Regulation of IgA differentiation in CH12LX B cells by lymphokines. IL-4 induces membrane IgM-positive CH12LX cells to express membrane IgA and IL-5 induces membrane IgA-positive CH12LX cells to secrete IgA

In these studies we utilized the Ag (SRBC)-reactive B cell line CH12LX to study isotype switching. CH12LX cells are a stable population of B cells mainly bearing membrane IgM (mIgM) (98 to 99%) with a small population of B cells bearing membrane IgA (mIgA) (1 to 2%). LPS induced a 5- to 10-fold increase in the secretion of both Ig, whereas a lymphokine-rich supernatant of D10 T cells induced a greater increase in the secretion of IgA than IgM. Analysis of the latter effect with recombinant lymphokines disclosed that rIL-4 induced an increase in the number of mIgA+ cells (6 to 15%) with minimal effect on IgA secretion, whereas IL-5 induced increased IgA secretion but had no effect on mIgA expression. The addition of both lymphokines induced increased mIgA expression and IgA secretion. No effect on mIgA expression or IgA secretion was seen with other lymphokines, including IL-1, IL-2, IL-3, IL-6, GM-CSF, and IFN-gamma. The rIL-4 effect on CH12LX cells represents true differentiation rather than selective proliferation for the following reasons: first, subclones of CH12LX cells respond to IL-4-containing T cell supernatant in the same fashion as the original cell line; second, culture of CH12LX cells with IL-4 causes the appearance of large numbers of dual-bearing mIgM/mIgA cells as well as mIgA+ cells and a dual-bearing mIgM/mIgA line was obtained by cloning CH12LX after stimulation with an IL-4-containing supernatant; third, sorted mIgA+ and mIgA- CH12LX cells had similar rates of proliferation in the presence or absence of IL-4. In further studies, it was found that IL-5 causes IgA secretion by mIgA+ but not mIgA- CH12LX cells indicating that it is acting as a post-isotype switch differentiation factor. These studies are consistent with the view that IL-4 and IL-5 act in a sequential fashion to induce IgA expression and secretion in CH12LX cells, IL-4 inducing differentiation of mIgM+ cells to mIgA+ cells and IL-5 enhancing the IgA secretion by the resulting mIgA-bearing cells.     

The roles of IL-4, TGF-beta and LPS in IgA switching.

CH12.LX B cells have been used as a lymphoma model of MHC restricted, antigen-dependent B cell differentiation. These B cells express surface IgM and secrete IgM. Most recently we have demonstrated that CH12.LX is a model of cytokine driven IgA differentiation. Recently, transforming growth factor beta (TGF-beta) has been shown to be a probable switch factor for IgA in LPS-stimulated mouse lymphocytes, therefore we chose CH12.LX B cells to study the effect of IL-4, TGF-beta and LPS in IgA isotype switching. Adding TGF-beta to the monoclonal cell line CH12.LX results in induction of mIgA expression but no enhancement of IgA secretion similar to the effect of IL-4. The addition of LPS serves as a non-specific stimulus to enhance the secretion of the expressed immunoglobulin, but has no IgA specific activity of its own. IL-4 and TGF-beta together are synergistic for mIgA expression. Pretreatment studies show that TGF-beta added after IL-4 is the same as TGF-beta alone whereas the converse is the same as adding both cytokines together. TGF-beta acts by increasing the steady state levels of alpha message, whereas northern analysis indicates that IL-4 does not induce alpha message the way TGF-beta does. These data confirm that TGF-beta by itself is an isotype switch factor for IgA. In addition, IL-4 and TGF-beta cause mIgA expression through different mechanisms. CH12.LX B cells serve as a valuable model to study the role of multiple signals required for mIgA expression and IgA secretion.     

