Marginal zone and B1 B cells unite in the early response against T-independent blood-borne particulate antigens

The rate of pathogen elimination determines the extent and consequences of an infection. In this context, the spleen with its highly specialized lymphoid compartments plays a central role in clearing blood-borne pathogens. Splenic marginal zone B cells (MZ), by virtue of their preactivated state and topographical location, join B1 B cells to generate a massive wave of IgM producing plasmablasts in the initial 3 days of a primary response to particulate bacterial antigens. Because of the intensity and rapidity of this response, combined with the types of antibodies produced, splenic MZ and B1 B cells endowed with a "natural memory" provide a bridge between the very early innate and the later appearing adaptive immune response.

Posttranslational association of immunoglobulin heavy chain binding protein with nascent heavy chains in nonsecreting and secreting hybridomas

A rat monoclonal antibody specific for immunoglobulin (Ig) heavy chain binding protein (BiP) has allowed the examination of the association of BiP with assembling Ig precursors in mouse B lymphocyte-derived cell lines. The anti-BiP monoclonal antibody immunoprecipitates BiP along with noncovalently associated Ig heavy chains. BiP is a component of the endoplasmic reticulum and binds free intracellular heavy chains in nonsecreting pre-B (mu+, L-) cell lines or incompletely assembled Ig precursors in (H+, L+) secreting hybridomas and myelomas. In the absence of light chain synthesis, heavy chains remain associated with BiP and are not secreted. The association of BiP with assembling Ig molecules in secreting hybridomas is transient and is restricted to the incompletely assembled molecules which are found in the endoplasmic reticulum. BiP loses affinity and disassociates with Ig molecules when polymerization with light chain is complete. We propose that the association of BiP with Ig heavy chain precursors is a novel posttranslational processing event occurring in the endoplasmic reticulum. The Ig heavy chains associated with BiP are not efficiently transported from the endoplasmic reticulum to the Golgi apparatus. Therefore, BiP may prevent the premature escape and eventual secretion of incompletely assembled Ig molecules.

Preferential utilization of the most JH-proximal VH gene segments in pre-B-cell lines

The most JH-proximal VH gene segments are used highly preferentially to form VHDJH rearrangements in pre-B-cell lines. This result demonstrates that the rate at which immunoglobulin VH gene segments recombine is influenced by their chromosomal organization, and that the initial repertoire of VH genes expressed in pre-B cells is strikingly different from that seen in mature populations.

Rearrangement of antigen receptor genes is defective in mice with severe combined immune deficiency

A process unique to lymphocyte differentiation is the rearrangement of genes encoding antigen-specific receptors on B and T cells. A mouse mutant (C.B-17scid) with severe combined immune deficiency, i.e., that lacks functional B and T cells, shows no evidence of such gene rearrangements. However, rearrangements were detected in Abelson murine leukemia virus-transformed bone marrow cells and in spontaneous thymic lymphomas from C.B-17scid mice. Most of these rearrangements were abnormal: approximately 80% of Igh rearrangements deleted the entire Jh region, and approximately 60% of TCR beta rearrangements deleted the entire J beta 2 region. The deletions appeared to result from faulty D-to-J recombination. No such abnormal rearrangements were detected in transformed tissues from control mice. The scid mutation may adversely affect the recombinase system catalyzing the assembly of antigen receptor genes in developing B and T lymphocytes.

Marginal zone B cells exhibit unique activation, proliferative and immunoglobulin secretory responses

Mouse splenic B cells can be separated based on their distinctive expression of cell surface antigens. Marginal zone (MZ) B cells are CD38high CD21high CD23low/-, while follicular (FO) B cells are CD21int CD23int and newly formed (NF) B cells are CD21dim/- CD23-. Exploiting these phenotypic distinctions, we isolated the three B cell subsets and assessed their other phenotypic differences and functional capabilities in vitro. FO B cells proliferate more than the other B cell subsets in response to either IgM or CD38 cross-linking. MZ B cells proliferate better than FO B cells when stimulated with lipopolysaccharide (LPS), sub-optimal levels of LPS and CD38 cross-linking or CD40 ligation. When NF, FO and MZ B cells were stimulated with either LPS or LPS and interleukin-4, MZ B cells secreted more IgM and IgG3 than the other two subsets. Similarly, calcium fluxes measured within MZ B cells are larger in amplitude and more sustained after B cell receptor cross-linking than those found in the other two subsets. Collectively, these results indicate that CD38high CD21high CD23low/- MZ B cells are specially suited to play a unique role in response to antigens delivered to the MZ area.

