The generation of antibody diversity: its dependence on antigenic stimulation

Reverse Transcriptase Mechanism of Somatic Hypermutation: 60 Years of Clonal Selection Theory

The evidence for the reverse transcriptase mechanism of somatic hypermutation is substantial and multifactorial. In this 60th anniversary year of the publication of Sir MacFarlane Burnet's Clonal Selection Theory, the evidence is briefly reviewed and updated.

Affinity maturation of lymphocyte receptors and positive selection of T cells in the thymus

In this review we have re-evaluated the dominant paradigm that TcR V genes do not somatically mutate. We highlight the many structural and functional similarities between Ig and TcR antigen-specific receptors on B and T cells. We have reviewed the factors influencing the somatic and germline evolution of IgV regions in B cells, have evaluated in detail various models which could be invoked to explain the pattern of variation in both transcribed and non-transcribed segments of germline IgV-gene DNA sequences, and applied this perspective to the TcR V beta and V alpha genes. Whilst specific TcRs recognize a complex of a short antigenic peptide bound to MHC Class I or II glycoprotein, and Ig receptors can recognize both oligopeptides and conformational determinants on undegraded polypeptides, they both employ heterodimer variable regions (Fabs) utilizing all three CDRs in epitope binding. We conclude that a plausible case can be made for the possibility that rearranged TcR V genes may undergo some type of somatic hypermutation process during T-cell development in the thymus (concurrent with or after the positive selection phase) thus allowing a repertoire of TvR alpha beta heterodimers to be both positively and negatively selected by the same set of ligands (self MHC + self peptide) in the thymus.

Adoptive transfer of murine host protection to salmonellosis with T-cell growth factor-dependent, Salmonella-specific T-cell lines

A spent medium antigen was prepared from the avirulent RIA strain of Salmonella typhimurium. Lymph node cells isolated from female BALB/c mice injected subcutaneously with the spent medium antigen exhibited antigen-specific proliferation. By using these cells and T-cell growth factor, continuous spent medium antigen-specific, Thy 1.2-sensitive lines were generated. These cells exhibited antigen-specific proliferation in vitro and were effective in inducing significant (P less than 0.01) host protection when adoptively transferred to naive syngeneic mice.

Antigen binding to lymphoid cells from unimmunized mice. VII. Restriction of antigen-binding capacity by maturing B lymphocytes

Neonatal spleen populations have been studied for antigen-binding capacity in an attempt to determine if the frequency of double antigen-binding cells changes as the population matures. Just after birth, the frequencies of total ABC, double ABC and Ig-bearing cells were similar to the frequencies seen in the adult bone marrow. During the first week after birth, the proportion of total ABC and Ig-bearing cells rose sharply so that by 8 days after birth, these cells were about half of adult levels. The proportion of total double ABC, after a brief rise at day 1, remained constant throughout the test period. Thus, the proportion of total ABC which are doubles gradually decreases with age. Sedimentation velocity studies indicate that double ABC's tend to co-sediment with cells which require a period of maturation before they can respond to a thymus-independent antigen in irradiated hosts. Single ABC, on the other hand, tend to co-sediment with cells which are immediately responsive to antigen, or require a shorter maturational period before they become responsive to antigen. All of these data, taken together with our other work, suggest the possibility that multiple ABC are clonally-derived lymphocyte populations which become more restricted in their antigen-binding capacity as they mature.

Antigenic modification, rosette-forming cells, and Salmonella typhimurium resistance in outbred and inbred mice

To assess the separate contributions of host T cells and the physical state of the antigen in the development of effective. Salmonella resistance, glutaraldehyde-treated and untreated protein- and ribonucleic acid-rich extracts (E-RNA extracts) of virulent Salmonella typhimurium SR-11 or attenuated S. typhimurium RIA were used to immunize Salmonella-resistant Salmonella-susceptible strains of mice for the purpose of determining whether antigen-specific T-cell or B-cell responses were formed and, if so, which responses predominated. The resistance imparted to each mouse strain after vaccination with S. typhimurium RIA was used as the standard for comparison. The inbred mouse strains C57BL/6 and DBA/2 and their F(1) hybrid (strain BDF(1)), outbred ICR Swiss mice, and endotoxin-resistant C3H/HeJ mice were examined for the capacity to develop resistance to lethal Salmonella infections, as well as the ability to generate antigen-reactive T cells. Only the BDF(1), C3H/HeJ, and ICR Swiss mice were able to develop resistance to challenge infections mediated by the virulent SR-11 strain of S. typhimurium after vaccination with the living, attenuated RIA strain of S. typhimurium or immunization with E-RNA extracts. We developed an assay to identify the antigen-reactive rosette-forming lymphocytes present in lymph nodes and spleens of immunized mice. Levels of 0.2% or higher of theta antigen-bearing, antigen-reactive rosette-forming cells were found in the lymph nodes or spleens or both of only the BDF(1), C3H/HeJ, and ICR Swiss mice (i.e., in the "Salmonella responder" strains). Mouse strains C57BL/6 and DBA/2, which failed to develop resistance to lethal infections after immunization with the S. typhimurium RIA vaccine or with the E-RNA extracts, lacked effective numbers of antitheta antigen-sensitive rosette-forming cells. Modification of the effective E-RNA extracts by polymerization with glutaraldehyde resulted in a marked diminution in their abilities to induce resistance to salmonellosis in the two responder mouse strains tested (BDF(1) and ICR Swiss), even though detectable levels of antibody were induced.