Differential glycosylation of two glycoproteins synthesized by murine B cells in response to IL-4 plus IL-5

We sought to determine whether selected cytokines, known to stimulate profoundly B-cell activation and differentiation, also have as yet unrecognized effects upon the glycosylation of secreted Ig and/or membrane-associated proteins. The glycosylation of both secreted IgM and membrane-bound MHC Class-I synthesized by CH12LX cells was detected by enzyme-lectin conjugates in immunoabsorption assays. Stimulation of B cells with IL-4 plus IL-5 significantly decreases the terminal glycosylation of secreted IgM, whereas LPS has a minor effect, despite the fact that both stimuli are equipotent for IgM secretion. Neither LPS nor IL-4 plus IL-5 affect MHC Class-I expression. However, IL-4 plus IL-5 substantially increases the terminal glycosylation of MHC Class-I produced from both mIgM(+)and mIgA(+)CH12LX cells. LPS has no or a modest effect on the terminal glycosylation of MHC Class-I produced from CH12LX cells. These results suggest that Th(2)-derived cytokines differentially influence the glycosylation of secreted and membrane-associated glycoproteins of B cells. In turn, this might elucidate the basis of aberrant glycosylation reported in conditions such as IgA nephropathy, cancer and rheumatoid arthritis.     

T cell-induced expression of membrane IgG by 70Z/3 B cells

To study T cell regulation of B cell isotype differentiation, we determined the capacity of clonal T cell populations (hybridomas derived by fusing BW5147 with Con A-activated Peyer's patch (PP) and spleen T cells) to induce "downstream" isotype expression by the pre-B cell lymphoma 70Z/3. In initial studies, we found that 70Z/3 B cells cultured in the presence of LPS (1 microgram/ml) expressed membrane IgM (mIgM) but not membrane IgG (mIgG). In contrast, 70Z/3 B cells cultured with HAJ-3 T cells, a PP-derived T cell hybridoma (as well as other similarly derived PP and spleen hybridomas), or with HAJ-3 T cells plus LPS do express mIgG. Such expression occurred in spite of mitomycin C-induced blockage of cell proliferation, and is observed in 70Z/3 B cell subclones cultured with HAJ-3 T cells. For these reasons, it is not due to selective expansion of a small pre-switched mIgG-bearing 70Z/3 B cell subpopulation. In other studies it was shown that 70Z/3 B cells expressing mIgG after induction by HAJ-3 T cells continue to express mIgM and do not secrete IgG. Finally, exposure of 70Z/3 B cells to the macrophage factor IL 1 and the T cell factors IL 2, BSF-pl, and BCGF-II present in EL-4 cell supernatants did not result in mIgG expression. On the basis of these studies, we conclude that a clonal B cell population expressing mIgM can be induced by T cells to co-express mIgG. Because the B cells do not express mIgG unless exposed to T cells, this represents a T cell-induced isotype switch.     

Lack of Fc epsilon RII expression by murine B cells after in vivo immunization is directly associated with Ig secretion and not Ig isotype switching.

The "low affinity" Fc receptor for IgE (Fc epsilon RII) has been reported to be absent from normal murine and human B cells that express a membrane (m)Ig isotype other than mIgM or mIgD in vivo. This would suggest that Fc epsilon RII expression is specifically lost after in vivo Ig isotype switching. We demonstrate that during a murine immune response to the bacterium Brucella abortus, to goat anti-mouse IgD (G alpha M delta) antibody, or to infection with the nematode parasites Nippostrongylus brasilienis or Heligmosomoides polygyrus, Fc epsilon RII expression is low or absent on virtually all B cells secreting IgM, IgG1, IgG2a, and IgE. However, up to 50% of B cells that express mIgG1 after G alpha M delta injection continue to express Fc epsilon RII. These mIgG1 + Fc epsilon RII+ cells secrete little, if any, IgG1 when placed in vitro, in contrast to their mIgG1 + Fc epsilon RII- counterparts. The mIgG1 + Fc epsilon RII+ cells may be a transitional cell population, because they undergo substantial loss of Fc epsilon RII in culture, unlike mIgM+ Fc epsilon RII+ cells, which maintain constant levels of Fc epsilon RII throughout a comparable culture period. Thus, low or absent expression of Fc epsilon RII after immunization in vivo is directly associated with B cell differentiation to Ig production in the presence or absence of Ig isotype switching. However, all post-switched B cells may eventually lack Fc epsilon RII expression, independently of their differentiative state.     