Autoreactivity by design: innate B and T lymphocytes

Innate B and T lymphocytes are a subset of lymphocytes that express a restricted set of semi-invariant, germ-line-encoded, autoreactive antigen receptors. Although they have long been set apart from mainstream immunological thought, they now seem to represent a distinct immune-recognition strategy that targets conserved stress-induced self-structures, rather than variable foreign antigens. Innate lymphocytes regulate a range of infectious, tumour and autoimmune conditions. New studies have shed light on the principles and mechanisms that drive their unique development and function, and show their resemblance to another subset of innate lymphocytes, the natural killer cells.

Developmentally controlled expression of immunoglobulin VH genes

Although antibody diversity arises mainly from apparently random combinatorial and somatic mutational mechanisms acting upon a limited number of germline antibody genes, the antibody repertoire develops in an ordered fashion during mammalian ontogeny. A series of early pre-B and B-lymphocyte cell lines were examined to determine whether an ordered rearrangement of gene families of the variable region of immunoglobulin heavy chains (VH) may be the basis for the programmed development of the antibody response. The results indicated that the VH repertoire of fetal B-lineage cells is largely restricted to the VH 7183 gene family and that subsequent recruitment of additional VH gene families occurs during neonatal development. These results have important implications in understanding the ontogeny of immune function.

B1 cells: similarities and differences with other B cell subsets

Since their discovery, B1 B cells' origins and developmental pathways have eluded characterization. In the past year, focus on B1 B cells has shifted dramatically from developmental to functional aspects of these cells. Most advances have been made in describing the physiological activities of B1 cells, including their migration, activation by antigen and role in both autoimmunity and malignancy.

Assembly and secretion of heavy chains that do not associate posttranslationally with immunoglobulin heavy chain-binding protein

Heavy chain-binding protein (BiP) associates posttranslationally with nascent Ig heavy chains in the endoplasmic reticulum (ER) and remains associated with these heavy chains until they assemble with light chains. The heavy chain-BiP complex can be precipitated by antibody reagents against either component. To identify sites on heavy chain molecules that are important for association with BiP, we have examined 30 mouse myelomas and hybridomas that synthesize Ig heavy chains with well characterized deletions. Mutant Ig heavy chains that lack the CH1 domain could not be demonstrated to associate with BiP, whereas mutant Ig heavy chains with deletions of the CH2 or CH3 domain were still able to associate with BiP. In two light chain negative cell lines that produced heavy chains with deletions of the CH1 domain, free heavy chains were secreted. When Ig assembly and secretion were examined in mutants that did not associate with BiP, and were compared with normal parental lines, it was found that the rate of Ig secretion was increased in the mutant lines and that the Ig molecules were secreted in various stages of assembly. In one mutant line (CH1-) approximately one-third of the secreted Ig molecules were incompletely assembled, whereas the Ig molecules secreted by the parental line were completely assembled. Our data show the CH1 domain to be important for association with BiP and that when this association does not occur, incompletely assembled heavy chains can be secreted. This implies a role for BiP in preventing the transport of unassembled Ig molecules from the ER.

Positive selection from newly formed to marginal zone B cells depends on the rate of clonal production, CD19, and btk

Using immunoglobulin heavy chain transgenic mice, we show that B cell clones reaching the long-lived pool are heterogeneous: some are enriched in the CD21(high) compartment (mostly marginal zone [MZ]), others reside primarily in the follicles (FO). Altering the composition of the B cell receptor through N region additions decreases the rate of clonal production and the MZ enrichment. This process can be recapitulated by purified CD21(low) B cells and is due to a preferential clonal survival that requires a functional btk tyrosine kinase. We also show that generation of the MZ population is dependent on CD19. These findings suggest that the MZ B cell repertoire is positively selected and have functional implications for antigenic responses effected by B cells from this microenvironment.

