgroups of 7S gamma-globulin type proteins were readily distinguished. The first, the Vi subgroup, consisted of ten of 64 myeloma proteins studied. The second, the We group, contained the majority of myeloma proteins. The third, the Ge subgroup, included three of 50 myeloma proteins. The fourth remains ill-defined and appears heterogeneous. Counterparts for both the Vi and the Ge subgroup, were found in the Fr II gamma-globulin and in the normal gamma-globulin of all of a large number of individual sera studied. The unique antigenic character of both groups was localized to the H chains, although different determinants were involved for different antisera. An essential role of intact disulfide bonds was apparent with certain rabbit antisera. In addition to the special antigenic characteristics, the Ge subgroup showed in each instance a fast mobility for the F fragments produced by papain which was not found for other myeloma proteins.

STRUCTURAL ASPECTS OF HUMAN ERYTHROCYTE AUTOANTIBODIES. I. L CHAIN TYPES AND ELECTROPHORETIC DISPERSION

The 7S gamma-globulins causing erythrocyte autosensitization in 20 patients were isolated by elution and examined for homogeneity or heterogeneity of their L chain types and electrophoretic dispersion. The isolated erythrocyte autoantibodies from 12 patients contained only 1 detectable L chain type. Two of these "monotypic" populations showed appreciable restriction of electrophoretic dispersion, while 2 others more nearly resembled the electrophoretic heterogeneity of normal gamma-globulins. The autoantibodies from the other 8 patients exhibited L chains of both types. The single "bitypic" population so tested was relatively polydisperse electrophoretically. As a comparison, anti-Rh(o) isoantibodies from 5 of 6 donors without known hematologic disease showed bitypic reactions, and 2 of these isoantibody populations were relatively polydisperse electrophoretically. One Rh isoantibody is described which contained only 1 demonstrable L chain type. The structural similarities to "paraproteins" observed in a significant proportion of these erythrocyte autoantibodies raise the possibility of their origin from a restricted population of antibody forming cells, and may have implications concerning the pathogenesis of erythrocyte autosensitization.

STUDIES ON HUMAN ANTIBODIES. I. STARCH GEL ELECTROPHORESIS OF THE DISSOCIATED POLYPEPTIDE CHAINS

Specific precipitates and purified human antibodies were reduced and alkylated and subjected to starch gel electrophoresis in 8 M urea to dissociate and separate the L and H polypeptide chains. Dissociated antibodies to dextran, levan, teichoic acid, blood group A substance, and tetanus toxoid showed sharp bands corresponding to L polypeptide chains. The patterns differed for antibodies of different specificities. Some differences were also seen among antibodies of the same specificity from different individuals. Purified antidextran antibodies showed particularly simple patterns resembling those of purified human γ myeloma proteins. In some cases only one L chain band was present.

IMMUNOLOGICAL AND BIOCHEMICAL STUDIES ON SERUM HAPTOGLOBIN

Antibody to purified type 1-1 human serum haptoglobin was obtained in rabbits. Using this antiserum it was demonstrated that the antigenic determinants of the haptoglobin molecule reside in both α- and β-chains. No immunological difference between the normal α^1F- and α^1S-chains of haptoglobin could be detected. A number of healthy individuals whose haptoglobin pattern appeared normal by starch gel electrophoresis were shown to possess a haptoglobin which could be distinguished immunologically from the common haptoglobin types. These immunological atypical haptoglobins have been observed in individuals of types 1-1, 2-1, and 2-2. Combined immunological and chemical studies of purified haptoglobin have enabled the construction of a tentative model for the structure of human type 1-1 haptoglobin.

NATURALLY OCCURRING HUMAN ANTIBODY REACTING WITH BENCE JONES PROTEINS

Agglutinating substances having characteristics of naturally occurring macroglobulin antibodies to human Bence Jones proteins have been identified in human sera. By means of hemagglutination and hemagglutination inhibition techniques, common determinants have been demonstrated on the light (L) polypeptide chains of pooled normal human γ₂-globulin and on some Bence Jones proteins of group 1 but not of group 2. Individual human sera serve to delineate subgroups of the two major antigenic groups of the Bence Jones proteins by agglutinating cells coated by one but not another protein of the same antigenic group. The complexity of subgroups, especially of group 2, is established by testing a panel of Bence Jones proteins of the same group for their ability to inhibit hemagglutination. By this means it appeared that different sera recognized different group-specific determinants of cells coated with a single Bence Jones protein. The capacity of the L polypeptide chains and proteolytic fragments of γ₂-globulin to inhibit the hemagglutination reaction between Bence Jones protein or L chain-coated cells and human sera was examined. These studies demonstrated that the determinants, toward which agglutinators of human serum are directed, appear to be blocked in intact γ₂-globulin and in all fragments in which H chain remains in proximity to L chain. It would appear that the presence of H chains bound to L chains by non-covalent bonds completely obstructs the reactivity of the involved L chain groups. The agglutinating capacity of a serum toward Bence Jones proteins or L chains of γ₂-globulin appeared to be independent of its agglutinating capacity for cells coated with intact γ₂-globulin. No correlation of the presence in serum of agglutinators for Bence Jones proteins or L chains with health or disease has been established.