Rapid loss of IgM expression by normal murine B cells undergoing IgG1 and IgE class switching after in vivo immunization

Injection of mice with polyclonal goat anti-mouse IgD antibody (G alpha M delta) stimulates a potent T cell-dependent immune response characterized by large increases in serum IgG1 and IgE concentrations and by the generation of substantial numbers of membrane (m)IgG1+ B cells. The onset of this response occurs 6 days after G alpha M delta injection and peaks by day 7 to 8. Utilizing two color fluorescence analysis and cell sorting we demonstrate that most mIgG1-expressing B cells lack mIgM during the period of onset of Ig isotype switching (day 6). Both IgG1 and IgE are produced predominantly by mIgM- cells. On day 6, IgG1 and IgE are secreted predominantly by cells expressing mIgG1 and mIgE, respectively. By day 8, a majority of the IgG1 secretion occurs among the mIgG1- cells but virtually all IgE secretion continues to come from the mIgE+ population. B cells that strongly express mIgG1 secrete little IgM or IgE. Freshly harvested B cells expressing mIgG1, 6 days after G alpha M delta injection, have undergone substantial deletion of CH mu-specific DNA in contrast to their mIgG1- counterparts. Hence, the great majority of B cells that switch to the IgG1 or IgE isotypes in vivo rapidly lose their expression of IgM.     

In vitro vomitoxin exposure alters IgA and IgM secretion by CH12LX B cells

The CH12LX cell line was used as a clonal model to assess the direct effects of vomitoxin on IgM and IgA secretion in B cells. When vomitoxin was included in LPS-driven CH12LX B cell cultures, it had multiple effects on Ig secretion. Whereas vomitoxin doses of 115 and 120 ng/ml caused 50% inhibition(ID₅₀) of IgA and IgM production, respectively, toxin concentrations in the 5 to 50 ng/ml range slightly stimulated IgA production. However, low vomitoxin doses did not induce switching of membrane IgM+ CH12LX B cells to membrane IgA+. Total cell number was unaffected at vomitoxin concentrations up to 100 ng/ml but dropped markedly at 200 ng/ml (ID₅₀=170 ng/ml). Using the MTT reduction assay as another measure of viability and cell function, vomitoxin was also inhibitory (ID₅₀=130 ng/ml). Both thymidine incorporation and leucine incorporation were also inhibited by the toxin with estimated ID₅₀s being 120 and 110 ng/ml, respectively. The results indicate that although at high doses, vomitoxin inhibits proliferation, Ig secretion and DNA/protein synthesis in the clonal B cell model, the toxin marginally stimulated IgA secretion at lower doses.     

Role of the interleukin 2 receptor in differentiation of a clone of Ly-1+ B cells

CH12.LX, an in vitro subclone of a murine B cell lymphoma that makes IgM reactive with sheep erythrocytes (SRBC), has cell surface receptors for the lymphokine interleukin 2 (IL 2). The binding of recombinant murine IL 2 to these receptors did not stimulate CH12.LX cells to differentiate and secrete antibody. However, the binding of either of two monoclonal antibodies (Mab) specific for the IL 2 receptor increased the proportion of CH12.LX cells that secrete hemolytic IgM. The effect did not require the presence of antigen. One of the Mab, 3C7, is known to block the binding of IL2 to its receptor on T cells, whereas the other, 7D4, which also reacts with the IL 2 receptor, does not block the binding of IL 2. The differentiation of CH12.LX induced by 3C7, but not that induced by 7D4, was inhibited by recombinant IL 2. Neither IL 2 (up to 200 U/ml) nor 3C7 (up to 10 micrograms/ml) had any significant influence on incorporation of [3H]thymidine; 7D4 at 10 micrograms/ml decreased thymidine incorporation by about 60%. Mitomycin C and hydroxyurea, which both inhibit the incorporation of [3H]thymidine into CH12.LX cells, also both induce antibody secretion. In both cases, the concentration necessary to cause differentiation is substantially lower than that needed to cause detectable inhibition of thymidine uptake. We conclude that the IL 2 receptor on CH12.LX cells is a functional signal transducing molecule, and we discuss the possible inverse relationship between proliferation and differentiation.     