Naturally occurring anti-idiotypic antibodies in myasthenia gravis patients

Aberrant regulation of the immune system can lead to the development of autoimmune diseases such as myasthenia gravis. Autoantibodies against the nicotinic acetylcholine receptor (AChR) are found in the serum of myasthenia gravis patients and trigger a reduction of AChR at the muscle endplate resulting in increased muscle fatiguability. It is possible that the autoimmune response results from altered idiotype anti-idiotype network interactions. Here we have used a monoclonal anti-AChR antibody (ACR-24, gamma 1, kappa) in an enzyme-linked immunoabsorbent assay (ELISA) to measure anti-AChR immunoglobulin in human sera. In this assay, ACR-24 is attached to microtitre plates followed by the addition of solubilized human AChR which is bound by the immobilized ACR-24. However, during the development of this assay, it was observed that certain myasthenic patients appeared to have antibodies which bound to ACR-24 alone. This unexpected finding suggested that we had discovered naturally occurring anti-idiotype antibodies in myasthenic sera.

Idiotype-directed interactions during ontogeny play a major role in the establishment of the adult B cell repertoire

IgM hybridomas derived from perinatal B cells show a high degree of auto-reactivity and many had demonstrable anti-idiotypic reactivities by binding studies. Selected multispecific antibodies were also shown to have potent idiotype-specific biological activities and if administered at appropriate stages of development could dramatically alter the responses of these mice when challenged with appropriate antigens in adult life. The results obtained suggest that idiotype-directed interactions between neonatal B cells play an important role in the early establishment of the B cell repertoire which is subsequently expressed in adult mice.

Evidence for selection of a population of multi-reactive B cells into the splenic marginal zone

Antibody reactivity to self-antigens is a normal component of the immune system. To study the mechanism by which self-reactive B cells are generated and maintained, we analyzed B cell development in transgenic mice that express a rearranged VH81X heavy chain from the pre-immune repertoire. In these mice, > 95% of B cells express the transgene in association with a variety of kappa light chains but V kappa 1 C being the dominant light chain. These transgenic B cells with identical V kappa 1C-J kappa 5 joins do not normally secrete IgM in vivo, but antibodies derived from these B cells, through LPS activation in vitro or after hybridoma immortalization, are self-reactive and recognize an ubiquitous epitope(s) on intracytoplasmic proteins from different tissues. They have the phenotype and localization pattern of long-lived marginal zone B cells and their development in vivo is blocked by injection of soluble VH81X-V kappa 1CJ kappa 5 IgM antibody. The observations in this transgenic mouse provide evidence for positive selection of a population of self-reactive B cells. These B cells enter the peripheral pool of B cells where they localize in the marginal zone of the spleen and, in contrast to other transgene-expressing B cells, do not secrete IgM antibody.

Functional characterization of monoclonal auto-anti-idiotype antibodies isolated from the early B cell repertoire of BALB/c mice

A large number of hybridomas were constructed by fusion of B cells from perinatal liver and spleen. Many of these showed multispecificity, high interconnectivity and anti-idiotype (Id) activity. Several of these were subjected to a detailed analysis to evaluate their influence on the developing immune system. A hybridoma BD2 (mu,kappa), derived from 2-day-old liver, was shown to have anti-T15 and anti-J558 activity by inhibition enzyme-linked immunosorbent assay and by in vivo administration. BD2 reduced primary T15 and J558 Id in adult BALB/c by 50%. In contrast, timed administration of this antibody during neonatal periods resulted in enhancement of responses to phosphorylcholine (PC) and alpha (1----3)-linked dextran (Dex) when these mice were challenged as adults. Another hybridoma DB3 (mu,kappa), derived from a lipopolysaccharide-stimulated fetal liver, reacts with GB4-10 (anti-T15) and not with PC. It also reacts with BD2. It is thus anti-anti-Id with respect to T15 and J558. Early administration of this antibody also led to an enhancement of anti-PC and anti-Dex responses, apparently via expansion of a set of intermediate anti-Id BD2-like B cells. In adult mice it suppressed responses to both antigens. A third hybridoma FC4 (mu,kappa), derived from 3-day-old spleen, reacts with GB4-10 as well as EB3-7 (anti-J558). Introduction of this antibody into neonatal mice enhanced anti-Dex responses while in adults it caused suppression of T15 Id. The results presented here suggest a possible role for neonatal anti-Id B cells in the primary activation of antigen-reactive B cells by Id selection.

