T cell dependent differentiation of human B cells: direct switch from IgM to IgE, and sequential switch from IgM via IgG to IgA production.

Ig production by splenic human B cells that express different surface Ig isotypes were analysed in limiting dilution cultures. Therefore, FACS sorted IgM+, IgG+ and IgA1+ B cells were stimulated with PMA-activated EL4 thymoma cells as helper cells in the presence of IL-2 and IL-4. We found that at least every second B cell responded in vitro and secreted the antibody corresponding to its surface Ig isotype. IgE secreting cells developed from surface IgM+ D+ cells (1/31 to 1/167), but not from IgG+ or IgA1+ cells (much less than 1/5000). Negative signalling of the IgM+ B cells by addition of anti-IgM antibodies into the cultures reduced the number of single IgM producing cells by greater than 85%, and completely inhibited IgE switch. In contrast, anti-IgG and anti-IgA antibodies did not reduce the IgE response. The results indicate a direct switch from IgM to IgE secretion in vitro. In contrast to IgE, IgA secreting cells developed from IgM+D+ (1/30 to 1/51) and from IgG+ B cells (1/14 to 1/25). Negative signalling of the IgG+ B cell subset within total B cells by anti-IgG antibodies suppressed the development of IgG as well as IgA producing cells, but did not inhibit IgM and IgE responses. This indicates a sequential switch from IgM via IgG to IgA. Taken together, this study indicates that IgE secreting cells are derived directly from IgM+D+ B cells by non-sequential switching, whereas IgA producing cells preferentially develop by sequential switching via IgG+ B cells.     

Circulating and pokeweed mitogen-induced immunoglobulin-secreting cells in systemic lupus erythematosus.

Peripheral blood mononuclear cells (PBM) obtained from patients with active untreated systemic lupus erythematosus (SLE) were evaluated both for the number of cells spontaneously secreting immunoglobulin (Ig) as well as for their capacity to generate immunoglobulin-secreting cells (ISC) in vitro in response to pokeweed mitogen (PWM). ISC were enumerated by a reverse haemolytic plaque assay designed to quantify the number of cells secreting IgG, IgM and IgA. PBM obtained from eight patients with active untreated SLE contained markedly increased numbers of ISC compared to age-, sex-, and race-matched normal control PBM. SLE PBM contained a mean of 13,805 +/- 3266 ISC per 10(6) cells, of which 74% secreted IgG, 10% IgM and 22% IgA, while normal PBM contained a mean of 779 +/- 143 ISC per 10(6) cells, with 57% secreting IgG, 25% IgM and 33% IgA. PBM obtained from SLE patients were also examined for their ability to generate ISC in vitro in response to PWM. SLE PBM were markedly deficient in their capacity to respond to PWM with the differentiation of ISC. This diminished responsiveness could not be ascribed to serum factors, the presence of increased numbers of cells with suppressive capacity or the absence of potentially responsive B cells. Rather, a deficiency of helper T cell activity appeared to be responsible. This was indicated by the observation that PWM responsiveness could be restored to SLE PBM by co-culturing them with purified mitomycin C-treated normal T cells.     

Age-associated changes in proliferative and differentiative response of human B cells and production of T cell-derived factors regulating B cell functions

The abilities of highly purified B cells to repeat replication for clonal expansion and to differentiate into immunoglobulin secreting cells (ISC) were examined in the aged and young groups. B cells from the aged showed twofold less proliferative response to B cell mitogen Cowan 1 (SAC) than those from the young. The original clone size of SAC responding B cells determined by colchicine block and [3H] thymidine [( 3H] TdR) uptake was not significantly reduced in the aged whereas the ability to repeat replication to expand clonal size was significantly reduced. B cells from aged and young persons were induced into ISC by combined stimulation with SAC and partially purified B cell differentiation factor (BCDF) free of IL-2 activity. ISCs for IgG and IgA were rather increased or at least not reduced in number in the aged as compared with those in the young. We also determined the IL-2 and BCDF activity produced by T cells from aged and young persons. Upon PHA stimulation, the aged T cells produced tenfold less IL-2 activity and threefold higher BCDF activity than did young T cells. Approximately threefold increase in spontaneous secretion of BCDF activity by aged T cells was found as compared with young T cells. The inverse correlation between the IL-2 activity and BCDF activity was found when both activities were determined in the same samples.     