Regulation of IgA secretion by T cell clones derived from the human gastrointestinal tract

The role of T cells in Ig isotype regulation is still unclear. To address this question, we generated mitogen-stimulated T cell clones from normal human lymphoid follicles of the gut-associated lymphoid tissue (appendix). Both the T cell clones and clonal supernatants provided preferential help for IgA secretion by PWM-stimulated B cells. Many of these CD3+, CD4+, 4B4+, DR+ helper clones co-expressed Fc-gamma and Fc-alpha R, but there was poor correlation between the expression of Fc-alpha R and IgA help (p = 0.31). Most of the T cell clones helped both IgM+A- and IgM-A+ B cell populations to secrete IgA, suggesting that they mediate switch of isotype-uncommitted B cells as well as post-switch expansion of IgA-committed B cells; however, some of the T cell clones helped IgM+A- B cell populations much more than IgM-A+ B cell populations, suggesting that, in this case, the regulatory effect is predominantly at the level of B cell switch. In all, these results show that the mucosal immune system contains individual T cells which are capable of positively regulating IgA-specific isotype differentiation at two levels of B cell development, thus allowing for efficient generation of IgA-secreting B cells.     

The role of IL-5 in IgA B cell differentiation

IL-5 enhances secretion of IgA by B cells. The stage of B cell differentiation at which IL-5 enhances IgA secretion and the mechanism by which it exerts this effect are unknown. We examined these issues by separating Peyer's patch (PP) B cells into membrane IgA (mIgA)-positive and mIgA-negative cells with panning or cell sorting. When LPS was used to activate these cells, mIgA-positive PP B cells were induced by IL-5 (either as crude T cell supernatant or rIL-5 to secrete large amounts of IgA. In contrast mIgA-negative PP B cells showed no significant amount of IgA secretion with IL-5. In addition, rIL-5 did not cause expression of mIgA by mIgM-bearing B cells. The mechanism involved in enhancement of IgA secretion was evaluated by utilizing an ELISPOT assay to quantitate IgA secreting cells. Both unsorted PP B cells and mIgA-positive PP B cells, when incubated with IL-5, showed an increase in the number of IgA-secreting cells that was proportional to the increase in total secreted IgA. However, LPS-activated PP mIgA-positive B cells, when incubated with rIL-5, showed no increase in proliferation, as measured by [3H]thymidine incorporation indicating that the increase in IgA-secreting cells after incubation with IL-5 occurred not as a result of proliferation but rather through promotion of terminal differentiation. Thus, IL-5 acts as a differentiation factor on B cells which have already undergone isotype switch to IgA B cells, promoting differentiation into IgA-secreting cells with resultant increased IgA secretion.     

Isotype switching in human B lymphocyte malignancies occurs by DNA deletion: evidence for nonspecific switch recombination

The mechanism and specificity of isotype switching operative in human B lymphocytes was investigated by a determination of immunophenotype and immunoglobulin heavy and light chain gene status in a panel of human Ig-, IgM, IgG, and IgA B cell malignancies. Regardless of specific tumor type or switched immunophenotype, isotype switching was accompanied by the rearrangement of the expressed CH gene downstream of VDJH, with concomitant deletion of upstream CH genes in all cases. On the allelically excluded chromosome, 25% of the IgG or IgA tumors have retained C mu, and 75% have deleted C mu. The 5' recombination breakpoints for both productive and excluded alleles lie within or near S mu, 3' of the enhancer. No correlation between the extent of allelically excluded CH deletions and the isotype produced by the tumor was observed. Excluded chromosome deletion endpoints were found 5', equal to, or 3' of productive chromosome deletion endpoints. Furthermore, we have identified at least one IgM+ tumor that has undergone abortive CH gene deletions and have observed several unanticipated switch region deletions and potential translocations. The data suggest that isotype switching in human B cells occurs by a nonsubclass- and nonclass-specific switch recombinase.     