Murine Peyer's patch T cell clones. Characterization of antigen-specific helper T cells for immunoglobulin A responses

We successfully cloned antigen-specific T cells from murine gut-associated lymphoreticular tissue, i.e., Peyer's patches, which are dependent upon T cell growth factor and independent of antigen for continuous growth. These clones exhibit helper activity for IgA responses to sheep erythrocytes (SRBC) and have been designated T helper (Th) A. Two broad categories of Th A clones have been maintained in continuous culture. The first group supports IgM and largely IgA anti-SRBC plaque-forming cell (PFC) responses in both normal and SRBC-primed splenic B cell cultures, whereas the second group supports low IgM, IgG1, and IgG2 and high IgA PFC responses. Subclones derived from single cells maintain the parent helper properties when propagated in culture for long periods (greater than 7 mo). Cloned Th A cells are antigen specific and do not support polyclonal or immune responses to other thymus dependent antigens in normal B cell cultures. Th A cells require full histocompatibility for helper functions because addition of cloned Th A cells to B cell cultures from other H-2 types does not result in IgA responses. Cloned Th A cells are Thy-1.2+ and Lyt-1+ and Lyt-2-, Ig-, and I-A-. Th A cells bear Fc receptors for IgA and do not possess receptors for IgM or IgG isotypes. Thus, T cells that primarily promote IgA isotype responses have been isolated in high frequency from murine PP, an anatomical site of major importance for induction and regulation of the IgA response.

Isotype specificity of helper T cell clones. Peyer's patch Th cells preferentially collaborate with mature IgA B cells for IgA responses

The nature of the IgA B cell precursors that receive preferential help from selected clones of T helper cells from mouse Peyer's patches (PP Th A) were studied. Activation of the PP Th A clones required the presence of antigen, sheep erythrocytes (SRBC), in a culture system supporting development of antibody-secreting plasma cells. Two types of PP Th A cells were used. Both gave vigorous IgA responses; the first also supported low IgM, and the second low IgM and IgG subclass antibody responses. Removal of sIgA+ B cells from either splenic or PP B cell cultures selectively depleted precursors of IgA antibody producers. Cultures of purified sIgA+ B cells, cloned PP Th A cells and SRBC, selectively yielded IgA antibody producers. Finally, PP Th A cells did not support IgA responses in B cell cultures derived from spleens of young mice (days 1-25), and full IgA responses were not seen until the donor mice were 6-7 wk of age. These results suggest that cloned T helper cells can recognize and collaborate with mature, IgA committed B cells.

An embryonic source of Ly1 but not conventional B cells

Ly1+ B cells differ from conventional B cells with respect to their anatomical localization, cell surface marker expression, and antibody repertoire suggesting that they may constitute a functionally distinct subset of B cells. To determine whether Ly1+ B cells also have a developmentally distinct site of origin we grafted various fetal primordia into adult severe combined immunodeficient (scid) mice and analyzed their potential to give rise to T and B cells. We demonstrated that fetal omentum, but not spleen or thymus grafts, reconstituted exclusively Ly1 B cells (including the Ly1 sister population) as well as a population of IgM and IgA producing plasma cells in the spleen and gut, respectively. Although thymus grafts regularly reconstituted T cells, thymus plus fetal omentum cografts gave rise to a population of Ly1+ B cells as well as T cells which were also derived from omentum. However, in neither omentum nor omentum plus thymus cografts were conventional B cells detected. These results provide the first evidence that Ly1 B cells but not conventional B cells are generated from the fetal omentum.

