H-2-restricted T-B cell interactions involved in polyspecific B cell responses mediated by soluble antigen

Seroreactivity of the Severe Acute Respiratory Syndrome Coronavirus 2 Recombinant S Protein, Receptor-Binding Domain, and Its Receptor-Binding Motif in COVID-19 Patients and Their Cross-Reactivity With Pre-COVID-19 Samples From Malaria-Endemic Areas

Despite the global interest and the unprecedented number of scientific studies triggered by the COVID-19 pandemic, few data are available from developing and low-income countries. In these regions, communities live under the threat of various transmissible diseases aside from COVID-19, including malaria. This study aims to determine the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) seroreactivity of antibodies from COVID-19 and pre-COVID-19 samples of individuals in Mali (West Africa). Blood samples from COVID-19 patients (n = 266) at Bamako Dermatology Hospital (HDB) and pre-COVID-19 donors (n = 283) from a previous malaria survey conducted in Dangassa village were tested by ELISA to assess IgG antibodies specific to the full-length spike (S) protein, the receptor-binding domain (RBD), and the receptor-binding motif (RBM436-507). Study participants were categorized by age, gender, treatment duration for COVID-19, and comorbidities. In addition, the cross-seroreactivity of samples from pre-COVID-19, malaria-positive patients against the three antigens was assessed. Recognition of the SARS-CoV-2 proteins by sera from COVID-19 patients was 80.5% for S, 71.1% for RBD, and 31.9% for RBM (p < 0.001). While antibody responses to S and RBD tended to be age-dependent, responses to RBM were not. Responses were not gender-dependent for any of the antigens. Higher antibody levels to S, RBD, and RBM at hospital entry were associated with shorter treatment durations, particularly for RBD (p < 0.01). In contrast, higher body weights negatively influenced the anti-S antibody response, and asthma and diabetes weakened the anti-RBM antibody responses. Although lower, a significant cross-reactive antibody response to S (21.9%), RBD (6.7%), and RBM (8.8%) was detected in the pre-COVID-19 and malaria samples. Cross-reactive antibody responses to RBM were mostly associated (p < 0.01) with the absence of current Plasmodium falciparum infection, warranting further study.

Sites that direct nuclear compartmentalization are near the 5' end of the mouse immunoglobulin heavy-chain locus

VDJ rearrangement in the mouse immunoglobulin heavy chain (Igh) locus involves a combination of events, including a large change in its nuclear compartmentalization. Prior to rearrangement, Igh moves from its default peripheral location near the nuclear envelope to an interior compartment, and after rearrangement it returns to the periphery. To identify any sites in Igh responsible for its association with the periphery, we systematically analyzed the nuclear positions of the Igh locus in mouse non-B- and B-cell lines and, importantly, in primary splenic lipopolysaccharide-stimulated B cells and plasmablasts. We found that a broad approximately 1-Mb region in the 5' half of the variable-gene region heavy-chain (Vh) locus regularly colocalizes with the nuclear lamina. The 3' half of the Vh gene region is less frequently colocalized with the periphery, while sequences flanking the Vh gene region are infrequently so. Importantly, in plasmacytomas, VDJ rearrangements that delete most of the Vh locus, including part of the 5' half of the Vh gene region, result in loss of peripheral compartmentalization, while deletion of only the proximal half of the Vh gene region does not. In addition, when Igh-Myc translocations move the Vh genes to a new chromosome, the distal Vh gene region is still associated with the nuclear periphery. Thus, the Igh region that interacts with the nuclear periphery is localized but is likely comprised of multiple sites that are distributed over approximately 1 Mb in the 5' half of the Vh gene region. This 5' Vh gene region that produces peripheral compartmentalization is the same region that is distinguished by requirements for interleukin-7, Pax5, and Ezh2 for rearrangement of the Vh genes.

Histone hypomethylation is an indicator of epigenetic plasticity in quiescent lymphocytes

Post-translational modifications of histone amino termini are thought to convey epigenetic information that extends the coding potential of DNA. In particular, histone lysine methylation has been implicated in conveying transcriptional memory and maintaining lineage fidelity. Here an analysis of histone lysine methylation in quiescent (G(0)) and cycling lymphocytes showed that methylation of histone H3 at lysines 4 (H3K4), 9 (H3K9), 27 (H3K27) and histone H4 at lysine 20 is markedly reduced in resting B lymphocytes as compared with cycling cells. Quiescent B cells also lacked heterochromatin-associated HP1beta and Ikaros at pericentric chromatin and expressed low levels of Ezh2 and ESET histone methyl transferases (HMTases). Nuclei from resting B or T cells were approximately three times more efficiently reprogrammed in nuclear transfer assays than cells in which HMTase expression, histone methylation and HP1beta binding had been restored following mitotic stimulation. These results showing local and global changes in histone lysine methylation levels in vivo demonstrate that constitutive heterochromatin organization is modified in resting lymphocytes and suggest that histone hypomethylation is a useful indicator of epigenetic plasticity.

Nonequivalent nuclear location of immunoglobulin alleles in B lymphocytes

Individual B lymphocytes normally express immunoglobulin (Ig) proteins derived from single Ig heavy chain (H) and light chain (L) alleles. Allelic exclusion ensures monoallelic expression of Ig genes by each B cell to maintain single receptor specificity. Here we provide evidence that at later stages of B cell development, additional mechanisms may contribute to prioritizing expression of single IgH and IgL alleles. Fluorescent in situ hybridization analysis of primary splenic B cells isolated from normal and genetically manipulated mice showed that endogenous IgH, kappa and lambda alleles localized to different subnuclear environments after activation and had differential expression patterns. However, this differential recruitment and expression of Ig alleles was not typically seen among transformed B cell lines. These data raise the possibility that epigenetic factors help maintain the monoallelic expression of Ig.

Soluble CD14 enriched in colostrum and milk induces B cell growth and differentiation

Induction of resting B cell growth and differentiation requires a complex series of temporally coordinated signals that are initiated on contact with activated helper T cells. These signals complement one another, each rendering the B cell susceptible to factors supporting progressive activation. Here, we demonstrate that soluble CD14 (sCD14) bypasses the physiological sequelae of events that limit B cell activation. B cell growth and differentiation in vitro is induced by both native and recombinant forms of sCD14 at nanomolar concentrations. sCD14-mediated cellular activation does not require membrane CD14 expression, depends on a region of CD14 that is not involved in lipopolysaccharide binding, and requires functional Toll-like receptor 4. Consistent with biological activity of sCD14 in vitro, its administration to neonatal mice enhances Ig secretion. The results presented establish sCD14 as a naturally occurring soluble B cell mitogen of mammalian origin.

Dynamic repositioning of genes in the nucleus of lymphocytes preparing for cell division

We show that several transcriptionally inactive genes localize to centromeric heterochromatin in the nucleus of cycling but not quiescent (noncycling) primary B lymphocytes. In quiescent cells, centromeric repositioning of inactive loci was induced after mitogenic stimulation. A dynamic repositioning of selected genes was also observed in developing T cells. Rag and TdT loci were shown to relocate to centromeric domains following heritable gene silencing in primary CD4+8+ thymocytes, but not in a phenotypically similar cell line in which silencing occurred but was not heritable. Collectively, these data indicate that the spatial organization of genes in cycling and noncycling lymphocytes is different and that locus repositioning may be a feature of heritable gene silencing.