IgM- and IgD-bearing peripheral blood lymphocytes differentiate to IgM but not IgG or IgA immunoglobulin-secreting cells

Human peripheral blood mononuclear cells were depleted of surface IgM⁺ or IgD⁺ cells and assayed for mitogen-induced differentiation to immunoglobulin-secreting cells (ISC) of IgM, IgG and IgA classes. Stimulatory agents included T cell-dependent poke weed mitogen, B cell mitogen Staphylococcus aureus bacteria strain Cowan I, and a combination of the two which gives uniform, high levels of ISC from all normal donors. Depletion of either IgM- or IgD-bearing B lymphocytes resulted in loss of cells bearing the other Ig class and blocked most of the mitogenic reactivity to anti-IgM and anti-IgD. Proliferative responses to Cowan I in these depleted populations were about 20% that of unfractionated mononuclear cells. Depletion of T cells increased the mitogenic response to Cowan I and to the two antibody preparations, showing that they are T-independent mitogens. Depletion of IgD⁺ cells caused partial loss of mitogen-induced IgM ISC (22%-60% of unseparated controls) but no loss of IgG or IgA ISC. Depletion of IgM-bearing cells caused complete loss of IgM ISC, but no loss of IgG or IgA ISC. We previously demonstrated that anti-IgM antibody blocked mitogen induction of Ig secretion of these three classes in spleen cells, but only IgM secretion in blood mononuclear cells. Together, the results suggest that the majority of cells in normal blood responding to mitogens to mature to IgG or IgA production belong to IgM^?, IgD^? B cell subsets, in contrast to precursors of secreting cells for these isotypes in the spleen. Thus, these blood precursors appear to be more mature than the corresponding spleen cells.     

IL-4 and IL-2 upregulate the expression of antigen B7, the B cell counterstructure to T cell CD28: an amplification mechanism for T-B cell interactions.

We recently generated mAb 104 which is specific for the B cell activation antigen Ag B7. With this we studied the regulation of Ag B7 expression on normal tonsillar B lymphocytes as well as the activities of B7+ and B7- activated B cells. SAC and to a lesser extent anti-IgM antibody upregulated Ag B7 and this was further enhanced by IL-2 and most notably IL-4. Ag B7 was expressed on virtually all sIgG+ and sIgA+ B cells and approximately half of the sIgD+ and sIgM+ B cells. SAC-stimulated B7+ B cells proliferated and produced IgM, IgG and IgA in response to IL-2 and IgM and IgG in response to IL-4. SAC-stimulated B7- B cells proliferated and produced only IgM in response to IL-2 and IL-4. Considering that Ag B7 has recently been shown to be the counterstructure of the T cell CD28 and that CD28 triggering strongly enhances cytokine production by T cells, it is likely that the CD28/B7 interaction represents an important amplification phenomenon in T-B cell interaction leading to humoral immune responses. The preferential expression of Ag B7 on IgG and IgA committed cells suggests that CD28/B7 interaction may be more specific to secondary antibody responses provided by memory T and B cells.     

Selective enhancement of human IgE production in vitro by synergy of pokeweed mitogen and mercuric chloride

IgE production in vitro was investigated in cultures of human peripheral blood mononuclear cells (MNC) from non-atopic donors with pokeweed mitogen (PWM), mercuric chloride (HgCl2), or both. PWM alone induced a few IgE immunoglobulin secreting cells (ISC) detected by reverse plaque forming cells (PFC) and many IgG, IgM, and IgA PFC. HgCl2 alone failed to produce significant numbers of ISC of any class. PWM plus HgCl2 caused a selective increase of IgE PFC without affecting IgG, IgM, and IgA PFC. Co-cultures of B cells plus mitomycin C (MMC) treated T cells stimulated by PWM alone produced more IgG, IgM, IgA and IgE PFC than those of B cells plus T cells. However, PWM plus HgCl2 produced significantly more IgE PFC selectively in those cultures. This effect was observed in the cells of most of the donors, but a few donors showed different responses.     