Monoclonal vs. heterogeneous anti-H-8 antibodies in the analysis of the anti-phosphorylcholine response in BALB/c mice

Biological activities of monoclonal A/J antibodies to the T15 idiotype in BALB/c mice were compared to heterogeneous antibodies raised by conventional immunization procedures. Two monoclonal antibodies, AB1-2 and GB4-10, which are of the gamma 1, chi class, appeared to have identical specificities by binding criteria and reacted similarly to conventional antibodies in their abilities to induce neonatal suppression, inhibit plaque-forming cell induction by phosphorylcholine (PC) antigens and to inhibit specifically, anti-PC plaque-forming cells. However, in functional analyses of anti-PC responses in various strains of mice, discrepancies were noted in the T15 responses as defined by monoclonal antibodies and conventional antisera. This heterogeneity was also observed in adult mice suppressed with the GB4-10 monoclonal antibody. These animals eventually produced an anti-PC responses of AB1-2 idiotype but lacking the GB4-10 marker. These results show that the T15 IgM anti-PC response in BALB/c and other strains of mice is heterogeneous and probably consists of a family of clones. Particular clones can be precisely eliminated by the use of appropriate monoclonal antibodies, and the anti-PC response that eventually recovers is still T15+ but lacking the suppressed clones.

Ontogeny of Ia and IgD on IgM-bearing B lymphocytes in mice

We used immunofluorescence to examine the developmental relationship of Ia and IgD on B cells. Pre-B cells in fetal liver did not express Ia. Only very few surface IgM-positive (sIgM+) B cells in fetal spleen were found to be Ia+ and were weakly stained for Ia. After birth there was a linear increase in the proportion of sIgM+ spleen cells which expressed Ia, reaching 95% by 9 days. Adult bone marrow also contains a sizeable proportion of sIgM+ Ia- cells. Unstimulated cells from fetal or newborn liver and spleen expressed Ia at the same rate in culture. Anti-Ia antisera suppressed the LPS-induced differentiation of IgM and IgG plasma cells in cultures of neonatal lymphocytes. Ia was also detected on IgM and IgG plasma cells in vitro suggesting that lipopolysaccharide (LPS)-stimulated B cells by may express Ia antigens, induced by LPS, or appearing as part of normal differentiation. IgD did not appear on sIgM+ cells until 3 days of age and then rose slowly to reach adult levels later than Ia antigens.

Monoclonal antibodies against protease-sensitive pneumococcal antigens can protect mice from fatal infection with Streptococcus pneumoniae

Monoclonal antibodies were raised against surface determinants of Streptococcus pneumoniae by hyperimmunizing X-linked immunodeficient (xid) CBA/N mice with the heat-killed rough strain R36A. 17 hybridomas produced antibody that bound intact R36A and did not cross-react with phosphocholine, an antigen common in the cell wall of all S. pneumoniae. The antibody produced by at least two of these hybridomas, Xi64 (IgM) and Xi126 (IgG2b), could protect mice from a lethal intravenous challenge of type 3 S. pneumoniae strains WU2 and A66 and of the type 2 strain D39. The minimum amount of antibody required to protect xid mice from 100 WU2 was 4.5 micrograms/mouse for Xi64 and 2.6 micrograms/mouse for Xi126,. Free phosphocholine, C-polysaccharide, and type 3 capsular polysaccharide all failed to inhibit the binding of Xi64 or Xi126 to R36A. These antibodies appeared to bind surface polypeptides, since treatment of R36A with either pepsin or trypsin, or of R36A lysate with trypsin, effectively eliminated the ability of Xi64 and Xi126 to bind antigens in these preparations. Binding studies indicated that these two antibodies recognized different epitopes that were expressed on several but not all serotypes of pneumococci.