mIgM:mIgD ratios on B cells: mean mIgD expression exceeds mIgM by 10-fold on most splenic B cells

The relative frequency of mIgM and mIgD molecules on B cell surfaces is important in determining, in large part, the isotype involvement in antigen binding and signal transduction. Although it is generally assumed that on most mature B cells, mIgM and mIgD occur in roughly equal quantities, no formal analysis of this question has been reported. In this report, we describe such an analysis based on the quantitation of anti-Fab or anti-kappa specific immunofluorescence of splenic B cells before or after capping with rabbit anti-IgD or anti-IgM antibodies or both. The results indicate that, whereas mean expression of IgD exceeds IgM on splenocytes by threefold, members of the major B cell subpopulation (60 to 70% of cells) express 10-fold more IgD than IgM.     

Ig isotype switching in B lymphocytes. Isolation and characterization of clonal variants of the murine Ly-1+ B cell lymphoma, CH12, expressing isotypes other than IgM

We have selected and cloned variant cells from the murine B cell lymphoma, CH12, that produce a variety of other Ig isotypes in addition to or in place of the original IgM and IgD. Variants were selected by flow cytometry and automated cloning and isotype production was analyzed by membrane immunofluorescence and ELISA of culture fluids. Variants have been isolated that produce the single isotypes IgA, IgG2b, and IgG3, as well as variants that produce more than one isotype simultaneously, i.e., IgM, IgD, and IgA; IgG2b and IgA; IgG3 and IgA. All isotypes have been seen as cell surface proteins and all except IgD have been found in culture supernatants. All isotypes display the same idiotype and Ag-binding specificity for phosphatidyl choline as the original IgM and all are translated from the same VDJH and VJ kappa gene assemblies. Production of more than one isotype by a variant clone is due to simultaneous production of all the isotypes by each cell within the clone. The finding that the variants producing more than one isotype are all tetraploid suggests the interesting possibility that each isotype is derived from an independently switching chromosome. All isotype variants can be stimulated by LPS to secrete the appropriate Ig isotype at an increased rate similar to the IgM expressing parent. The variants differ in stability; some have remained stable for more than 9 months in culture, whereas other have undergone further isotype switching. The facts that some isotypes have not been seen, that multistep switching has occurred, and that many variants produce IgA in addition to another isotype are discussed in relation to current notions of isotype switching mechanisms.     

Modulation of IgM secretion and H chain mRNA expression in CH12.LX.C4.5F5 B cells by adrenocorticotropic hormone

The murine B cell line CH12.LX.C4.5F5 (CH12 (5F5) expresses adrenocorticotropin (ACTH) receptors, which can modulate IgM secretion by these cells. Interestingly, the response to ACTH was concentration dependent, inducing IgM secretion at subnanomolar amounts and suppressing secretion at micromolar amounts. With the use of an enzyme-linking immunospot assay it was possible to demonstrate that the ACTH-induced increase in IgM secretion by CH12 (5F5) cells was caused at least in part by an increase in the number of cells secreting IgM. CH12 (5F5) cells activated with suboptimal concentrations of LPS demonstrated a similar biphasic response. ACTH at concentrations of 10(-13) to 10(-9) M augmented IgM secretion in LPS-activated cells as much as sixfold, whereas 10(-6) M ACTH slightly decreased LPS-induced IgM secretion. At the mRNA level, subnanomolar concentrations of ACTH increased microH chain mRNA expression up to twofold in unstimulated or LPS-stimulated CH12 (5F5) cells. Taken together, these studies show that physiologically relevant concentrations of ACTH can interact directly with receptors on these B lymphocytes to enhance IgM secretion and microH chain mRNA expression. Although ACTH does increase intracellular cAMP levels in CH12 (5F5) B cells, it is unlikely that the induction of this second messenger pathway is by itself responsible for the ACTH induced B cell differentiation. The concentration of ACTH necessary to stimulate significant intracellular cAMP increases was 10- to 100-fold higher than that required to increase IgM secretion. Furthermore, CH12 (5F5) cells treated with varying concentrations of 8-bromo cAMP or cholera toxin were inhibited in their ability to secrete IgM. These results strongly suggest that the enhancing effects of ACTH on CH12 (5F5) IgM secretion are via mechanisms independent of those mediated by cAMP.     