CORROBORATION OF RECENT MODELS OF THE GAMMA-G IMMUNOGLOBULIN MOLECULE

The relationships between the polypeptide chains of γG immunoglobulin and fragments of the molecule produced by papain and pepsin have been investigated. Specific procedures were employed including peptide mapping of tryptic hydrolysates and analysis of molecules reconstituted from chains labeled with different iodine isotopes. By these means, the Fab fragment was shown unequivocally to consist of the light chain and a portion of the heavy chain, the Fd fragment. The Fc fragment was found to be comprised of the residual portions of the heavy chain. These findings support the gross arrangement of chains embodied in recent models of the γG immunoglobulin molecule. The present studies have also provided additional information on the susceptibility of γG immunoglobulin to proteolytic cleavage. It was found that the portion of heavy chains corresponding to the Fd fragment was extensively cleaved by papain.

STRUCTURAL STUDIES OF HUMAN IMMUNOGLOBULINS. DIFFERENCES IN THE FD FRAGMENTS OF THE HEAVY CHAINS OF G MYELOMA PROTEINS

1. Comparison of peptide maps of the Fc fragments of normal G immunoglobulins and 11 G myeloma proteins of the We (b) type showed them to be very similar except for differences associated with the Gm type. Some additional differences were noted, however, in the Fc fragments of three Vi (c) myeloma proteins. 2. Peptide maps of heavy chains from the same G myeloma proteins differed from each other and from normal heavy chains. In general, the myeloma chains contained a larger number of well defined spots; some of these were common to normal heavy chains while others were unique to each protein. Others, present in normal heavy chains, were lacking in the myeloma proteins. 3. Comparison of the heavy chains and Fc fragments from the same protein suggests that much of the variability of different myeloma proteins and, presumably, antibodies resides in the Fd fragment. 4. Further support for this is given by the finding that the antigenic specificity of 3 myeloma proteins also appeared to reside in the Fd fragments.

A NEW CLASS OF HUMAN IMMUNOGLOBULINS. I. A UNIQUE MYELOMA PROTEIN

The unique myeloma protein from S. J., a patient with multiple myeloma, was isolated and characterized. It resembled other myeloma proteins in many respects. The S. J. myeloma protein migrated in a distinct peak in the slow β-globulin region on zone electrophoresis, appeared as a single band on starch gel electrophoresis, and sedimented at 7.04S in the ultracentrifuge. Papain and cysteine treatment produced Fc (fast) and Fab (slow) fragments. Reduction and alkylation of the myeloma protein produced heavy and light chains in a ratio of approximately 3:1. The S. J. myeloma protein had type L (type II) light chains. These were antigenically similar to the Bence Jones protein also found in this patient. The S. J. myeloma protein was unique in the properties of its heavy chains. The myeloma protein (and its heavy chains and Fc pieces) did not contain antigenic determinants specific for IgG, IgA, or IgM. The myeloma protein (and its heavy chains), however, did contain antigenic determinants which are characteristic of a new class of immunoglobulin. The S. J. myeloma protein was unusual also in its effect on the metabolism of normal IgG and in the electrophoretic mobility of the Fc fragment produced by papain digestion. No evidence was obtained to indicate that the entire heavy polypeptide of the S. J. protein was a grossly abnormal product of malignant cell metabolism. The unique properties of the S. J. myeloma protein (and its heavy chains) are believed to represent, in large measure, properties to be found in a small part of the normal immunoglobulin population.