The B-cell transmembrane protein CD72 binds to and is an in vivo substrate of the protein tyrosine phosphatase SHP-1

BACKGROUND

Signals from the B-cell antigen receptor (BCR) help to determine B-cell fate, directing either proliferation, differentiation, or growth arrest/apoptosis. The protein tyrosine phosphatase SHP-1 is known to regulate the strength of BCR signaling. Although the B-cell co-receptor CD22 binds SHP-1, B cells in CD22-deficient mice are much less severely affected than those in SHP-1-deficient mice, suggesting that SHP-1 may also regulate B-cell signaling by affecting other signaling molecules. Moreover, direct substrates of SHP-1 have not been identified in any B-cell signaling pathway.

RESULTS

We identified the B-cell transmembrane protein CD72 as a new SHP-1 binding protein and as an in vivo substrate of SHP-1 in B cells. We also defined the binding sites for SHP-1 and the adaptor protein Grb2 on CD72. Tyrosine phosphorylation of CD72 correlated strongly with BCR-induced growth arrest/apoptosis in B-cell lines and in primary B cells. Preligation of CD72 attenuated BCR-induced growth arrest/death signals in immature and mature B cells or B-cell lines, whereas preligation of CD22 enhanced BCR-induced growth arrest/apoptosis.

CONCLUSIONS

We have identified CD72 as the first clear in vivo substrate of SHP-1 in B cells. Our results suggest that tyrosine-phosphorylated CD72 may transmit signals for BCR-induced apoptosis. By dephosphorylation CD72. SHP-1 may have a positive role in B-cell signaling. These results have potentially important implications for the involvement of CD72 and SHP-1 in B-cell development and autoimmunity.

CD22 is a negative regulator of B-cell receptor signalling

BACKGROUND

. Antibody responses are triggered by binding of antigen to the B-cell antigen receptor (BCR). The strength of the resulting signal determines the outcome of the response, which may vary from the induction of tolerance to the antigen, to the production of specific high-affinity antibodies. Additional cell-surface proteins assist the BCR in its function, and can facilitate or inhibit an antibody response. CD22 is a BCR-associated transmembrane protein, the cytoplasmic tail of which contains three immunoreceptor tyrosine-based inhibitory motifs. These motifs are phosphorylated upon BCR-crosslinking, and can bind the tyrosine phosphatase SHP-1, a putative negative regulator of signalling from the BCR. In order to assess the role of CD22 in vivo, we have generated CD22(-/-) mice by targeted gene inactivation.

RESULTS

. In CD22(-/-) mice, B-cell development is normal. There are normal numbers of peripheral B cells, but these have a more mature phenotype. In addition, recirculating B cells are absent from the bone marrow. However, the distribution of the two B-cell subtypes, B-1 and B-2, is normal. After BCR-crosslinking in vitro, splenic CD22(-/-) B cells show an increased Ca2+ influx and a lower survival due to an increased induction of apoptosis. In contrast, there is an increased proliferative response to the B-cell mitogen lipopolysaccharide (LPS). A shorter average lifespan in the B-cell compartment is also found in vivo. Furthermore, T-cell independent immune responses are impaired, whereas T-cell dependent responses are normal.

CONCLUSIONS

. The absence of CD22 expression lowers the signalling threshold for BCR-crosslinking and can thus influence the fate of the B cell. We propose that the low threshold leads to hyperresponsiveness of the B cells and a chronic basal activation. In this model, engagement of the receptor without T-cell help leads to an increased induction of apoptosis, thus explaining the shorter lifespan of CD22(-/-) B cells and the low response to T-cell independent antigens. The alteration in B-cell phenotype and the higher levels of LPS-reactivity are attributable to the chronic basal stimulation.

Soluble CD40 ligand induces expression of CD25 and CD23 in resting human tonsillar B lymphocytes

In this report, we describe the dose-dependent increase in both CD25 and CD23 levels on resting human B cells in response to CD40 ligation, as mediated by soluble CD40 ligand (sCD40L) or anti-CD40 antibody. In combination with interleukin (IL)-4, sCD40L had limited additive effects on CD25 expression, but significantly enhanced CD23 expression on tonsillar B cells. Interferon-gamma (IFN-gamma) exerted no inhibitory effect upon increases in CD25 or CD23 driven by CD40 ligation with sCD40L or anti-CD40 antibody. These data suggest that the induction of CD25 and CD23 genes by IL-4 is mediated, at least in part, by an IFN-gamma-sensitive component, whereas gene activation driven via CD40 ligation involves signaling pathways which are not sensitive to IFN-gamma.

Follicular dendritic cells help resting B cells to become effective antigen-presenting cells: induction of B7/BB1 and upregulation of major histocompatibility complex class II molecules

This study was designed to investigate whether follicular dendritic cells (FDC) can activate B cells to a state in which they can function as effective antigen-presenting cells (APC). High buoyant density (i.e., resting) B cells specific for 2,4-dinitro-fluorobenzene (DNP) were incubated with DNP-ovalbumin (OVA) bearing FDC, after which their capacity to process and present to an OVA-specific T cell clone was assessed. The efficacies of alternative sources of antigen and activation signals in the induction of B cell APC function were compared with those provided by FDC. Only FDC and Sepharose beads coated with anti-immunoglobulin (Ig)kappa monoclonal antibody provided the necessary stimulus. FDC carrying inappropriate antigens also induced B cell APC function in the presence of exogenous DNP-OVA. However, in circumstances where soluble DNP-OVA was limiting, FDC bearing complexes containing DNP, which could crosslink B cell Ig receptors, induced the most potent APC function. Analysis by flow cytometry revealed that within 24 h of coculture with FDC, a significant percentage of B cells increased in size and expressed higher levels of major histocompatibility complex class II. By 48 h, an upregulation of the costimulatory molecule, B7/BB1, occurred, but only when exposed to the FDC bearing DNP. Taken together, the results demonstrate that FDC have the capacity to activate resting B cells to a state in which they can function as APC for T cells. The stimuli that FDC provide may include: (a) an antigen-dependent signal that influences the upregulation of B7/BB1; and (b) possibly a signal independent of crosslinking mIg that results in Ig internalization. The relevance of these findings to the formation of germinal centers and maintenance of the humoral response is discussed.

Differential regulation of actin polymerization following activation of resting T lymphocytes from young and aged mice

Actin polymerization accompanies receptor-mediated responses and is correlated with motility-related events. In T lymphocytes, there is a lateral redistribution of surface receptors into caps and aggregation of actin-myosin in cytoplasmic subcaps, and these are impaired in T cells from aged individuals. This study documents marked changes in age-related cytoskeletal actin filament function which may account for the reduced motility. Basal levels of filamentous actin (F-actin) are significantly higher in purified G(o) T cells from aged C57BL/6 mice, due to a preferential increase in the CD8+ subpopulation. Following activation of the resting T cells with Concanavalin A (Con A), F-actin depolymerized in cells from young mice for 2 min, followed by rapid polymerization, reaching a plateau 200% above resting levels. In cells from 15-17-month-old mice, an attenuated depolymerization phase was seen for 45 sec, followed by little polymerization. No depolymerization or polymerization phases occurred in cells from aged mice. Phorbol 12 myristate 13-acetate (PMA), which activates protein kinase C (PKC), bypassing receptor mediated signals, induced actin polymerization to 57% of the levels of that after Con A stimulation in cells from both young and old animals and partially eliminated the differences in actin filament assembly due to age. Perturbation of the cytoskeleton with cytochalasin E (CE) potentiated proliferation of Con A-stimulated T cells from aged mice but did not completely restore the deficit attributed to immunosenescence. The results show an age-related impairment of cytoskeletal functions and suggest that differences in early signal transduction events contribute to the decrements in surface receptor motility and subsequent proliferation of T lymphocytes from older individuals.