Inheritance of acquired immunological tolerance to foreign histocompatibility antigens in mice

CBA mice were rendered tolerant of major histocompatibility antigens of A/J mice by neonatal injection of 100 x 10(6) lymphoid cells of (CBA X A/J)F1 followed by repeated injections of F1 cells at 2-week intervals throughout the study. When adult (8 weeks old), 10 tolerant or normal CBA males were mated to normal CBA females. Spleen cells of the progeny were tested for their ability to mount a cytotoxic T lymphocyte response in vitro against A/J antigens or against C57BL/6J and B10.A (2R) antigens in a cell-mediated lympholysis (CML) assay. A significant proportion (50-60%) of first-generation offspring of tolerant fathers failed to produce detectable anti-A/J cytotoxic responses but responded in the normal range to stimulation by C57BL/6J or B10.A (2R). Second-generation offspring derived from mating animals born of tolerant male parents--either brother x sister matings (incross or matings to normal CBA mice (outcross)--also showed a high proportion (20-40%) with diminished anti-A/J CML responses when similarly tested in vitro. Thus, a specific acquired somatic characteristic in the immune system (tolerance to major histocompatibility antigens) induced in male mice shows significant transmission to first- and second-generation offspring.

Some limits to post-antigen generation of diversity: failure to detect variants in clones of hapten-specific antibody-forming cells (AFC) developing in culture from direct AFC-progenitor B cells

A search was made for variants in clones of hapten-specific antibody-forming cells (AFC) arising by stimulation of mature B cells with either thymus-independent hapten-POL (polymerized bacterial flagellin) conjugates, or the polyclonal activator lipopolysaccharides. Enriched, hapten-binding B cells or unfractionated spleen cells were cultivated for 3-4 days at limiting dilution in the presence of thymus filler cells, and the AFC in each microculture well were then assayed for plaque formation on various hapten-sheep red cell monolayers. No variants were found from (4-hydroxy-3-iodo-5-nitrophenyl) acetyl (NIP) to 2,4-dinitrophenyl specificity, nor from fluorescein (FLU) to NIP specificity. No variants were found in avidity for FLU hapten. All 374 clones examined, including clones of up to 300 AFC, appeared to be homogeneous in antibody specificity and plaque morphology under our conditions. These results differ from published findings using erythrocytes as antigens. Reasons for this discrepancy are discussed, including differences in sensitivity differences in immunological similarity between the test antigens, and in the particular B cell subsets involved.

Time and antigen dose-dependent variations of IgM antibody affinity

Affinity of IgM antibodies elicited in mice by a single injection of dinitrophenylated dextran was measured by equilibrium dialysis. Maturation of affinity was found to occur with time, the rate of increase being higher after larger antigen doses. Maturation was followed by a decrease of affinity with time, the fall being more pronounced after lower antigen doses. These findings are more in line with the theory of antigen-induced diversity rather than with the maturation theory.

Ontogeny of the antibody-forming cell line in mice. I. Kinetics of appearance of mature B cells

The development of functionally mature B cells in the liver and spleen of fetal and neonatal CBA mice has been followed. A mature B cell is defined as one giving rise to antibody-producing progeny within 2 days in culture. Very few such B cells are found during fetal life. After birth there is a rapid rise in the numbers reaching 1/3 of adult levels in the 7-day-old spleen.

Ontogeny of B cells in the chicken. I. Sequential development of clonal diversity in the bursa