Inbreeding trends and pedigree analysis of Irish dairy and beef cattle populations

The objective of this study was to determine the inbreeding levels and to analyze the pedigree of Irish purebred populations of Charolais, Limousin, Hereford, Angus, and Simmental beef cattle, as well as the Holstein-Friesian dairy breed. Pedigree analyses included quantifying the depth of known pedigree, average generation intervals, effective population size, the effective number of founders, ancestors, and founder genomes, as well as identifying the most influential animals within the current population of each breed. The annual rate of increase in inbreeding over the past decade was 0.13% (P < 0.001) in the Hereford, 0.06% (P < 0.001) in the Simmental, and 0.10% (P < 0.001) in the Holstein-Friesian breeds. Inbreeding in the other breeds remained relatively constant over the past decade. Herefords had the greatest mean inbreeding in 2004, at 2.19%, whereas Charolais had the lowest, at 0.54%. Over half of each purebred population in 2004 was inbred to some degree; the population with the greatest proportion of animals inbred was the Hereford breed (85%). All 6 breeds displayed a generation interval of approximately 6 yr in recent years. In the pure-bred females born in 2004, the 3 most influential animals contributed between 11% (Limousin) and 24% (Hereford) of the genes. Effective population size was estimated for the Hereford, Simmental, and Holstein-Friesian only, and was 64, 127, and 75, respectively. The effective number of founders varied from 55 (Simmental) to 357 (Charolais), whereas the effective number of ancestors varied from 35 (Simmental and Hereford) to 82 (Limousin). Thus, despite the majority of animals being inbred, the inbreeding level across breeds is low but rising at a slow rate in the Hereford, Simmental, and Holstein-Friesian.

In vivo suppression of perinatal multispecific B cells results in a distortion of the adult B cell repertoire

The isolation of multispecific B cell hybridomas with a variety of anti-idiotype (anti-Id) activities from the lymphoid organs of fetal and neonatal BALB/c mice suggested that the development of the immune system may depend on Id interactions among autologous B cells. In vitro analysis of antibodies secreted by these hybridomas showed extensive sharing of an idiotope defined by the monoclonal antibody FD5-1. Early and timed administration of this antibody during the perinatal period results in a distortion of the phosphorylcholine (PC) and alpha (1----3)dextran (Dex)-specific B cell precursor compartment of the developing repertoire and is reflected by a drastic reduction of antibody responses to these antigens when challenged as adults. These observations provide strong evidence for the involvement of the early appearing multispecific B cells in Id interactions that bring about the uniform development of the normal adult B cell repertoire. Interference with these interactions at critical stages of developmental results in permanent deficiencies in the adult B cell repertoire.

The human fetal omentum: a site of B cell generation

The fetal mouse omentum has been shown to be a source of precursors that exclusively reconstitutes Ly1+ B cells and the closely related Ly1- sister population, but not conventional B cells or T cells. We have extended these studies to compare B cell development in the human fetal omentum, liver, and spleen, and to demonstrate that the pro/pre-B cell compartment (CD24+, sIgM-) is detected in the omentum and liver but not spleen as early as 8 wk of gestation. From 8 to 12 wk of gestation, the proportions of IgM+ cells that were pre-B cells (cIgM+/sIgM-) in the omentum and liver were 53 +/- 15% and 45 +/- 13%, respectively, and IgM+ cells were not detectable in the spleen. After 12 wk, the percentage of pre-B cells was unchanged in the fetal liver (41 +/- 10%) but decreased significantly in the omentum (25 +/- 14%); pre-B cells were now detected in the spleen but at much lower percentages (2 +/- 3%) than either the omentum or liver. The nuclear enzyme, Tdt, was detected in approximately 25% of the CD24+ cells in the omentum and liver during the 8-12-wk time period, however, Tdt+ cells were not detected in the spleen. Approximately 40% of the mature B cells found in the omentum and spleen were CD5+ compared with only 20% in the liver. These results demonstrate that the fetal omentum, like the fetal liver and bone marrow, is a primary site of B cell development.