The induction of resting B cell differentiation does not require T cell contact

A T cell clone as well as immediately ex vivo CD4+ lymph node T cells are shown to support the differentiation of co-cultured resting B cells in the absence of T cell-B cell contact. Antibodies specific for class II products of the major histocompatibility complex inhibit the transactivation of resting but not activated B cells. This differential inhibition pattern indicates that the responses obtained from resting B cell populations are not due to their contamination with B cell blasts. Further, supernatants prepared from an activated T cell clone induce resting B cell differentiation. Two lines of evidence suggest that the activity contained in these supernatants can be attributed to interleukin (IL)-5. Activity is neutralized by monoclonal anti-IL-5; and both recombinant and affinity-purified IL-5 induce the differentiation of immediately ex vivo resting and activated B cells with comparable efficiency. Taken together, these results demonstrate that contact with T cells does not provide prerequisite signals for the induction of resting B cell differentiation.

IgM antigen receptor complex contains phosphoprotein products of B29 and mb-1 genes

Membrane immunoglobulin M (mIgM) and mIgD are major B-lymphocyte antigen receptors, which function by internalizing antigens for processing and presentation to T cells and by transducing essential signals for proliferation and differentiation. Although ligation of mIgM or mIgD results in rapid activation of a phospholipase C and a tyrosine kinase(s), these receptors have cytoplasmic tails of only three amino acid residues (Lys-Val-Lys), which seem ill suited for direct physical coupling with cytoplasmic signal transduction structures. In this report, we identify the alpha, beta, and gamma components of the mIgM-associated phosphoprotein complex, which may play a role in signal transduction. Proteolytic peptide mapping demonstrated that the IgM-alpha chain differs from Ig-beta and Ig-gamma. The chains were purified, and amino-terminal sequencing revealed identity with two previously cloned B-cell-specific genes. One component, IgM-alpha, is a product of the mb-1 gene, and the two additional components, Ig-beta and Ig-gamma, are products of the B29 gene. Immunoblotting analysis using rabbit antibodies prepared against predicted peptide sequences of each gene product confirmed the identification of these mIgM-associated proteins. The deduced sequence indicates that these receptor subunits lack inherent protein kinase domains but include common tyrosine-containing sequence motifs, which are likely sites of induced tyrosine phosphorylation.

Strong T cell tolerance in parent----F1 bone marrow chimeras prepared with supralethal irradiation. Evidence for clonal deletion and anergy

T cell tolerance induction was examined in long-term H-2-heterozygous parent----F1 chimeras prepared with supralethal irradiation (1,300 rad). Although these chimeras appeared to be devoid of host-type APC, the donor T cells developing in the chimeras showed marked tolerance to host-type H-2 determinants. Tolerance to the host appeared to be virtually complete in four assay systems: (a) primary mixed lymphocyte reactions (MLR) of purified lymph node (LN) CD8+ cells (+/- IL-2); (b) primary MLR of CD4+ (CD8-) thymocytes; (c) skin graft rejection; and (d) induction of lethal graft-vs.-host disease by CD4+ cells. Similar tolerance was observed in chimeras given double irradiation. The only assay in which the chimera T cells failed to show near-total tolerance to the host was the primary MLR of post-thymic CD4+ cells. In this assay, LN CD4+ cells regularly gave a significant antihost MLR. The magnitude of this response was two- to fourfold less than the response of normal parental strain CD4+ cells and, in I-E(-)----I-E+ chimeras, was paralleled by approximately 70% deletion of V beta 11+ cells. Since marked tolerance was evident at the level of mature thymocytes, tolerance induction in the chimeras presumably occurred in the thymus itself. The failure to detect host APC in the thymus implies that tolerance reflected contact with thymic epithelial cells (and/or other non-BM-derived cells in the thymus). To account for the residual host reactivity of LN CD4+ cells and the incomplete deletion of V beta 11+ cells, it is suggested that T cell contact with thymic epithelial cells induced clonal deletion of most of the host-reactive T cells but spared a proportion of these cells (possibly low affinity cells). Since these latter cells appeared to be functionally inert in the thymus (in contrast to LN), we suggest that the thymic epithelial cells induced a temporary form of anergy in the remaining host-reactive thymocytes. This anergic state disappeared when the T cells left the thymus and reached LN.

STB1, a mouse lymphocyte marker found on T cell and B cell subpopulations

A set of monoclonal antibodies has been raised to cell surface markers which are either hyperexpressed, or exclusive to activated murine lymphocytes. One antigen is present on all B cells of the Ly1 lineage and defines a novel subpopulation of classical B cells, most of which are activated B cells. In Western blots, this antigen appears as a single glycoprotein of 95 kDa molecular weight but of greater interest, is its expression at high levels on all mature peripheral T cells but on only 30% of thymocytes. This marker is expressed at high levels on all CD4 and CD8 thymocytes, most double-negative (CD4-CD8-) thymocytes, and at low levels on double-positive (CD4+CD8+) thymocytes. It may prove useful as a developmental marker for subdividing the double-positive (CD4+CD8+) and double negative (CD4-CD8-) thymocyte populations. Taken together these results indicate that mab 3.17 recognises a novel lymphocyte antigen expressed on subsets of T and B cells, which is hyperexpressed on lymphocyte activation.

Interleukin 5 regulation of peritoneal B-cell proliferation and antibody secretion

The influence of recombinant interleukin 5 (rIL-5) on murine peritoneal B-cell proliferation and antibody secretion was examined. Larger, low buoyant density peritoneal B cells proliferated better with rIL-5 than the smaller resting B cells. this was also true for splenic B cells; however, comparison of the respective populations showed the large peritoneal B-cell responses to be superior. Limiting dilution analyses showed that from 25% to about 40% of large peritoneal B cells proliferated in response to rIL-5 when lipopolysaccharide (LPS) was present. No detectable difference in the fraction of proliferating splenic B cells was seen in the presence of rIL-5. These results are consistent with expression of IL-5 receptors on about 70% of low-density peritoneal B cells as determined by fluorescent staining with anti-Il-5 receptor monoclonal antibody (MoAb). IL-5 also enhanced spontaneous and mitogen-driven IgM secretion by both peritoneal and splenic B lymphocytes; the increases exhibited by peritoneal B cells, however, were at least twice those exhibited by splenic B cells. Spontaneous and mitogen-driven secretion of auto-antibodies to bromelain-treated mouse erythrocytes (BrMRBC) by peritoneal B cells were also increased by this interleukin. Furthermore, rIL-5 enhanced peritoneal B-cell plaque-forming cell (PFC) responses to TNP-LPS but not to TNP-Ficoll. Both an anti-IL-5R MoAb and an anti-IL-5 MoAb blocked the rIL-5-induced enhancement of proliferation and auto-antibody PFC responses. Hence, IL-5 appears to be important for the regulation of proliferation and antibody secretion by many murine peritoneal B cells.