Defective IL-6 secretion in HIV-infected haemophilia patients.

To study the role of IL-6 in HIV-induced B cell defects, in vitro B cell responses and IL-6 secretion were determined simultaneously in 67 haemophilia patients. Twenty-three patients were HIV- (Group 1), 27 HIV+ stage CDC II, III (Group 2), and 17 were HIV+ stage CDC IV (Group 3). Pokeweed mitogen (PWM) was used for T cell-dependent and Staphylococcus aureus Cowan I (SAC I) for T cell-independent B cell stimulation. B cell differentiation was assessed in a reverse haemolytic plaque assay and by ELISA determination of IgG and IgM in culture supernatants. An ELISA was used to measure IL-6 in plasma and culture supernatants. HIV- patients showed impaired immunoglobulin-secreting cell (ISC) responses after T cell-independent and T cell-dependent stimulation (P < 0.0001 and P < 0.01, respectively), whereas IL-6 secretion, IgM and IgG responses were comparable to those in healthy controls. HIV+ patients at stage CDC II, III or IV demonstrated significantly reduced mitogen-stimulated IL-6 secretion (P < 0.05, PWM; P < or = 0.001, SAC I) as well as impaired ISC and IgG responses (P < 0.01, PWM; P < or = 0.0001, SAC I). CDC IV patients showed reduced IgM responses in addition (P < 0.02, PWM; P < 0.0005, SAC I). Plasma IL-6 levels were elevated both in HIV+ patients (CDC II, III patients: 165 +/- 73 pg/ml, P < 0.005; CDC IV patients: 58 +/- 18 pg/ml, P < 0.0001) and in HIV- patients (283 +/- 65 pg/ml, P < 0.0001) which appeared to be a T cell effect induced by treatment with haemophilia factor concentrates. Our data provide evidence for different types of B cell deficiencies in HIV- patients (impaired ISC response only) and HIV+ patients (impaired ISC as well as IL-6 and IgM/IgG responses). The defective IL-6 secretion in HIV+ patients is likely to affect terminal B cell differentiation and this may explain the reduced immunoglobulin secretion in these patients in response to antigenic challenge.     

Disodium cromoglycate enhances ongoing immunoglobulin production in vitro in human B cells

The effect of disodium cromoglycate (DSCG) upon human immunoglobulin (Ig) isotypes and IgG subclasses production by purified B cells was studied. DSCG enhanced IgM, IgG1, IgG2, IgG3, IgG4 and IgA production in a dose-dependent fashion, while DSCG failed to induce IgE production at any concentrations tested by purified B cells. When B cells were separated into small resting and large activated B cells, DSCG failed to induce Ig production from small resting B cells in the presence or absence of Staphylococcus aureus Cowan strain I (SAC). In contrast, in large activated B cells DSCG significantly enhanced all types of Ig production (two-to threefold), especially IgG4 production (seven-to 11-fold), except IgE, which large B cells did not produce. The enhancement of IgG subclass production was not subclass switching, since DSCG failed to enhance IgG1 production in B cells depleted of surface IgG1+ cells (sIgG1+ cells). Similarly, DSCG did not enhance IgG2, IgG3 or IgG4 production from sIgG2-, sIgG3- or sIgG4- B cells, respectively, Interleukin-4 (IL-4) or interleukin-6 (IL-6) also enhanced Ig production except IgG4 from large activated B cells. The enhancing effect of DSCG was not mediated by IL-4 or IL-6 since anti-IL-4 or anti-IL-6 antibody failed to block the DSCG-induced enhancement. DSCG also enhanced IgG2 and IgM production from human B-cell lines GM-1500 and CBL, respectively. These results suggest that DSCG directly and preferentially stimulates activated B cells which are producing Ig and, in addition, enhances their Ig production.     