THE ROLE OF DISULFIDE BONDS IN THE COMPLEMENT-FIXING AND PRECIPITATING PROPERTIES OF 7S RABBIT AND SHEEP ANTIBODIES

The number of total disulfide bonds in rabbit and sheep 7S gamma globulin, before and after treatment with 2-mercaptoethanol, has been measured by amperometric titration. Mercaptan reduction could diminish the complement-fixing efficiency of 7S rabbit gamma globulin by no more than 90 per cent without any significant decrease in maximal complement-fixing ability. This was associated with the reduction of only 7 disulfide bonds, including 1 interchain disulfide bond [S—S₍₁₎]. The reduction of 7 disulfide bonds in 7S sheep gamma globulin was associated with a 64 per cent decrease in complement-fixing efficiency, while reduction of 10 to 11 disulfide bonds decreased the complement-fixing efficiency by 90 per cent. Reduction of more disulfide bonds was not associated with any further decrease in complement-fixing efficiency, nor of any decrease in maximal complement-fixing ability. The reduction of more than 10 to 11 disulfide bonds in 7S rabbit and sheep gamma globulins was associated with a decrease in precipitating ability. The disulfide bonds which are more labile to mercaptan reduction appear to be associated with complement-fixing efficiency while the disulfide bonds which are more resistant to mercaptan reduction appear to be associated with precipitating ability. The measurement of interpolypeptide chain disulfide bonds has been discussed. One easily reduced interchain disulfide bond [S—S₍₁₎] appears to be important for the complement-fixing efficiency of 7S rabbit antibody. The integrity of this same bond is essential for the precipitating ability of 5S rabbit antibody and may also be important for its complement-fixing ability.

PREPARATION AND PROPERTIES OF THE PEPTIDE CHAINS OF NORMAL HUMAN 19 S GAMMA-GLOBULIN (IGM)

1. A method is described for preparing pure samples of 19s gamma-globulin (IgM) from normal human serum by using successive steps of dialysis, density-gradient ultracentrifugation, chromatography on DEAE-cellulose, and gel filtration on Sephadex G-200. The yield of IgM (20-25mg./100ml. of serum) was equivalent to about one-quarter of that present in normal serum. 2. Analysis of the separated peptide chains of normal IgM and IgG (7s gamma-globulin) showed considerable differences in the amino acid composition of A chains from the two proteins; their respective B chains, on the other hand, were similar in composition. The carbohydrate of both proteins is confined almost entirely to the A chains; the IgM A chain contains about four times as much carbohydrate as the IgG A chain. 3. These findings support the view that the different classes of human immunoglobulin have B chains that are identical and A chains that are chemically distinct.

RECOMBINATION OF ANTIBODY POLYPEPTIDE CHAINS IN THE PRESENCE OF ANTIGEN

Conditions were developed by which the separated H and L chains of gamma₂ globulins recombined to form four-chained molecules in good yields. In the absence of antigen, anti-2,4-dinitrophenyl (anti-DNP) H chains randomly reassociated with a mixture of antibody and non-specific gamma₂ globulin L chains. In the presence of a specific hapten, however, the antibody H chains preferentially interacted with the anti-DNP L chains. Antibody H chain-antibody L chain recombinants formed in the presence of hapten were more active than the corresponding recombinants formed in the absence of hapten. Speculations are made regarding the possible mechanisms and biological significance of these effects.

SINGLE CELL STUDIES ON 19S ANTIBODY PRODUCTION

Rats were immunized with Salmonella adelaide flagella. By zone centrifugation of serum samples in sucrose gradients, it was shown that, as in many other systems of antibody formation, the first response was the formation of 19S, mercaptoethanol (ME)-sensitive antibody. This was quickly replaced by 7S, ME-insensitive antibody. Popliteal lymph node cell suspensions were prepared, and cells with antibody on their surface were identified by the method of bacterial adherence. By micromanipulation such cells were washed, placed into microdroplets, examined under high-power phase contrast and broken to release intracellular antibody. These droplets were then studied in either of two ways. In the first method, each droplet was halved and one half treated with ME. Then both halves were titrated for immobilizing antibody through serial twofold dilution of the half microdroplets. Droplets showing destruction of antibody by ME were classified as 19S; those showing no reduction in titer as 7S; and those showing significant (>1 log(2)) reduction as double producers; i.e., cells containing both 7S and 19S antibodies. In the second method, droplets were divided into 4 equal quarters, for testing after treatment with either ME, or a specific rabbit anti-rat 7S globulin serum, or both. In these experiments, cells showing some remaining antibody after treatment with either reagent, but not after treatment with both reagents, were classified as double producers. Of 144 cells tested, 123 contained readily detectable amounts of antibody. These comprised 42 19S cells, 64 7S cells, and 17 double producers. The double producers were frequent at times when the switchover from 19S to 7S antibody production was occurring. All except 4 of the cells in the study could clearly be identified as members of the plasma cell series. Though 7S cells became more frequent as the cell population matured, no clear-cut correlation between cell immaturity and 19S production could be obtained. In the primary response many fully mature plasma cells contained only 19S antibody; conversely, in the secondary response many blasts contained 7S antibody. No morphological difference between 19S and 7S cells could be found. The results suggested that many cells or cell clones go through a sequence whereby each forms first 19S and later 7S antibody with identical combining sites.