Interleukin 5 regulation of peritoneal Ly-1 B lymphocyte proliferation, differentiation and autoantibody secretion

The in vitro effects of recombinant interleukin (IL) 5 on proliferation and maturation of mouse Ly-1 B cells were studied. Most freshly isolated peritoneal Ly-1 B cells expressed high levels of IL5 receptor (R). Limiting dilution analyses showed that mitogens could reveal IL5 responsiveness in more than half of low density peritoneal Ly-1 B cells. IL 5 was able not only to increase the proportion of these Ly-1 B cells induced to proliferate, but it also shifted the clone size distribution of proliferating cells towards larger clone sizes. Splenic Ly-1 B cells also proliferated in response to mitogens plus IL5. Spontaneous and polyclonal activator-induced plaque-forming cell responses of Ly-1 B cells were increased by IL5. Furthermore, IL5 increased the frequency of peritoneal Ly-1 B cells induced to secrete certain autoantibodies. IL5 was certainly the agent responsible since its effects on both proliferation and differentiation were inhibited by either anti-IL5R monoclonal antibodies or by anti-IL5 monoclonal antibodies. Hence, Ly-1 B cells, IL5 and the IL5R appear to constitute a system of cellular proliferation, differentiation and some autoantibody production. Strategies specifically targeting the interleukin and receptor elements of this system might afford external control of these cellular responses.

Immunoglobulin-specific T-B cell interaction. I. Presentation of self immunoglobulin determinants by B lymphocytes

Immunoglobulin (Ig)-specific T-B cell interactions have been studied in the model of T cell recognition of the kappa chain Ig kappa-1b allotype in Ig kappa-1-congeneic rat strains. An efficient presentation of endogenous Ig allotypic determinants by irradiated spleen cells from (WAG.1b x August)F1 (RT-1u/c; Ig kappa-1b/1a) rats to Ig kappa-1b-specific lymph node T cells from Ig kappa-1-congeneic (WAG x August)F1 (RT-1u/c; Ig kappa-1a) rats was demonstrated. This presentation was found to be sensitive to high irradiation doses (greater than 1000 rad). By fractionation of Ig kappa-1b+ F1 spleen cells on Percoll density gradient we have shown that a radioresistant, low-density fraction, consisting mainly of macrophages (M phi) and dendritic cells, triggers only weak Ig kappa-1b-specific T cell response. The high level of response was observed against radiosensitive spleen cell fractions of intermediate and high density, suggesting that B cells were the main antigen-presenting cells (APC) of Ig kappa-1b determinants of endogenous Ig. This conclusion was confirmed in the experiments using purified B cells from Ig kappa-1b-bearing rats. Earlier we have shown that the responsiveness of August (RT-1c; Ig kappa-1a) and WAG (RT-1u; Ig kappa-1a) rats to Ig kappa-1b in vivo is controlled by the dominant allele of an RT-1-linked Ir gene. August and (August X WAG)F1 rats were found to be responders to Ig kappa-1b while WAG rats were nonresponders. The same pattern of Ir gene-controlled reactivity was demonstrated using an Ig kappa-1b-specific T cell proliferation assay. Ig kappa-1b-specific F1 T cell response was only observed when Ig kappa-1b+ B cells or IgG (Ig kappa-1b)-pulsed M phi-bearing responder major histocompatibility complex (MHC) haplotype were used as the APC. Anti-RT-1 monoclonal antibody inhibition studies suggested that the RT-1Bc molecule is the main restricting element of T cell recognition of Ig kappa-1b+ B cell as well as exogenous IgG (Ig kappa-1b). We have demonstrated allelic exclusion of Ig kappa-1b presenting function by negatively and positively selecting for Ig kappa-1b+ and Ig kappa-1a+ B cells from heterozygous F1(Ig kappa-1b/1a) rats. This clearly indicate that the B cells presented exclusively Ig kappa-1b allotypic determinants of their own Ig.(ABSTRACT TRUNCATED AT 400 WORDS)

B cell activation by synthetic lipopeptide analogues of bacterial lipoprotein bypassing phosphatidylinositol metabolism and proteinkinase C translocation

Synthetic analogues of bacterial lipoprotein induce proliferation of murine small resting B lymphocytes. We investigated the role of proteinkinase C (PKC) activation in lipopeptide-induced B cell stimulation. Using a standardized extraction procedure, there was no change in membrane bound and soluble PKC activity upon stimulation with lipopeptide. However, omitting Ca2+ chelators from the standard extraction medium resulted in a decrease of membrane bound PKC activity after stimulation. Lipopeptide failed to induce phosphoinositide degradation and the generation of the two second messengers cAMP and cGMP. To test whether guanosinetriphosphate-binding proteins are involved in lipopeptide-induced signal transfer we investigated the effect of LiCl, choleratoxin and pertussistoxin on B lymphocyte proliferation. LiCl and pertussistoxin did not inhibit cell activation, whereas choleratoxin reduced the proliferation rate at concentrations higher than 0.5 micrograms/ml. Similar results were observed when LPS was used as mitogen, whereas the anti-immunoglobulin-induced B cell activation was inhibited by all three compounds. Our results show, that B cell activation by bacterial lipopeptides bypasses phosphatidylinositol metabolism and PKC translocation.

Differential activation of cytokine genes in normal CD4-bearing T cells is stimulus dependent

Studies of cloned CD4+ T cell lines have shown that they can be separated into two distinct subsets with distinctions in their functional capabilities and by the differential release of either interleukin 2 (IL 2) (TH1/inflammatory type) or IL 4 (TH2/helper type) upon activation. To establish if in vivo-derived CD4+ T cells can exhibit distinct subsets we have investigated whether normal CD4+ T cells demonstrate differential expression of IL 2 and IL 4 mRNA, and secretion of IL 2 and IL 4 after primary stimulation in vitro. Utilizing the technique of in situ hybridization IL 2 and IL 4 gene expression in individual CD4+ T cells was readily detectable after concanavalin A (Con A) phytohemagglutinin (PHA) or pokeweed mitogen (PWM)-mediated activation. The frequencies of activated T cells producing IL 2 and IL 4 mRNA after Con A or PHA activation were approximately equivalent (30-40% of cells); however, after PWM activation the number of CD4+ T cells expressing IL 4 mRNA (78%) was more than twofold greater than the number of cells producing IL 2 mRNA (30%). Maximal levels of IL 2 gene expression occurred 24 h after mitogen activation whereas the highest levels of IL 4 mRNA were not detected until 48 h after mitogen activation. Similar distinctions in the kinetics of IL 2 and IL 4 secretion after mitogen activation were also found demonstrating good concordance in the observed expression of IL 2 and IL 4 mRNA and the levels of secreted lymphokines detected by bioassay. Most importantly, we have shown by in situ hybridization analysis that the majority of individual CD4+ T cells produce only IL 2 or IL 4 mRNA, and not both, after primary activation in vitro. By contrast, most CD4+ T cells activated in the presence of PMA and ionophore express both IL 2 and IL 4 mRNA. Our studies demonstrate that in normal, non-clonal populations of CD4+ T cells, the production of IL 2 and IL 4 is independently regulated in the majority of cells and appears to be stimulus dependent.