IgM and IgG but not cytokine secretion is restricted to the CD27+ B lymphocyte subset.

In a recent study we reported that CD27 is expressed on a subpopulation of human B lymphocytes and presented circumstantial phenotypic evidence that CD27 expression may be acquired late during B cell differentiation. Here we present functional data showing that, after in vitro stimulation, CD27+ but not CD27- B cells secrete large amounts of both IgM and IgG. Using double immunofluorescence staining of CD27 and IgD, three functionally different B cell subsets representing distinct stages of B cell differentiation can be isolated: 1) the CD27- IgD+ B cells, which do not secrete appreciable Ig; 2) the CD27+IgD+ B cells, which exclusively secrete IgM; and 3) the CD27+IgD- B cells, which comprise the IgG-producing cells. Furthermore, costimulation of CD27- B cells with low m.w. B cell growth factor, in the presence or in the absence of a CD40 mAb, does not induce these cells to become Ig-secreting cells. Although CD27- B cells hardly secrete Ig of any isotype in response to Staphylococcus aureus+IL-2, these cells proliferate vigorously and express the IL-2R alpha chain (CD25) under these stimulatory conditions. Furthermore, both CD27- and CD27+ B cells are capable of producing similar amounts of IL-6 and TNF-alpha. Taken together, these findings indicate that CD27 is a unique non-Ig surface marker discriminating naive from primed B lymphocytes. Furthermore, the capacity to proliferate and to secrete the B cell differentiation factors IL-6 and TNF-alpha already exists at an early B cell differentiation stage at which the cells lack CD27 expression and are not induced to produce Ig.     

Development and distribution of B lineage cells in the domestic cat: analysis with monoclonal antibodies to cat mu-, gamma-, kappa-, and lambda-chains and heterologous anti-alpha antibodies

To trace the development and distribution of B lineage cells in the domestic cat (Felis catus), we have produced monoclonal antibodies against mu-, gamma-, kappa-, and lambda-chains of feline immunoglobulins (Ig). Goat antibodies against human mu-, alpha-, and lambda-chains, which are reactive with shared determinants on their feline counterparts, were used in conjunction with the panel of mouse monoclonal antibodies. Cytoplasmic mu+ pre-B cells and surface IgM+ B lymphocytes were observed in 42 day fetal liver in which pre-B cells were more abundant than IgM+ B cells. Pre-B cells also were found in bone marrow in young cats, and continued to be generated in the marrow throughout life. In the spleen, adult levels of B cells were attained by 12 wk of age, at which time the frequencies of surface IgM+, IgG+, and lambda+ cells were 49, 3, and 40%, respectively. The distributions of Ig isotypes also were determined among plasma cells as a function of age and tissue localization. IgM plasma cells were predominant in the bone marrow of 1-wk-old cats, whereas IgG plasma cells were the prevalent isotype in adult bone marrow. In the mesenteric lymph nodes of adult animals, the frequency distributions of IgM, IgG, and IgA plasma cells were similar to the frequency distributions of IgM, IgG, and IgA isotypes among bone marrow plasma cells. IgA+ plasma cells predominated in the intestinal lamina propria, in which IgG+ and IgM+ plasma cells were relatively infrequent. In the tissues of both young and adult animals, the ratio of lambda:kappa expression was approximately 3:1. We conclude that the pattern of B cell development in the cat resembles that found in other mammals, except that the kappa to lambda ratio is reversed.     