Human interleukin-5 induces staphylococcal A Cowan 1 strain-activated human B cells to secrete IgM

Studies on the role of human interleukin (IL)-5 in B cell growth and differentiation have yielded conflicting results. To clarify this issue, we studied the role of purified recombinant IL-5 on activated human B cells which were depleted of Tcells and adherent cells. Human IL-5 augments IgM secretion, but not IgG or IgA secretion of purified human B cells activated with staphylococcal A Cowan 1 strain (SAC). However, the period of B cell activation with SAC is critical for the B cell to respond to IL-5. After 24 h of SAC activation, human B cells are responsive to the IL-5 signal, but with longer periods of activation, IL-5 responsiveness diminishes. This may explain some of the previous conflicting results. The IgM enhancement was not seen when B cells were activated with pokeweed mitogen. In addition, human recombinant IL-4 synergized with IL-5 in augmenting IgM secretion by SAC-activated B cells, while IL-5 synergized with IL-2 to augment IgM, IgG and IgA secretion by SAC-activated B cells. As the purified IL-5 was derived from a COS-1 cell supernatant, and COS-1 cells secrete IL-6, we examined whether a polyclonal IL-6 antibody blocked the IgM-enhancing activity of IL-5. IL-6 antibody did not block the IL-5 enhancement of IgM secretion, but a monoclonal antibody to IL-5 inhibited the human IL-5 activity on human B cells. These results demonstrate that human IL-5 augments IgM secretion of SAC-activated human B-cells. In addition, this lymphokine synergizes with IL-4 and IL-2 in supporting Ig secretion.     

The effect of phenytoin in vitro on normal human mononuclear cells and on human lymphoblastoid B cell lines of different Ig isotype specificities

The anticonvulsant drug phenytoin, in less than cytotoxic concentrations, caused significant reductions in Ig secretion by unstimulated or EBV-stimulated normal MNC, as measured by PFC or secretion of Ig into the culture medium. Isotype-specific LBL varied in their sensitivity, the secretion of IgA (1 line) and IgG (3 lines) being reduced by phenytoin near therapeutic concentrations, whereas that of IgM (1 line) was resistant. Six-day exposure of MNC to phenytoin caused no selective depletion of or enrichment for B cells, monocytes or T cell subsets. The results suggest that the reduction in serum Ig levels reported in phenytoin-treated epileptic patients is, at least in part, due to a direct effect of the drug on the B lymphocyte. However, among EBV-activated normal MNC, those secreting IgA were no more sensitive to the drug than those secreting IgG or IgM, and other factors may, therefore, operate to cause the preferential reduction in serum IgA in phenytoin-treated patients.     

Interleukin-4 suppresses immunoglobulin production by peripheral blood lymphocytes of patients with common variable immunodeficiency (CVI) induced by supernatants of T cell clones.

Supernatants of both CD4+ and CD8+ alloreactive T cell clones induced IgM, IgG and IgA synthesis by peripheral blood lymphocytes (PBL) of healthy donors in vitro. These supernatants were also tested on their capacity to induce immunoglobulin production by PBL of four patients with CVI and one patient with CVI and thymoma. A low degree of IgM, IgG and IgA production was induced in one patient with CVI. In the patient with CVI and thymoma, induction of IgG and IgA synthesis was in the normal range, whereas IgM production was reduced. In the three other patients only a low production of IgM was induced. Interestingly, pre-incubation of the PBL for 24 h with interleukin-4 (IL-4) suppressed immunoglobulin production both by PBL of the patients with CVI and healthy donors. The strongest inhibitory effects were observed on IgA synthesis. These data indicate that B cells of three patients with CVI can not be induced to switch to IgG or IgA producing cells in vitro. In contrast, B cells of the patient with CVI and thymoma were able to respond to the relevant B cell growth and differentiation factors present in the T cell clone supernatants, suggesting that the T cells of this patient may fail to produce these factors. However, the proliferative responses of the T cells to phytohaemagglutinin (PHA), concanavalin A (Con A) and pokeweed mitogen (PWM), were normal in all five patients tested. In addition, the interleukin-2 (IL-2) and interferon-gamma (IFN-gamma) production by PBL of the five patients was also in the normal range. Although only a small number of patients was tested, these results support the view that defects in both regulatory T cell functions and/or intrinsic B cell defects may contribute to the pathogenesis of CVI.     

Is the activated T helper stimulus able to induce Ig isotype switching in cultured human B cells?