IL-4, but not IL-5, can act synergistically with B cell activating factor (BCAF) to induce proliferation of resting B cells

B cell activating factor (BCAF) was initially identified in the supernatant of the murine T helper cell clone 52-3 (52-3 SN) because of its ability to promote activation and proliferation of resting B cells in the absence of any other costimulus. In this paper, we show that 52-3 T helper cells also secrete IL-4 and IL-5 and we have analyzed the influence of these two lymphokines on B cell proliferation induced by BCAF-containing 52-3 SN. Using the neutralizing anti-IL-4 monoclonal antibody 11B11, we observed partial inhibition of B cell proliferation. 52-3 SN free of IL-4 prepared using an immunoabsorbent column was still able to induce significant B cell proliferation. Although recombinant IL-4 alone does not induce B cell proliferation, it increased the proliferation induced by IL-4-free 52-3 SN. Kinetic studies showed that IL-4 is required at the start of B cell cultures in order to exert optimal synergistic effects. In contrast, anti-IL-5 monoclonal antibody NC17 did not affect the B cell proliferative activity of 52-3 SN whether or not IL-4 was present. When 52-3 SN was tested on dextran-sulfate-activated B cells, IL-5 and BCAF activities were detected but only the IL-5 activity was neutralized by monoclonal antibody NC17. These results demonstrate that (i) BCAF-containing SN can induce proliferation of resting B cells independently of IL-4 and IL-5, and (ii) IL-4, but not IL-5, can act synergistically with BCAF to induce B cell proliferation.

Ligand-induced desensitization of B-cell membrane immunoglobulin-mediated Ca2+ mobilization and protein kinase C translocation

Binding of ligand to B-cell membrane immunoglobulin (mIg) can lead to activation of a number of distinct biologic responses, including altered expression of genes encoding c-fos, c-myc, and Ia, as well as proliferation and immunologic tolerance. Tolerance could reflect a functional uncoupling of receptors from systems that generate intracellular second messengers (i.e., receptor desensitization). To better understand the molecular basis of immune regulation, we examined the ability of mIg to function as a signal transducer after the cell's initial contact with mIg-binding ligand. The results show that ligand binding to as little as 2-10% of mIgM or mIgD renders the cell unresponsive to ligand binding to the reciprocal isotype as judged by Ca2+ mobilization and protein kinase C translocation responses. This heterologous receptor desensitization lasts longer than 24 hr and does not reflect loss of receptor from the cell surface. Studies with the calcium ionophore ionomycin, 1,2-dioctanoyl-sn-glycerol, and the protein kinase inhibitor staurosporine indicate that both protein kinase C-dependent and protein kinase C-independent (staurosporine-insensitive) mechanisms mediate heterologous desensitization after mIg crosslinking.

MHC class I antigens as surface markers of adult erythrocytes during the metamorphosis of Xenopus

An alloantiserum produced against Xenopus MHC class I antigens has been used to distinguish different erythrocyte populations at metamorphosis. By analysis using a fluorescence-activated cell sorter (FACS) analyzer, tadpole (stage 55) and adult erythrocytes have distinct volume differences and tadpole cells have no MHC antigens on the cell surface. Both tadpole and adult erythrocytes express a "mature erythrocyte" antigen marker, recognized by its monoclonal antibody (F1F6). During metamorphosis, immature erythrocytes, at various stages of differentiation, which express adult levels of cell-surface MHC antigens by 12 days after tail resorption, are found in the bloodstream. These immature cells are biosynthetically active, produce adult hemoglobin, and mature by 60 days after the completion of metamorphosis. Percoll gradient-density fractionation has shown that all of the cells in the new erythrocyte series express adult levels of MHC antigens but there is only a gradual increase in the amount of "mature erythrocyte" antigen. Tadpole erythrocytes, which are biosynthetically active during larval stages, produce small amounts of surface MHC antigens before the metamorphic climax and then become metabolically inactive. They are completely cleared from the circulation by 60 days after metamorphosis. Erythrocytes from tadpoles arrested in their development for long periods of time express intermediate levels of MHC antigens, suggesting a "leaky" expression of these molecules in the tadpole cells. The most abundant erythrocyte cell-surface proteins from tadpoles and adults, as judged by two-dimensional gel electrophoresis, are very different.

Biochemical characterization and biological effects of partially purified B cell-activating factor (BCAF)

B cell-activating factor (BCAF) has been characterized and partially purified from the supernatant of the murine tumor T cell line 373. BCAF has an apparent molecular mass of 15 to 20 kDa when analyzed by Superose 12 fast protein liquid chromatography (FPLC) gel filtration and a pI of 4.5 to 5.0 when analyzed by FPLC chromatofocusing. Concentrated supernatant was applied to a nickel chelating column and unadsorbed active material was further purified by two sequential C4 reverse-phase high-performance liquid chromatography steps. This purification procedure allowed a complete separation of interleukin 2 and interleukin 4 activities from BCAF. This partially purified BCAF induces Ia expression, cell size increase and proliferation of small resting B cells. Furthermore, we have shown that the activity of partially purified BCAF is insensitive to treatment by monoclonal antibodies specific for interleukin 4 and interleukin 5.

Inhibitory effect of anti-class II antibodies on human B-cell activation

The role of class II antigens for B-cell activation was analyzed using purified human B cells and anti-class II monoclonal antibodies. The stimulation of purified B cells with Staphylococcus aureus Cowan I induced proliferation and differentiation into immunoglobulin-producing cells in the presence of interleukin-1 and T-cell-derived factors (B-cell growth factor and B-cell differentiation factor). The addition of anti-class II monoclonal antibodies inhibited B-cell responses. However, anti-class I monoclonal antibody did not inhibit B-cell responses. When mitomycin C and cycloheximide-treated B cells were added to the induction culture of B cells as the stimulator, B-cell responses were enhanced in a dose-dependent manner. Furthermore, the stimulator B cells also partially restored the suppressed B-cell responses which were induced by the pretreatment of B cells with anti-class II antibody. This enhancing effect of stimulator B cells on B-cell responses was inhibited by the pretreatment of stimulator B cells with anti-class II antibody. The treatment of B cells with anti-class II antibody and complement depleted the activity of both responder B cells and stimulator B cells. These results suggest that cellular interaction among B cells exists in the B-cell activation induced with Staphylococcus aureus, Cowan I and anti-class II antibody inhibits B-cell activation by interfering in this cellular interaction.

T helper cell-dependent induction of resting B cell differentiation need not require cognate cell interactions

We have analyzed the role of cognate interaction with helper T cells (Th) in support of resting B cell differentiation to plaque formation. Co-culture of histoincompatible resting B cells and resting Th cells resulted in the induction of plaque-forming cells when dimeric but not monomeric fragments of anti-T cell receptor (TcR) antibody were added to culture. The efficiency of B cell activation was comparable to that supported by lipopolysaccharide and lectin-mediated Th-B cell conjugate formation. Further, if resting Th cells were preactivated with antigen and histocompatible antigen-presenting cells, the requirement for addition of anti-TcR to mixtures of histoincompatible Th and B cells was obviated. These results demonstrate that TcR-mediated Th recognition of major histoincompatibility complex class II/antigen composites on the resting B cell membrane does not provide obligate signals for B cell differentiation to plaque formation. We are left with two possibilities. Either the entire process of Th cell-dependent induction of resting B cell differentiation is mediated by soluble lymphokines or if Th-B cell contact is mandatory, it is mediated through nonpolymorphic cell surface determinants.