Recombinant murine IL-5 induces high rate IgA synthesis in cycling IgA-positive Peyer's patch B cells

Recent studies have shown that purified IL-5 from T cell lines and clones enhances IgA synthesis in LPS-triggered splenic B cell cultures, and that this effect is augmented by IL-4. In this study we have examined the ability of rIL-5 and rIL-4 to support spontaneous Ig synthesis in normal Peyer's patch (PP) B cell cultures. The rIL-4 supported proliferation of the HT-2 and in vivo adapted BCL-1 cell lines, increased Ia expression on normal spleen B cells, co-stimulated splenic B cell proliferation in the presence of anti-mu and enhanced IgG1 synthesis in LPS triggered splenic B cell cultures. The rIL-5 supported BCL-1 proliferation, co-stimulated splenic B cell proliferation in the presence of dextran sulfate, and increased IgA synthesis in LPS-stimulated splenic B cell cultures. Markedly enhanced IgA responses occurred in PP B cell, but not splenic B cell cultures supplemented with rIL-5 in the absence of an added B cell trigger. However, rIL-4 alone did not enhance IgA synthesis or increase the IgA synthesis of PP B cell cultures stimulated with rIL-5. The rIL-5 receptive PP B cells were present in the blast cell subpopulation, inasmuch as a low density fraction isolated on Percoll gradients accounted for the enhanced IgA synthesis. Further, cell cycle analysis of whole PP B cells using propidium iodide in conjunction with staining for surface B220, demonstrated that approximately 12 to 16% of the B cells were in the S and G2/M stages of cell cycle, the remainder being in Go + G1. The surface IgM+ B cells were predominantly in Go + G1, whereas the sIgA+ B cell subpopulation was enriched for cells in the S and G2/M compartments. The PP B cell subset responsible for enhanced IgA synthesis in the presence of rIL-5 was sIgA-positive because FACS-depletion of the sIgA+ B cells resulted in the total loss of rIL-5 enhanced IgA synthesis. Further, when PP B cells were enriched for sIgA+ B cells by cell sorting, these cells responded to rIL-5 with increased IgA synthesis in a dose-dependent manner. When the actual numbers of IgA secreting cells were assessed in PP B cell cultures with supplemental rIL-5, no significant increase in total IgA-producing cells was noted when compared with B cells cultured without rIL-5.(ABSTRACT TRUNCATED AT 400 WORDS)     

Regulation of the IgM and IgA anti-dextran B1355S response: synergy between IFN-gamma, BCGF II, and IL 2

T cell help is required for the induction of the humoral antibody response to dextran B1355S, a type II thymus-independent bacterial polysaccharide antigen. In the present study we have identified three B cell growth and differentiation factors that can substitute for T cells in the induction of IgM and IgA antibody responses to alpha(1,3) glucan determinants on dextran B1355S. Dextran B1355S stimulated murine B cell cultures supplemented with a combination of murine recombinant interferon-gamma (IFN-gamma) and a late-acting B cell growth and differentiation factor, BCGF II, produced both IgA and IgM anti-alpha(1,3) dextran plaque-forming cells (PFC). Interleukin 2 (IL 2) was not required for those responses. In contrast, recombinant IFN-gamma and recombinant IL 2 in combination supported the induction of IgA but not IgM anti-alpha(1,3) dextran PFC. In all cases, depletion of surface IgA-bearing B cells significantly decreased IgA but not IgM anti-dextran responses, indicating that the B cells responding to those lymphokines already were committed to IgA expression. These studies indicate that B cell growth and differentiation factors can exhibit differential effects on the induction of IgA compared with IgM responses.