It Is confirmed that large amounts of IgM, IgG, and IgA areproduced when human B cells are cultured with T cells activatedby immobilized CD3 antibody (CD3 system). IL-2 was essential;lowerlevels of Ig production with different isotype ratios were obtainedif IL-4 or IL-6 replaced IL-2. Depletion of sIgG⁺ or sIgA⁺ cellsfrom the B population to be cultured markedly reduced productionof IgG or IgA. Culturesof B cells selected with the pan-B markersCD19, CD72, or CD21 contained similar levels of Ig of all threeisotypes, whereas B cells selected for sIgM or sIgD expressionproduced IgM but very little IgG or IgA indicating that littleisotype switching was occurring. Production of IgG or IgA fromcells expressing these isotypes was more efficient than productionof IgM from IgM⁺ IgD⁺ cells. These results are considered inthe light of the demonstration by others of the production ofmultiple isotypes from single sIgM⁺-selected B cells. Clonedhuman T cells from a single donor induced production of allthree isotypes, but the proportions varied indicating that thepotent T-B cell interactions inducing B cell activation mayoverride and conceal the operation of isotype specific cellinteractions. Some T clones used at an optimal dose were aseffective untreated as X-irradiated, whereas with other clonesmaximumIg production was not achieved without irradiation.     

Immunoglobulin and cytokine production by neonatal lymphocytes.

Growth and differentiation of cord blood B cells were studied using T cell-depleted populations. In the absence of in vitro activation, cord blood B cells proliferated in response to cytokines including interleukin-2 (IL-2) and interleukin-4 (IL-4); anti-mu-stimulated cord B cells had a lesser response to IL-2 than adult cells. IgM synthesis by cord blood B cells was enhanced by interleukin-6 (IL-6) and decreased by IL-2. In cultures activated by Staphylococcus aureus Cowan I (SAC), cord blood B cells produced lesser increases in IgM than adult B cells regardless of the cytokine added. Cord blood B cells produced no IgG or IgA with any cytokine preparation with or without SAC activation. Supernatants of cord blood T cells pulse-stimulated with phytohaemagglutinin and phorbol myristate acetate contained less IL-2 and IL-6 and had less growth and differentiation activity than adult T cell supernatants. The results confirm a limited cord blood B cell response and also suggest a limitation in production of B cell stimulatory lymphokines by cord blood T cells.     

Expression of IL-5 receptors on B cells: Role in mucosal B cell responses

IL-5 plays an important role in mucosal B cell responses. An in vitro model of IgA B cell differentiation, CH12LX, has been used to demonstrate that IL-5R are expressed on both IgM+ and IgA+ B cells and that IL-5 will enhance secretion of both IgM and IgA by CH12LX B cells, depending on the surface isotype of individual cells. Further, the enhanced secretion of Ig in response to IL-5 is associated with increased steady-state levels of Ig mRNA. Finally, normal Peyer's patch B cells are also capable of expressing IL-5R in response to appropriate stimuli (i.e. anti-IgM- or anti-IgD-dextran). Thus, mucosal B cell responses to IL-5 are brought about by interaction of IL-5 with specific receptors expressed on the surface of those cells.     

In vitro production of immunoglobulins of various classes and subclasses by cord blood B cells in African neonates: modeling and assessment of determination

Cord blood B cells obtained from neonates of healthy Senegalese mothers were assayed in vitro for their capacity to fully differentiate and secrete immunoglobulins (Ig) of various classes and subclasses. Stimulation of mononuclear cells with SAC particles or anti-micro antibodies in the presence of IL-4, or with IL-2 and IL-10 induced a strong production of IgG, provided that an additional CD40/CD40L signal was present, in contrast to adult cell cultures. Cord blood mononuclear cells differentially stimulated with various cytokines in order to lead to Ig heavy chain switching and production of the various classes/subclasses consistently produced IgG1, IgG3, IgG4, IgE and IgA. This system has been applied to immune cells from African neonates that have not been extensively studied previously. Estimation of Ig production as OD ratios could be applied to cultures where cord blood B cells are stimulated with defined antigens of human pathogens to which the fetus immune system was primed in utero.     