A noncognate interaction with anti-receptor antibody-activated helper T cells induces small resting murine B cells to proliferate and to secrete antibody

Culture of small resting allogeneic B cells (of an irrelevant haplotype) with two clones of T helper (Th) cells that were activated by the F23.1 anti-T cell receptor antibody led to the activation of B cells to proliferate and to secrete antibody. Th cell supernatants by themselves had no effect on resting B cells (even in the presence of intact F23.1 antibody), but could induce antibody secretion by anti-Ig-preactivated B cells. Both F23.1+ clones (E9.D4 and 4.35F2) and one F23.1- clone (D2.2) could synergize with supernatants from activated E9.D4 T cells to induce B cell activation. F(ab')2 fragments of F23.1 induced E9.D4 to activate B cells as efficiently as intact F23.1 and B cell populations that had been incubated with F23.1 were not activated when cultured with E9.D4, although T cells recognized cell-presented F23.1 and were weakly activated. Reduction of the density of F23.1 adsorbed to plastic resulted in weak T cell activation, and these T cells did not induce B cell responses. Haptenated B cell populations, although recognized by E9.D4, were not activated. Separation of T and B cells by a 0.4-micron membrane prevented T-dependent B cell activation, although Th cell-derived B cell-activating lymphokines would be assayed across these membranes. These results suggest a polyclonal noncognate B cell activation that depends on physical contact between B cells and activated T cells. The requirement for a cognate interaction of Th with B cells for the production and delivery of B help can therefore be overcome by activating Th cells with high densities of T cell receptor ligands.

The molecular interactions with helper T cells which limit antigen-specific B cell differentiation

Helper T (Th) cell-dependent activation requirements for 2,4,6-trinitrophenyl (TNP)-specific resting B cells obtained from mice transgenic for Sp-6 mu, kappa genes were analyzed. Carrier-specific T cell help required linked recognition of TNP carrier and was functionally restricted by the B cell major histocompatibility complex. However, histoincompatible T cell-B cell conjugates formed by bridging surface immunoglobulin and Th cell receptor for antigen (TcR) through TNP-conjugated anti-TcR antibodies resulted in the efficient differentiation of TNP-specific B cells. Thus, Th cell-dependent cognate recognition of B cells is not obligatory. Specific conjugate formation could be obviated by using unconjugated fragments of anti-TcR antibodies. If dimeric, these fragments supported the Th cell-dependent differentiation of co-cultured histoincompatible resting B cells. Unconjugated monomeric fragments were ineffective, demonstrating the necessity for TcR cross-linking. Resting B cells from Sp-6+ mice rendered TNP-conjugated monomeric fragments of anti-TcR antibodies effectively multivalent, thereby satisfying conditions for the activation of co-cultured Th cells. The results demonstrate that Th cells do not transduce activation signals through TcR recognition of B cell membrane-associated ligand which limit the induction of B cell differentiation. Cross-linking of TcR on Th cells is required, sufficient and can be induced through interaction with the antigen-specific B cell surface.

Early changes in quiescent B cell physiology subsequent to cognate and bystander interaction with helper T cells

We have assessed early changes in quiescent B cells following cognate and bystander interaction with cloned helper T cells. Variables monitored include Ia expression, blastogenesis, G0 to G1 transition, and progression through cycle. We have also assessed the antigen specificity, Ia restriction, and dependence on membrane immunoglobulin crosslinking of both generation and delivery of effectors that mediate B cell responses. The results demonstrate that antigen presentation by quiescent B cells to T cells resulting in the generation of effectors that activate B cells is Ia-restricted and dependent on antigen and an (mIgM and mIgD) crosslinking signal. However, once generated, T cell effector functions act independently of Ia haplotype to promote Ia expression, blastogenesis, and G0 to G1 transition by most small B cells. Although these responses can be mediated by T cell supernatants, further progression of B cells through S, G2 and M is only efficient when Th cells are present in cultures. Thus, results suggest that one or more Ia unrestricted, labile and/or cell-associated factors, not active in most conventional T cell supernatants, are necessary to stimulate proliferation of small B cells.

Supernatant from a cloned helper T cell stimulates resting B cells to express transferrin and IL-2 receptors

We describe the properties of the supernatant from a murine cloned helper T cell (clone 52.3) which is able to polyclonally activate most resting B cells in the absence of any additional stimulus. We hypothesize that an activity which we call BCAF (B-cell-activating factor(s] exists in our supernatant which can activate resting B cells alone or in conjunction with other lymphokines. In the present report, we investigate changes in the surface antigen pattern induced on resting B cells by BCAF-containing supernatant. Analysis of the cells by flow cytometry shows that transferrin receptor and IL-2 receptor expression increase on a large fraction of B cells after 2 days of activation by the T-helper-cell clone supernatant. Monoclonal anti-transferrin receptor antibody inhibits cell division but does not affect blastogenesis, while IL-2 has no effect in our experimental system. Our present results confirm that BCAF-containing supernatants can act on most resting B cells and replace helper T cells in inducing B-cell activation and proliferation.

Antigen-induced and polyclonal B-cell responses in human peripheral blood lymphocyte cultures

This work was designed to delineate the anti-hapten antibody (Ab) response induced by trinitrophenol-polyacrylamide (TNP-PAA) beads from the nonspecific B-cell response which concomitantly occurs in human peripheral blood mononuclear cell (PBMC) cultures. Indeed human PBMC produce consistent amounts of immunoglobulins when cultured at high cell density in the presence of fetal bovine serum, regardless of the presence of antigen. In contrast, the stimulation of such cultures by TNP-PAA leads to an Ab response characterized by the following: cells secreting anti-hapten Ab at a high rate (detected by a plaque-forming cel (PFC) assay); a 10-30 times enhancement in the number of hapten-specific binding cells (detected by a rosette-forming cell (RFC) assay); the production of anti-TNP IgM Ab (detected by an ELISA assay). The anti-TNP response is specifically triggered by the particulate antigen, as shown by the following: The TNP-PAA antigen induces a clear-cut increase in the amount of anti-TNP Ab whereas it only marginally increases that of total IgM. The anti-TNP Ab response is specifically abolished when anti-TNP RFC are depleted from the PBMC preparation before the initiation of the cultures. The anti-TNP Ab response is specifically abolished when PBMC are triggered by TNP-PAA in the concomitant presence of a soluble TNP-protein conjugate. These results demonstrate the ability of polymeric antigens to specifically activate human peripheral blood B cells.

Neither interleukin 2 nor gamma interferon directly promote growth or differentiation of mouse B cells

The roles of interleukin 2 (IL-2) and gamma interferon (IFN-gamma) as direct mediators of B-cell growth and differentiation were analysed. Products of cloned genes were used in both cases. The use of flow cytometric assays coupled with density fractionation of responding splenic B-cell populations enabled both the characterization of B cells responding to various stimuli and the estimation of their frequency. B cells responding to non-IL-2 related lymphokines promoting growth and differentiation were restricted to low buoyant density fractions. In addition, these cells expressed densities of IL-2 receptor determinants comparable to those found on T cells, although, IL-2 did not support their growth or differentiation. The inability to demonstrate any direct effect of either IL-2 or IFN-gamma on B cells in any state of activation suggests that their physiological roles are mediated through additional cell types.

Autoreactive or self restricted?