The initial development and distribution of lymphocytes expressing surface IgM (sIgM) and of specific antigen-binding cells (ABC) were studied in the chicken in an attempt to gain information on the process by which B-cell diversity is generated. The antigens used were sheep erythrocytes (SE), keyhole limpet hemocyanin (KLH), and poly-L(Tyr, Glu)poly-D,L-Ala-poly-L-Lys (TGAL). The results indicate that generation of the total sIgM-positive population begins in the bursa and that specific clones of ABC develop in a fixed sequential pattern which is not influenced by either deprivation of or deliberate exposure to exogenous antigens. Cells bearing sIgM by immunofluorescence (IgM-positive cells) were detected first in the bursa on the 12th day of incubation, KLH-ABC and TGAL-ABC by the 16th day, and SE-ABC on the 18th day. The doubling time of the sIgM-positive population of bursal cells was determined to be approximately 10 h before significant antigen-independent seeding to the spleen began a few days before hatching. Clonal expansion of SE-ABC in the bursa also appeared to be antigen independent as was the initial development of SE-ABC in the blood and spleen which ceased abruptly after bursectomy at hatching. Specific ABC were observed to develop in multiple bursal follicles as small foci of ABC among the much larger total population of sIgM-positive cells within an individual follicle. Intravenously infused SE-ABC homed to the embryonic spleen but not to the bursa. The results are interpreted as favoring a hypothetical model in which individual stem cells give rise to multiple clones of B cells by a predetermined pattern of sequential expression of variable region genes.

Sharing of idiotypic specificities between different antibody populations from an individual rabbit

Rabbits hyperimmunized with tobacco mosaic virus synthesize very heterogeneous antibodies. Despite this, specific anti-idiotypic sera recognizing a large part (70%) of these antibodies can be raised in rabbits matched for allotypic specificities a1, a2, a3, b4, b5, b6, c7, and b9. Different rabbits synthesize antibodies with different idiotypic specificities. However, in the serum of a single rabbit antibody fractions of different isoelectric pH share some idiotypic specificities. The results show that, at least in certain cases, antibodies against one antigen are not simply a random collection of immunoglobulin which happen to fit with this antigen, but that some definite relationship exists between the products of different clones which have been activated by antigen. These findings are discussed in the light of network concepts of the immune system.

Antibody response to phosphorylcholine in vitro. I. Studies on the frequency of precursor cells, average clone size and cellular cooperation

The anti-phosphorylcholine (PC) antibodies synthesized by BALB/c spleen cells in microcultures upon immunization with heat-killed vaccine of Pneumococci R36A (Pn) are directed exclusively to the PC epitope. These antibodies are of very restricted avidity and 88% of the responding clones express the idiotype characteristic of the TEPC-15 PC-binding myeloma. This idiotypic restriction appears to be due to the absence of clones capable of expressing other idiotypes, rather than to "clonal dominance". The estimated frequency of precursor cells for the PC epitope is 1 X 10(-5) to 2.5 X 10(-5). These precursors give rise to clones with an average size of 9 plaque-forming cells. When the logarithm of the number of negative wells was plotted against the number of spleen cells/well, the fraction of nonresponding cultures decreased exponentially as the number of spleen cells was increased. This indicated that only one cell type was limiting in our assay, presumably a B cell. Furthermore, treatment of spleen cells with AKR anti serum completely abolished the response to sheep red cells without affecting the response to PC. It is concluded that PC is a T cell-independent antigen. Of interest was the finding that PC requires adherent (A) cells and this is a particular characteristic of PC, since most T cell-independent antigens have been found not to require A cells. Reasons for the possible homogeneity of the response to PC are also discussed.

Fine specificity of the H-2 linked immune response gene for the gallinaceous lysozymes

An immune response (Ir) gene is described which controls the ability of mice to respond to seven very closely related gallinaceous egg white lysozymes (GEL). This Ir-GEL gene locus is linked to the major histocompatibility locus of the mouse and operates at the level of the T cell. Responsiveness to the nonimmunogenic prototype hen egg white lysozyme (HEL) is dominant and is unrelated to age or sex of the animals, or to dose of protein administered. Ninety percent of C57BL/6 mice are absolute nonresponders to the nonimmunogenic lysozymes in complete Freund's adjuvant. The remaining mice exhibit severely restricted responses, with different anti-HEL clonotypes appearing in individual mice. The fine specificity of the Ir-GEL locus is evident in the discrimination of as few as two amino acid differences in a single region of the lysozyme molecule. This very precise distinction determines whether there will, or will not, be any response to the multideterminant molecule.

Generation of antibody diversity. IV. Variation within single clones of antibody-forming cells developing in vivo

Previous experimental work demonstrated that clonal variation occurs in vitro. The present experiments were designed to test for clonal variation in vivo. B cells were transferred at limiting dilution, with antigen, into irradiated recipients. Seven days later spleens were assayed for plaque-forming cell (PFC) colonies. Control experiments showed that these PFC colonies were clones, that is, they were derived from a single B cell precursor. When the clones were analyzed for heterogeneity of the PFC population, using cross-reactivity on various mixtures of red blood cells as a method of detecting differences in antibody specificity, from 23-83% of the clones contained variants. By adjusting the amounts of helper activity and antigen available to a developing clone, we have been able to influence this variation; high levels of help and/or antigen favor pure clones, while low levels of either produce mainly mixed clones.