Distribution of IgM, IgA and IgG secreting cells in the tissues of normal and tumour-bearing mice.

The levels of IgM, IgA and IgG secreting cells were examined in control, Corynebacterium parvum-stimulated and tumour-bearing, normal and athymic (Nu/Nu) mice. The percentage of IgA to IgM or IgG secreting cells is relatively higher in peripheral blood than in the spleen or peritoneum of normal mice. Within tumours, irrespective of their degree of vascularization and immunogenicity, the pattern of Ig secreting cells in similar to that seen in peripheral blood and different from that in spleen and peritoneum even in athymic mice. Intraperitoneal injection of C. parvum changes the relative percentages of Ig secreting cells in the peritoneal cavity to resemble that seen in the peripheral blood and tumours. It appears that Ig secreting cells extravasate from peripheral blood in a non-isotype specific manner into sites of chronic stimulation.     

Immunoglobulin isotype production by cycling human B lymphocytes in response to recombinant cytokines and anti-IgM

Actively cycling populations of purified human tonsilar B lymphocytes were examined for their capacity to secrete IgM, IgA, IgE and IgG of all four subclasses in direct response to recombinant cytokines; in some experiments, monoclonal antibody to IgM (anti-mu) was included in order to explore the influence of antigen receptor ligation on immunoglobulin (Ig) production. Enhanced IgM release was seen on culture of the cycling cells with either interleukin-2 (IL-2), IL-4 or interferon-alpha (IFN-alpha). IL-2 and IFN-alpha also augmented IgA production, whereas IL-4 had no effect on this isotype. IL-4 did, however, encourage the production of the IgG subclasses IgG1, IgG2 and IgG3, while IL-2 augmented IgG1 and IgG3 release and IFN-alpha increased IgG1 levels. IgG4 production, and that of IgE, failed to be perturbed by any of the cytokines assayed. Neither IL-1 alpha, IL-1 beta, IL-5 nor IFN-gamma significantly altered the profile of Ig isotype release. When confronted with anti-mu, cycling B cells demonstrated a marked suppression in IgM production. Suppression could not be overcome by the addition to culture of the normally IgM-promoting IL-4. Concomitant with the reduction in IgM levels, an increase in IgG release was observed. This was comprised of elevations in IgG1 and IgG3. Although not influencing IgA release directly, anti-mu was found to promote increased IgA production in co-culture with either IL-2 or IFN-alpha. The findings are discussed in the context of recent findings on Ig isotype control in both human and murine systems.     

Surface immunoglobulin ligands and cytokines differentially affect proliferation and antibody production by human CD5+ and CD5- B lymphocytes.

Normal human peripheral blood B lymphocytes were separated into CD19+ CD5+ and CD19+ CD5- subsets by dual-color FACS sorting. In most experiments the cells were activated with Staphylococcus aureus Cowan I (SAC) and cultured in the absence or presence of recombinant human IL-1 alpha, IL-2, or IL-6, or combinations of these cytokines. Unstimulated CD5+ and CD5- B cells showed a comparable, low level of incorporation of [3H]thymidine into DNA. SAC stimulated proliferation of CD5+ and CD5- B cells, and this proliferation was augmented by IL-2 in the case of CD5- B cells. Anti-mu beads stimulated some proliferation of the CD5- subset and augmented SAC-induced proliferation of these cells. In contrast, anti-mu beads did not stimulate proliferation of the CD5+ subset and had no effect on SAC-induced proliferation of these cells. CD5+ B cells activated by anti-mu beads were stimulated to proliferate in the presence of IL-4, but not in the presence of IL-2. These observations support the interpretation that two signals are required for proliferation of CD5+ B cells. Using a two-step culture system, SAC activation itself did not induce Ig production by either subset of purified B cells. However, it primed the cells for antibody production in the presence of IL-2. IL-1 and IL-6 by themselves augmented antibody formation by these cells slightly, if at all. However, IL-6, and to a lesser extent IL-1, augmented antibody production in the presence of IL-2. Under the culture conditions used CD5- B cells produced IgM, IgG, and IgA whereas the CD5+ B cells produced almost exclusively IgM. The expression on B cells of surface activation markers was analyzed after culture for 2 days with SAC or anti-mu beads. In both subsets expression of Leu-23 and Leu-21 was increased, with some differences in intensity (Leu-23 greater in CD5+ cells, Leu-21 greater in CD5- cells). SAC increased IL-2R expression to a greater extent than anti-mu beads. In neither subset was expression of CD23 increased. These observations are discussed in the context of the possible role of the CD5+ subset of B lymphocytes as components of a system of natural immunity.     