Putative major histocompatibility complex (MHC) class II-specific T cell clones were isolated from nominal antigen-primed T cell populations propagated in vitro under antigen selection. However, unlike the proliferative responses of nominal antigen-specific, MHC class II-restricted T cell clones derived from the same population, the former's MHC-specific proliferative responses required that the culture medium be supplemented with serum. The role of xenogeneic antigens in the generation of this category of "autoreactive" T cells is discussed.

Dependency of B cells on the presence of adherent cells, or factors derived from them, for the production of autoantibodies in vitro in the absence of cell division

Peritoneal cells from untreated mice secrete autoantibodies after 3-4 days of in vitro culture, although the cells do not divide. Here, peritoneal cells enriched for B cells to contain 95% surface Ig-bearing cells, did not secrete autoantibodies after 3 days of in vitro culture unless plastic-adherent cells derived from the peritoneal cavity were cultured with the B cells. Cell-free media, taken from peritoneal adherent cells that had been cultured for 3 days in vitro, when added a final concentration of 50% in fresh culture medium to purified B cells, substituted for the presence of accessory cells. In contrast to cultures of unfractionated peritoneal cells, little increase in precursor frequency was detected when enriched B cells were cultured in the presence of LPS/DXS. However, the addition of adherent cells, supernatants derived from adherent cells, or cytokines produced by a T-cell hybrid EL4, resulted in an increased precursor frequency when LPS/DXS was added to the culture medium. Three macrophage cell lines, P388-D1, J774, and PU-5-IR, when added to purified B cells. augmented the autoantibody precursor frequency detected in vitro. This is strong evidence that potentially autoreactive B cells require one or more types of accessory cells in order to differentiate into autoantibody secretors during culture in vitro. Further, the results provide indirect evidence that interleukin 1 may be a crucial molecule in the differentiation of B cells.

Trans-stimulation of T cells: characterization of targets and involvement in loss of alloreactivity

The rapid loss of alloreactivity within populations of antigen-primed, in vitro propagated T cells cannot be explained by the appearance of suppressor cells nor by the dilution effect of the proliferative antigen-specific T cells alone. The involvement of trans-stimulation in loss of alloreactivity, i.e. the recruitment of non-antigen-specific T cells into proliferation in an antigen-dependent and specific fashion, was assessed. Susceptibility to trans-stimulation was found to correlate directly with state of activation at the outset of assay. Large T cells (low buoyant density) but not small T cells (high buoyant density) are susceptible to trans-stimulation. Moreover, in vitro pre-activation of small T cells by mitogen confers susceptibility to trans-stimulation. Analysis of the alloreactivity in Percoll fractions of antigen-primed lymph node T cells revealed activity in both large- and small-T-cell fractions with some enrichment in the latter. The small T cells, refractive to trans-stimulation, are diluted out of the population within the early weeks of antigen-mediated in vitro propagation, accounting for a rapid loss of considerable alloreactivity. The loss of all detectable alloreactivity within antigen-selected populations suggests that the state of activation conferring sensitivity to trans-stimulation must be maintained, and that neither the antigen nor the culture conditions employed met this requirement.

Properties of purified T cell subsets. I. In vitro responses to class I vs. class II H-2 alloantigens

In light of the widely accepted view that Ia-restricted L3T4+ T helper cells play a decisive role in controlling the differentiation of Lyt-2+ cells, experiments were designed to examine whether Lyt-2+ cells can respond to antigen in the absence of L3T4+ cells. The results showed that highly purified Lyt-2+ cells gave high primary mixed lymphocyte reactions (MLR) to various class I differences, including both mutant and allelic differences; responses to class II (Ia) differences were generally undetectable with Lyt-2+ cells. The intensity of MLR to class I differences was not affected by addition of anti-L3T4 monoclonal antibodies (mAb) to the cultures or by removing T cells from the stimulator populations. Negative selection experiments showed that Lyt-2+ cells could respond to class I differences across Ia barriers. MLR of purified Lyt-2+ cells peaked on days 3-4 and then fell sharply; background responses with syngeneic stimulators (auto-MLR) were virtually absent. Parallel experiments with purified L3T4+ cells showed that this subset responded in MLR only to class II (Ia) and not class I differences, reached peak responses only on day 6 rather than days 3-4, and often gave high auto-MLR. Within the first 3-4 d of culture, MLR were generally higher with Lyt-2+ cells than L3T4+ cells. Although no evidence could be found that Ia-restricted L3T4+ cells were required for the response of Lyt-2+ cells, presentation of antigen by Ia+ cells appeared to be essential. Thus, responses were ablated by pretreating stimulator cells with anti-Ia mAb plus C'. Significantly the failure of Lyt-2+ cells to respond to anti-Ia plus C'-treated stimulators could not be restored by adding syngeneic spleen cells; addition of IL-2 led to only a minor (15%) restoration of the response. It is suggested that Ia+ cells provide an obligatory second signal required by Lyt-2+ cells.

Anti-immunoglobulin M induces both B-lymphocyte proliferation and differentiation in Xenopus laevis

Anti-IgM induced the proliferation of spleen lymphocytes of the amphibian Xenopus laevis as determined by 3H-thymidine uptake. The responding cells were B lymphocytes, since lymphocyte populations enriched in surface-Ig-positive cells exhibited an increased proliferative response, and spleen cells from larvally thymectomized animals still responded to anti-IgM. Immunofluorescence analysis and gel electrophoresis of biosynthetically labeled Ig polypeptides revealed that lymphoblasts induced by anti-IgM differentiated into plasmablasts that synthesized and secreted mainly IgM and small amounts of IgY. The in vitro differentiation of B lymphocytes also occurred in spleen cells obtained from thymectomized animals. These findings are in contrast with those obtained in mammals and suggest that the differentiation of B lymphocytes in X. laevis is subject to different regulatory mechanisms.

A self-Ia reactive T cell clone directly stimulates every hundredth B cell and helps antigen-specific B cell responses

A self-I-A reactive T cell clone has been isolated that proliferates in the presence of irradiated syngeneic spleen cells in mouse serum. The cells directly stimulate approximately every hundredth B cell, which includes small B cells, into both proliferation and antibody formation in the absence of added antigen. There is evidence to suggest that cells of this type are generally present in murine T cell populations. The cells may recognize self-I-A as such. We failed to obtain evidence, but cannot exclude that they recognize an antibody idiotype in addition. The clone also augments antigen-specific antibody responses in vivo and in vitro in the presence of antigen. In the in vitro response to a T-independent TI-2 antigen, the T cells in co-culture with B cells induced specifically the production of IgG1 antibodies, thus mimicking the function of major histocompatibility complex-restricted, antigen-specific helper T cells. However, IgG1 production was not observed when splenic T cells were added to this system. Thus, self-I-A reactive T cells, generally inactive in the system, may locally help B cell responses to antigens which the T cells do not themselves recognize. In such instances one might attribute to the T cells antigen or idiotype-binding specificities which the cells in reality do not display.

Recombinant human interleukin 2 directly provides signals for the proliferation and functional maturation of murine B lymphocytes

In this study the effect of recombinant human interleukin 2 (rec.hIL-2) on the proliferation and maturation of B lymphocytes was investigated. It was found that the presence of rec.hIL 2 results in proliferation of mitogen (LPS)-activated B cell blasts. In addition, it is shown that highly enriched murine B cells can be induced by rec.hIL-2 to proliferate and to develop into antibody-secreting cells (PFC) in the presence of antigen (SRBC). When tested for its effect on B cell preparations enriched for resting (small) or activated (blasted) B lymphocytes, it was found that rec.hIL 2 provides signals for both B cell populations to develop into PFC. In contrast, induction of proliferation by the same lymphokine source was only seen in blasted B cells. The data indicate that IL 2 is involved in the generation of B effector cells by directly acting on their precursors thereby providing differentiation as well as proliferation signals.

On the molecular basis of T-helper-cell function. IV. B-lymphocyte-promotor factors: on their mode of action, biochemical nature and possible relationship to molecules involved in specific T-helper-cell activity

Some further aspects of B-lymphocyte-promotor factor (B-LPF) activity have been studied. This activity was present in the supernatants of certain helper-T-cell lines, and it induced polyclonal activation of Ig+ B cells into Ig-secreting cells. It was found that B-LPF induced polyclonal, terminal B-cell differentiation (1) in T-cell- and macrophage-depleted spleen cell populations, (2) in both Lyb-5- and Lyb-5+ cells as well as in small and blast-like splenic B cells, and (3) in normal rather than memory B cells. B-LPF function was neither restricted to major histocompatibility complex gene products nor to immunoglobulin allotypes. B-LPF-like activity was also produced by some B-cell lymphomas/hybrids and by the P388-D1 macrophage line. B-LPF activity was found in three MW fractions: (I) greater than 180,000 (pI greater than 7.0 and 4.5-5.5); (II) 50,000-70,000 (pI greater than 7.0; 6.0-6.5, and 4.5-5.5); and (III) 10,000-15,000 (pI greater than 7.0). All three MW forms of B-LPF activity carried antiserum 6036-defined and AB-1.9.3 monoclonal antibody-defined determinants, and they reacted with chicken gammaglobulin (CGG)-Sepharose but not with human serum albumin-Sepharose. These data indicate that the three MW forms of B-LPF activity are associated/dissociated forms of a 10,000-15,000 MW form (subunit) rather than three different molecular species with B-LPF activity. A comparative study between antigen-specific helper factors and B-LPF was hampered by the finding that the helper-T-cell hybridomas used (e.g., T85-109-45/1) only produced B-LPF in our hands. Previously, it has been described that these helper-T-cell hybrids produced CGG-specific, I-Ak-restricted helper factors. However, one surprising observation was that B-LPF produced by both T85 hybrid cells and L12 T lymphoma cells was absorbed and could be eluted from CGG-Sepharose columns. The relationship of B-LPF to other nonspecific and apparently specific T-helper-cell products is discussed in particular in the light of the observations that many immunologically active molecules are built up from 10,000-12,000 molecular weight domain-like polypeptide structures.

Capacity of small B cell-enriched populations to stimulate mixed lymphocyte reactions: marked differences between irradiated vs. mitomycin C-treated stimulators

To investigate whether small B cells can stimulate mixed lymphocyte reactions (MLR), highly purified populations of large vs. small B cell fractions were tested for their capacity to evoke MLR across Mls vs. H-2 barriers. Large B cell fractions stimulated high MLR to Mls and H-2 determinants, irrespective of whether the stimulators were exposed to irradiation or pretreated with mitomycin C. In accord with the findings of others, irradiated small B cell fractions proved to be very poor stimulators of MLR. Significantly, however, mitomycin C-treated small B cell fractions elicited high MLR, particularly to Mls determinants. The finding that small B cell fractions treated with irradiation are poor stimulators of T cells correlates with the known radiosensitivity of B cells. In this respect, the widely held view that small B cells do not have antigen-presenting cell (APC) function rests largely on studies with irradiated B cells. The present finding that T cells respond well to small B cells treated with mitomycin C, however, indicates that small B cell fractions do have APC function. Whether the APC function of small B cells reflects a response to resting B cells per se rather than to cells undergoing activation in vitro, however, remains to be ascertained.

Major histocompatibility complex-restricted, polyclonal B cell responses resulting from helper T cell recognition of antiimmunoglobulin presented by small B lymphocytes

Anti-Ig has been widely used as a model for antigen receptor-mediated B cell activation. B cells activated with mitogenic concentrations of anti-Ig (approximately 10 micrograms/ml) become responsive to a set of T cell-derived, antigen-nonspecific helper factors that enable the B cells to proliferate, and, in some cases, mature to Ig secretion. In the present experiments, we show that anti-Ig can also be used as a model for major histocompatibility complex (MHC)-restricted, antigen-specific T-B cell collaboration. We used murine helper T cell lines and T cell hybridomas specific for a protein antigen, the F(ab')2 fragment of normal rabbit IgG. Small B cells are very efficient at presenting rabbit anti-IgM or rabbit anti-IgD to these rabbit Ig-specific T cell lines and hybridomas, and the responding (initially) small B cells, appear to be the only antigen-presenting cells required. Efficient presentation depends upon binding of rabbit antibody to mIg on the B cell surface. MHC-restricted recognition of rabbit Ig determinants on the B cell surface results in a polyclonal B cell response. This response is qualitatively different from the well-studied response to blastogenic concentrations of anti-Ig plus stable, T cell-derived helper factors, since it (a) requires 1,000-fold lower concentrations of anti-Ig, (b) involves helper T cell functions other than, or in addition to, the local production of the same stable helper factors, and (c) is largely MHC-restricted at the T-B cell level.

Effects of mu-specific antibodies on B cell growth and maturation

Over a wide range of concentrations affinity-purified rabbit anti-mouse mu chain antibodies, or their F(ab')2 fragments, inhibit the appearance of immunoglobulin-secreting cells (plaque-forming cells; PFC) in lipopolysaccharide-stimulated murine spleen cell cultures without affecting proliferation. Both IgM and IgG PFC are inhibited although the number of blasts bearing surface IgG remains unaltered. The IgM and IgG PFC response could be reconstituted to normal levels in cell cultures suppressed by mu-specific antibodies by the addition of supernatants from in vitro propagated helper T cell clones, or from EL4 lymphoma cells induced with phorbol ester. Interleukin 1-containing P388 supernatant, or recombinant DNA-derived murine interferon-gamma, did not reconstitute the PFC response in cell cultures suppressed by mu-specific antibodies, indicating that other factors are responsible for these effects. When spleen cell cultures, pre-activated with either lipopolysaccharide or monoclonal mouse mu-specific antibodies coupled to Sepharose, were exposed to EL4 supernatants in the presence of soluble mu-specific antibodies, maturation to secretion was inhibited while proliferation was not. The implications of these findings on assay systems for B cell growth and maturation factors are discussed.

Antigen-specific helper T-cell clone supernatant is sufficient to induce both polyclonal proliferation and differentiation of small resting B lymphocytes

Gradient-purified resting B lymphocytes can be polyclonally stimulated by antigen-specific major histocompatibility complex (MHC)-restricted helper T lymphocytes as well as by antigen-activated helper T-cell supernatant. In contrast to what has been described so far, we show that helper T-cell supernatant (in the absence of any other added stimulus, such as that provided by anti-mu antibodies) is sufficient to induce both proliferation of resting B cells and their differentiation into IgM-secreting cells. The stimulation induced by the helper T-cell supernatant takes place in serum-free medium and is not MHC-restricted. Our findings strongly support the existence of a B-cell activating factor acting on the resting B cell and causing it to enter the G1 phase of the cell cycle in a MHC-unrestricted manner.