Mucosal homeostasis: role of interleukins, isotype-specific factors and contrasuppression in the IgA response

Oral ingestion of antigen elicits immune responses at mucosal sites where humoral immunity is largely due to antibodies of the IgA isotype. This is often accompanied by suppression of systemic responses to the same antigen, a state termed oral tolerance. This IgA response is regulated by interactions between T cell subsets found at IgA inductive tissues, i.e., the gut-associated lymphoreticular tissue (GALT) or Peyer's patches (PP). PP T helper (Th) cells support IgA responses, and interleukins 5 (IL-5) and IL-6 can augment secretion of this isotype. Subsets of Th cells may also express Fc receptors for IgA (Fc alpha R) and secrete Fc alpha R as an IgA-binding factor (IBF alpha). Membrane-derived Fc alpha R is a glycoprotein of 38,000 M.W. and this molecule induces selective increases in IgA secreting cells (as determined by the ELISPOT assay) in PP B cell cultures. Fc alpha R+ T cell lines have been shown to secrete IBF alpha as well as IL-5 both of which promote IgA synthesis. Recombinant IL-5 (rIL-5) and rIL-6 induce IgA synthesis mainly by PP B cell blasts, and principally act on surface IgA-positive (sIgA+) B cells for these responses. Another form of mucosal regulation is provided by T contrasuppressor (Tcs) cells, which abrogate oral tolerance when adoptively transferred to mice and restore systemic responsiveness to the antigen sheep erythrocyte (SRBC). Tcs cells from mice systemically primed with SRBC support IgM and IgG subclass responses, while Tcs cells from orally primed mice support IgM, IgG subclass and IgA anti-SRBC responses. These Tcs cells are CD3+, CD4-, 8- and are antigen-specific. These regulatory cells may use the gamma-delta (gamma-delta) form of T cell receptor for antigen recognition.     

1,25-Dihydroxyvitamin D3-mediated inhibition of human B cell differentiation.

We have examined the mechanisms of 1,25-dihydroxyvitamin D3 (D3)-mediated inhibition of human B cell differentiation to immunoglobulin (Ig) secreting cells. B lymphocytes were purified from human tonsils and peripheral blood mononuclear cells. Mononuclear cells were separated into adherent cells and nonadherent cells. Cells were stimulated with Staphylococcus aureus Cowen I (SAC) or pokeweed mitogen (PWM) for 7 days and immunoglobulin production was measured by ELISA assay, 1,25-dihydroxyvitamin D3 was added at different times during cultures. 1,25-Dihydroxyvitamin D3, in a dose-dependent manner, inhibited both SAC and PWM-induced Ig production by mononuclear cells. The maximum inhibition was observed when 1,25-dihydroxyvitamin D3 was added at the beginning of culture, but inhibition could still be observed when 1,25-dihydroxyvitamin D3 was added on day 4 of cultures. The inhibitory effect of 1,25-dihydroxyvitamin D3 on Ig production was significantly greater on mononuclear cells than on nonadherent cells. Addition of in vitro purified IL-1 to nonadherent cells enhanced 1,25-dihydroxyvitamin D3-induced inhibition of Ig production. 1,25-Dihydroxyvitamin D3 also inhibited the expression of IL-2 receptors on B cells activated with SAC. 1,25-dihydroxyvitamin D3 did not inhibit Ig production by SAC preactivated B cell blasts in response to T cell supernatants. These data suggest that vitamin D3 inhibits Ig production by inhibiting IL-2 receptor expression on B cells and via its effect on adherent macrophages. Vitamin D3 does not influence the effect of differentiation factors on activated B cells that have already expressed growth/differentiation factor receptors.